Surface finishing of internal geometries for manifolds

Abstract

At least one implementation describes an electropolishing apparatus comprising a cathode having a width and a length. In at least one implementation, a dielectric is around cathode. In at least one implementation, the dielectric extends at least partially along the length of cathode. In at least one implementation, cathode comprises a superelastic metal alloy. In at least one implementation, the dielectric comprises a functionally graded material.

Description

CLAIM FOR PRIORITY

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 503,941, filed May 23, 2024, titled “SURFACE FINISHING OF INTERNAL GEOMETRIES FOR MANIFOLDS,” which is incorporated by reference in its entirety for all purposes.BACKGROUND

[0002] Process tools are used to perform treatments such as deposition and etching of film on semiconductor wafer substrates. These process tools can comprise a vacuum chamber in which chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes can be performed. Precision deposition processes such as ALD use precise delivery of precursor gases and vapors (collectively, process gases) into vacuum chamber through a gas distribution showerhead within vacuum chamber. These processes employ process gases comprising an inert carrier gas and one or more vaporized precursor substances. Process gas can be formed by vaporizing one or more liquid precursors and then mixing vapors with carrier gas.

[0003] Ultra-high purity (UHP) (e.g., >99.999% pure, or 5N purity) process gases can be exclusively employed in some semiconductor manufacture processes. Such high purity gases risk contamination by delivery systems for semiconductor process tools if gases flow within conduits that are not highly polished to UHP standards. While great care can be taken to purge gas delivery lines and distribution manifolds to remove residual gases and vapors between process cycles, residual contaminants can remain on inadequately polished interior surfaces of tubing and manifold flow paths. Even small degrees of surface roughness can provide microscopic pores and cracks within which gases can adsorb or precursor vapors can condense. These can leach into fresh UHP gases employed in subsequent processes. As many gas delivery lines and manifolds can be manufactured for non-UHP applications, gas delivery systems meeting UHP standards are expensive and are performed in highly specialized fabrication facilities. Due to difficulties in polishing interior surfaces of curved tubing and gas lines to UHP standards, a reliable and efficient electropolishing apparatus for highly curved metal tubing is desired.BRIEF DESCRIPTION OF DRAWINGS

[0004] Material described herein is illustrated by way of example and not by way of limitation in accompanying figures. For simplicity and clarity of illustration, elements illustrated in figures are not necessarily drawn to scale and exact locations. For example, dimensions of some elements can be exaggerated relative to other elements for clarity. Also, various physical features can be represented in their simplified “ideal” forms and geometries for clarity of discussion, but it is nevertheless to be understood that practical implementations can only approximate illustrated ideals. For example, smooth surfaces and square intersections can be drawn in disregard of finite roughness, corner-rounding, and imperfect angular intersections characteristic of structures formed by nanofabrication techniques. Further, where considered appropriate, reference labels have been repeated among figures to indicate corresponding or analogous elements.

[0005] FIG. 1 illustrates a cross-sectional view of a metal tubing internal electropolishing process, according to at least one implementation of disclosure.

[0006] FIG. 2 illustrates a portion of an electropolishing apparatus according to at least one implementation of disclosure.

[0007] FIG. 3A illustrates a 3D profile view of cathode, according to at least one implementation of disclosure.

[0008] FIG. 3B illustrates a cross-sectional view of cathode assembly shown in FIG. 3A, according to at least one implementation of disclosure.

[0009] FIG. 3C illustrates a top 3D view of a cathode assembly, according to at least one implementation of disclosure.

[0010] FIG. 3D illustrates a cross-sectional view of cathode assembly shown in FIG. 3C, according to at least one implementation of disclosure.

[0011] FIG. 4 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0012] FIG. 5 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0013] FIG. 6 illustrates a cross-sectional view of a cathode assembly, comprising a sheath and a cathode centered within the sheath, in accordance with at least one implementation.

[0014] FIG. 7 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0015] FIG. 8A illustrates an end-on view of a cathode assembly, according to at least one implementation of disclosure.

[0016] FIG. 8B illustrates an end-on view of a cathode assembly, according to at least one implementation of disclosure.

[0017] FIG. 8C illustrates a cross-sectional view of cathode assembly shown in FIG. 8A, according to at least one implementation of disclosure.

[0018] FIGS. 9A-9D illustrate an exemplary method for operating an electropolishing apparatus according to at least one implementation of disclosure.DETAILED DESCRIPTION

[0019] Disclosed herein is an electrochemical apparatus for polishing interior surfaces of metal tubing having a tortuous path, such as sharp bends, branching, and changes in diameter, in accordance with at least one implementation. In at least one implementation, electrochemical polishing apparatus comprises a cathode embedded within a sheath comprising a dielectric material. In at least one implementation, cathode can be an elongated cylindrical rod, a solid or stranded wire or other suitable elongated structure, comprising a conductive material. In at least one implementation, non-conductive sheath may extend at least a portion of length of cathode. In at least one implementation, a sheath may cover a portion of an electrochemically active surface of cathode. In at least one implementation, a suitable sheath can comprise multiple separate segments that are distributed as spacers along the length of a cathode. In at least one implementation, a suitable dielectric sheath may prevent cathode held within it from contacting an inner wall of a tube undergoing internal electropolishing (e.g., an anode). Such contact may potentially create a short circuit between cathode and anode. In at least one implementation, a suitable sheath can comprise a porous structure. In at least one implementation, a cathode comprises a superelastic alloy to enable cathode to assume curvature (e.g., a bend) of a section of metal tubing. In at least one implementation, the superelastic nature of alloy material can enable such a cathode to assume its original straight shape without plastic deformations (e.g., permanent kinking) when bending stress is relieved. In at least one implementation, dielectric material can possess engineered external geometry and internal pore structures that can be fabricated by additive manufacturing processes such as three-dimensional (3D) printing.

[0020] Here, “dielectric” can generally refer to a sheath (e.g., a continuous cylindrical sheath) that can extend along the length of a rod-shaped cathode. In at least one implementation, the sheath can guide cathode through a tortuous path that, for example, can comprise one or more 90-degree bends along a section of metal tubing. In at least one implementation, the dielectric sheath can be engineered to hold cathode substantially at the center of metal tubing while cathode advances or retreats within metal tubing. In at least one implementation, the dielectric sheath can be engineered to enable sufficient stiffness to maintain radial position of cathode while cathode follows sharp bends of metal tubing. In at least one implementation, the dielectric sheath can also have sufficient flexibility to bend at sharp bends of tubing, yielding to lateral forces during contact with curved portions of interior wall.

[0021] In at least one implementation, a dielectric sheath, such as a jacket, beads, or spacers around cathode can prevent mechanical contact between cathode and interior wall of tubing, as noted previously. In at least one implementation, a dielectric sheath can mechanically center cathode within interior of metal tubing, holding cathode substantially equidistant from interior wall of metal tubing. In at least one implementation, a dielectric sheath can guide cathode substantially along central axis of both straight, curved and branched sections of metal tubing.

[0022] In at least one implementation, a suitable dielectric comprises a material having a functionally graded stiffness, for example, exhibiting a radial stiffness gradation. In at least one implementation, a radial stiffness gradient can exhibit maximal stiffness near cathode, while stiffness can decrease along radial dimension. In at least one implementation, a suitable stiffness gradient can be gradual. In at least one implementation, a suitable stiffness gradient can be stepped (e.g., a sudden change in stiffness at a distance from center). In at least one implementation, a suitable dielectric can be a porous sheath, whose porosity can be radially uniform, whereas material can also be compositionally graded to exhibit greatest stiffness at center and least stiffness at periphery. In at least one implementation, a stiffness gradient can result from a radial distribution of Young's modulus, for example, of material composition.

[0023] Reducing stiffness from center to periphery can optimize mechanical resistance of porous dielectric sheath to lateral forces resulting from contact with interior wall of tubing. This effect can be advantageous as porous dielectric sheath encounters bends in tubing, where lateral forces can be greatest. In at least one implementation, diminishing gradation of stiffness can be engineered to enable porous dielectric sheath to readily bend at sharp angles (e.g., 90 degrees), while substantially maintaining cathode along central axis of metal tubing. For example, a more porous exterior of a porous sheath can yield more readily to lateral forces encountered at bends of tubing by partially collapsing to yield at curvature of bends. At the same time, an inner portion of porous sheath can exhibit greater rigidity. In at least one implementation, a more rigid center can deform axially to bend, but exhibit slight radial deformation. In at least one implementation, a structural porosity gradient can be accompanied by a compositional gradient where the Young's modulus is varied.

[0024] In at least one implementation, maintenance of cathode at center of tubing where it is substantially equidistant from interior wall can be used to ensure uniform radial current density from all points of interior wall to cathode within electropolishing zone. In at least one implementation, if the cathode is off-axis (e.g., moved away from central axis of tubing and non-equidistant), a radial distribution of current distribution between cathode and interior wall of tubing will not be angularly uniform. Thus, a portion of interior wall that is closer to cathode can be over polished, resulting in pitting or recessing, while a more distal portion of interior wall can be under polished, resulting in a non-uniform UHP finish on interior wall. In at least one implementation, a radial symmetry of current distribution within electropolishing zone can ensure uniform polishing of interior wall as cathode is translated axially within tubing.

[0025] In at least one implementation, a suitable dielectric material can be structured to promote maximum mass transfer of electrolytes to and from cathode. In at least one implementation, a porous sheath can comprise a continuous body extending along cathode, covering substantially the entire length of cathode. In at least one implementation, the porous sheath comprises substantially straight pores and a uniform radial porosity distribution, where pores can extend perpendicular to cathode. In at least one implementation, pores can extend obliquely to cathode. In at least one implementation, where porosity is uniform, a suitable dielectric material can be compositionally graded to imbue a radial stiffness gradient. In at least one implementation, pores can be engineered to have desired diameters and densities for optimal mass transfer of electrolytes to and from cathode.

[0026] In at least one implementation, a cathode can be translated within tubing at a rate enabling convection of electrolyte into and out of openings, spaces or pores within sheath. In at least one implementation, a cathode can be set into reciprocating motion to agitate electrolyte within a piece of tubing undergoing internal electropolishing. In at least one implementation, the exterior surface (or outer portion) of a sheath can comprise epi-structures such as raised mesas, bumps or fibers. In at least one implementation, epi-structures can be engineered on an exterior surface of a sheath to minimize mechanical contact of a sheath with interior wall of tubing, while permitting substantial exposure of interior wall to electrolyte. In at least one implementation, epi-structures such as mesas and bumps can be shaped as miniature scoops. In at least one implementation, scoop-shaped epi-structures can have concave faces to direct flow of electrolytes into pores of a sheath comprising such epi-structures, when such as sheath is translated within a piece of tubing. In at least one implementation, pores can be oriented obliquely within a porous sheath, where a first subset of pores can be oriented oppositely to a second subset of pores. In at least one implementation, obliquely opposed pores can intersect at surface of a cathode, enabling flow of bulk electrolyte through pores as cathode is moved along tubing in one direction or other. In at least one implementation, pore density can be adjusted to maximize exposed surface area of cathode. In at least one implementation, scoop-shaped bumps at entrance to an individual pore can enhance convective flow of electrolytes into pore. In at least one implementation, the shape of scoops can be engineered to maximize flow into each pore.

[0027] In at least one implementation, a suitable dielectric comprises independent spacers distributed at intervals along a length of a cathode. In at least one implementation, multiple spacers can be separated from one another by bare sections of cathode. In at least one implementation, spacers can comprise annular shapes, cross shapes, and star shapes. In at least one implementation, as spacers can enable a substantial portion of cathode surface to be exposed to electrolyte, spacers can comprise solid structures that do not have an internal porosity. In at least one implementation, spacers can have openings that enable axial flow of electrolytes across structure. For example, in at least one implementation, spacers can have a star shape, possessing spaces between arms to permit axial flow of electrolytes across thickness of a spacer. In at least one implementation, spacers can have openings permitting electrolytes to flow across the structure.

[0028] In at least one implementation, cathode can comprise a superelastic metal alloy for following tortuosity of a piece of tubing. In at least one implementation, superelastic property of cathode can impart a shape memory so that cathode can revert to its original form. In at least one implementation, default (e.g., original) shape of cathode can be straight, curved, spiral shaped, etc. In at least one implementation, a superelastic alloy employed for the cathode can be compositionally tuned to undergo a solid-state phase transition from a rigid crystalline phase to a superelastic crystalline phase (e.g., a martensite / austenite phase transition) at a specified temperature, such as operating temperature of electropolishing process. For example, an electropolishing process may be performed at room temperature (e.g., 20° C.) or at elevated temperatures. In at least one implementation, a phase transition can be triggered by relatively low levels of mechanical stress imposed on the cathode. In at least one implementation, stress can be induced by axial and transverse (e.g., lateral) forces acting on the cathode as it encounters bends in tubing. In at least one implementation, stress threshold can be tuned by adjustment of ratio of elements composing alloy. In at least one implementation, alloy can comprise nickel and titanium (e.g., Nitinol). In at least one implementation, adjustment of nickel and titanium composition ratio can determine transition temperature and threshold of mechanical stress to induce transition. In at least one implementation, the shape memory property of the cathode material enables it to revert to its original shape when relieved of mechanical stress.

[0029] In at least one implementation, a superelastic metal alloy is employed as a cathode material can enable the cathode to have a shape memory. The shape memory effect can enable the cathode to straighten and resume its original shape after passing through a bend of tubing. In at least one implementation, stress experienced by the cathode when passing through a bend is relieved when it passes into a straight portion of tubing following the bend. In at least one implementation, the cathode can assume a straight shape, for example, without plastic deformation (e.g., permanent kinks). When encountering a following bend, the cathode may experience mechanical stress anew, triggering a transition to a superelastic state (e.g., austenite phase). In at least one implementation, after electropolishing process is complete, the cathode can be withdrawn from tubing and revert to its original shape (e.g., as a straight rod) without plastic deformation. In at least one implementation, other cathode form factors can include non-linear shapes, such as simple curved shapes, serpentine shapes, etc.

[0030] In at least one implementation, in addition to having property of superelasticity, the cathode can have sufficient conductivity and chemical inertness to be suitable as an electrode employed for electropolishing. In at least one implementation, to fulfill this property, a superelastic alloy can comprise suitable alloy systems such as, but not limited to, nickel and titanium, copper and zinc, copper, zinc and aluminum, gold, copper, and zinc, and gold and cadmium.

[0031] In at least one implementation, in following description, well-known methods and devices are shown in block diagram form, rather than in detail, to avoid obscuring present disclosure. Reference throughout this specification to “an implementation” or “one implementation” or “some implementations” means that a particular feature, structure, function, or characteristic described in connection with implementation is included in at least one implementation of disclosure. Thus, appearances of phrase “in an implementation” or “in one implementation,”“in at least one implementation,” or “some implementations” in various places throughout this specification are not necessarily referring to same implementation of disclosure. Furthermore, particular features, structures, functions, or characteristics can be combined in any suitable manner in one or more implementations. For example, a first implementation can be combined with a second implementation anywhere particular features, structures, functions, or characteristics associated with two implementations are not mutually exclusive.

[0032] Here, “coupled” and “connected,” along with their derivatives, can be used to describe functional or structural relationships between components. These terms are not intended as synonyms for each other. Rather, in particular implementations, term “connected” can be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. Here, “coupled” can be used to indicate that two or more elements are in either direct or indirect (with other intervening elements between them) physical, electrical or in magnetic contact with each other, and / or that two or more elements co-operate or interact with each other (e.g., as in a cause-and-effect relationship). Here, “coupled” can also generally refer to direct attachment of one electronic component to another. An electric or magnetic field can couple one component to another, where field is controlled by one component to influence other in some manner.

[0033] Here, “over,”“under,”“between,” and “on” can generally refer to a relative position of one component or material with respect to other components or materials where such physical relationships are noteworthy. Unless these terms are modified with “direct” or “directly,” one or more intervening components or materials can be present. Similar distinctions are to be made in context of component assemblies. As used throughout this description, and in claims, a list of items joined by term “at least one of” or “one or more of” can mean any combination of listed terms.

[0034] Here, “adjacent” can generally refer to a position of a thing being next to (e.g., immediately next to or close to with one or more things between them) or adjoining another thing (e.g., abutting it).

[0035] Here, “electrochemical” can generally refer to chemical reactions taking place at an electrified working electrode immersed in an electrolyte containing elements or compounds that are susceptible to undergo reactions when encountering electrified electrode. In at least one implementation, if electrified with a positive potential relative to a referenced electrode in same electrolyte, working electrode is referred to as an anode. In at least one implementation, if electrified by a negative potential relative to a referenced electrode in same electrolyte, electrode is referred to as a cathode.

[0036] Here, “cathode” can generally refer to an electrochemical electrode having a negative potential relative to a referenced electrode in same electrolyte.

[0037] Here, “cathode assembly” can generally refer to an assembly of objects including a cathode, as defined herein. In at least one implementation, a cathode assembly includes a dielectric sheath or plurality of dielectric spacers, both of which are defined below, in which a rod-shaped cathode can be embedded.

[0038] Here, “electrolyte” can generally refer to a liquid solution comprising ionized molecules or atoms.

