Continuous 3D printing of composites for microstructures

Abstract

A method of continuous 3D printing to print an article is provided. The method includes displacing a nozzle of the 3D printer in a nozzle displacement direction; extruding the ink from the nozzle concurrently with the displacement of the nozzle to form an extrudate on the porous substrate; and displacing the rotating magnetic field in the nozzle displacement direction simultaneously with the displacement of the nozzle. The ink includes a plurality of magnetically responsive particles and a colloidal gel. The colloidal gel may include fumed silica in an aqueous solution.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to Singapore Patent Application No. 10202250710M filed on August 11, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to the field of additive manufacturing, and more particularly to 3D printing of microstructures. Background Art

[0004] Direct ink writing is an extrusion-based additive manufacturing technology that involves the extrusion of viscoelastic ink from a nozzle and the layer-by-layer deposition of the ink to build a 3D structure on a computer-controlled translation platform. In conventional direct ink writing, the ink must be a paste with high viscosity and elasticity so that the printed material can have the shape retention properties required to build multiple layers. Conventional direct ink writing methods cannot produce 3D printed products with anisotropic properties. Summary of the invention

[0005] In one aspect, the present application discloses a method for continuous 3D printing to print an object. The method includes displacing a nozzle of a 3D printer along a nozzle displacement direction; simultaneously with the displacement of the nozzle, extruding ink from the nozzle to form an extrudate on a porous substrate; and simultaneously with the displacement of the nozzle, displacing a rotating magnetic field along the nozzle displacement direction. The magnetic field is a rotating magnetic field characterized by having a rotation speed greater than 1 Hz. The ink includes a plurality of magnetically responsive micron particles and a colloidal gel. The colloidal gel may include fumed silica nanoparticles in an aqueous solution.

[0006] On the other hand, the present application discloses an ink for use in a continuous 3D printing method. The ink includes a plurality of magnetically responsive micron particles. The magnetically responsive particles include: a plurality of microplatelets; and a plurality of magnetically responsive nanoparticles adsorbed on the surfaces of each of the plurality of microplatelets. The ink may be selected from a material characterized by a viscosity of up to 1000 Pa.s and a yield stress of up to 45 Pa for a preferred operating range of uniform arrangement capability (of the magnetically responsive micron particles) and shear thinning properties. In some embodiments, the ink includes water; and 4% by weight (wt%) of fumed silica. In some embodiments, the ink is characterized by a viscosity of less than 1000 Pa.s and a yield stress of less than 45 Pa. In some embodiments, examples of 3D printable inks include, but are not limited to, a component comprising 20 wt% of mA12O3, the component being characterized by a viscosity of 200 Pa.s and a yield stress of 4 Pa.

[0007] In another aspect, the present application discloses an apparatus for use in a continuous 3D printing method. The apparatus includes a 3D printer having a nozzle; and a magnet coupled to the nozzle. The magnet can rotate relative to the nozzle to provide a rotating magnetic field. The magnet and the nozzle can be displaced together along a nozzle displacement path to provide a gradient magnetic field, the gradient magnetic field being characterized by having a magnetic field strength that decreases as the distance from the nozzle increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1A is a schematic diagram of a device and method for continuous 3D printing according to an embodiment of the present disclosure;

[0009] Figure 1B is an electron micrograph of Al2O3 microplatelets (mAl2O3) modified with superparamagnetic iron oxide nanoparticles as an example of magnetically responsive particles in ink according to an embodiment of the present disclosure;

[0010] Figure 2A is a schematic side view of an apparatus according to another embodiment;

[0011] Figure 2B yes Figure 2A A schematic diagram of a top view of the device shown;

[0012] Figure 3A is a perspective schematic diagram of another embodiment of the device;

[0013] Figure 3B yes Figure 3A A schematic top view of the device shown;

[0014] Figure 4is a schematic diagram showing the time for magnetically consistent alignment, accumulation time, and time associated with 3D printing as a function of viscosity;

[0015] Figure 5 is a schematic diagram showing magnetic alignment time, accumulation time, and time associated with 3D printing as a function of the number of printed layers;

[0016] Fig. 6A The kinetics of uniform arrangement of individual magnetic flakes in resins of different background viscosities (resins with increasing concentrations of fumed SiO2 at 0wt%, 5wt%, 6wt%, 7wt%, 7.5wt% and 8wt%, respectively) are shown, with multiple magnetic flakes demonstrating the ability to uniformly arrange in resins with SiO2 content up to 7.5wt%;

[0017] Figure 6B is the yield stress σ of SiO2 with different weight percentage (wt%) to the ink y The influence curve of

[0018] Figure 6C It is shown that in a polymer resin with 5 wt% SiO2, the uniform arrangement degree (D L / D W ) and the consistent arrangement time t m Multiple curves as a function of Fe3O4 coating;

[0019] Fig.6D It shows that for the microsheets with 14% volume percentage (vol%) of Fe3O4 coating, the uniform arrangement degree (D L / D W ) and the consistent arrangement time t m Multiple curves as a function of the weight percentage (wt%) of SiO2 in the resin;

[0020] Fig. 6E are images showing different coherent arrangement dynamics of individual magnetic microplatelets in resins of different background viscosities (resins with increasing concentrations of gas-prepared SiO2 of 0wt%, 5wt%, 6wt%, 7wt%, 7.5wt% and 8wt%), indicating that the microplatelets are able to be coherently arranged in resins with SiO2 contents up to 7.5wt%;

[0021] Fig. 7A shows multiple curves of viscosity as a function of shear rate for aqueous inks containing 4 wt % SiO2 and increasing concentrations of mA12O3;

[0022] Figure 7Bis a graph showing the effect of different weight percentages (wt%) of mA12O3 on the yield stress of an ink containing water and 4wt% SiO2;

[0023] Figure 7C A plurality of curves showing the relationship between shear stress and shear rate for inks having concentrations of mA12O3 ranging from 15 wt% to 35 wt%;

[0024] Fig.7D Shown for Figure 7C The ink shown has multiple curves of elastic modulus G' and viscous modulus G'' as a function of shear stress;

[0025] Fig. 7E Shows the consistent arrangement quality of the ink (D L / D W ) and multiple curves of zero shear viscosity as a function of microplatelet loading;

[0026] Figure 7F It is shown that for 50 mT and 100 mT magnetic fields, t m Theoretical calculations as a function of ink viscosity;

[0027] Figure 7G The accumulation time t is shown when printing on a gypsum substrate using an ink containing 20 wt% mA12O3 at 23°C, 40°C and 50°C. j As a function of the number of printed layers n;

[0028] FIG. 8A to FIG. 8E is a schematic diagram and corresponding electron micrograph of a 3D printed microstructure obtained using an aqueous ink containing 20wt% mA12O3 and 4wt% SiO2;

[0029] Fig. 9 are images of 3D-printed samples with square, star, and circle shapes;

[0030] Fig.10 is an image of a porous, 3D-printed sample;

[0031] Fig.11 is an image of a sintered sample with magnetic response properties;

[0032] Fig.12 A 3D printed object with magnetically responsive shape-changing functionality is shown;

[0033] Fig.13 This is an electron micrograph of a 3D-printed graphite sample with consistent vertically arranged microstructures;

[0034] Fig.14The electrical conductivity of the 3D printed graphite composite is shown, with the conductivity measured (a) on the surface of the sample having a consistent horizontal (lateral) arrangement, (b) along the surface of the consistent vertical (axial) arrangement, and (c) perpendicular to the surface of the consistent vertical (axial) arrangement, and Fig.14 Also shown is the conductivity of the graphite composite reduced by reduction at a higher temperature of 1050°C after 4 hours, as indicated by the shaded area;

[0035] Fig.15 Examples showing the application of 3D printed magnetoelectric composites for remote actuation; and

[0036] Fig.16 An example of another application of 3D printed graphite composites for electromagnetic shielding is shown. DETAILED DESCRIPTION

[0037] References to "one embodiment," "another embodiment," or "an embodiment" (or similar references) in this specification mean that a particular feature, structure, or characteristic described with respect to that embodiment is included in at least one embodiment. Therefore, the appearance of "in one embodiment" or "in an embodiment" or similar phrases in different places in this specification do not necessarily refer to the same embodiment. In addition, the described features, structures, or characteristics may be combined in any appropriate manner in one or more embodiments. In the following description, many specific details are provided to give a comprehensive understanding of the embodiments. Those skilled in the art will recognize that different embodiments may be practiced without one or more specific details, or by other methods, components, materials, etc. In other cases, to avoid confusion, some or all known structures, materials, or operations may not be shown or described in detail.

[0038] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular forms "a," "an," and "an" may be construed to include the plural forms "one or more" unless the context clearly indicates otherwise.

[0039] Terms such as "first" and "second" used in the specification and claims are used only for the purpose of brevity and clarity, and do not necessarily imply priority or order unless the context requires otherwise. The terms "about" and "substantially" applied to numerical values ​​include both the exact value and reasonable differences that those skilled in the art will understand, and the terms "generally" and "substantially" should be understood in a similar manner unless otherwise specified.

[0040] In the present disclosure, the terms "three-dimensional printing" (3D printing) and additive manufacturing may be used interchangeably to refer to a method of making a predetermined product by applying material, rather than conventional subtractive methods such as milling. Examples of 3D printing methods include (and are not limited to) continuous 3D printing methods, such as inkjet printing, ink direct writing, etc. Continuous 3D printing can be described as a method of providing material in the form of an extrudate from a traveling nozzle (rather than providing the material as a powder bed). Continuous 3D printing also includes a method in which the target shape of a solid object is substantially determined by one or more layers of material deposited or printed according to a corresponding computer-aided design (CAD) model. It will be appreciated by those skilled in the art that conventional molding processes must address issues such as uneven distribution / flow of materials within the mold cavity, the formation of voids or air pockets, and flash, and that conventional molding processes may be particularly challenging for relatively complex shapes. In contrast, continuous 3D printing can provide greater freedom to build new shapes outside the limitations of conventional molds.

[0041] For simplicity, the terms "extrusion", "jetting", "deposition", "writing", "printing", and "printing" are used interchangeably to refer to the delivery of 3D printing materials on a substrate or previously printed layers in a process such as direct ink writing. For simplicity, the term "porous substrate" used herein refers to one or both of the substrate and the previously printed layer.

[0042] When describing a process or some mechanisms, the present disclosure may refer to "steps" to assist understanding and facilitate reference only. Those skilled in the art will appreciate that in actual operation and in different embodiments, different "steps" may overlap, run in parallel, or be different, thereby distinguishing into discrete, multiple ordered parts in different combinations or arrangements.

[0043] According to various embodiments of the present disclosure, example methods, articles made therefrom, devices, materials, etc., are described herein with the aid of figures and test data / results, which are only to aid understanding and are not limiting.

[0044] 3D printing equipment

[0045] Figure 1A 1 is a schematic diagram showing an apparatus 100 and a method 500 according to an embodiment of the present disclosure. The apparatus 100 includes a 3D printer 110 having a nozzle 120. During operation, in the presence of a rotating magnetic field (H), while the ink 300 is extruded from the nozzle 120 and deposited on the substrate 150 or on a previously printed layer, the apparatus 100 moves along the nozzle displacement direction 209 at a nozzle velocity V pThe nozzle 120 is moved to perform continuous 3D printing. Preferably, the rotating magnetic field is characterized by a rotation speed higher than / faster than 1 Hertz (Hz). During the entire continuous 3D printing process, the device 100 provides a stronger magnetic field at a location near the nozzle 120 relative to the magnetic field far from the nozzle 120. The ink 300 includes magnetically responsive particles 320. Figure 1B An image of an exemplary magnetically responsive particle 320 is shown. In this example, the magnetically responsive particle 320 includes Fe3O4 nanoparticles attached to the surface of Al2O3 microplatelets to form magnetic aluminum oxide particles (mAl2O3). The method 500 includes continuous 3D printing on a substrate 150 / previously printed layer, wherein the substrate / previously printed layer has sufficient porosity to assist or enable water removal from the extrudate 200. The substrate is a substrate selected to have a porosity that promotes relatively rapid drying, which in turn is embodied to enable the extrudate to maintain its "as-deposited" shape. The deposited ink or extrudate 200 undergoes a time of magnetically consistent alignment (510), followed by a time of accumulation (520), and then a time of drying or curing (530).

[0046] Further references Figure 2A and Figure 2B The apparatus 100 is described. The apparatus 100 of the present disclosure comprises a 3D printer 110 having a nozzle 120 for extruding a continuous extrudate 200 of ink. The 3D printer 110 is selected as a printer that implements control of its nozzle 120 over a 3D nozzle displacement path. FIG. 2A to FIG. 3B In the illustrated embodiment, the proposed magnetic field distribution (a stronger magnetic field near the nozzle 120 relative to a magnetic field away from the nozzle 120) is provided by a magnet 130 coupled to the nozzle 120. The 3D printer may include a magnet 130 (e.g., a permanent magnet) coupled to a motor 140 and coupled to the 3D printer 110 so that the magnet 130 can rotate (or stop rotating) when the magnet 130 follows the nozzle 120 along the nozzle displacement path. In some other embodiments, such as Figure 3A and Figure 3B As schematically shown, the nozzle 120 may be positioned behind the magnet 130 relative to the nozzle displacement direction 209 .