[0039] Here, “electropolish” can generally refer to an electrochemical process for electrochemically planarizing metal surfaces by partial dissolution of microscopic structures related to surface roughness.

[0040] Here, “elastic metal alloy” or “superelastic metal alloy” can generally refer to a metal alloy system that has property of elasticity (or superelasticity), or property to undergo large amounts of elastic strain when under mechanical stress.

[0041] Here, “tubing” can generally refer to a metallic tubular pipe or conduit that has diameter ranging from approximately 1 mm to approximately 5 cm but can be greater. In at least one implementation, wall of tubing can comprise aluminum, stainless steel alloys, copper, or brass. In at least one implementation, metal tubing can be distinguished from pipe in that it is generally not threaded. In at least one implementation, coupling between sections of metal tubing and components such as valves, mass flow controllers, gauges and the like can be done by use of compression fittings.

[0042] Here, “bend” can generally refer to sharp curvature in a section of metal tubing. In at least one implementation a bend can have a 90-degree angle.

[0043] Here, “functionally graded material” can generally refer to a material having a graded property, such as stiffness. In at least one implementation, gradation of property is generally engineered by special manufacturing methods, such as layer-by-layer building methods as performed by 3D printing, for example.

[0044] Here, “opening” can generally refer to a hole or an aperture in surface of an object.

[0045] Here, “pore” can generally refer to a small opening that extends a distance that can be many times its characteristic diameter below surface of an object. In at least one implementation, pores can be on order of a millimeter in diameter to microscopic. Path along which a pore extends can be straight or tortuous.

[0046] Here, “pore network” can generally refer to a network of pores within a material. Individual pores within the network can be interconnected, forming a continuous network. In at least one implementation, pore network can be analogous to a root system of a tree, for example.

[0047] Here, “fractal” can generally refer to group of physically connected objects, Examples of common fractal structures are tree branches, leaf veins, circulatory systems in animals, dendritic networks such as nerve and root networks, rock outgrowths, snowflakes, and ice crystals. In at least one implementation, fractal objects have a fundamental geometric shape, which is repeated by attaching smaller versions of same shape (secondary size) to fundamental object, then attaching, or outgrowth of, still smaller versions (e.g., tertiary size) to secondary objects, ad infinitum. In at least one implementation, characteristic dimensions of successively smaller objects scale down in size in same mathematic proportions with each level of reduction.

[0048] Here, “fractal network” can generally refer to a network of interconnected branches, branches having derived from fractal branching from larger members of network.

[0049] Here, “sheath” can generally refer to a coat or containing structure in which an object inserts or is embedded. In at least one implementation, a sheath is a dielectric material shaped cylindrically in which a rod-shaped cathode is embedded.

[0050] Here, “epi-structure” can generally refer to one or more structures on a surface of an object, such as bumps, mesas or filaments on a sheath.

[0051] Here, “bump” can generally refer to a protrusion above the surface of an object. In at least one implementation, bump can have a generally rounded or hemispherical shape. Mesas, which are similar protrusions having flat tops, can be bumps.

[0052] Here, “filament” can generally refer to a fibrous epi-structure, such as a fiber or bristle sheath or portion of a sheath.

[0053] Here, “spacer” can generally refer to a disc-like section of dielectric that is distributed along the length of a rod-shaped object. In at least one implementation, a spacer is a disc-shaped dielectric structure that can be distributed along the length of a rod-shaped cathode.

[0054] Unless otherwise specified in explicit context of their use, terms “substantially equal,”“about equal,” and “approximately equal” can generally mean that there is no more than incidental variation between two things so described. In art, such variation is typically no more than + / −10% of referred value.

[0055] FIG. 1 illustrates a cross-sectional view of a metal tubing internal electropolishing process comprising electropolishing apparatus 100, according to some implementations of disclosure. In at least one implementation, electropolishing apparatus 100 can be part of a larger electropolishing apparatus (not shown), for example, comprising an automated cathode insertion mechanism and / or agitation mechanism. In at least one implementation, electropolishing apparatus 100 comprises cathode 102. In at least one implementation, cathode 102 has a circular cylindrical form factor, but can have other suitable cross-sectional form factors, such as elliptical or rectilinear cross sections. In at least one implementation, cathode 102 can comprise a large gauge (e.g., American wire gauge (AWG) #10 (f~2.5 mm) or larger) solid or stranded wire. In at least one implementation, cathode 102 can be a wire spiral. In at least one implementation, cathode 102 can be an elongated (circular) rod-shaped electrode, having a minimum aspect ratio of at least 2:1 (e.g., having a length that is at least two-fold its width, or diameter).

[0056] In at least one implementation, cathode 102 comprises a suitable superelastic metal alloy. A superelastic metal alloy is an alloy that undergoes a solid-state phase transition from a normal metal to a highly elastic metal (e.g., a martensitic to austenitic phase transition) when stressed with small lateral or longitudinal forces, as described below. In at least one implementation, cathode 102 comprises a metal alloy that undergoes a superelastic solid phase transition at an operating temperature of electropolishing process. For example, an operating temperature of a particular electropolishing process may be approximately 15° C. Another electropolishing process may be performed at higher temperatures. A wire or rod-shaped cathode 102 comprising a superelastic metal may easily bend and snake along a tortuous path within tubing 106 at or above operating temperatures employed in the electropolishing process, enabling electropolishing of metal tubing having one or more bends, as described below. In at least one implementation, tubing bends can have bend angles ranging from obtuse to acute angles (e.g., to less than 90 degrees).

[0057] In at least one implementation, spacers 104 may be included to center cathode 102 within tubing 106 (e.g., workpiece), such that cathode 102 is substantially aligned with a central axis of tubing 106. Centering cathode 102 along center of tubing 106 may ensure that cathode 102 remain as equidistant from interior wall 108 as practicable to enable a symmetrical electropolish of interior wall 108. For example, an off-center cathode may cause an over-polish (e.g., dissolution) of nearby portions of interior wall 108, while more distal portions may be under-polished. In at least one implementation, spacers 104 comprise a dielectric electrically insulative material, described below. In at least one implementation, spacers 104 are disc shaped. In at least one implementation, spacers may be bead shaped. In at least one implementation, spacers 104 can be immovably affixed on cathode 102 at desired intervals.

[0058] In at least one implementation, spacers 104 can comprise one or more dielectric materials, such as one or more suitable organic polymers. In at least one implementation, spacers 104 can comprise a compliant periphery to allow cathode 102 to follow a tortuous path within tubing 106 and a relatively hard inner region to maintain a minimum distance between cathode 102 and interior wall 108 of tubing 106. As tubing 106 serves as an anode in an electropolishing process, contact or close proximity of cathode 102 and interior wall 108 is to be avoided to prevent a short circuit or electrical arcing. In at least one implementation, a tortuous path may include bends, branch points, as well as increases and decreases in diameter of tubing 106. In at least one implementation, tubing 106 comprises a metal such as, but not limited to, stainless steel alloys, aluminum alloys or copper alloys. In at least one implementation, tubing 106 comprises interior wall 108 that is subject to the electropolishing process. Dielectric spacers 104 are shown in FIG. 1 for exemplary purposes. In at least one implementation, electropolishing apparatus 100 may comprise a continuous porous dielectric sheath around cathode 102 that provides a similar function as spacers 104, as described below. In at least one implementation, such a dielectric sheath may have similar mechanical properties as spacers 104.

[0059] In at least one implementation, cathode 102 can be coupled to a mechanical drive mechanism (not shown) that can push cathode 102 into tubing 106 and withdraw it when the electropolishing process is complete. In at least one implementation, cathode 102 may be part of a stand-alone apparatus that is manually controlled. In at least one implementation, electrolyte 110 may be translated back and forth within tubing 106 by reciprocating motion, to agitate electrolyte 110 within tubing 106. For example, horizontal arrows illustrated in FIG. 1 indicate possible movement of cathode 102 along tubing 106 suitable forms of agitation may be employed. In at least one implementation, cathode 102 can be electrically coupled to a potentiostat or other suitable power supply (not shown) that can provide electrical power to cathode 102 for driving an electropolishing process.

[0060] In at least one implementation, tubing 106 can be filled with a suitable electropolishing electrolyte 110 (light grey shading). In at least one implementation, electrolyte 110 may be pumped into interior of tubing 106. In at least one implementation, tubing 106 and electropolishing apparatus 100 can be immersed within a vat or bath of electrolyte 110. In at least one implementation, movement of cathode 102 can be in either direction. In at least one implementation, spacers 104 can have openings that permit flow of electrolyte 110 across spacers 104 (indicated by arrows) as cathode 102 is advanced or retracted along tubing 106.

[0061] In at least one implementation, as cathode 102 is electrified to a desired potential or set of potentials by a power supply (not shown), an electropolishing of interior wall 108 can ensue. In at least one implementation, a power supply can be a potentiostat (e.g., a power supply able to maintain a constant electrochemical potential at a working electrode when coupled to a reference electrode), which can impose a negative potential (e.g., a voltage) on cathode 102 and a positive potential on tubing 106 (anode). In at least one implementation, potentials can be referenced to a reference electrode (not shown) immersed in same electrolyte. In at least one implementation, electropolishing process dissolves asperities and other salient features on the surface of interior wall 108, evening out surface topography toward a flat surface. In at least one implementation, interior wall 108 can have an initial measurable mean (e.g., average) surface roughness Ra that has a value exceeding standards for ultra-high purity (UHP) tubing (e.g., Ra>1 μm). In at least one implementation, an industrial UHP standard for surface roughness of electropolished interior surfaces of stainless-steel tubing employed in gas handling service can be Ra=0.25 μm or less.

[0062] In at least one implementation, electrochemical conditions, including composition of electrolyte 110 and voltage applied to cathode 102, can be chosen to optimize electropolishing process. In at least one implementation, electropolishing process can adopt electropolishing conditions for stainless steel alloys set forth by the American Society for Testing and Materials (ASTM) standard B912. In at least one implementation, an electropolishing voltage maintained by a potentiostat or power supply can determine a current density between interior wall 108 and cathode 102. In at least one implementation, the current density can be optimized to obtain target value of Ra within an acceptable time frame. In at least one implementation, an electropolishing process can comprise multiple voltages applied in succession to cathode 102.

[0063] While a straight segment of tubing 106 is shown, in at least one implementation, tubing 106 can have sharp bends and other curvature.

[0064] FIG. 2 illustrates electropolishing apparatus assembly 200, comprising cathode 202 within sheath 204, in accordance with at least one implementation. In at least one implementation, sheath 204 comprises a porous flexible dielectric material. In at least one implementation, sheath 204 has a substantially cylindrical form factor to fit into cylindrical tubing. In at least one implementation, sheath 204 has a rectilinear form factor, such as a square cross section, for insertion into square tubing. In at least one implementation, sheath 204 is functionally graded radially, such that a peripheral portion of sheath 204 is more compliant than an a relatively hard interior portion of sheath 204. In at least one implementation, electropolishing apparatus assembly 200 can be inserted into tubing 206. In at least one implementation, tubing 206 has multiple 90-degree bends 208, 210, 212, 214, 216 and 218, as shown in FIG. 2.

[0065] To facilitate contact of electrolyte with interior wall 220 of tubing 206, in at least one implementation, sheath 204 comprises a plurality of epi-structures. In at least one implementation, epi-structures comprise raised regions, such as mesas or bumps on an exterior surface of sheath 204 (not shown), described below. In at least one implementation, epi-structures comprise fibers or bristles, described below. In at least one implementation, epi-structures on sheath 204 can be engineered to minimally contact interior wall 220 to enable electrolyte to readily flow or diffuse to interior wall 220. In at least one implementation, electrolytes can also infiltrate into pore structures within sheath 204 to reach cathode 202. In at least one implementation, pore structures within sheath 204 can be engineered to maintain lines-of-sight (LOS) between cathode and interior wall 220. Lines of sight (LOS) can be straight direct paths for current flow between cathode 202 and interior wall 220. Efficacity of electropolishing of complex workpieces is greatest if a large degree of line of sight is provided between cathode and target surfaces of an anode.

[0066] In at least one implementation, sheath 204 can maintain cathode 202 substantially along the central axis of metal tubing 206. As noted above, centering of cathode 202 can maintain uniform electropolishing of interior wall 220. In at least one implementation, sheath 204 can be functionally engineered for optimal flexibility and stiffness, as described below.

[0067] In at least one implementation, cathode 202 can comprise a metallic alloy capable of a superelastic phase transition (e.g., a transition from a martensitic phase to an austenitic phase) at operating temperature of an electropolishing process. As noted above, a superelastic phase transition of cathode 202 can enable cathode 202 to pass through bends 208-218 without undergoing plastic deformation (e.g., a shape memory effect; no permanent kinking). In at least one implementation, small axial and lateral stress forces can be experienced by cathode 202 when passing through bends 208-218, triggering a superelastic phase transition, rendering portions of cathode 202 supple when transiting through bends 208-218. For example, superelasticity can enable portions of cathode 202 transiting within bends 208-218 to bend elastically. In at least one implementation, 90-degree or larger bending of cathode 202 can be enabled, followed by resuming its original shape without plastic deformation (e.g., kinking) within straight segments of tubing 206. Elastic strain can be high (e.g., 200,000 parts per million (ppm), or approximately 20% of its initial length with relatively low stress of several megapascals). In at least one implementation, a metal alloy material of cathode 202 may be engineered to undergo a superelastic phase transition when encountering lateral and longitudinal forces generated by contact between sheath 204 and interior wall 220 during insertion of electropolishing apparatus assembly 200 into tubing 206. In at least one implementation, a metal alloy material of cathode 202 may be engineered to undergo a superelastic phase transition at temperatures at or near operating temperatures of one or more electropolishing processes.

[0068] During insertion or withdrawal of cathode 202, segments of cathode 202 passing out of bends 208-218, portions of cathode 202, such as bent portion 222 within bend 218, may no longer encounter significant mechanical forces within straight portions of tubing 206. Thus, while within straight portions of tubing 206 (e.g., straight tubing segment 224 following bent portion 222), cathode 202 can return to a non-elastic state (e.g., a transition from austenitic phase to martensitic phase). While some ordinary (e.g., non-superelastic) stainless steel or non-ferrous alloys can have sufficient flexibility to bend with sharp angles such as 90-degree angles of bends 208-218, they can experience significant plastic deformation. For example, a straight wire formed from spring steel may not recover its original shape after bending through first bend encountered (e.g., bend 208) when inserted into tubing 206. Such a cathode can become permanently kinked, and therefore difficult to advance further or retract within tubing 206.

[0069] FIG. 3A illustrates a 3D profile view of cathode assembly 300, comprising sheath 302 and cathode 304 embedded within sheath 302, in at least one implementation. In at least one implementation, sheath 302 has a circular cylindrical shape. In at least one implementation, cathode 304 extends substantially along a central axis of symmetry of sheath 302.

[0070] In at least one implementation, sheath 302 can comprise a polymeric material having a desired stiffness. In at least one implementation, polymeric material can have a uniform composition throughout the entire length of sheath 302. In at least one implementation, a polymeric material can comprise a fluoropolymer, such as polytetrafluoroethylene (PTFE), or a hydrocarbon such as polypropylene, a polyester such as polyethylene terephthalate (PET), a polyamide such as Nylon, or other suitable polymers. In at least one implementation, sheath 302 can be formed directly on cathode 304 or formed independently, for example by a 3D printing process. In at least one implementation, sheath 302 can have a radially graded composition for fine tuning stiffness. In at least one implementation, sheath 302 can comprise a radially graded polymer blend. In at least one implementation, manufacture of such a structure can comprise formation of multiple layers comprising a linear or non-linear gradation of blend composition. In at least one implementation, by such layering of compositionally distinct polymer blends, a linear or non-linear radial gradient of Young's modulus can be engineered. In at least one implementation, a stiffness gradient can be optimized by adjustment of number of layers and layer thicknesses, as well as material composition.

[0071] In at least one implementation, sheath 302 comprises a plurality of pores 306 (illustrated as white circles).

[0072] Mass transfer limitations may reduce overall electrochemical reaction rates and cause overpotentials to develop non-uniformly over cathode 304. Overpotentials may lead to non-uniform electropolishing of interior wall 312 due to non-uniform electrochemical reaction rates. In at least one implementation, pores 306 can have any suitable structure that enables rapid infusion of electrolytes through pores 306 to reach cathode 304 and for electrolytes to rapidly diffuse away from cathode 304. In at least one implementation, pores 306 can be straight pores that extend substantially linearly from surface 310 to cathode 304, where diameters are constant along the pore path. In at least one implementation, individual pores 306 that extend into interior of sheath 302 to cathode 304 along substantially linear paths can allow direct line of sight (LOS) between cathode 304 and interior wall 312 (denoted as a dashed line) of a tubing workpiece. In at least one implementation, pores 306 provide diffusion paths of electrolytes to and from cathode 304 during operation of cathode assembly 300. For enabling efficient electropolishing processes, pores 306 may be engineered to provide optimal line-of-sight diffusion paths between cathode 304 and interior wall 312. In at least one implementation, pore sidewall angles may be adjusted to form conical pores of decreasing diameter toward cathode 304. Conical pores can funnel diffusing electrolyte current from extended regions of interior wall 312 to cathode 304 in order to compensate pore diffusion limitations of mass transfer between cathode 304 and interior wall 312.

[0073] In at least one implementation, pores 306 are shown to have relatively large diameters of several millimeters (mm). In at least one implementation, pores 306 are also shown to be distributed in a regular pattern with a relatively low pore density (e.g., number of pores per unit area of surface 310). In at least one implementation, pores 306 can have submillimeter diameters and can be distributed in random patterns that have a higher pore density. In at least one implementation, pore density can also be optimized by adjustment of pore void volume relative to surrounding filled volume of sheath 302. In at least one implementation, pore density is adjusted to maximize cathode surface area while retaining mechanical integrity of sheath 302. For example, in at least one implementation, pores 306 may be engineered to leave 50% or greater of surface area of cathode 304 exposed to electrolyte, whereby cathode 304 can have direct line-of-sight (LOS) access to interior wall 312 for highest electropolishing efficacity.

[0074] While pores 306 are shown to be straight, in at least one implementation, pores 306 can also be engineered to extend along tortuous paths between surface 310 of sheath 302 and cathode 304. In at least one implementation, pores 306 can generally be organized in engineered structures that impart a radial gradation of stiffness within sheath 302. For example, in at least one implementation, pore structures can be organized such that graded stiffness of sheath 302 (e.g., quantified as a stiffness modulus) is smallest at the periphery, (e.g., near and at surface 310), and greatest at or near cathode 304. A graded stiffness can enable sheath 302 to readily yield to follow the curvature of sharp bends within a tubing bend (e.g., bends 208-218 in FIG. 2), yet be stiff enough to hold cathode 304 substantially along a center line of curvature of bend. For example, in at least one implementation, pores 306 can extend along substantially fractal paths between surface 310 and cathode 304, where pore diameters can increase with increasing depth below surface 310 (e.g., see FIG. 7).

[0075] Compositional gradation of Young's modulus may compensate for porosity distributions within sheath 302 that may not support desired functional gradation of mechanical properties of sheath 302. For example, in at least one implementation, the pore structure of sheath 302 can be uniform as shown. A uniform pore structure, whereby pores have constant diameter and uniform distribution, may not produce desired graded mechanical properties of sheath 302. For example, it may be desired that sheath 302 has a soft peripheral portion relative to its interior portion near cathode 304. To overcome this limitation, compositional gradation may be accomplished by compositional grading.

[0076] In at least one implementation, sheath 302 can have a functionally graded chemical composition from cathode 304 to surface 310 of sheath 302. In at least one implementation, a functional gradation of polymer blend can produce a radially graded Young's modulus. A gradation of polymer composition, for example, can enable a radially graded stiffness within sheath 302 comprising a uniform pore density. In at least one implementation, sheath 302 can be fabricated by an additive process, such as 3D printing, where a polymer composition can be compositionally graded. In at least one implementation, during additive manufacturing, material having varied composition can be added in layers, for example, by 3D printing. In at least one implementation, successive layers can comprise linearly or non-linearly varying polymer compositions, where stiffness (e.g., quantified as Young's modulus) of layers can be respectively linearly or non-linearly decreased during build of sheath 302

[0077] In at least one implementation, sheath 302 comprises an array of epi-structures to provide minimal contact with interior wall 312 and space for electrolyte infiltration between surface 310 of sheath 302 and interior wall 312. In at least one implementation, epi-structures can be raised structures 308, such as flat mesas or rounded bumps. In at least one implementation raised structures 308 are represented as rounded bumps spaced at regular or random intervals along surface 310 of sheath 302. While raised structures 308 are shown as bumps, flat-topped mesas may be equally represented by raised structures 308. In at least one implementation, raised structures 308 can be arranged in a square array pattern between pores 306, as illustrated in FIG. 3A. In at least one implementation, raised structures 308 can be arranged in any suitable pattern, such as a hexagonal staggered pattern. In at least one implementation, raised structures 308 can be hemispherical bumps as shown. In at least one implementation, hemispherical geometry of raised structures 308 can serve to minimize contact area of sheath 302 with interior wall 312. In at least one implementation, raised structures 308 can have other suitable geometries. In at least one implementation, raised structures 308 can have a conical geometry.

[0078] In at least one implementation, when engaged for electropolishing, raised structures 308 can serve as standoffs to center sheath 302 and cathode 304 within a tubing workpiece. In at least one implementation, distance d between individual raised structures 308 and height g of raised structures 308 can be engineered to create gap 314 over sheath 302, permitting a maximal surface area of interior wall 312 to be accessible to electrolyte. In at least one implementation, by adjustment of parameters d and g, gap 314 can allow electrolytes to be introduced at a desired flow rate between sheath 302 and interior wall 312. In at least one implementation, gap 314 can also facilitate permeation of electrolyte into pores 306.

[0079] FIG. 3B illustrates a cross-sectional view of cathode assembly 300 within tubing workpiece 316, showing pores 306 extending through sheath 302 along straight paths, in accordance with at least one implementation. Here, cross section in FIG. 3B is taken along cut A-A′ in FIG. 3A to show profiles of pores 306. In at least one implementation, pores 306 can have a conical shape, having diameter D that decreases toward cathode 304 along pore length L. A conical angle d of pores 306 may be adjusted to optimize current flow to cathode 304 and structural / mechanical properties of sheath 302. In at least one implementation, a conical pore shape may maximize flow of current between interior wall 312 and cathode 304, while decreasing void volume from periphery of sheath 302 to cathode 304, thereby creating a stiffness gradation of sheath 302. For example, higher void volume toward periphery of sheath 302 can decrease stiffness of sheath 302 in its periphery, while lower void volume near cathode 304 can increase stiffness in interior portion of sheath 302.

[0080] In at least one implementation, pores 306 extend a length L along straight paths below surface 310, reaching cathode 304. Line-of-sight (LOS) lines are indicated in FIG. 3B. It is understood that dimensions of pores 306 may not be drawn to scale, and that pores 306 may be significantly smaller than indicated. In at least one implementation, pores 306 are distributed in an azimuthally symmetrical manner around cathode 304, as shown. Pore density can be adjusted to expose maximum surface area of cathode 304 while maintaining structural integrity of sheath 302. In at least one implementation, the relative amount of exposed surface of cathode 304 and surface of cathode 304 covered by sheath material can be a function of D. In at least one implementation, D can be adjusted to optimize open cathode area for optimal electropolishing rate and covered cathode area for maximum integrity of cathode assembly 300. In at least one implementation, diameter D and density of pores 306, as well as pore distribution, can be adjusted such that at least 50% of surface area of cathode 304 is uncovered, (e.g., can be exposed to electrolyte), and have line-of-sight (LOS) access to interior wall 312. LOS can ensure that current flow between interior wall 312 and cathode 304 is unobstructed.

[0081] In at least one implementation, sheath 302 can comprise a polymer material having a graded or uniform composition. In at least one implementation, a radially graded composition can enable a radial stiffness gradient, as noted above. In at least one implementation, where sheath 302 comprises a uniform composition, a stiffness gradient can exist because a ratio of solid volume to void volume of pores 306 is reduced as it approaches cathode 304. While the illustrated implementation shows a uniform pore structure, some non-uniform pore structure implementations are described below.

[0082] FIG. 3C illustrates a top 3D view of cathode assembly 350, comprising sheath 352 and cathode 354 centered within sheath 352, in accordance with at least one implementation. In at least one implementation, sheath 352 comprises a plurality of pores 356 arranged in a regular array. In at least one implementation, sheath 352 further comprises an array comprising a plurality of scoop-shaped raised structures 358 extending above surface 360. In at least one implementation, raised structures 358 are distributed on surface 360 in a regular array. In at least one implementation, raised structures 358 can be collocated with pores 356. In at least one implementation, individual raised structures 358 are collocated between adjacent pores 356. In at least one implementation, as shown in inset, raised structure 358 comprises concave walls 362 and 364, adjacent to pores 356a and 356b, respectively. In at least one implementation, concave wall 362 and concave wall 364 face in opposite directions along an axis of sheath 352.

[0083] In at least one implementation, raised structures 358 can serve as standoffs to separate surface 360 from interior wall of tubing (e.g., interior wall 312, FIG. 3B), as described above. In at least one implementation, raised structures 358 can also serve as baffles to enhance transfer of electrolyte into pores 356 by functioning as scoops or paddles (e.g., of a water wheel). In at least one implementation, electrolyte can be agitated by small reciprocal motions of cathode assembly 350 while within a metal tube. In at least one implementation, concave wall 362 can concentrate flow of electrolyte into pore 356a when sheath 352 is translated toward the left side of figure. In at least one implementation, when sheath 352 is translated toward the right side of figure, concave wall 364 can concentrate flow of electrolyte into pore 356b.

[0084] FIG. 3D illustrates a cross-sectional view of cathode assembly 350, showing an internal structure of sheath 352, in accordance with at least one implementation. The cross section is taken from cut line A-A′ in FIG. 3C. In at least one implementation, pores extend from surface 360 at oblique angles. In at least one implementation, adjacent pores can have opposing orientations, as shown. In at least one implementation, bottoms of pores 356 can intersect interconnecting adjacent pores 356 to form V-shaped conduits. In at least one implementation, intersection of adjacent pores 356 can enhance contact of electrolytes with the surface of cathode 354 by increasing contact area. In at least one implementation, interconnection of pores 356 can enable entry and exit of electrolytes within pores 356 without relying mainly on diffusion to renew electrolytes at surface of cathode 354. In at least one implementation, during agitation, electrolytes can wash over exposed portions of cathode 354, enhancing mass transfer to and from cathode 354.

[0085] FIG. 4 illustrates a cross-sectional view of cathode assembly 400, comprising sheath 402 and cathode 404, in accordance with at least one implementation. In at least one implementation, sheath 402 comprises outer layer 406, which comprises a plurality of filaments 408, over core layer 410. In at least one implementation, filaments 408 can be bristles. In at least one implementation, core layer 410 can comprise a substantially solid structure. In at least one implementation, filaments 408 can extend radially from core layer 410, forming a substantially open structure within outer layer 406 (e.g., large open volume). In at least one implementation, outer layer 406 can have a significantly smaller stiffness with respect to core layer 410. In at least one implementation, an open volume between filaments 408 communicates with pores 412 within core layer 410. In at least one implementation, pores 412 can be organized in radial patterns, such as shown in FIG. 4. In at least one implementation, groups of pores 412 (between white areas, which represent solid regions between pore groups) converge at cathode 404, forming one large pore opening at surface of cathode 404. In at least one implementation, electrolytes can permeate space between filaments 408 to interface between outer layer 406 and core layer 410, entering pores 412. In at least one implementation, electrolyte can continue to diffuse toward cathode 404, filling pores 412.

[0086] In at least one implementation, filaments 408 have a fibrous structure. In at least one implementation, core layer 410 can have a significantly greater stiffness than outer layer 406. In at least one implementation, filaments 408 can have sufficient rigidity to aid in centering cathode 404. In at least one implementation, a stiffness gradient can be engineered by choice of structure and materials for two layers. In at least one implementation, exposed surface of cathode 404 can be adjusted by increasing or decreasing diameter and number of pores 412 within each group.

[0087] FIG. 5 illustrates a cross-sectional view of cathode assembly 500, comprising sheath 502 and cathode 504, in accordance with at least one implementation. In at least one implementation, sheath 502 can be like sheath 402 shown in FIG. 4, comprising outer layer 506, filaments (e.g., bristles) 508 and core layer 510. In at least one implementation, the structure of core layer 510 can be a variation of the example of core layer 410 shown in FIG. 4. In at least one implementation, spaces between filaments 508 can comprise open volume within outer layer 506 that communicates with pores 512 within core layer 510.

[0088] In at least one implementation, groups of pores 512 are arranged at periphery of core layer 510, converging after partial extension through core layer 510. In at least one implementation, groups of pores 512 converge to channels 514 and 516 (black shapes in core layer 510), which expand in diameter as they approach cathode 504. In at least one implementation, channels 514 are tributary channels of channels 516. In at least one implementation, during operation, electrolytes within open volume (white areas in outer layer 506) between filaments 508 can flow into pores 512 and into channels 514. In at least one implementation, channels 514 can feed channels 516, where electrolytes can contact exposed surfaces of cathode 504 at bases of channels 516. In at least one implementation, pore structure within core layer 510 can be fractal, where dimensions of channels 514 and 516 can be fractally related. In at least one implementation, other suitable geometrical relationships can be employed. In at least one implementation, exposed surfaces of cathode 504 and volume of solid material within core layer 510 (e.g., white regions) can be optimized to produce an engineered stiffness gradient within sheath 502.

[0089] FIG. 6 illustrates a cross-sectional view of cathode assembly 600, comprising sheath 602 and cathode 604 centered within sheath 602, in accordance with at least one implementation. In at least one implementation, sheath 602 comprises a plurality of filaments (e.g., bristles) 606 extending from cathode 604 toward tubing 608. In at least one implementation, filaments 606 may be divided into at least two subsets, whereby a first subset comprises long filaments 606A and a second subset comprises short filaments 606B. In at least one implementation, short filaments 606B may have a length L1 that is 20% or less than length L2 of long filaments 606A. In at least one implementation, length L1 of short filaments 606B may be 10% or less than length L2 of long filaments 606A. In at least one implementation, length L1 of short filaments 606B may be 5% or less than length L2 of long filaments 606A. In at least one implementation, ends 610 of long filaments 606A contact inner wall 612 of tubing to center cathode 604 within tubing 608 and provide mechanical resistance to lateral forces when navigating bends within tubing 608. In at least one implementation, length L2 of long filaments 606A may be substantially equal to inner radius of tubing 608 minus radius of cathode 604. As noted above, long filaments 606A can become stiffer as they are compressed. For example, long filaments 606A may stiffen as they are compressed when cathode assembly 600 negotiates a bend within tubing 608, providing increasing mechanical resistance between cathode 604 and inner wall 612 of tubing 608 to prevent a short circuit or arcing between cathode 604 and tubing 608 (anode). In at least one implementation, long filaments 606A may be engineered to center cathode 604 within tubing 608 within a desired tolerance (e.g., within 10% of a diameter of tubing 608). In at least one implementation, long filaments 606A may be engineered to maintain a minimum distance between cathode 604 and inner wall 612 while negotiating a gradual bend or a sharp bend (e.g., 30 degrees or more), for example.

[0090] In at least one implementation, short filaments 606B may be collocated with long filaments 606A. For example, in at least one implementation, short filaments 606B may be collocated between adjacent long filaments 606A, as shown. Short filaments 606B may provide an additional mechanical barrier to mitigate direct contact between cathode 604 and inner wall 612 of tubing 608 should long filaments 606A fail to maintain a minimum distance, as noted above. While in at least one implementation long filaments 606A and short filaments 606B have same diameter and comprise same material, by virtue of their shorter length, short filaments 606B can have a larger spring constant than long filaments 606A. Short filaments 606B may resist greater lateral forces than long filaments 606A, providing a hard barrier to contact between cathode 604 and inner wall 612.

[0091] In at least one implementation, long filaments 606A and short filaments 606B can be directly tethered to cathode 604. In at least one implementation, long filaments 606A and short filaments 606B may be engineered to provide sufficient exposed cathode surface area and line of site (LOS) to inner wall 612. For example, in at least one implementation, diameters and densities of long filaments 606A and short filaments 606B may be engineered to provide at least 50% exposed cathode surface area.

[0092] FIG. 7 illustrates a cross-sectional view of cathode assembly 700, comprising sheath 702 and cathode 704 centered within sheath 702, in accordance with at least one implementation. In at least one implementation, sheath 702 comprises pluralities of pores 706a and 706b in a fractal pore network extending between cathode 704 and surface 708. In at least one implementation, sizes of pores 706a decrease significantly as the pore network approaches surface 708. In at least one implementation, pore densities rapidly increase approaching surface 708. In at least one implementation, along fractal paths, diameters of pores 706a and 706b can be smallest at surface 708, while density of pores 706a can be largest at surface 708. In at least one implementation, change of pore structure can be gradual with depth below surface 708. In at least one implementation, as fractal paths approach cathode 704, the trend can reverse, where densities of pores 706b can be least near cathode 704, while pore diameters can be greatest at cathode 704.

[0093] In at least one implementation, within core zone 710, pores 706b are few, but can be relatively large in comparison to pores 706a within peripheral zone 712. In at least one implementation, successive levels of pores 706a can branch from larger pores deeper within peripheral zone 712. At surface 708, pore densities can be maximal within sheath 702. In at least one implementation, pore densities within peripheral zone 712 can be graded by virtue of fractal branching patterns of the pore structure. In at least one implementation, stiffness of peripheral zone 712 of sheath 702 can also be graded due to increasing pore density, where stiffness can decrease toward surface 708. In at least one implementation, sheath 702 can have a sponge-like structure and softness within peripheral zone 712.