[0047] The motor 140 is preferably coupled to a portion of the 3D printer 110 that supports the nozzle 120 so that the magnet 130 is displaced along the nozzle displacement path similar to the nozzle 120. The magnet 130 is preferably coupled to the 3D printer 110 so that the axis of rotation of the magnet (rotation axis 139) can be controllably oriented relative to the nozzle axis 129 (the direction in which the nozzle extrudes ink). The magnet 130 is preferably coupled to the 3D printer 110, wherein the magnet 130 is spaced apart from the nozzle 120. The magnet 130 is preferably coupled to the 3D printer 110 so that the magnet 130 is spaced apart from the extrudate 200 by a spacing 137, which can be controllably changed to change, for example, the magnetic field to which the extrudate 200 is subjected. The magnet 130 is preferably coupled to the nozzle 120 so that the magnetic field strength at the nozzle 120 or nozzle tip 122 remains substantially constant throughout a single run of continuous 3D printing. The magnet 130 can be arranged to be independent of the nozzle speed V p The magnet 130 can be switched between a non-rotating state and a rotating state in accordance with the conditions of the nozzle displacement and / or the ink extrusion rate at the nozzle tip 122. The provided magnetic field distribution enables the application of a rotating magnetic field to cause the micro-sheets to be arranged biaxially in a plane of the magnet 130. Preferably, the rotation frequency of the magnetic field is high enough to promote the biaxially consistent arrangement caused by the balance between the viscous force and the magnetic moment. The position and orientation of the magnet 130 can be adjusted to adjust the magnetic field strength and the consistent arrangement direction as needed.

[0048] In one exemplary test, a neodymium magnet (having dimensions of, for example, 35 millimeters (mm) x 10 mm x 10 mm) is fixed to a rotating shaft of a DC gear motor (e.g., motor 140). Magnet 130 is coupled to 3D printer 110, wherein magnet 130 is spaced about 1 cm from a tip (nozzle tip) 122 of a nozzle of the 3D printer. The magnetic field strength to which the extruded ink is subjected is about 100 mT (milli-Tesla). The printing thickness of each layer is about 0.2 mm. The printing speed is about 1 mm / s (1 millimeter per second). The flow rate of the ink is about 400%. About 5 mL of ink is provided. The ink can be extruded through a needle or nozzle of about 1.2 mm diameter.

[0049] Preferably, a porous substrate 150 is provided. In other words, during the 3D printing process, the substrate 150 provided to support the printed object can be a substrate selected to draw water away from the ink printed on the substrate 150. The substrate 150 can have a total porosity of 60% and a density of about 1.2 g / mL. In one example, the substrate 150 is made by dispersing 8 grams (g) of gypsum powder in 8 g of water and drying in an oven at 48° C. overnight. Preferably, the substrate 150 provides a substantially flat surface 502 ( Figure 1A / Figure 2A) for the deposition of materials, enabling the continuous 3D printing process to take advantage of its freedom to build a wide variety of objects of different shapes / structures / features without the expense and limitations of molds.

[0050] Method and product thereof

[0051] For brevity, the method 500 may be referred to as magnetic direct ink writing (M-DIW), although those skilled in the art will appreciate that the inks proposed herein for use in M-DIW may render conventional direct ink writing (DIW) inoperable.

[0052] The 3D printing method 500 proposed herein includes extruding ink through a nozzle of a 3D printer simultaneously with the movement of the nozzle and the magnet (or magnetic field distribution) along the nozzle displacement path. A first layer of the extrudate can be deposited on a porous substrate (e.g., a gypsum substrate). Subsequent multiple layers of the extrudate can be deposited on one or more previously formed layers of the same 3D printing material. The 3D printing process can be continuous, i.e., multiple layers can be printed in one continuous process without suspending printing. For example, there is no need to suspend before printing a second layer on top of the first layer. In other words, during the entire printing process of the object, there is no need to suspend printing intermittently in order to cure or dry the printed material to achieve the purpose of printing subsequent layers.

[0053] Simultaneously with the printing of the ink presented herein, the magnetic field distribution follows the nozzle along a nozzle displacement path, for example, the magnet 130 and the nozzle 120 move simultaneously along the same or similar path during the printing process. In some embodiments, the magnet can be after the nozzle, before the nozzle, or beside the nozzle relative to the nozzle displacement direction 209. Depending on a preset local anisotropy, the magnet 130 can be set to rotate about a rotation axis 139, and the rotation axis 139 is angularly displaced relative to both the nozzle axis 129 and the nozzle displacement direction 209. The method 500 involves displacing a magnetic field along the nozzle displacement path of the 3D printing nozzle 120, wherein the magnet 130 rotates relative to the nozzle 120. The method 500 can be described in terms of subjecting the printed ink or extrudate 200 to the action of a magnetic field (H), which is applied at a speed that is proportional to the nozzle velocity (V p ) corresponding rate decreases.

[0054] Reference again Figure 1A or Figure 2A, the printed ink can form a continuous line of extrudate 200 printed according to the nozzle displacement path. When printing, the printed ink can undergo a step of magnetically aligning 510, followed by a step of accumulating 520 and a step of drying 530. In some cases, these steps can be performed in parallel. The solidified material (i.e., the printed ink after drying) is observed to be anisotropic, with particles aligned or oriented in a similar direction. In contrast, the ink before extrusion from the nozzle is a liquid, in which the magnetic particles are in different random orientations.

[0055] In sync with the extrusion of ink 300 out of the nozzle, and even before the ink 300 is deposited (before the ink reaches the substrate or previously printed layer), the magnetically responsive particles 320 in the ink 300 can be subjected to the magnetic field and begin to orient themselves so that the magnetically responsive particles 320 are substantially approximately oriented or aligned. In other words, the magnetically responsive particles in the newly extruded ink undergo magnetic alignment 510 by rotating the magnetic field. As the extruded ink is deposited, the deposited ink continues to be subjected to the magnetic field. The relative orientation of the rotating magnet 130 and the substrate 150 / previous layer is such that the substantially aligned magnetically responsive particles 320 are oriented in a desired orientation. By rotating the magnet 130 relative to the nozzle axis 129, the nozzle displacement direction 209, or the nozzle velocity (V p ) to set the rotation axis 139 of the magnet 130, the desired orientation can be controlled. As the nozzle 120 moves further away from a volume of deposited ink, the magnetically responsive particles 320 in the deposited ink are subjected to increasingly weaker magnetic field strengths.