[0094] In at least one implementation, a plurality of epi-structures, shown as bumps 714, extend from surface 708. In at least one implementation, bumps 714 can be arranged in any suitable pattern on surface 708. In at least one implementation, bumps 714 can have a rounded or hemispherical shapes to minimize contact area with an interior wall of a section of metal tubing, while providing space between surface 708 and inner wall. In at least one implementation, other suitable shapes, such as a conical form factor, can be employed. In at least one implementation, bumps 714 can enable minimal contact of sheath 702 with an interior wall of a metal tubing to be electropolished. In at least one implementation, bumps 714 can be engineered to optimize electrolyte flow between surface 708 and inner wall. In at least one implementation, during operation, pores 706a can be infused with electrolytes flowing over surface 708. In at least one implementation, electrolytes can readily diffuse to deeper levels within peripheral zone 712. Smaller pores can feed successively larger pores, finally feeding pores 706b within core zone 710. In at least one implementation, pores 706b within core zone 710 communicate with cathode 704. As such, pores 706b can have relatively large diameters to cover large portions of a cathode surface. In at least one implementation, while pores 706b can be relatively large in comparison to pores 706a within peripheral zone 712, core zone 710 can be relatively stiff with respect to peripheral zone 712, due to relatively small numbers of pores 706b.

[0095] FIG. 8A illustrates an end-on view of cathode assembly 800, comprising spacer 802 and cathode 804, where spacer 802 can comprise a short section of sheath material, in accordance with at least one implementation. In at least one implementation, multiple spacers 802 can be distributed at intervals over cathode 804 (e.g., see FIG. 8C). In at least one implementation, spacer 802 can provide similar functions as any of the sheaths described in this disclosure (e.g., sheath 302). In at least one implementation, spacer 802 can primarily be employed to center cathode 804 within a section of metal tubing, maintaining cathode 804 substantially equidistant from interior wall 810 (represented by dashed circle) of tubing. As shown in FIG. 8C, spacer 802 can have a small thickness relative to length of cathode 804, leaving a large portion of cathode 804 directly exposed to electrolytes, in at least one implementation. In at least one implementation, spacer 802 has a uniform composition and stiffness. In at least one implementation, spacer 802 can comprise a material (e.g., a polymer composition) that has a desired Young's modulus. In at least one implementation, spacer 802 can comprise a solid body that lacks a porosity, as pores are not a primary means to bring electrolytes to cathode 804, as would be case for a sheath (e.g., sheath 302).

[0096] In at least one implementation, spacer 802 comprises a plurality of epi-structures, represented here as bumps 806 to provide minimal contact area with interior wall 810 of a section of tubing 812. In at least one implementation, minimal contact area can minimize friction between spacer 802 and interior wall 810 when cathode assembly 800 is translated through metal tubing. In at least one implementation, spacer 802 can comprise multiple openings 808. In at least one implementation, openings 808 can enable flow of electrolytes across spacer 802 during translation of cathode assembly 800, for example. In at least one implementation, openings 808 can be engineered to optimize electrolyte flow across spacer 802. In at least one implementation, number of openings 808 and diameter D1 of openings 808 can be adjusted to maximize flow of electrolyte. In at least one implementation, adjustment of number and diameter of openings 808 can be employed to optimize stiffness of spacers 802. In at least one implementation, spacers 802 can exhibit an optimal stiffness to facilitate movement of cathode assembly 800 through bends in tubing 812, while maintaining cathode 804 substantially equidistant from interior wall 810.

[0097] FIG. 8B illustrates an end-on view of cathode assembly 850, comprising spacer 852 and cathode 854, in accordance with one implementation. In contrast to sheaths described above, spacer 852 has a relatively small axial extent (e.g., thickness) relative to its diameter. In at least one implementation, spacer 852 can have a diameter to thickness aspect ratio of 1:1 or greater. In at least one implementation, a plurality of spacers 852 can be distributed at intervals along length of cathode 854, leaving a significant percentage of surface of cathode 854 directly exposed to electrolyte. In at least one implementation, plurality of spacers 852 can maintain cathode 854 substantially equidistant from interior wall 860 of tubing 862.

[0098] In at least one implementation, spacer 852 can comprise a suitable nonporous polymer material. In at least one implementation, material can have a desired Young's modulus and modulus of elasticity, for example, enabling a balance between stiffness and compliance. In at least one implementation, spacer material can be radially graded to permit a more rigid core and softer periphery.

[0099] In at least one implementation, spacer 852 comprises a plurality of lobes 856. In at least one implementation, lobes 856 are arranged in a symmetric star pattern. In at least one implementation, lobes 856 can have any suitable shape and angular distribution. In at least one implementation, lobes 856 have rounded ends to perform a similar function for minimizing contact area with interior wall 810, as do bumps 806 described for spacer 802 (see FIG. 7). In at least one implementation, spaces in between lobes 856 can channel flow of electrolytes through spacer 852, enabling bulk electrolytes to wash over exposed segments of cathode 854 between spacers 852.

[0100] FIG. 8C illustrates a cross-sectional view of cathode assembly 800. It is understood that cathode assembly 850, shown in FIG. 8B, can also be represented in FIG. 8C. In at least one implementation, spacers 852 can be distributed along cathode 854 at intervals d1, as shown in illustrated example, where d1 can be several times thickness t. In at least one implementation, spacers 852 can have a small axial extent, or thickness t, relative to their diameter D2. In at least one implementation, lobes 856 contact interior wall 860 of tubing 862. While in cross-sectional view of FIG. 8C, openings such as openings 808 shown in FIG. 8A are not indicated, it is understood that they are present to enable flow of electrolyte across spacers 852.

[0101] FIGS. 9A-9D illustrate an exemplary method for operating an electropolishing apparatus for electrochemical polishing of interior walls of tubing according to some implementations of disclosure. FIG. 9A illustrates a cross-sectional view of cathode assembly 900 partially inserted into a first segment of tubing 902. In at least one implementation, cathode assembly 900 can be part of a larger electropolishing apparatus (not shown). In at least one implementation, metal tubing 902 comprises multiple 90-degree bends, such as bends 904, 906 and 908, through which cathode assembly 900 passes. In at least one implementation, cathode assembly 900 comprises sheath 910 and cathode 912 embedded within sheath 910. In at least one implementation, cathode assembly 900 can comprise guide tip 914 to aid negotiation of bends 904-908 by cathode assembly 900.

[0102] In at least one implementation, cathode 912 can comprise a superelastic metal alloy, such as nickel-titanium alloys (e.g., Nitinol, approximately 50% nickel and 50% titanium). Other suitable superelastic alloys are described above. Such alloys can undergo superelastic phase transitions (e.g., a martensite-austenite phase transition) by imposing mild stress on material. Elastic strain resulting from mild stresses can be as much as 18%. In at least one implementation, stresses can be tuned to range up to several hundred megapascals (MPa) at a desired temperature by alteration of alloy systems, composition ratio, and / or thermo-mechanical conditioning (e.g., annealing). Thus, portions of cathode 912 experiencing mechanical stresses induced by interaction with interior wall 916 can immediately transition to a superelastic phase.

[0103] In FIG. 9B, cathode assembly 900 has advanced within tubing 902. Guide tip 914 is pressed against interior wall 916 of tubing 902. Cathode 912 begins to bend in response to impinging axial compressive stresses as well as transverse stresses on cathode 912 by contact of guide tip 914 on interior wall 916 at bend 904. In at least one implementation, at least lead portion 918 (delineated by dashed box) of cathode 912 can react to impinging stresses by undergoing a phase transition to a superelastic phase (e.g., a martensite-austenite phase transition). In at least one implementation, elastic property enables cathode 912 to readily follow curvature of bend 904 without experiencing plastic deformation.

[0104] In FIG. 9C, lead portion 920 of cathode assembly 900 (including front portion of sheath 910, delineated by dashed box) has passed through bend 904 and continues to advance through straight segment 922 of tubing 902. Relieved of stresses encountered within bend 904, lead portion of cathode 912 (e.g., lead portion 918) reverts to a straight rod. In at least one implementation, the superelastic property enables cathode 912 to exhibit a shape memory, allowing it to revert to its original shape when not experiencing stress. In at least one implementation, the shape memory property of cathode 912 obviates plastic deformation, thereby avoiding kinks and other permanent deformations in cathode 912.

[0105] In at least one implementation, sheath 910 can be advanced through bend 904. In at least one implementation, sheath 910 can be a functionally graded structure, as described above. In at least one implementation, sheath 910 can exhibit a radially graded stiffness, enabling sheath 910 to be sufficiently compliant to negotiate 90-degree curvature of bend 904 while maintaining cathode 912 substantially equidistant from interior wall 916. In at least one implementation, balance between stiffness and compliance can be engineered according to methods described above. In at least one implementation, porosity within sheath 910 can be engineered to create a radial stiffness gradient, where stiffness decreases from center to periphery of sheath 910. In at least one implementation, gradient can be monotonic (e.g., gradual) or stepwise. In at least one implementation, sheath 910 can be engineered to exhibit a radial gradient of Young's modulus. This can be accomplished by growing sheath 910 in layers comprising a polymer bend, for example. In at least one implementation, the composition of the polymer blend can be systematically varied in successive layers. In at least one implementation, stiffness can be graded by combining compositional gradients with structural gradients. In at least one implementation, porosity within sheath 910 can be radially graded along with composition.

[0106] In FIG. 9D, cathode assembly 900 has advanced through bends 906 and 908. At this stage, electrolytes can be introduced by flooding into tubing 902. In at least one implementation, electrolytes can initially fill gaps between interior wall 916 and surface of sheath 910, then permeate pore structure within sheath 910. In at least one implementation, while not shown in figure, sheath 910 can be replaced by spacers (e.g., spacers 802 or 852, FIGS. 8A and 8B). In at least one implementation, spacers can also have engineered stiffness to comply with tight curvature of bends 904-908. In at least one implementation, most of cathode 912 can be directly exposed to electrolyte.

[0107] In at least one implementation, after introduction of electrolyte into tubing 902, cathode assembly 900 can be set into a small amplitude reciprocal motion by a mechanism within electropolishing apparatus. In at least one implementation, cathode assembly 900 is advanced and retracted small distances within tubing 902. In at least one implementation, reciprocal motion can agitate electrolyte to enhance mass transfer into and out of pores within sheath 910.

[0108] The following are additional examples provided in view of the above-described implementations. Here, one or more features of example, in isolation or in combination, can be combined with one or more features of one or more other examples to form further examples also falling within the scope of the disclosure. As such, one implementation can be combined with one or more other implementation without changing scope of disclosure.

[0109] Example 1 is an electropolishing apparatus, comprising a cathode, wherein the cathode has a length and a diameter; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises a superelastic metal alloy.

[0110] Example 2 is an electropolishing apparatus of any example herein, particularly example 1, wherein the length of the cathode is at least twice the diameter of the cathode.

[0111] Example 3 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

[0112] Example 4 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of pores that extend from the cathode to an exterior surface of the sheath.

[0113] Example 5 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a straight path.

[0114] Example 6 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a tortuous path.

[0115] Example is an electropolishing apparatus of any example herein, particularly example 6, wherein the plurality of pores is interconnected in a pore network that extends between the cathode and the exterior surface of the sheath, wherein ones of the plurality of pores include a pore diameter and a pore length, wherein the pore diameter and the pore length are to decrease with increasing distance from the cathode and are smallest near the exterior surface of the sheath.

[0116] Example 8 is an electropolishing apparatus of any example herein, particularly example 7, wherein the pore network comprises a fractal network comprising interconnected pores extending from the cathode to the exterior surface of the sheath, wherein the pore diameter and the pore length decrease non-linearly with distance from the cathode.

[0117] Example 9 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of compliant epi-structures.

[0118] Example 10 is an electropolishing apparatus of any example herein, particularly example 9, wherein the plurality of compliant epi-structures comprises a plurality of raised structures distributed on a surface of the sheath.

[0119] Example 11 is an electropolishing apparatus of any example herein, particularly example 10, wherein the plurality of raised structures comprises one or more concave surfaces.

[0120] Example 12 is an electropolishing apparatus of any example herein, particularly example 10, wherein the plurality of raised structures comprises a plurality of filaments.

[0121] Example 13 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments extends at least partially between the surface of the cathode to an exterior surface of the sheath.

[0122] Example 14 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments comprises a first subset of filaments having a first length, and a second subset of filaments having a second length, wherein the second length is 20% or less of the first length.

[0123] Example 15 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments extends between an exterior surface of the sheath to an inner region of the sheath, wherein the inner region comprises a porous solid dielectric material.

[0124] Example 16 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a plurality of spacers distributed along the length of the cathode, and wherein portions of the cathode are exposed between individual ones of the plurality of spacers.

[0125] Example 17 is an electropolishing apparatus of any example herein, particularly example 16, wherein individual ones of the plurality of spacers comprise a plurality of openings.

[0126] Example 18 is an electropolishing apparatus of any example herein, particularly example 16, wherein individual ones of the plurality of spacers comprise one or more arms.

[0127] Example 19 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a polymer blend, wherein a composition of the polymer blend is graded to produce a gradient of Young's modulus between the cathode and a surface of the dielectric.

[0128] Example 20 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a fluoropolymer, a polyester, a polyamine or a polypropylene.

[0129] Example 21 is an electropolishing system comprising a metal tubing and an electropolishing apparatus comprising: a cathode, wherein the cathode has a length; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy, and wherein the dielectric comprises a functionally graded material, wherein a cathode assembly comprises the cathode and the dielectric, wherein the cathode assembly is positioned within the metal tubing.

[0130] Example 22 is an electropolishing system of any example herein, particularly example 21, wherein the metal tubing has a curvature comprising one or more bends along the metal tubing.

[0131] Example 23 is an electropolishing system of any example herein, particularly example 21, further comprising a power supply electrically coupled to the cathode.

[0132] Example 24 is an electropolishing system of any example herein, particularly example 21, further comprising an electrolyte within the metal tubing.

[0133] Example 25 is an electropolishing system of any example herein, particularly example 21, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

[0134] Example 26 is a method for electropolishing comprising providing an electropolishing apparatus, wherein the electropolishing apparatus comprises: a cathode, wherein the cathode has a length; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy and the dielectric comprises a functionally graded material, wherein a cathode assembly comprises the cathode and the dielectric, wherein the method further comprises: inserting the cathode assembly into a tubing, wherein the tubing comprises one or more bends; and agitating an electrolyte in the tubing.

[0135] Example 27 is a method for electropolishing of any example herein, particularly example 26, wherein inserting the cathode assembly into the tubing comprises passing the cathode assembly through the one or more bends.

[0136] Example 28 is a method for electropolishing of any example herein, particularly example 26, wherein agitating the electrolyte in the tubing comprises translating the cathode assembly by reciprocating motion.

[0137] Besides what is described herein, various modifications can be made to disclose implementations and implementations thereof without departing from their scope. Therefore, illustrations of implementations herein should be construed as examples, and not restrictive to scope of present disclosure.

Examples

example 3

[0111 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

example 4

[0112 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of pores that extend from the cathode to an exterior surface of the sheath.

example 5

[0113 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a straight path.

CLAIM FOR PRIORITY

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 503,941, filed May 23, 2024, titled “SURFACE FINISHING OF INTERNAL GEOMETRIES FOR MANIFOLDS,” which is incorporated by reference in its entirety for all purposes.BACKGROUND

[0002] Process tools are used to perform treatments such as deposition and etching of film on semiconductor wafer substrates. These process tools can comprise a vacuum chamber in which chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes can be performed. Precision deposition processes such as ALD use precise delivery of precursor gases and vapors (collectively, process gases) into vacuum chamber through a gas distribution showerhead within vacuum chamber. These processes employ process gases comprising an inert carrier gas and one or more vaporized precursor substances. Process gas can be formed by vaporizing one or more liquid precursors and then mixing vapors with carrier gas.

[0003] Ultra-high purity (UHP) (e.g., >99.999% pure, or 5N purity) process gases can be exclusively employed in some semiconductor manufacture processes. Such high purity gases risk contamination by delivery systems for semiconductor process tools if gases flow within conduits that are not highly polished to UHP standards. While great care can be taken to purge gas delivery lines and distribution manifolds to remove residual gases and vapors between process cycles, residual contaminants can remain on inadequately polished interior surfaces of tubing and manifold flow paths. Even small degrees of surface roughness can provide microscopic pores and cracks within which gases can adsorb or precursor vapors can condense. These can leach into fresh UHP gases employed in subsequent processes. As many gas delivery lines and manifolds can be manufactured for non-UHP applications, gas delivery systems meeting UHP standards are expensive and are performed in highly specialized fabrication facilities. Due to difficulties in polishing interior surfaces of curved tubing and gas lines to UHP standards, a reliable and efficient electropolishing apparatus for highly curved metal tubing is desired.BRIEF DESCRIPTION OF DRAWINGS

[0004] Material described herein is illustrated by way of example and not by way of limitation in accompanying figures. For simplicity and clarity of illustration, elements illustrated in figures are not necessarily drawn to scale and exact locations. For example, dimensions of some elements can be exaggerated relative to other elements for clarity. Also, various physical features can be represented in their simplified “ideal” forms and geometries for clarity of discussion, but it is nevertheless to be understood that practical implementations can only approximate illustrated ideals. For example, smooth surfaces and square intersections can be drawn in disregard of finite roughness, corner-rounding, and imperfect angular intersections characteristic of structures formed by nanofabrication techniques. Further, where considered appropriate, reference labels have been repeated among figures to indicate corresponding or analogous elements.

[0005] FIG. 1 illustrates a cross-sectional view of a metal tubing internal electropolishing process, according to at least one implementation of disclosure.

[0006] FIG. 2 illustrates a portion of an electropolishing apparatus according to at least one implementation of disclosure.

[0007] FIG. 3A illustrates a 3D profile view of cathode, according to at least one implementation of disclosure.

[0008] FIG. 3B illustrates a cross-sectional view of cathode assembly shown in FIG. 3A, according to at least one implementation of disclosure.

[0009] FIG. 3C illustrates a top 3D view of a cathode assembly, according to at least one implementation of disclosure.

[0010] FIG. 3D illustrates a cross-sectional view of cathode assembly shown in FIG. 3C, according to at least one implementation of disclosure.

[0011] FIG. 4 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0012] FIG. 5 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0013] FIG. 6 illustrates a cross-sectional view of a cathode assembly, comprising a sheath and a cathode centered within the sheath, in accordance with at least one implementation.

[0014] FIG. 7 illustrates a cross-sectional view of a cathode assembly, according to at least one implementation of disclosure.

[0015] FIG. 8A illustrates an end-on view of a cathode assembly, according to at least one implementation of disclosure.

[0016] FIG. 8B illustrates an end-on view of a cathode assembly, according to at least one implementation of disclosure.

[0017] FIG. 8C illustrates a cross-sectional view of cathode assembly shown in FIG. 8A, according to at least one implementation of disclosure.

[0018] FIGS. 9A-9D illustrate an exemplary method for operating an electropolishing apparatus according to at least one implementation of disclosure.DETAILED DESCRIPTION

[0019] Disclosed herein is an electrochemical apparatus for polishing interior surfaces of metal tubing having a tortuous path, such as sharp bends, branching, and changes in diameter, in accordance with at least one implementation. In at least one implementation, electrochemical polishing apparatus comprises a cathode embedded within a sheath comprising a dielectric material. In at least one implementation, cathode can be an elongated cylindrical rod, a solid or stranded wire or other suitable elongated structure, comprising a conductive material. In at least one implementation, non-conductive sheath may extend at least a portion of length of cathode. In at least one implementation, a sheath may cover a portion of an electrochemically active surface of cathode. In at least one implementation, a suitable sheath can comprise multiple separate segments that are distributed as spacers along the length of a cathode. In at least one implementation, a suitable dielectric sheath may prevent cathode held within it from contacting an inner wall of a tube undergoing internal electropolishing (e.g., an anode). Such contact may potentially create a short circuit between cathode and anode. In at least one implementation, a suitable sheath can comprise a porous structure. In at least one implementation, a cathode comprises a superelastic alloy to enable cathode to assume curvature (e.g., a bend) of a section of metal tubing. In at least one implementation, the superelastic nature of alloy material can enable such a cathode to assume its original straight shape without plastic deformations (e.g., permanent kinking) when bending stress is relieved. In at least one implementation, dielectric material can possess engineered external geometry and internal pore structures that can be fabricated by additive manufacturing processes such as three-dimensional (3D) printing.

[0020] Here, “dielectric” can generally refer to a sheath (e.g., a continuous cylindrical sheath) that can extend along the length of a rod-shaped cathode. In at least one implementation, the sheath can guide cathode through a tortuous path that, for example, can comprise one or more 90-degree bends along a section of metal tubing. In at least one implementation, the dielectric sheath can be engineered to hold cathode substantially at the center of metal tubing while cathode advances or retreats within metal tubing. In at least one implementation, the dielectric sheath can be engineered to enable sufficient stiffness to maintain radial position of cathode while cathode follows sharp bends of metal tubing. In at least one implementation, the dielectric sheath can also have sufficient flexibility to bend at sharp bends of tubing, yielding to lateral forces during contact with curved portions of interior wall.

[0021] In at least one implementation, a dielectric sheath, such as a jacket, beads, or spacers around cathode can prevent mechanical contact between cathode and interior wall of tubing, as noted previously. In at least one implementation, a dielectric sheath can mechanically center cathode within interior of metal tubing, holding cathode substantially equidistant from interior wall of metal tubing. In at least one implementation, a dielectric sheath can guide cathode substantially along central axis of both straight, curved and branched sections of metal tubing.

[0022] In at least one implementation, a suitable dielectric comprises a material having a functionally graded stiffness, for example, exhibiting a radial stiffness gradation. In at least one implementation, a radial stiffness gradient can exhibit maximal stiffness near cathode, while stiffness can decrease along radial dimension. In at least one implementation, a suitable stiffness gradient can be gradual. In at least one implementation, a suitable stiffness gradient can be stepped (e.g., a sudden change in stiffness at a distance from center). In at least one implementation, a suitable dielectric can be a porous sheath, whose porosity can be radially uniform, whereas material can also be compositionally graded to exhibit greatest stiffness at center and least stiffness at periphery. In at least one implementation, a stiffness gradient can result from a radial distribution of Young's modulus, for example, of material composition.

[0023] Reducing stiffness from center to periphery can optimize mechanical resistance of porous dielectric sheath to lateral forces resulting from contact with interior wall of tubing. This effect can be advantageous as porous dielectric sheath encounters bends in tubing, where lateral forces can be greatest. In at least one implementation, diminishing gradation of stiffness can be engineered to enable porous dielectric sheath to readily bend at sharp angles (e.g., 90 degrees), while substantially maintaining cathode along central axis of metal tubing. For example, a more porous exterior of a porous sheath can yield more readily to lateral forces encountered at bends of tubing by partially collapsing to yield at curvature of bends. At the same time, an inner portion of porous sheath can exhibit greater rigidity. In at least one implementation, a more rigid center can deform axially to bend, but exhibit slight radial deformation. In at least one implementation, a structural porosity gradient can be accompanied by a compositional gradient where the Young's modulus is varied.

[0024] In at least one implementation, maintenance of cathode at center of tubing where it is substantially equidistant from interior wall can be used to ensure uniform radial current density from all points of interior wall to cathode within electropolishing zone. In at least one implementation, if the cathode is off-axis (e.g., moved away from central axis of tubing and non-equidistant), a radial distribution of current distribution between cathode and interior wall of tubing will not be angularly uniform. Thus, a portion of interior wall that is closer to cathode can be over polished, resulting in pitting or recessing, while a more distal portion of interior wall can be under polished, resulting in a non-uniform UHP finish on interior wall. In at least one implementation, a radial symmetry of current distribution within electropolishing zone can ensure uniform polishing of interior wall as cathode is translated axially within tubing.

[0025] In at least one implementation, a suitable dielectric material can be structured to promote maximum mass transfer of electrolytes to and from cathode. In at least one implementation, a porous sheath can comprise a continuous body extending along cathode, covering substantially the entire length of cathode. In at least one implementation, the porous sheath comprises substantially straight pores and a uniform radial porosity distribution, where pores can extend perpendicular to cathode. In at least one implementation, pores can extend obliquely to cathode. In at least one implementation, where porosity is uniform, a suitable dielectric material can be compositionally graded to imbue a radial stiffness gradient. In at least one implementation, pores can be engineered to have desired diameters and densities for optimal mass transfer of electrolytes to and from cathode.

[0026] In at least one implementation, a cathode can be translated within tubing at a rate enabling convection of electrolyte into and out of openings, spaces or pores within sheath. In at least one implementation, a cathode can be set into reciprocating motion to agitate electrolyte within a piece of tubing undergoing internal electropolishing. In at least one implementation, the exterior surface (or outer portion) of a sheath can comprise epi-structures such as raised mesas, bumps or fibers. In at least one implementation, epi-structures can be engineered on an exterior surface of a sheath to minimize mechanical contact of a sheath with interior wall of tubing, while permitting substantial exposure of interior wall to electrolyte. In at least one implementation, epi-structures such as mesas and bumps can be shaped as miniature scoops. In at least one implementation, scoop-shaped epi-structures can have concave faces to direct flow of electrolytes into pores of a sheath comprising such epi-structures, when such as sheath is translated within a piece of tubing. In at least one implementation, pores can be oriented obliquely within a porous sheath, where a first subset of pores can be oriented oppositely to a second subset of pores. In at least one implementation, obliquely opposed pores can intersect at surface of a cathode, enabling flow of bulk electrolyte through pores as cathode is moved along tubing in one direction or other. In at least one implementation, pore density can be adjusted to maximize exposed surface area of cathode. In at least one implementation, scoop-shaped bumps at entrance to an individual pore can enhance convective flow of electrolytes into pore. In at least one implementation, the shape of scoops can be engineered to maximize flow into each pore.

[0027] In at least one implementation, a suitable dielectric comprises independent spacers distributed at intervals along a length of a cathode. In at least one implementation, multiple spacers can be separated from one another by bare sections of cathode. In at least one implementation, spacers can comprise annular shapes, cross shapes, and star shapes. In at least one implementation, as spacers can enable a substantial portion of cathode surface to be exposed to electrolyte, spacers can comprise solid structures that do not have an internal porosity. In at least one implementation, spacers can have openings that enable axial flow of electrolytes across structure. For example, in at least one implementation, spacers can have a star shape, possessing spaces between arms to permit axial flow of electrolytes across thickness of a spacer. In at least one implementation, spacers can have openings permitting electrolytes to flow across the structure.

[0028] In at least one implementation, cathode can comprise a superelastic metal alloy for following tortuosity of a piece of tubing. In at least one implementation, superelastic property of cathode can impart a shape memory so that cathode can revert to its original form. In at least one implementation, default (e.g., original) shape of cathode can be straight, curved, spiral shaped, etc. In at least one implementation, a superelastic alloy employed for the cathode can be compositionally tuned to undergo a solid-state phase transition from a rigid crystalline phase to a superelastic crystalline phase (e.g., a martensite / austenite phase transition) at a specified temperature, such as operating temperature of electropolishing process. For example, an electropolishing process may be performed at room temperature (e.g., 20° C.) or at elevated temperatures. In at least one implementation, a phase transition can be triggered by relatively low levels of mechanical stress imposed on the cathode. In at least one implementation, stress can be induced by axial and transverse (e.g., lateral) forces acting on the cathode as it encounters bends in tubing. In at least one implementation, stress threshold can be tuned by adjustment of ratio of elements composing alloy. In at least one implementation, alloy can comprise nickel and titanium (e.g., Nitinol). In at least one implementation, adjustment of nickel and titanium composition ratio can determine transition temperature and threshold of mechanical stress to induce transition. In at least one implementation, the shape memory property of the cathode material enables it to revert to its original shape when relieved of mechanical stress.

[0029] In at least one implementation, a superelastic metal alloy is employed as a cathode material can enable the cathode to have a shape memory. The shape memory effect can enable the cathode to straighten and resume its original shape after passing through a bend of tubing. In at least one implementation, stress experienced by the cathode when passing through a bend is relieved when it passes into a straight portion of tubing following the bend. In at least one implementation, the cathode can assume a straight shape, for example, without plastic deformation (e.g., permanent kinks). When encountering a following bend, the cathode may experience mechanical stress anew, triggering a transition to a superelastic state (e.g., austenite phase). In at least one implementation, after electropolishing process is complete, the cathode can be withdrawn from tubing and revert to its original shape (e.g., as a straight rod) without plastic deformation. In at least one implementation, other cathode form factors can include non-linear shapes, such as simple curved shapes, serpentine shapes, etc.

[0030] In at least one implementation, in addition to having property of superelasticity, the cathode can have sufficient conductivity and chemical inertness to be suitable as an electrode employed for electropolishing. In at least one implementation, to fulfill this property, a superelastic alloy can comprise suitable alloy systems such as, but not limited to, nickel and titanium, copper and zinc, copper, zinc and aluminum, gold, copper, and zinc, and gold and cadmium.

[0031] In at least one implementation, in following description, well-known methods and devices are shown in block diagram form, rather than in detail, to avoid obscuring present disclosure. Reference throughout this specification to “an implementation” or “one implementation” or “some implementations” means that a particular feature, structure, function, or characteristic described in connection with implementation is included in at least one implementation of disclosure. Thus, appearances of phrase “in an implementation” or “in one implementation,”“in at least one implementation,” or “some implementations” in various places throughout this specification are not necessarily referring to same implementation of disclosure. Furthermore, particular features, structures, functions, or characteristics can be combined in any suitable manner in one or more implementations. For example, a first implementation can be combined with a second implementation anywhere particular features, structures, functions, or characteristics associated with two implementations are not mutually exclusive.

[0032] Here, “coupled” and “connected,” along with their derivatives, can be used to describe functional or structural relationships between components. These terms are not intended as synonyms for each other. Rather, in particular implementations, term “connected” can be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. Here, “coupled” can be used to indicate that two or more elements are in either direct or indirect (with other intervening elements between them) physical, electrical or in magnetic contact with each other, and / or that two or more elements co-operate or interact with each other (e.g., as in a cause-and-effect relationship). Here, “coupled” can also generally refer to direct attachment of one electronic component to another. An electric or magnetic field can couple one component to another, where field is controlled by one component to influence other in some manner.

[0033] Here, “over,”“under,”“between,” and “on” can generally refer to a relative position of one component or material with respect to other components or materials where such physical relationships are noteworthy. Unless these terms are modified with “direct” or “directly,” one or more intervening components or materials can be present. Similar distinctions are to be made in context of component assemblies. As used throughout this description, and in claims, a list of items joined by term “at least one of” or “one or more of” can mean any combination of listed terms.

[0034] Here, “adjacent” can generally refer to a position of a thing being next to (e.g., immediately next to or close to with one or more things between them) or adjoining another thing (e.g., abutting it).

[0035] Here, “electrochemical” can generally refer to chemical reactions taking place at an electrified working electrode immersed in an electrolyte containing elements or compounds that are susceptible to undergo reactions when encountering electrified electrode. In at least one implementation, if electrified with a positive potential relative to a referenced electrode in same electrolyte, working electrode is referred to as an anode. In at least one implementation, if electrified by a negative potential relative to a referenced electrode in same electrolyte, electrode is referred to as a cathode.

[0036] Here, “cathode” can generally refer to an electrochemical electrode having a negative potential relative to a referenced electrode in same electrolyte.

[0037] Here, “cathode assembly” can generally refer to an assembly of objects including a cathode, as defined herein. In at least one implementation, a cathode assembly includes a dielectric sheath or plurality of dielectric spacers, both of which are defined below, in which a rod-shaped cathode can be embedded.

[0038] Here, “electrolyte” can generally refer to a liquid solution comprising ionized molecules or atoms.

[0039] Here, “electropolish” can generally refer to an electrochemical process for electrochemically planarizing metal surfaces by partial dissolution of microscopic structures related to surface roughness.

[0040] Here, “elastic metal alloy” or “superelastic metal alloy” can generally refer to a metal alloy system that has property of elasticity (or superelasticity), or property to undergo large amounts of elastic strain when under mechanical stress.

[0041] Here, “tubing” can generally refer to a metallic tubular pipe or conduit that has diameter ranging from approximately 1 mm to approximately 5 cm but can be greater. In at least one implementation, wall of tubing can comprise aluminum, stainless steel alloys, copper, or brass. In at least one implementation, metal tubing can be distinguished from pipe in that it is generally not threaded. In at least one implementation, coupling between sections of metal tubing and components such as valves, mass flow controllers, gauges and the like can be done by use of compression fittings.

[0042] Here, “bend” can generally refer to sharp curvature in a section of metal tubing. In at least one implementation a bend can have a 90-degree angle.

[0043] Here, “functionally graded material” can generally refer to a material having a graded property, such as stiffness. In at least one implementation, gradation of property is generally engineered by special manufacturing methods, such as layer-by-layer building methods as performed by 3D printing, for example.

[0044] Here, “opening” can generally refer to a hole or an aperture in surface of an object.

[0045] Here, “pore” can generally refer to a small opening that extends a distance that can be many times its characteristic diameter below surface of an object. In at least one implementation, pores can be on order of a millimeter in diameter to microscopic. Path along which a pore extends can be straight or tortuous.

[0046] Here, “pore network” can generally refer to a network of pores within a material. Individual pores within the network can be interconnected, forming a continuous network. In at least one implementation, pore network can be analogous to a root system of a tree, for example.

[0047] Here, “fractal” can generally refer to group of physically connected objects, Examples of common fractal structures are tree branches, leaf veins, circulatory systems in animals, dendritic networks such as nerve and root networks, rock outgrowths, snowflakes, and ice crystals. In at least one implementation, fractal objects have a fundamental geometric shape, which is repeated by attaching smaller versions of same shape (secondary size) to fundamental object, then attaching, or outgrowth of, still smaller versions (e.g., tertiary size) to secondary objects, ad infinitum. In at least one implementation, characteristic dimensions of successively smaller objects scale down in size in same mathematic proportions with each level of reduction.

[0048] Here, “fractal network” can generally refer to a network of interconnected branches, branches having derived from fractal branching from larger members of network.