[0056] The proposed method 500 facilitates or enables accumulation 520 to occur simultaneously with the loss of water from the deposited ink, for example, the loss of water from the deposited ink to the porous substrate. Accumulation refers to the phenomenon that approximately uniformly arranged particles pack themselves more and more tightly as water or solution (in which particles are distributed) is removed. Water can be drawn away by the capillary action of the porous substrate so that water is at least partially lost from the solution or ink. Accumulation can include "dropping" of particles due to gravity and the reduction of surrounding / supporting water or solution in which the particles are distributed. Accumulation can involve the insertion (slotting) of particles between other approximately uniformly arranged particles as the surrounding water or solution is lost (by, for example, adsorption, capillary action, evaporation, etc.).

[0057] Drying or curing 530 may continue after accumulation. Preferably, drying is assisted by the porous substrate 150 and / or heating. The printed article may be heated or sintered, that is, after the article has been 3D printed, the printed article may be heated or sintered (rather than sintering the intermediate workpiece after printing each layer and before printing the next layer). In one example, for an article made of a 3D-printed alumina-based composite, the article may be sintered in air at 1600°C in a box furnace (e.g., a box furnace commercially available from Nabertherm, Switzerland) for about two hours. Preferably, the heating curve includes a heating rate of 2.5°C per minute, with an intermediate platform of about five hours at 500°C. In another example, for an article made of 3D-printed graphite, the graphite composite may be reduced at 1050°C, at a heating rate of 5°C per minute, in a tube furnace (e.g., a Protégé XST split tube furnace), in an argon atmosphere for four hours. In examples using mineral-based additives (eg, SiO2), sintering can beneficially form a mineral matrix to bind the microplatelets.

[0058] The arrangement of the microstructures in the present method 500 can be described by the following steps: (i) in a newly deposited ink, the microflakes are aligned under the action of a magnetic field (step 510); (ii) water is removed from the deposited ink with the aid of a porous substrate, which results in the accumulation of microflakes and promotes shape retention (step 520); and (iii) the printed ink layer is cured by drying (step 530). In the curing step, the accumulated deposited layer is further dried by a porous base (e.g., a porous substrate or a previously printed and dried / cured layer) to strengthen the assembly and allow subsequent multiple layers to be deposited according to the same process. For the subsequent multiple layers, the underlying layer acts as a substrate and water is removed by capillary forces.

[0059] Ink

[0060] Embodiments of the present disclosure include ink 300 suitable for continuous 3D printing of articles including ceramic-based articles. The ink is characterized by having a viscosity or fluidity that allows the ink to be continuously delivered or extruded through a nozzle, and the ink is also characterized by having a viscosity that allows the extruded ink to substantially maintain a preset shape (e.g., in terms of the height, contact angle, etc. of the extrudate) before the extrudate is fully solidified. In other words, the ink is characterized by having a shape-retaining property or a shape-retaining property after the ink is deposited on a porous substrate. The shape-retaining property of the ink is also characterized by being magnetically responsive at the microstructure level while maintaining the shape of the printed state at the macroscopic level. Preferably, the ink includes magnetically responsive particles that can be arranged biaxially in a rotating magnetic field of relatively weak magnetic field strength (weaker magnetic field), that is, biaxially in any direction of at least two directions, and the magnetically responsive particles are, for example, microplatelets. Preferably, the magnetically responsive particles are microparticles coated with magnetic nanoparticles, and the magnetic nanoparticles have a surface charge opposite to the surface charge of the microparticles. In other words, non-magnetic particles can be selected and made magnetically responsive by adsorption of magnetic nanoparticles thereon. The ink is characterized by having a curing time (after extrusion), the curing time is long enough to allow a uniform arrangement of magnetically responsive particles, and the curing time is short enough to allow another layer of ink to be printed on the previously printed ink, i.e., multi-layer 3D printing. In the present disclosure, the term "microparticle" refers to particles with linear dimensions in the range of, for example, 0.1 μm to 100 μm, micrometer (μm) level, as will be understood by those skilled in the art. The term "microsheet" refers to a relatively thin sheet having two opposing major surfaces, such as substantially in the form of a sheet or thinner sheet, so that the microsheet can be described as a "two-dimensional" (2-D) shape or a substantially planar shape. The term "nanoparticle" refers to particles with linear dimensions in the range of, for example, 1 nm to 200 nm, nanometer (nm) level, as will be understood by those skilled in the art.

[0061] The ink composition can be determined or adjusted based on time-based parameters and viscosity within a printable and directable viscosity range, such as Figure 4 and Figure 5 The diagram shown is schematically shown. Figure 4 is the time t for magnetically consistent alignment m , time t used for accumulation j and the time t associated with 3D printing p Schematic diagram of the viscosity of the ink as a function of the viscosity of the ink (ink viscosity). As the magnetic susceptibility χ increases, the ink exhibits a magnetically consistent arrangement time t m To make the ink printable and orientable, the ink viscosity is adjusted to a printable and orientable viscosity range. Figure 5 Yes m,t j With t p Schematic diagram as a function of the number of printed layers n. The ink viscosity can be adjusted (with the aid of fumed silica, for example) so that (for the number of layers to be printed) the accumulation time and the magnetically coherent alignment time are within a printable and orientable range.

[0062] Advantageously, the particles can be selected from a wide variety of different materials, including non-magnetic materials. The magnetically responsive particles in the ink can be functionalized particles. Examples of particles include but are not limited to metal flakes, carbon-based materials, boron nitride microflakes, microfibers, etc. The amount of magnetically responsive nanoparticles (e.g., superparamagnetic iron oxide nanoparticles (SPIONs)) can be adjusted according to the size and other intrinsic properties of the selected microflakes.

[0063] Those skilled in the art will not select the ink of the present disclosure for conventional 3D printing without creative labor, because although the ink is extrudable, it is not constructible due to the low yield stress that will make it have too high fluidity. In other words, the printed ink will not be able to maintain shape and has no ability to bear the weight of multiple layers. For example, in conventional ink direct writing, the ink is required to be a paste with high viscosity and elasticity so that the printed material can have the shape retention characteristics required for 3D printing. In contrast, the present disclosure proposes a preferred ink component for ink direct writing, wherein the ink is characterized by having a high zero shear viscosity of about 200Pa.s and a low yield stress of about 4Pa. In some embodiments of the present disclosure, the ink component for ink direct writing is characterized by having a relatively high zero shear viscosity, wherein the zero shear viscosity is less than 200Pa.s. In some embodiments of the present disclosure, the ink component for ink direct writing is characterized by having a relatively low yield stress, wherein the yield stress is less than 4Pa.

[0064] In some embodiments, the concentrations of magnetically responsive microflakes and colloidal gel in the ink are balanced to obtain an ink that is viscous enough to print, but not too viscous to allow the microflakes to rotate and align with the magnetic field. The higher zero shear viscosity allows the ink to be extruded through a finer nozzle without severe die swell. In addition, the higher zero shear viscosity ensures uniformity of the ink without sedimentation. The lower yield stress allows the microflakes to move under the action of the magnetic moment and achieve magnetic orientation of the microflakes.

[0065] The method of continuous 3D printing (also known as magnetic ink direct writing or M-DIW) involves, after extruding the magnetically responsive ink in a relatively low viscosity form, removing a relatively low viscosity solvent (e.g., water) from the ink in a controlled manner during the printing process. In some examples, the removal of water is achieved by printing the ink on a porous substrate (e.g., gypsum). After the water is removed, microplatelets accumulate at the gypsum surface, which forms a percolated layer.

[0066] Preferably, the ink meets contradictory requirements in terms of viscosity. At lower viscosities, back-pressure prevents continuous extrusion, whereas at higher viscosities, the force required for extrusion is too high for the equipment. In addition, for consistent arrangement of microplatelets, the following time relationship is preferably met:

[0067]

[0068] where t m is the time required for the microchips to be magnetically aligned, t p is the time related to the printing speed, and t j is the accumulation time of ink ( Figure 4 ). Time t p is the time when the magnetic field is strong enough to orient the micro-flakes just after the ink is deposited. In fact, the magnet moves with the nozzle. Therefore, as the nozzle moves at a speed V p As the ink moves away, the magnetic field in the deposited ink becomes weaker and weaker. p It can be defined as follows:

[0069]

[0070] in is the distance the nozzle travels at which the magnetic field is high enough to align the microplatelets in the deposited ink. The magnetic field strength profile is set by the magnets used. Preferably, a higher printing speed is used, which corresponds to a shorter This is for t m Constraint is imposed because the magnetic field at one point in the deposited ink is preferably high enough for rapid and consistent alignment of the microflakes. m It can be given by the following formula:

[0071]

[0072] Where V is the volume of the microsheet, is the Perrin friction factor, is the viscosity of the fluid, is the dielectric constant of vacuum, a and b are the average thickness and average diameter of the microsheets, d is the thickness of the magnetic coating, and χ is the magnetic susceptibility of the microsheets. For a given ink composition, according to formula (3), and Proportional to. Reduce t m And ensure that t m <t p The preferred method is to increase χ by adding a magnetic coating to the microplate. The accumulation time t after the ink is deposited on the printed substrate j Given by:

[0073]

[0074] where R is the hydrostatic resistance, J is the ratio between the volume of the solidified layer and the volume of the drawn liquid, ΔP is the capillary suction, and x is the distance from the substrate.

[0075] As shown in formula (4), t j With η o In general, with the increase of t m Compared to particle suspensions, j However, calculation of Eq. (4) shows that the first printed layer (smaller x and higher ΔP) has a very short accumulation time and may not be able to achieve t m <t j The strong hydrodynamic forces generated at the surface of the porous substrate inevitably lead to a uniform horizontal arrangement of the structure.

[0076] In addition, t j increases with x, and hence with the number of layers (n) ( Figure 5 ). Too long t j Will need to set lower V p , because multiple layers require a longer time to accumulate before the next layer is deposited. j This can indicate poor shape retention when 3D printing the next layer. Therefore, choosing an ink with a higher zero shear viscosity and a lower yield stress is not an intuitive choice.

[0077] Characterization of ink

[0078] For ease of understanding, the relationship between η0 and t is further described for the ink based on mA12O3 microplatelets. m In the tests conducted, mA12O3 with an average diameter of 5 μm was suspended at different concentrations in a background fluid with a viscosity of η0. The time t for the microplatelets to align in a magnetic field of 100 mT was measured. m. We also change the magnetic susceptibility χ of the microplate to reduce t m ( FIG. 6A to FIG. 6D ). For testing purposes, a resin (zero drying time) was used instead of water to investigate a wider viscosity range. The resin is a liquid epoxy resin with a low zero shear viscosity η0 of about 20 Pa.s. Adding 5wt% to 8wt% SiO2 to the resin increases η0 from 30 Pa.s to 700 Pa.s, and its yield stress σ y Increase from 0Pa to 45Pa ( Fig. 6A and Figure 6B ). At the same time, the elastic modulus G' and the viscous modulus G'' increase by two orders of magnitude.

[0079] The Al2O3 microsheets were coated with up to 18.6 vol% SPIONs relative to the microsheets. m An inverted optical microscope was used to observe the dynamic response of mA12O3 under a 100 mT magnetic field, which was rotated at a frequency of 2 Hz. The degree of uniformity D was determined by taking the ratio of the long and short diameters of the ellipse from the Fast Fourier Transforms (FFT) of the micrograph at the half-width of the Lorentz fits. L / D W . D L / D W = 1 means no consistent arrangement, but D L / D W >1 indicates the presence of magnetically coherent arrangement. As expected from Equation 3, increasing the amount of SPIONs on the microchip increases its magnetic response and leads to a shorter coherent arrangement time t m In pure resin and resin containing 5wt% SiO2 ( Figure 6C ), by increasing the SPIONs coating from 5 vol% to 18 vol%, while maintaining D L / D W >2, higher consistent arrangement quality, t m From 6 seconds to 0.1 seconds. The results show that t m As the SiO2 content increases ( Fig.6D In the resin containing 7 wt% SiO2, optical microscopy images show that uniform arrangement can still occur, where t m is 4 seconds. In the resin containing 8wt% SiO2, the optical microscopy image ( Fig. 6E) shows that no alignment occurs. Even with an 18 vol% coating, the resin containing 8 wt% SiO2 does not allow magnetic alignment to occur. Therefore, the study of the effect of the background liquid on the magnetic orientation of the microsheets shows that when the yield stress σ of the liquid is y When the pressure is less than 45Pa, mA12O3 can be arranged uniformly under a weaker magnetic field.