[0049] Here, “sheath” can generally refer to a coat or containing structure in which an object inserts or is embedded. In at least one implementation, a sheath is a dielectric material shaped cylindrically in which a rod-shaped cathode is embedded.

[0050] Here, “epi-structure” can generally refer to one or more structures on a surface of an object, such as bumps, mesas or filaments on a sheath.

[0051] Here, “bump” can generally refer to a protrusion above the surface of an object. In at least one implementation, bump can have a generally rounded or hemispherical shape. Mesas, which are similar protrusions having flat tops, can be bumps.

[0052] Here, “filament” can generally refer to a fibrous epi-structure, such as a fiber or bristle sheath or portion of a sheath.

[0053] Here, “spacer” can generally refer to a disc-like section of dielectric that is distributed along the length of a rod-shaped object. In at least one implementation, a spacer is a disc-shaped dielectric structure that can be distributed along the length of a rod-shaped cathode.

[0054] Unless otherwise specified in explicit context of their use, terms “substantially equal,”“about equal,” and “approximately equal” can generally mean that there is no more than incidental variation between two things so described. In art, such variation is typically no more than + / −10% of referred value.

[0055] FIG. 1 illustrates a cross-sectional view of a metal tubing internal electropolishing process comprising electropolishing apparatus 100, according to some implementations of disclosure. In at least one implementation, electropolishing apparatus 100 can be part of a larger electropolishing apparatus (not shown), for example, comprising an automated cathode insertion mechanism and / or agitation mechanism. In at least one implementation, electropolishing apparatus 100 comprises cathode 102. In at least one implementation, cathode 102 has a circular cylindrical form factor, but can have other suitable cross-sectional form factors, such as elliptical or rectilinear cross sections. In at least one implementation, cathode 102 can comprise a large gauge (e.g., American wire gauge (AWG) #10 (f~2.5 mm) or larger) solid or stranded wire. In at least one implementation, cathode 102 can be a wire spiral. In at least one implementation, cathode 102 can be an elongated (circular) rod-shaped electrode, having a minimum aspect ratio of at least 2:1 (e.g., having a length that is at least two-fold its width, or diameter).

[0056] In at least one implementation, cathode 102 comprises a suitable superelastic metal alloy. A superelastic metal alloy is an alloy that undergoes a solid-state phase transition from a normal metal to a highly elastic metal (e.g., a martensitic to austenitic phase transition) when stressed with small lateral or longitudinal forces, as described below. In at least one implementation, cathode 102 comprises a metal alloy that undergoes a superelastic solid phase transition at an operating temperature of electropolishing process. For example, an operating temperature of a particular electropolishing process may be approximately 15° C. Another electropolishing process may be performed at higher temperatures. A wire or rod-shaped cathode 102 comprising a superelastic metal may easily bend and snake along a tortuous path within tubing 106 at or above operating temperatures employed in the electropolishing process, enabling electropolishing of metal tubing having one or more bends, as described below. In at least one implementation, tubing bends can have bend angles ranging from obtuse to acute angles (e.g., to less than 90 degrees).

[0057] In at least one implementation, spacers 104 may be included to center cathode 102 within tubing 106 (e.g., workpiece), such that cathode 102 is substantially aligned with a central axis of tubing 106. Centering cathode 102 along center of tubing 106 may ensure that cathode 102 remain as equidistant from interior wall 108 as practicable to enable a symmetrical electropolish of interior wall 108. For example, an off-center cathode may cause an over-polish (e.g., dissolution) of nearby portions of interior wall 108, while more distal portions may be under-polished. In at least one implementation, spacers 104 comprise a dielectric electrically insulative material, described below. In at least one implementation, spacers 104 are disc shaped. In at least one implementation, spacers may be bead shaped. In at least one implementation, spacers 104 can be immovably affixed on cathode 102 at desired intervals.

[0058] In at least one implementation, spacers 104 can comprise one or more dielectric materials, such as one or more suitable organic polymers. In at least one implementation, spacers 104 can comprise a compliant periphery to allow cathode 102 to follow a tortuous path within tubing 106 and a relatively hard inner region to maintain a minimum distance between cathode 102 and interior wall 108 of tubing 106. As tubing 106 serves as an anode in an electropolishing process, contact or close proximity of cathode 102 and interior wall 108 is to be avoided to prevent a short circuit or electrical arcing. In at least one implementation, a tortuous path may include bends, branch points, as well as increases and decreases in diameter of tubing 106. In at least one implementation, tubing 106 comprises a metal such as, but not limited to, stainless steel alloys, aluminum alloys or copper alloys. In at least one implementation, tubing 106 comprises interior wall 108 that is subject to the electropolishing process. Dielectric spacers 104 are shown in FIG. 1 for exemplary purposes. In at least one implementation, electropolishing apparatus 100 may comprise a continuous porous dielectric sheath around cathode 102 that provides a similar function as spacers 104, as described below. In at least one implementation, such a dielectric sheath may have similar mechanical properties as spacers 104.

[0059] In at least one implementation, cathode 102 can be coupled to a mechanical drive mechanism (not shown) that can push cathode 102 into tubing 106 and withdraw it when the electropolishing process is complete. In at least one implementation, cathode 102 may be part of a stand-alone apparatus that is manually controlled. In at least one implementation, electrolyte 110 may be translated back and forth within tubing 106 by reciprocating motion, to agitate electrolyte 110 within tubing 106. For example, horizontal arrows illustrated in FIG. 1 indicate possible movement of cathode 102 along tubing 106 suitable forms of agitation may be employed. In at least one implementation, cathode 102 can be electrically coupled to a potentiostat or other suitable power supply (not shown) that can provide electrical power to cathode 102 for driving an electropolishing process.

[0060] In at least one implementation, tubing 106 can be filled with a suitable electropolishing electrolyte 110 (light grey shading). In at least one implementation, electrolyte 110 may be pumped into interior of tubing 106. In at least one implementation, tubing 106 and electropolishing apparatus 100 can be immersed within a vat or bath of electrolyte 110. In at least one implementation, movement of cathode 102 can be in either direction. In at least one implementation, spacers 104 can have openings that permit flow of electrolyte 110 across spacers 104 (indicated by arrows) as cathode 102 is advanced or retracted along tubing 106.

[0061] In at least one implementation, as cathode 102 is electrified to a desired potential or set of potentials by a power supply (not shown), an electropolishing of interior wall 108 can ensue. In at least one implementation, a power supply can be a potentiostat (e.g., a power supply able to maintain a constant electrochemical potential at a working electrode when coupled to a reference electrode), which can impose a negative potential (e.g., a voltage) on cathode 102 and a positive potential on tubing 106 (anode). In at least one implementation, potentials can be referenced to a reference electrode (not shown) immersed in same electrolyte. In at least one implementation, electropolishing process dissolves asperities and other salient features on the surface of interior wall 108, evening out surface topography toward a flat surface. In at least one implementation, interior wall 108 can have an initial measurable mean (e.g., average) surface roughness Ra that has a value exceeding standards for ultra-high purity (UHP) tubing (e.g., Ra>1 μm). In at least one implementation, an industrial UHP standard for surface roughness of electropolished interior surfaces of stainless-steel tubing employed in gas handling service can be Ra=0.25 μm or less.

[0062] In at least one implementation, electrochemical conditions, including composition of electrolyte 110 and voltage applied to cathode 102, can be chosen to optimize electropolishing process. In at least one implementation, electropolishing process can adopt electropolishing conditions for stainless steel alloys set forth by the American Society for Testing and Materials (ASTM) standard B912. In at least one implementation, an electropolishing voltage maintained by a potentiostat or power supply can determine a current density between interior wall 108 and cathode 102. In at least one implementation, the current density can be optimized to obtain target value of Ra within an acceptable time frame. In at least one implementation, an electropolishing process can comprise multiple voltages applied in succession to cathode 102.

[0063] While a straight segment of tubing 106 is shown, in at least one implementation, tubing 106 can have sharp bends and other curvature.

[0064] FIG. 2 illustrates electropolishing apparatus assembly 200, comprising cathode 202 within sheath 204, in accordance with at least one implementation. In at least one implementation, sheath 204 comprises a porous flexible dielectric material. In at least one implementation, sheath 204 has a substantially cylindrical form factor to fit into cylindrical tubing. In at least one implementation, sheath 204 has a rectilinear form factor, such as a square cross section, for insertion into square tubing. In at least one implementation, sheath 204 is functionally graded radially, such that a peripheral portion of sheath 204 is more compliant than an a relatively hard interior portion of sheath 204. In at least one implementation, electropolishing apparatus assembly 200 can be inserted into tubing 206. In at least one implementation, tubing 206 has multiple 90-degree bends 208, 210, 212, 214, 216 and 218, as shown in FIG. 2.

[0065] To facilitate contact of electrolyte with interior wall 220 of tubing 206, in at least one implementation, sheath 204 comprises a plurality of epi-structures. In at least one implementation, epi-structures comprise raised regions, such as mesas or bumps on an exterior surface of sheath 204 (not shown), described below. In at least one implementation, epi-structures comprise fibers or bristles, described below. In at least one implementation, epi-structures on sheath 204 can be engineered to minimally contact interior wall 220 to enable electrolyte to readily flow or diffuse to interior wall 220. In at least one implementation, electrolytes can also infiltrate into pore structures within sheath 204 to reach cathode 202. In at least one implementation, pore structures within sheath 204 can be engineered to maintain lines-of-sight (LOS) between cathode and interior wall 220. Lines of sight (LOS) can be straight direct paths for current flow between cathode 202 and interior wall 220. Efficacity of electropolishing of complex workpieces is greatest if a large degree of line of sight is provided between cathode and target surfaces of an anode.

[0066] In at least one implementation, sheath 204 can maintain cathode 202 substantially along the central axis of metal tubing 206. As noted above, centering of cathode 202 can maintain uniform electropolishing of interior wall 220. In at least one implementation, sheath 204 can be functionally engineered for optimal flexibility and stiffness, as described below.

[0067] In at least one implementation, cathode 202 can comprise a metallic alloy capable of a superelastic phase transition (e.g., a transition from a martensitic phase to an austenitic phase) at operating temperature of an electropolishing process. As noted above, a superelastic phase transition of cathode 202 can enable cathode 202 to pass through bends 208-218 without undergoing plastic deformation (e.g., a shape memory effect; no permanent kinking). In at least one implementation, small axial and lateral stress forces can be experienced by cathode 202 when passing through bends 208-218, triggering a superelastic phase transition, rendering portions of cathode 202 supple when transiting through bends 208-218. For example, superelasticity can enable portions of cathode 202 transiting within bends 208-218 to bend elastically. In at least one implementation, 90-degree or larger bending of cathode 202 can be enabled, followed by resuming its original shape without plastic deformation (e.g., kinking) within straight segments of tubing 206. Elastic strain can be high (e.g., 200,000 parts per million (ppm), or approximately 20% of its initial length with relatively low stress of several megapascals). In at least one implementation, a metal alloy material of cathode 202 may be engineered to undergo a superelastic phase transition when encountering lateral and longitudinal forces generated by contact between sheath 204 and interior wall 220 during insertion of electropolishing apparatus assembly 200 into tubing 206. In at least one implementation, a metal alloy material of cathode 202 may be engineered to undergo a superelastic phase transition at temperatures at or near operating temperatures of one or more electropolishing processes.

[0068] During insertion or withdrawal of cathode 202, segments of cathode 202 passing out of bends 208-218, portions of cathode 202, such as bent portion 222 within bend 218, may no longer encounter significant mechanical forces within straight portions of tubing 206. Thus, while within straight portions of tubing 206 (e.g., straight tubing segment 224 following bent portion 222), cathode 202 can return to a non-elastic state (e.g., a transition from austenitic phase to martensitic phase). While some ordinary (e.g., non-superelastic) stainless steel or non-ferrous alloys can have sufficient flexibility to bend with sharp angles such as 90-degree angles of bends 208-218, they can experience significant plastic deformation. For example, a straight wire formed from spring steel may not recover its original shape after bending through first bend encountered (e.g., bend 208) when inserted into tubing 206. Such a cathode can become permanently kinked, and therefore difficult to advance further or retract within tubing 206.

[0069] FIG. 3A illustrates a 3D profile view of cathode assembly 300, comprising sheath 302 and cathode 304 embedded within sheath 302, in at least one implementation. In at least one implementation, sheath 302 has a circular cylindrical shape. In at least one implementation, cathode 304 extends substantially along a central axis of symmetry of sheath 302.

[0070] In at least one implementation, sheath 302 can comprise a polymeric material having a desired stiffness. In at least one implementation, polymeric material can have a uniform composition throughout the entire length of sheath 302. In at least one implementation, a polymeric material can comprise a fluoropolymer, such as polytetrafluoroethylene (PTFE), or a hydrocarbon such as polypropylene, a polyester such as polyethylene terephthalate (PET), a polyamide such as Nylon, or other suitable polymers. In at least one implementation, sheath 302 can be formed directly on cathode 304 or formed independently, for example by a 3D printing process. In at least one implementation, sheath 302 can have a radially graded composition for fine tuning stiffness. In at least one implementation, sheath 302 can comprise a radially graded polymer blend. In at least one implementation, manufacture of such a structure can comprise formation of multiple layers comprising a linear or non-linear gradation of blend composition. In at least one implementation, by such layering of compositionally distinct polymer blends, a linear or non-linear radial gradient of Young's modulus can be engineered. In at least one implementation, a stiffness gradient can be optimized by adjustment of number of layers and layer thicknesses, as well as material composition.

[0071] In at least one implementation, sheath 302 comprises a plurality of pores 306 (illustrated as white circles).

[0072] Mass transfer limitations may reduce overall electrochemical reaction rates and cause overpotentials to develop non-uniformly over cathode 304. Overpotentials may lead to non-uniform electropolishing of interior wall 312 due to non-uniform electrochemical reaction rates. In at least one implementation, pores 306 can have any suitable structure that enables rapid infusion of electrolytes through pores 306 to reach cathode 304 and for electrolytes to rapidly diffuse away from cathode 304. In at least one implementation, pores 306 can be straight pores that extend substantially linearly from surface 310 to cathode 304, where diameters are constant along the pore path. In at least one implementation, individual pores 306 that extend into interior of sheath 302 to cathode 304 along substantially linear paths can allow direct line of sight (LOS) between cathode 304 and interior wall 312 (denoted as a dashed line) of a tubing workpiece. In at least one implementation, pores 306 provide diffusion paths of electrolytes to and from cathode 304 during operation of cathode assembly 300. For enabling efficient electropolishing processes, pores 306 may be engineered to provide optimal line-of-sight diffusion paths between cathode 304 and interior wall 312. In at least one implementation, pore sidewall angles may be adjusted to form conical pores of decreasing diameter toward cathode 304. Conical pores can funnel diffusing electrolyte current from extended regions of interior wall 312 to cathode 304 in order to compensate pore diffusion limitations of mass transfer between cathode 304 and interior wall 312.

[0073] In at least one implementation, pores 306 are shown to have relatively large diameters of several millimeters (mm). In at least one implementation, pores 306 are also shown to be distributed in a regular pattern with a relatively low pore density (e.g., number of pores per unit area of surface 310). In at least one implementation, pores 306 can have submillimeter diameters and can be distributed in random patterns that have a higher pore density. In at least one implementation, pore density can also be optimized by adjustment of pore void volume relative to surrounding filled volume of sheath 302. In at least one implementation, pore density is adjusted to maximize cathode surface area while retaining mechanical integrity of sheath 302. For example, in at least one implementation, pores 306 may be engineered to leave 50% or greater of surface area of cathode 304 exposed to electrolyte, whereby cathode 304 can have direct line-of-sight (LOS) access to interior wall 312 for highest electropolishing efficacity.

[0074] While pores 306 are shown to be straight, in at least one implementation, pores 306 can also be engineered to extend along tortuous paths between surface 310 of sheath 302 and cathode 304. In at least one implementation, pores 306 can generally be organized in engineered structures that impart a radial gradation of stiffness within sheath 302. For example, in at least one implementation, pore structures can be organized such that graded stiffness of sheath 302 (e.g., quantified as a stiffness modulus) is smallest at the periphery, (e.g., near and at surface 310), and greatest at or near cathode 304. A graded stiffness can enable sheath 302 to readily yield to follow the curvature of sharp bends within a tubing bend (e.g., bends 208-218 in FIG. 2), yet be stiff enough to hold cathode 304 substantially along a center line of curvature of bend. For example, in at least one implementation, pores 306 can extend along substantially fractal paths between surface 310 and cathode 304, where pore diameters can increase with increasing depth below surface 310 (e.g., see FIG. 7).

[0075] Compositional gradation of Young's modulus may compensate for porosity distributions within sheath 302 that may not support desired functional gradation of mechanical properties of sheath 302. For example, in at least one implementation, the pore structure of sheath 302 can be uniform as shown. A uniform pore structure, whereby pores have constant diameter and uniform distribution, may not produce desired graded mechanical properties of sheath 302. For example, it may be desired that sheath 302 has a soft peripheral portion relative to its interior portion near cathode 304. To overcome this limitation, compositional gradation may be accomplished by compositional grading.