[0080] mA12O3 microplatelets with 14 vol% SPIONs coating appear to be aligned only when suspended in a background liquid with a zero shear viscosity of less than 200 Pa.s and a yield stress of less than 4 Pa, where t m It's 4 seconds.

[0081] The rheological properties of the ink must be considered and not the rheological properties of the background liquid, as the microplatelets will begin to interact with each other, increasing the viscosity of the ink. In the following, the colloidal gel or background liquid in the ink was chosen to be water containing 4 wt% fumed silica. This background has a large fluidity (liquid) and the individual microplatelets can be easily aligned. A 14 vol% SPIONS coating for mA12O3 was chosen and different concentrations of mA12O3 were tested (see Table 1).

[0082] Table 1. Ink composition

[0083]

[0084] The viscosity of the ink at zero shear increases from 10 Pa.s at a loading of 15 wt% of the microsheet to 3000 Pa.s at a loading of 35 wt% ( Fig. 7A All inks exhibit shear-thinning behavior ( Fig. 7A ), and its yield stress increases from 4Pa for the ink containing 20wt% mA12O3 to 45Pa for the ink containing 35wt% mA12O3 ( Figure 7B and Figure 7C In addition, the ink containing more than 15 wt% of microplatelets exhibited solid-like behavior at lower shear rates (G' > G'') ( Fig.7D ). This indicates that some shape retention is achieved for these inks. In contrast, the inks containing 0 wt% and 15 wt% mA12O3 exhibited liquid-like behavior (G' ≈ G''), indicating that there is no strong interaction between the SiO2 nanoparticles and the mA12O3 microplatelets ( Figure 7C and Fig.7D These results indicate the formation of a percolating network structure in the ink starting from 20 wt% mA12O3.

[0085] Based on the test results, magnetic alignment is expected to occur for all inks with a yield stress below 45 Pa, except for a concentration of 35 wt %. At 35 wt %, the higher yield stress indicates that the microplatelets interact more strongly, which prevents the microplatelets from aligning under the weaker magnetic field used in the tests.

[0086] To verify that the magnetic alignment occurred as predicted, D was measured from electron micrographs rather than optical images. L / D W ratio( Fig. 7E ). The orientation of the alignment was set so that the microflakes were oriented vertically relative to the horizontal substrate. This orientation was chosen because it is the least achievable due to the need to overcome gravity and the hydrodynamic forces caused by capillary suction. As predicted, all inks aligned with each other, except for the ink with 35 wt% microflakes (see Fig. 7E ).

[0087] In some tests, the magnetic field strength was varied in space from 378 mT at the magnet surface to 6 mT at a distance of 3 cm. The magnet was set so that the magnetic field strength at the nozzle tip was 100 mT. At a constant speed V of 1 mm / s p For continuous 3D printing, the magnetic field strength applied to the deposited ink gradually decreases from 100 mT to 50 mT within 5 s. This printing speed is reasonable for 3D printing. p Preferably, t m By using, for example, magnets with different geometries and applying a wider spatial distribution of magnetic field strength above 50 mT, faster printing speeds can be achieved, thereby improving printing efficiency.

[0088] Since the high concentration of mA12O3 in the ink makes it difficult to see individual microplatelets under an optical microscope, t is calculated using equation (3) for magnetic field strengths of 100 mT and 50 mT. m ( Figure 7F ). When the concentration of microsheets is high, the microsheets tend to interact with each other and hinder each other's rotation, so the viscosity of the ink, that is, the viscosity of the background liquid and other microsheets, is used as η0 in the calculation formula (3). The prediction shows that for a magnetic field strength of 100mT, mA12O3 microsheets can be uniformly arranged in an ink with a viscosity of 2000Pa.s within 40 seconds, and for a magnetic field strength of 50mT, mA12O3 microsheets can be uniformly arranged within 140s. In practice, the calculated curve is used to determine the time required to obtain t m <t p= The required ink viscosity for 5s. At 100mT, inks with viscosities up to 400Pa.s can be arranged consistently. This corresponds to inks containing 20wt% to 25wt% mA12O3. This shows that inks containing up to 20wt% mA12O3 can be arranged consistently during the M-DIW process.

[0089] The ink containing 20 wt% mA12O3 using the present method exhibited a yield stress of 5 Pa and good shape retention. j Measured as a function of the number of printed layers ( Figure 7G ). At 23°C, t j increases with the number of layers, as expected from equation (4). As more layers are deposited and the print thickness increases, water removal slows down due to the increase in tortuosity. j < 2 s < t m , and thus no consistent arrangement control in the first layer can be obtained, which is consistent with the test results. After 10 layers, t j 1 min, which is challenging for thicker parts. To increase the number of layers, heat can be applied to the substrate to increase the t j Halved.

[0090] To demonstrate that the control of the microstructure in the 3D printed parts is effective, an ink containing 20 wt% mA12O3 was used for testing, and the direction of the rotating magnetic field was adjusted during the printing process ( FIG. 8A to FIG. 8E ). In the multiple layers printed above the first layer, micro-sheets with uniform horizontal arrangement and micro-sheets with uniform vertical arrangement whose base surface is perpendicular or parallel to the printing direction can be realized ( FIG. 8A to FIG. 8C ). The quality of the consistent arrangement is good, among which D L / D W It is 1.9.

[0091] In a printed part, you can use Fig.8D ) and within a single layer ( Fig. 8E ) to obtain local orientation. Some slightly non-uniformly aligned regions of about 100µm can be observed between the well-aligned regions. These non-uniformly aligned regions are attributed to the manual change of the magnetic field orientation. Therefore, the ink containing 20wt% mA12O3 is an excellent ink for M-DIW. Interestingly, the ink containing 15wt% mA12O3 can be printed even at a lower zero shear viscosity of about 10Pa.s.

[0092] Ink manufacturing

[0093] In some examples, the ink includes magnetically responsive aluminum oxide microflakes (mAl2O3). A suspension can be made by dispersing 10 grams (g) of Al2O3 (aluminum oxide) microflakes in 400 mL of distilled water for about 10 min (minutes). An example of a suitable Al2O3 microflake can have a lateral dimension (size) of about 5µm (micrometer) and a thickness of about 700nm (nanometer), such as Al2O3 microflakes available from Kinsei Matec Co., Ltd. In some examples, the aluminum oxide particles have a diameter in the range of 1µm to 10µm. A volume of ferrofluid is diluted in 100 mL of distilled water and added dropwise to the suspension. An example of a ferrofluid (cationic type) can include EMG 605 (commercially available from Ferrotec Corporation in the United States) containing 3.9% vol SPIONs (superparamagnetic iron oxide nanoparticles). The mixture can be left under stirring for about three days or a suitable period of time to allow the SPIONs to electrostatically adsorb to the Al2O3 microplatelets. The water can then be filtered out of the mixture and the microplatelets washed with water and ethanol and then dried (e.g., overnight) in an oven (e.g., IKA Oven 125) at about 48°C (degrees Celsius).

[0094] In some other examples, the ink includes magnetically responsive graphite (mGr) microplatelets. The graphite particles may have a diameter in the range of 1µm to 10µm. The graphite particles may be functionalized as described above. A suspension may be made by grinding 2 g of graphite in 20 mL of ethanol by ball milling. Ball milling may be performed (e.g., a Retsch PM 100 device) with grinding balls of 5 mm (millimeter) and 3 mm in diameter at a speed of 500 rpm (revolutions per minute) for 4 hours (at intervals of 5 minutes with 2 minutes of rest). After filtration, the ball-milled graphite microplatelets may be dispersed in water again by dropwise application of a diluted ferrofluid (diluted to 100 mL of water). The mixture may be in a state of stirring for about three days or a suitable period of time. The water is then filtered out of the mixture, and the microplatelets are washed with distilled water and ethanol, then dried in a freeze dryer (e.g., Martin Christ / Alpha 2-4), and then dried in an oven at about 48°C.

[0095] In various examples of the present disclosure, the ink includes magnetically responsive particles (preferably functionalized microplatelets) and a rheology modifier (also known as a colloidal gel) in an aqueous solution. Examples of rheology modifiers include, but are not limited to, materials consisting essentially of fumed silica (SiO2) nanoparticles.

[0096] Sample Products

[0097] Being able to 3D print colloidal gels or solutions containing oriented microplatelets is beneficial for several reasons. The present methods and inks beneficially enable the fabrication of ceramics and composites with controllable properties, for example, as presented using an ink containing 20 wt% mA12O3. The printed material is an assembly of inorganic particles with a relatively high solid loading. Using the proposed printable and orientable ink, the relative density of the resulting product measured after printing is about 31%. In the solid part, the concentration of microplatelets is a relatively high concentration of 70 vol%, with the remainder made of 20 vol% SiO2 and 10 vol% Fe3O4. In the final printed body, there are about 22 vol% microplatelets. This concentration of oriented, anisotropic microplatelets is not achievable with other 3D printing methods, which typically only reach 15 vol% and 4.7 vol%. The higher concentration of inorganics can obtain highly reinforced composites after impregnation with polymers or conversion to ceramics after sintering. In other words, the range of products that can be manufactured using the present method and ink is very wide.

[0098] Furthermore, the present disclosure enables control of microstructures and formation of localized properties in different portions of the same article, thereby making multifunctional articles. For example, one section of an article may be printed with vertically oriented microflakes to increase hardness and wear resistance, while another section of the article may have horizontally oriented microflakes to impart higher strength.

[0099] Using the ink and the proposed M-DIW process ( Figures 9 to 11 ), microstructured ceramics of different 3D shapes, porous grids and bulk samples were 3D printed. The ink contains 20wt% alumina and 4wt% fumed silica. The resolution of the layer height is 0.1 ± 0.02mm, and the line width is 1.2 ± 0.3mm. Up to 30 layers can be deposited, which leads to the formation of samples of about 3mm thickness. The printed parts are formed into ceramics by sintering at 1600°C. The shapes of the 3D printed samples with unique orientation can be maintained with good resolution without deformation after sintering. The high temperature treatment retains the presence of iron oxide (Fe3O4) in the ceramic, which has magnetic properties and can be easily attracted by magnets. This is an interesting property that can be used to make actuated structures. For example, the sintered ceramics can be combined with a softer polymer matrix to build a magnetically actuated gripper ( Fig.12 ).

[0100] Samples with a two-layer microstructure, such as horizontal and vertical layers, deform due to anisotropic contraction between the two layers. This deformability can be used to make ceramics with special shapes that cannot be made otherwise.

[0101] The ability to 3D print self-shaping ceramics provides a wide range of structures and a large design space. A self-shaping coil was 3D printed with two layers of micro-sheets arranged in a consistent horizontal manner and eight layers of micro-sheets arranged in a consistent vertical manner. Curved grid structures can also be obtained after sintering a grid structure with three layers of micro-structures arranged in a consistent transverse manner and three layers of micro-structures arranged in a consistent axial manner. The obtained ceramic can undergo significantly larger deformations compared to conventional self-shaping materials, which is attributed to the thinner dimensions and larger shrinkage anisotropy achieved by the present disclosure. After sintering, the shrinkage of the printed ink with 20wt% mA12O3 perpendicular to the micro-sheet base plane is 39.4 ± 1%, while the shrinkage parallel to the base plane is 22.3 ± 2.9%. This higher shrinkage can be attributed to the densification of the ceramic and the melting of the fumed silica during the sintering process.

[0102] In the fabricated samples, the sintered ceramics had a density of 3 g / cm³ and a total porosity of 23%. The ceramic objects retained their microstructure after sintering. This demonstrates the ability to 3D print biomimetic ceramics such as nacre-like alumina. The nacre-like ceramics fabricated by this continuous 3D printing exhibited a hardness of 7.86 ± 1.12 GPa on axially oriented (e.g. vertically oriented) microplatelets and a hardness of 7.38 ± 1.06 GPa on transversely oriented (e.g. horizontally oriented) microplatelets. Despite the anisotropic microstructure, the slight difference in hardness may be due to residual porosity and vitrified interfaces. The ceramics exhibited anisotropic flexural properties, where the flexural strength of the uniformly transversely arranged ceramics was 36% higher than that of the uniformly axially arranged ceramics, reaching 134 ± 6.5 MPa. Similarly, the flexural modulus of the uniformly transversely arranged samples was 50% higher than that of the uniformly axially arranged samples, reaching 152 ± 40 GPa. These good mechanical properties are derived from densification, good uniformly arranged microstructures, and strong interfaces between alumina grains and silica matrix. The element distribution diagram shows that iron (Fe) and silicon (Si) diffuse in Al2O3 during sintering. Compared with other nano-silica-reinforced alumina ceramics obtained by conventional 3D printing methods, the ceramics of the present application have about 3 times higher flexural strength.

[0103] Even in examples where the bending properties obtained using the present method are comparable to those obtained using conventional methods, the present method has the added benefit of enabling control over the orientation of the microstructures. Using conventional methods such as by vat photopolymerization and digital light processing, higher density alumina ceramics can be obtained. For example, dense alumina ceramics with a bending strength greater than 450 MPa can be obtained using conventional methods, but control of the microstructures is not achievable.