[0076] In at least one implementation, sheath 302 can have a functionally graded chemical composition from cathode 304 to surface 310 of sheath 302. In at least one implementation, a functional gradation of polymer blend can produce a radially graded Young's modulus. A gradation of polymer composition, for example, can enable a radially graded stiffness within sheath 302 comprising a uniform pore density. In at least one implementation, sheath 302 can be fabricated by an additive process, such as 3D printing, where a polymer composition can be compositionally graded. In at least one implementation, during additive manufacturing, material having varied composition can be added in layers, for example, by 3D printing. In at least one implementation, successive layers can comprise linearly or non-linearly varying polymer compositions, where stiffness (e.g., quantified as Young's modulus) of layers can be respectively linearly or non-linearly decreased during build of sheath 302

[0077] In at least one implementation, sheath 302 comprises an array of epi-structures to provide minimal contact with interior wall 312 and space for electrolyte infiltration between surface 310 of sheath 302 and interior wall 312. In at least one implementation, epi-structures can be raised structures 308, such as flat mesas or rounded bumps. In at least one implementation raised structures 308 are represented as rounded bumps spaced at regular or random intervals along surface 310 of sheath 302. While raised structures 308 are shown as bumps, flat-topped mesas may be equally represented by raised structures 308. In at least one implementation, raised structures 308 can be arranged in a square array pattern between pores 306, as illustrated in FIG. 3A. In at least one implementation, raised structures 308 can be arranged in any suitable pattern, such as a hexagonal staggered pattern. In at least one implementation, raised structures 308 can be hemispherical bumps as shown. In at least one implementation, hemispherical geometry of raised structures 308 can serve to minimize contact area of sheath 302 with interior wall 312. In at least one implementation, raised structures 308 can have other suitable geometries. In at least one implementation, raised structures 308 can have a conical geometry.

[0078] In at least one implementation, when engaged for electropolishing, raised structures 308 can serve as standoffs to center sheath 302 and cathode 304 within a tubing workpiece. In at least one implementation, distance d between individual raised structures 308 and height g of raised structures 308 can be engineered to create gap 314 over sheath 302, permitting a maximal surface area of interior wall 312 to be accessible to electrolyte. In at least one implementation, by adjustment of parameters d and g, gap 314 can allow electrolytes to be introduced at a desired flow rate between sheath 302 and interior wall 312. In at least one implementation, gap 314 can also facilitate permeation of electrolyte into pores 306.

[0079] FIG. 3B illustrates a cross-sectional view of cathode assembly 300 within tubing workpiece 316, showing pores 306 extending through sheath 302 along straight paths, in accordance with at least one implementation. Here, cross section in FIG. 3B is taken along cut A-A′ in FIG. 3A to show profiles of pores 306. In at least one implementation, pores 306 can have a conical shape, having diameter D that decreases toward cathode 304 along pore length L. A conical angle d of pores 306 may be adjusted to optimize current flow to cathode 304 and structural / mechanical properties of sheath 302. In at least one implementation, a conical pore shape may maximize flow of current between interior wall 312 and cathode 304, while decreasing void volume from periphery of sheath 302 to cathode 304, thereby creating a stiffness gradation of sheath 302. For example, higher void volume toward periphery of sheath 302 can decrease stiffness of sheath 302 in its periphery, while lower void volume near cathode 304 can increase stiffness in interior portion of sheath 302.

[0080] In at least one implementation, pores 306 extend a length L along straight paths below surface 310, reaching cathode 304. Line-of-sight (LOS) lines are indicated in FIG. 3B. It is understood that dimensions of pores 306 may not be drawn to scale, and that pores 306 may be significantly smaller than indicated. In at least one implementation, pores 306 are distributed in an azimuthally symmetrical manner around cathode 304, as shown. Pore density can be adjusted to expose maximum surface area of cathode 304 while maintaining structural integrity of sheath 302. In at least one implementation, the relative amount of exposed surface of cathode 304 and surface of cathode 304 covered by sheath material can be a function of D. In at least one implementation, D can be adjusted to optimize open cathode area for optimal electropolishing rate and covered cathode area for maximum integrity of cathode assembly 300. In at least one implementation, diameter D and density of pores 306, as well as pore distribution, can be adjusted such that at least 50% of surface area of cathode 304 is uncovered, (e.g., can be exposed to electrolyte), and have line-of-sight (LOS) access to interior wall 312. LOS can ensure that current flow between interior wall 312 and cathode 304 is unobstructed.

[0081] In at least one implementation, sheath 302 can comprise a polymer material having a graded or uniform composition. In at least one implementation, a radially graded composition can enable a radial stiffness gradient, as noted above. In at least one implementation, where sheath 302 comprises a uniform composition, a stiffness gradient can exist because a ratio of solid volume to void volume of pores 306 is reduced as it approaches cathode 304. While the illustrated implementation shows a uniform pore structure, some non-uniform pore structure implementations are described below.

[0082] FIG. 3C illustrates a top 3D view of cathode assembly 350, comprising sheath 352 and cathode 354 centered within sheath 352, in accordance with at least one implementation. In at least one implementation, sheath 352 comprises a plurality of pores 356 arranged in a regular array. In at least one implementation, sheath 352 further comprises an array comprising a plurality of scoop-shaped raised structures 358 extending above surface 360. In at least one implementation, raised structures 358 are distributed on surface 360 in a regular array. In at least one implementation, raised structures 358 can be collocated with pores 356. In at least one implementation, individual raised structures 358 are collocated between adjacent pores 356. In at least one implementation, as shown in inset, raised structure 358 comprises concave walls 362 and 364, adjacent to pores 356a and 356b, respectively. In at least one implementation, concave wall 362 and concave wall 364 face in opposite directions along an axis of sheath 352.

[0083] In at least one implementation, raised structures 358 can serve as standoffs to separate surface 360 from interior wall of tubing (e.g., interior wall 312, FIG. 3B), as described above. In at least one implementation, raised structures 358 can also serve as baffles to enhance transfer of electrolyte into pores 356 by functioning as scoops or paddles (e.g., of a water wheel). In at least one implementation, electrolyte can be agitated by small reciprocal motions of cathode assembly 350 while within a metal tube. In at least one implementation, concave wall 362 can concentrate flow of electrolyte into pore 356a when sheath 352 is translated toward the left side of figure. In at least one implementation, when sheath 352 is translated toward the right side of figure, concave wall 364 can concentrate flow of electrolyte into pore 356b.

[0084] FIG. 3D illustrates a cross-sectional view of cathode assembly 350, showing an internal structure of sheath 352, in accordance with at least one implementation. The cross section is taken from cut line A-A′ in FIG. 3C. In at least one implementation, pores extend from surface 360 at oblique angles. In at least one implementation, adjacent pores can have opposing orientations, as shown. In at least one implementation, bottoms of pores 356 can intersect interconnecting adjacent pores 356 to form V-shaped conduits. In at least one implementation, intersection of adjacent pores 356 can enhance contact of electrolytes with the surface of cathode 354 by increasing contact area. In at least one implementation, interconnection of pores 356 can enable entry and exit of electrolytes within pores 356 without relying mainly on diffusion to renew electrolytes at surface of cathode 354. In at least one implementation, during agitation, electrolytes can wash over exposed portions of cathode 354, enhancing mass transfer to and from cathode 354.

[0085] FIG. 4 illustrates a cross-sectional view of cathode assembly 400, comprising sheath 402 and cathode 404, in accordance with at least one implementation. In at least one implementation, sheath 402 comprises outer layer 406, which comprises a plurality of filaments 408, over core layer 410. In at least one implementation, filaments 408 can be bristles. In at least one implementation, core layer 410 can comprise a substantially solid structure. In at least one implementation, filaments 408 can extend radially from core layer 410, forming a substantially open structure within outer layer 406 (e.g., large open volume). In at least one implementation, outer layer 406 can have a significantly smaller stiffness with respect to core layer 410. In at least one implementation, an open volume between filaments 408 communicates with pores 412 within core layer 410. In at least one implementation, pores 412 can be organized in radial patterns, such as shown in FIG. 4. In at least one implementation, groups of pores 412 (between white areas, which represent solid regions between pore groups) converge at cathode 404, forming one large pore opening at surface of cathode 404. In at least one implementation, electrolytes can permeate space between filaments 408 to interface between outer layer 406 and core layer 410, entering pores 412. In at least one implementation, electrolyte can continue to diffuse toward cathode 404, filling pores 412.

[0086] In at least one implementation, filaments 408 have a fibrous structure. In at least one implementation, core layer 410 can have a significantly greater stiffness than outer layer 406. In at least one implementation, filaments 408 can have sufficient rigidity to aid in centering cathode 404. In at least one implementation, a stiffness gradient can be engineered by choice of structure and materials for two layers. In at least one implementation, exposed surface of cathode 404 can be adjusted by increasing or decreasing diameter and number of pores 412 within each group.

[0087] FIG. 5 illustrates a cross-sectional view of cathode assembly 500, comprising sheath 502 and cathode 504, in accordance with at least one implementation. In at least one implementation, sheath 502 can be like sheath 402 shown in FIG. 4, comprising outer layer 506, filaments (e.g., bristles) 508 and core layer 510. In at least one implementation, the structure of core layer 510 can be a variation of the example of core layer 410 shown in FIG. 4. In at least one implementation, spaces between filaments 508 can comprise open volume within outer layer 506 that communicates with pores 512 within core layer 510.

[0088] In at least one implementation, groups of pores 512 are arranged at periphery of core layer 510, converging after partial extension through core layer 510. In at least one implementation, groups of pores 512 converge to channels 514 and 516 (black shapes in core layer 510), which expand in diameter as they approach cathode 504. In at least one implementation, channels 514 are tributary channels of channels 516. In at least one implementation, during operation, electrolytes within open volume (white areas in outer layer 506) between filaments 508 can flow into pores 512 and into channels 514. In at least one implementation, channels 514 can feed channels 516, where electrolytes can contact exposed surfaces of cathode 504 at bases of channels 516. In at least one implementation, pore structure within core layer 510 can be fractal, where dimensions of channels 514 and 516 can be fractally related. In at least one implementation, other suitable geometrical relationships can be employed. In at least one implementation, exposed surfaces of cathode 504 and volume of solid material within core layer 510 (e.g., white regions) can be optimized to produce an engineered stiffness gradient within sheath 502.

[0089] FIG. 6 illustrates a cross-sectional view of cathode assembly 600, comprising sheath 602 and cathode 604 centered within sheath 602, in accordance with at least one implementation. In at least one implementation, sheath 602 comprises a plurality of filaments (e.g., bristles) 606 extending from cathode 604 toward tubing 608. In at least one implementation, filaments 606 may be divided into at least two subsets, whereby a first subset comprises long filaments 606A and a second subset comprises short filaments 606B. In at least one implementation, short filaments 606B may have a length L1 that is 20% or less than length L2 of long filaments 606A. In at least one implementation, length L1 of short filaments 606B may be 10% or less than length L2 of long filaments 606A. In at least one implementation, length L1 of short filaments 606B may be 5% or less than length L2 of long filaments 606A. In at least one implementation, ends 610 of long filaments 606A contact inner wall 612 of tubing to center cathode 604 within tubing 608 and provide mechanical resistance to lateral forces when navigating bends within tubing 608. In at least one implementation, length L2 of long filaments 606A may be substantially equal to inner radius of tubing 608 minus radius of cathode 604. As noted above, long filaments 606A can become stiffer as they are compressed. For example, long filaments 606A may stiffen as they are compressed when cathode assembly 600 negotiates a bend within tubing 608, providing increasing mechanical resistance between cathode 604 and inner wall 612 of tubing 608 to prevent a short circuit or arcing between cathode 604 and tubing 608 (anode). In at least one implementation, long filaments 606A may be engineered to center cathode 604 within tubing 608 within a desired tolerance (e.g., within 10% of a diameter of tubing 608). In at least one implementation, long filaments 606A may be engineered to maintain a minimum distance between cathode 604 and inner wall 612 while negotiating a gradual bend or a sharp bend (e.g., 30 degrees or more), for example.

[0090] In at least one implementation, short filaments 606B may be collocated with long filaments 606A. For example, in at least one implementation, short filaments 606B may be collocated between adjacent long filaments 606A, as shown. Short filaments 606B may provide an additional mechanical barrier to mitigate direct contact between cathode 604 and inner wall 612 of tubing 608 should long filaments 606A fail to maintain a minimum distance, as noted above. While in at least one implementation long filaments 606A and short filaments 606B have same diameter and comprise same material, by virtue of their shorter length, short filaments 606B can have a larger spring constant than long filaments 606A. Short filaments 606B may resist greater lateral forces than long filaments 606A, providing a hard barrier to contact between cathode 604 and inner wall 612.

[0091] In at least one implementation, long filaments 606A and short filaments 606B can be directly tethered to cathode 604. In at least one implementation, long filaments 606A and short filaments 606B may be engineered to provide sufficient exposed cathode surface area and line of site (LOS) to inner wall 612. For example, in at least one implementation, diameters and densities of long filaments 606A and short filaments 606B may be engineered to provide at least 50% exposed cathode surface area.

[0092] FIG. 7 illustrates a cross-sectional view of cathode assembly 700, comprising sheath 702 and cathode 704 centered within sheath 702, in accordance with at least one implementation. In at least one implementation, sheath 702 comprises pluralities of pores 706a and 706b in a fractal pore network extending between cathode 704 and surface 708. In at least one implementation, sizes of pores 706a decrease significantly as the pore network approaches surface 708. In at least one implementation, pore densities rapidly increase approaching surface 708. In at least one implementation, along fractal paths, diameters of pores 706a and 706b can be smallest at surface 708, while density of pores 706a can be largest at surface 708. In at least one implementation, change of pore structure can be gradual with depth below surface 708. In at least one implementation, as fractal paths approach cathode 704, the trend can reverse, where densities of pores 706b can be least near cathode 704, while pore diameters can be greatest at cathode 704.

[0093] In at least one implementation, within core zone 710, pores 706b are few, but can be relatively large in comparison to pores 706a within peripheral zone 712. In at least one implementation, successive levels of pores 706a can branch from larger pores deeper within peripheral zone 712. At surface 708, pore densities can be maximal within sheath 702. In at least one implementation, pore densities within peripheral zone 712 can be graded by virtue of fractal branching patterns of the pore structure. In at least one implementation, stiffness of peripheral zone 712 of sheath 702 can also be graded due to increasing pore density, where stiffness can decrease toward surface 708. In at least one implementation, sheath 702 can have a sponge-like structure and softness within peripheral zone 712.

[0094] In at least one implementation, a plurality of epi-structures, shown as bumps 714, extend from surface 708. In at least one implementation, bumps 714 can be arranged in any suitable pattern on surface 708. In at least one implementation, bumps 714 can have a rounded or hemispherical shapes to minimize contact area with an interior wall of a section of metal tubing, while providing space between surface 708 and inner wall. In at least one implementation, other suitable shapes, such as a conical form factor, can be employed. In at least one implementation, bumps 714 can enable minimal contact of sheath 702 with an interior wall of a metal tubing to be electropolished. In at least one implementation, bumps 714 can be engineered to optimize electrolyte flow between surface 708 and inner wall. In at least one implementation, during operation, pores 706a can be infused with electrolytes flowing over surface 708. In at least one implementation, electrolytes can readily diffuse to deeper levels within peripheral zone 712. Smaller pores can feed successively larger pores, finally feeding pores 706b within core zone 710. In at least one implementation, pores 706b within core zone 710 communicate with cathode 704. As such, pores 706b can have relatively large diameters to cover large portions of a cathode surface. In at least one implementation, while pores 706b can be relatively large in comparison to pores 706a within peripheral zone 712, core zone 710 can be relatively stiff with respect to peripheral zone 712, due to relatively small numbers of pores 706b.

[0095] FIG. 8A illustrates an end-on view of cathode assembly 800, comprising spacer 802 and cathode 804, where spacer 802 can comprise a short section of sheath material, in accordance with at least one implementation. In at least one implementation, multiple spacers 802 can be distributed at intervals over cathode 804 (e.g., see FIG. 8C). In at least one implementation, spacer 802 can provide similar functions as any of the sheaths described in this disclosure (e.g., sheath 302). In at least one implementation, spacer 802 can primarily be employed to center cathode 804 within a section of metal tubing, maintaining cathode 804 substantially equidistant from interior wall 810 (represented by dashed circle) of tubing. As shown in FIG. 8C, spacer 802 can have a small thickness relative to length of cathode 804, leaving a large portion of cathode 804 directly exposed to electrolytes, in at least one implementation. In at least one implementation, spacer 802 has a uniform composition and stiffness. In at least one implementation, spacer 802 can comprise a material (e.g., a polymer composition) that has a desired Young's modulus. In at least one implementation, spacer 802 can comprise a solid body that lacks a porosity, as pores are not a primary means to bring electrolytes to cathode 804, as would be case for a sheath (e.g., sheath 302).

[0096] In at least one implementation, spacer 802 comprises a plurality of epi-structures, represented here as bumps 806 to provide minimal contact area with interior wall 810 of a section of tubing 812. In at least one implementation, minimal contact area can minimize friction between spacer 802 and interior wall 810 when cathode assembly 800 is translated through metal tubing. In at least one implementation, spacer 802 can comprise multiple openings 808. In at least one implementation, openings 808 can enable flow of electrolytes across spacer 802 during translation of cathode assembly 800, for example. In at least one implementation, openings 808 can be engineered to optimize electrolyte flow across spacer 802. In at least one implementation, number of openings 808 and diameter D1 of openings 808 can be adjusted to maximize flow of electrolyte. In at least one implementation, adjustment of number and diameter of openings 808 can be employed to optimize stiffness of spacers 802. In at least one implementation, spacers 802 can exhibit an optimal stiffness to facilitate movement of cathode assembly 800 through bends in tubing 812, while maintaining cathode 804 substantially equidistant from interior wall 810.

[0097] FIG. 8B illustrates an end-on view of cathode assembly 850, comprising spacer 852 and cathode 854, in accordance with one implementation. In contrast to sheaths described above, spacer 852 has a relatively small axial extent (e.g., thickness) relative to its diameter. In at least one implementation, spacer 852 can have a diameter to thickness aspect ratio of 1:1 or greater. In at least one implementation, a plurality of spacers 852 can be distributed at intervals along length of cathode 854, leaving a significant percentage of surface of cathode 854 directly exposed to electrolyte. In at least one implementation, plurality of spacers 852 can maintain cathode 854 substantially equidistant from interior wall 860 of tubing 862.

[0098] In at least one implementation, spacer 852 can comprise a suitable nonporous polymer material. In at least one implementation, material can have a desired Young's modulus and modulus of elasticity, for example, enabling a balance between stiffness and compliance. In at least one implementation, spacer material can be radially graded to permit a more rigid core and softer periphery.

[0099] In at least one implementation, spacer 852 comprises a plurality of lobes 856. In at least one implementation, lobes 856 are arranged in a symmetric star pattern. In at least one implementation, lobes 856 can have any suitable shape and angular distribution. In at least one implementation, lobes 856 have rounded ends to perform a similar function for minimizing contact area with interior wall 810, as do bumps 806 described for spacer 802 (see FIG. 7). In at least one implementation, spaces in between lobes 856 can channel flow of electrolytes through spacer 852, enabling bulk electrolytes to wash over exposed segments of cathode 854 between spacers 852.

[0100] FIG. 8C illustrates a cross-sectional view of cathode assembly 800. It is understood that cathode assembly 850, shown in FIG. 8B, can also be represented in FIG. 8C. In at least one implementation, spacers 852 can be distributed along cathode 854 at intervals d1, as shown in illustrated example, where d1 can be several times thickness t. In at least one implementation, spacers 852 can have a small axial extent, or thickness t, relative to their diameter D2. In at least one implementation, lobes 856 contact interior wall 860 of tubing 862. While in cross-sectional view of FIG. 8C, openings such as openings 808 shown in FIG. 8A are not indicated, it is understood that they are present to enable flow of electrolyte across spacers 852.

[0101] FIGS. 9A-9D illustrate an exemplary method for operating an electropolishing apparatus for electrochemical polishing of interior walls of tubing according to some implementations of disclosure. FIG. 9A illustrates a cross-sectional view of cathode assembly 900 partially inserted into a first segment of tubing 902. In at least one implementation, cathode assembly 900 can be part of a larger electropolishing apparatus (not shown). In at least one implementation, metal tubing 902 comprises multiple 90-degree bends, such as bends 904, 906 and 908, through which cathode assembly 900 passes. In at least one implementation, cathode assembly 900 comprises sheath 910 and cathode 912 embedded within sheath 910. In at least one implementation, cathode assembly 900 can comprise guide tip 914 to aid negotiation of bends 904-908 by cathode assembly 900.

[0102] In at least one implementation, cathode 912 can comprise a superelastic metal alloy, such as nickel-titanium alloys (e.g., Nitinol, approximately 50% nickel and 50% titanium). Other suitable superelastic alloys are described above. Such alloys can undergo superelastic phase transitions (e.g., a martensite-austenite phase transition) by imposing mild stress on material. Elastic strain resulting from mild stresses can be as much as 18%. In at least one implementation, stresses can be tuned to range up to several hundred megapascals (MPa) at a desired temperature by alteration of alloy systems, composition ratio, and / or thermo-mechanical conditioning (e.g., annealing). Thus, portions of cathode 912 experiencing mechanical stresses induced by interaction with interior wall 916 can immediately transition to a superelastic phase.

[0103] In FIG. 9B, cathode assembly 900 has advanced within tubing 902. Guide tip 914 is pressed against interior wall 916 of tubing 902. Cathode 912 begins to bend in response to impinging axial compressive stresses as well as transverse stresses on cathode 912 by contact of guide tip 914 on interior wall 916 at bend 904. In at least one implementation, at least lead portion 918 (delineated by dashed box) of cathode 912 can react to impinging stresses by undergoing a phase transition to a superelastic phase (e.g., a martensite-austenite phase transition). In at least one implementation, elastic property enables cathode 912 to readily follow curvature of bend 904 without experiencing plastic deformation.

[0104] In FIG. 9C, lead portion 920 of cathode assembly 900 (including front portion of sheath 910, delineated by dashed box) has passed through bend 904 and continues to advance through straight segment 922 of tubing 902. Relieved of stresses encountered within bend 904, lead portion of cathode 912 (e.g., lead portion 918) reverts to a straight rod. In at least one implementation, the superelastic property enables cathode 912 to exhibit a shape memory, allowing it to revert to its original shape when not experiencing stress. In at least one implementation, the shape memory property of cathode 912 obviates plastic deformation, thereby avoiding kinks and other permanent deformations in cathode 912.

[0105] In at least one implementation, sheath 910 can be advanced through bend 904. In at least one implementation, sheath 910 can be a functionally graded structure, as described above. In at least one implementation, sheath 910 can exhibit a radially graded stiffness, enabling sheath 910 to be sufficiently compliant to negotiate 90-degree curvature of bend 904 while maintaining cathode 912 substantially equidistant from interior wall 916. In at least one implementation, balance between stiffness and compliance can be engineered according to methods described above. In at least one implementation, porosity within sheath 910 can be engineered to create a radial stiffness gradient, where stiffness decreases from center to periphery of sheath 910. In at least one implementation, gradient can be monotonic (e.g., gradual) or stepwise. In at least one implementation, sheath 910 can be engineered to exhibit a radial gradient of Young's modulus. This can be accomplished by growing sheath 910 in layers comprising a polymer bend, for example. In at least one implementation, the composition of the polymer blend can be systematically varied in successive layers. In at least one implementation, stiffness can be graded by combining compositional gradients with structural gradients. In at least one implementation, porosity within sheath 910 can be radially graded along with composition.

[0106] In FIG. 9D, cathode assembly 900 has advanced through bends 906 and 908. At this stage, electrolytes can be introduced by flooding into tubing 902. In at least one implementation, electrolytes can initially fill gaps between interior wall 916 and surface of sheath 910, then permeate pore structure within sheath 910. In at least one implementation, while not shown in figure, sheath 910 can be replaced by spacers (e.g., spacers 802 or 852, FIGS. 8A and 8B). In at least one implementation, spacers can also have engineered stiffness to comply with tight curvature of bends 904-908. In at least one implementation, most of cathode 912 can be directly exposed to electrolyte.

[0107] In at least one implementation, after introduction of electrolyte into tubing 902, cathode assembly 900 can be set into a small amplitude reciprocal motion by a mechanism within electropolishing apparatus. In at least one implementation, cathode assembly 900 is advanced and retracted small distances within tubing 902. In at least one implementation, reciprocal motion can agitate electrolyte to enhance mass transfer into and out of pores within sheath 910.

[0108] The following are additional examples provided in view of the above-described implementations. Here, one or more features of example, in isolation or in combination, can be combined with one or more features of one or more other examples to form further examples also falling within the scope of the disclosure. As such, one implementation can be combined with one or more other implementation without changing scope of disclosure.

[0109] Example 1 is an electropolishing apparatus, comprising a cathode, wherein the cathode has a length and a diameter; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises a superelastic metal alloy.

[0110] Example 2 is an electropolishing apparatus of any example herein, particularly example 1, wherein the length of the cathode is at least twice the diameter of the cathode.

[0111] Example 3 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

[0112] Example 4 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of pores that extend from the cathode to an exterior surface of the sheath.

[0113] Example 5 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a straight path.

[0114] Example 6 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a tortuous path.

[0115] Example is an electropolishing apparatus of any example herein, particularly example 6, wherein the plurality of pores is interconnected in a pore network that extends between the cathode and the exterior surface of the sheath, wherein ones of the plurality of pores include a pore diameter and a pore length, wherein the pore diameter and the pore length are to decrease with increasing distance from the cathode and are smallest near the exterior surface of the sheath.

[0116] Example 8 is an electropolishing apparatus of any example herein, particularly example 7, wherein the pore network comprises a fractal network comprising interconnected pores extending from the cathode to the exterior surface of the sheath, wherein the pore diameter and the pore length decrease non-linearly with distance from the cathode.

[0117] Example 9 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of compliant epi-structures.

[0118] Example 10 is an electropolishing apparatus of any example herein, particularly example 9, wherein the plurality of compliant epi-structures comprises a plurality of raised structures distributed on a surface of the sheath.

[0119] Example 11 is an electropolishing apparatus of any example herein, particularly example 10, wherein the plurality of raised structures comprises one or more concave surfaces.

[0120] Example 12 is an electropolishing apparatus of any example herein, particularly example 10, wherein the plurality of raised structures comprises a plurality of filaments.

[0121] Example 13 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments extends at least partially between the surface of the cathode to an exterior surface of the sheath.

[0122] Example 14 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments comprises a first subset of filaments having a first length, and a second subset of filaments having a second length, wherein the second length is 20% or less of the first length.

[0123] Example 15 is an electropolishing apparatus of any example herein, particularly example 12, wherein the plurality of filaments extends between an exterior surface of the sheath to an inner region of the sheath, wherein the inner region comprises a porous solid dielectric material.

[0124] Example 16 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a plurality of spacers distributed along the length of the cathode, and wherein portions of the cathode are exposed between individual ones of the plurality of spacers.

[0125] Example 17 is an electropolishing apparatus of any example herein, particularly example 16, wherein individual ones of the plurality of spacers comprise a plurality of openings.

[0126] Example 18 is an electropolishing apparatus of any example herein, particularly example 16, wherein individual ones of the plurality of spacers comprise one or more arms.

[0127] Example 19 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a polymer blend, wherein a composition of the polymer blend is graded to produce a gradient of Young's modulus between the cathode and a surface of the dielectric.

[0128] Example 20 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric comprises a fluoropolymer, a polyester, a polyamine or a polypropylene.

[0129] Example 21 is an electropolishing system comprising a metal tubing and an electropolishing apparatus comprising: a cathode, wherein the cathode has a length; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy, and wherein the dielectric comprises a functionally graded material, wherein a cathode assembly comprises the cathode and the dielectric, wherein the cathode assembly is positioned within the metal tubing.

[0130] Example 22 is an electropolishing system of any example herein, particularly example 21, wherein the metal tubing has a curvature comprising one or more bends along the metal tubing.

[0131] Example 23 is an electropolishing system of any example herein, particularly example 21, further comprising a power supply electrically coupled to the cathode.

[0132] Example 24 is an electropolishing system of any example herein, particularly example 21, further comprising an electrolyte within the metal tubing.

[0133] Example 25 is an electropolishing system of any example herein, particularly example 21, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

[0134] Example 26 is a method for electropolishing comprising providing an electropolishing apparatus, wherein the electropolishing apparatus comprises: a cathode, wherein the cathode has a length; and a dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy and the dielectric comprises a functionally graded material, wherein a cathode assembly comprises the cathode and the dielectric, wherein the method further comprises: inserting the cathode assembly into a tubing, wherein the tubing comprises one or more bends; and agitating an electrolyte in the tubing.

[0135] Example 27 is a method for electropolishing of any example herein, particularly example 26, wherein inserting the cathode assembly into the tubing comprises passing the cathode assembly through the one or more bends.

[0136] Example 28 is a method for electropolishing of any example herein, particularly example 26, wherein agitating the electrolyte in the tubing comprises translating the cathode assembly by reciprocating motion.

[0137] Besides what is described herein, various modifications can be made to disclose implementations and implementations thereof without departing from their scope. Therefore, illustrations of implementations herein should be construed as examples, and not restrictive to scope of present disclosure.

Examples

example 3

[0111 is an electropolishing apparatus of any example herein, particularly example 1, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

example 4

[0112 is an electropolishing apparatus of any example herein, particularly example 3, wherein the sheath comprises a plurality of pores that extend from the cathode to an exterior surface of the sheath.

example 5

[0113 is an electropolishing apparatus of any example herein, particularly example 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a straight path.

Claims

1. An electropolishing apparatus, comprising:a cathode, wherein the cathode has a length and a diameter; anda dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises a superelastic metal alloy.

2. The electropolishing apparatus of claim 1, wherein the length of the cathode is at least twice the diameter of the cathode.

3. The electropolishing apparatus of claim 1, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

4. The electropolishing apparatus of claim 3, wherein the sheath comprises a plurality of pores that extend from the cathode to an exterior surface of the sheath.

5. The electropolishing apparatus of claim 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a straight path.

6. The electropolishing apparatus of claim 4, wherein ones of the plurality of pores extend from the cathode to the exterior surface of the sheath along a tortuous path.

7. The electropolishing apparatus of claim 6, wherein the plurality of pores is interconnected in a pore network that extends between the cathode and the exterior surface of the sheath, wherein ones of the plurality of pores include a pore diameter and a pore length, wherein the pore diameter and the pore length are to decrease with increasing distance from the cathode and are smallest near the exterior surface of the sheath.

8. The electropolishing apparatus of claim 7, wherein the pore network comprises a fractal network comprising interconnected pores extending from the cathode to the exterior surface of the sheath, wherein the pore diameter and the pore length decrease non-linearly with distance from the cathode.

9. The electropolishing apparatus of claim 3, wherein the sheath comprises a plurality of compliant epi-structures.

10. The electropolishing apparatus of claim 9, wherein the plurality of compliant epi-structures comprises a plurality of raised structures distributed on a surface of the sheath.

11. The electropolishing apparatus of claim 10, wherein the plurality of raised structures comprises one or more concave surfaces.

12. The electropolishing apparatus of claim 10, wherein the plurality of raised structures comprises a plurality of filaments.

13. The electropolishing apparatus of claim 12, wherein the plurality of filaments extends at least partially between the surface of the cathode to an exterior surface of the sheath.

14. The electropolishing apparatus of claim 12, wherein the plurality of filaments comprises a first subset of filaments having a first length, and a second subset of filaments having a second length, wherein the second length is 20% or less of the first length.

15. The electropolishing apparatus of claim 12, wherein the plurality of filaments extends between an exterior surface of the sheath to an inner region of the sheath, wherein the inner region comprises a porous solid dielectric material.

16. The electropolishing apparatus of claim 1, wherein the dielectric comprises a plurality of spacers distributed along the length of the cathode, and wherein portions of the cathode are exposed between individual ones of the plurality of spacers.

17. The electropolishing apparatus of claim 16, wherein individual ones of the plurality of spacers comprise a plurality of openings.

18. The electropolishing apparatus of claim 16, wherein individual ones of the plurality of spacers comprise one or more arms.

19. The electropolishing apparatus of claim 1, wherein the dielectric comprises a polymer blend, wherein a composition of the polymer blend is graded to produce a gradient of Young's modulus between the cathode and a surface of the dielectric.

20. The electropolishing apparatus of claim 1, wherein the dielectric comprises a fluoropolymer, a polyester, a polyamine, or a polypropylene.

21. A system, comprising:a metal tubing; andan electropolishing apparatus comprising:a cathode, wherein the cathode has a length; anda dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy, and wherein the dielectric comprises a functionally graded material,wherein a cathode assembly comprises the cathode and the dielectric, wherein the cathode assembly is positioned within the metal tubing.

22. The system of claim 21, wherein the metal tubing has a curvature comprising one or more bends along the metal tubing.

23. The system of claim 21, further comprising a power supply electrically coupled to the cathode.

24. The system of claim 21, further comprising an electrolyte within the metal tubing.

25. The system of claim 21, wherein the dielectric is a sheath around the cathode, wherein the sheath extends at least partially along the length of the cathode, and wherein the cathode extends substantially along a central axis of the sheath.

26. A method, comprising:providing an electropolishing apparatus, wherein the electropolishing apparatus comprises:a cathode, wherein the cathode has a length; anda dielectric around the cathode, wherein the dielectric extends at least partially along the length of the cathode, wherein the cathode comprises an elastic metal alloy and the dielectric comprises a functionally graded material, wherein a cathode assembly comprises the cathode and the dielectric, wherein the method further comprises:inserting the cathode assembly into a tubing, wherein the tubing comprises one or more bends; andagitating an electrolyte in the tubing.

27. The method of claim 26, wherein inserting the cathode assembly into the tubing comprises passing the cathode assembly through the one or more bends.

28. The method of claim 26, wherein agitating the electrolyte in the tubing comprises translating the cathode assembly by reciprocating motion.

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