[0104] The proposed M-DIW printing method is not limited to alumina-based inks. Graphite microplatelets were successfully used instead of alumina to produce functional materials with anisotropic properties. Magnetically responsive graphite (mGr) microplatelets can be uniformly arranged in resins with up to 7wt% SiO2, which corresponds to a background viscosity of 100Pa.s ( Fig.13 ). The magnetically responsive graphite (mGr) microflakes require about 25 s to align. This is completely different from the case of alumina microflakes. The longer aligning time is the result of less SPIONs coating. Graphite has a weaker ability to adsorb SPIONs than Al2O3. However, the magnetically responsive graphite (mGr) microflakes can align in the same viscous solution as mA12O3 microflakes.

[0105] In some tests, 4wt% SiO2 was suspended in water as the background liquid of the ink, and the weight percentage (wt%) of magnetically responsive graphite was varied. 10wt% magnetically responsive graphite (mGr), equivalent to 4.3vol%, produced a suitable ink with a viscosity of 78Pa.s and a yield stress of 2.5Pa. In 4wt% SiO2, the time for consistent arrangement was less than 5s, thus meeting the time requirement for 3D printing in Equation 1. Similar to alumina inks, grid structures and other 3D shapes can be printed at higher resolutions and exhibit magnetic attraction properties.

[0106] Furthermore, the graphite composites exhibit anisotropic conductivity originating from the microstructure. Fig.14The conductivity of the 3D printed graphite composite measured (a) on the surface of the sample in a uniform horizontal (lateral) arrangement, (b) on the surface of the sample in a uniform vertical (axial) arrangement, (c) on the surface perpendicular to the sample in a uniform vertical (axial) arrangement, and the conductivity of the 3D printed graphite composite measured on the surface of the graphite composite reduced at a higher temperature of 1050°C for 4 hours, as shown in the shaded area. For example, the conductivity measured on the surface of the 3D printed composite in a uniform lateral arrangement (e.g., uniform horizontal arrangement) using a four-probe is 5S / m, which is higher than that of the sample in a uniform axial arrangement (e.g., uniform vertical arrangement). After reduction using a heat treatment at 1050°C in an argon atmosphere, the conductivity increases by almost 5 times, up to 26S / m, and exhibits strong anisotropy. Compared with other 3D printed graphite materials, M-DIW can not only control the orientation, but also obtain a higher conductivity and exhibit anisotropy.

[0107] Lower conductivity can be beneficial for sensing applications, while higher conductivity can be used in electronic devices, batteries, etc. In the case of adjustable electromagnetic properties, the present method and ink enable the manufacture of magnetically controlled switches ( Fig.15 ) or the manufacture of electromagnetic shielding components in electronic equipment ( Fig.16 ). A conductive compound was 3D printed to join the two copper wires that drive the LEDs.

[0108] The continuous 3D printing proposed in this article enables an object to be made in one run (for example, a continuous 3D printing process using one ink to build multiple layers to make a 3D object), and enables the object to have different anisotropic properties in different parts of the same object. This makes it possible to manufacture a wide variety of products or objects in a fully automated production environment. A wide range of applications can be obtained in different fields, such as robots, actuators, sensors, electronic devices, supercapacitors and batteries. The technology can also reduce the number of parts that conventionally need to be produced separately and subsequently assembled.

[0109] In the present disclosure, M-DIW is disclosed as a versatile, simple, cost- and time-efficient 3D printing method for fabricating microstructured composites and ceramic materials with different chemical compositions and functionalities. The quality of the uniform arrangement obtained is similar to that obtained using other magnetic orientation methods, with a misorientation of about 20°, depending on the aspect ratio of the microplatelets used.

[0110] Compared to other magnetic printing methods, the printing time is significantly improved due to the continuous deposition of the ink. In conventional magnetic printing, it takes about 5 hours to print 51 layers. Using this M-DIW method, it only takes about 1.1 hours. Auxiliary water removal (through, for example, a porous substrate) enables relatively fast water removal from the deposited ink, which enables faster curing and good shape retention, i.e., the deposited ink can maintain the fidelity of the shape. This enables print thicknesses of several centimeters to be obtained.

[0111] Changes and modifications may be introduced without exceeding the scope of the present disclosure. For example, for printed articles with larger sizes / dimensions, an external heat source may be applied in combination with a porous substrate. To further accelerate the printing process, a larger magnet may be used so that the magnetic field is distributed over a larger area, thereby arranging the microsheets uniformly over a larger area. Depending on the desired microstructure design, a trade-off between orientation and higher printing speeds may be achieved in localized regions of smaller dimensions. In accordance with the principles described herein, ink components with other microsheet chemistries and dimensions may be developed by adjusting the magnetic coating, concentration, and viscosity of the background liquid. In addition to materials with higher solid loadings, softer composites with microstructures may be obtained afortiori by applying polymers or hydrogels in the ink, or by using a lower concentration of ink and impregnating the printed material after it dries. Alternatively, an oxide such as CaO may be applied as a flux. These optimizations may enable 3D printing of pearlescent-like alumina with higher strength and toughness. For conductive inks, the shape and exfoliation of graphite flakes can be adjusted to improve their percolation.

[0112] The use of SPION coated particles provides the option of converting any one or more of a wide variety of particles into magnetically responsive particles.The magnetic properties can be removed after the article has been manufactured by dissolving / removing the iron oxide using, for example, orthophosphoric acid.

[0113] According to various embodiments, a method for continuous 3D printing to print an object includes: displacing a nozzle of a 3D printer along a nozzle displacement direction at a nozzle speed; while the nozzle is displacing, extruding ink from the nozzle to form an extrudate on a porous substrate, wherein the ink includes a plurality of magnetically responsive particles and a colloidal gel; and while the nozzle is displacing, displacing a rotating magnetic field along the nozzle displacement direction.

[0114] According to various embodiments, the method includes rotating a magnet relative to the nozzle to provide a rotating magnetic field, the magnet being disposed adjacent to the nozzle; and simultaneously displacing the magnet and the nozzle in a nozzle displacement direction at a nozzle velocity.

[0115] According to various embodiments, the rotating magnetic field is characterized by having a rotation speed greater than 1 Hz.

[0116] According to various embodiments, the colloidal gel may include fumed silica nanoparticles in aqueous solution.

[0117] According to different embodiments, the extrudate may sequentially undergo the following changes: a step of performing magnetically consistent arrangement within a magnetically consistent arrangement time; a step of accumulating within an accumulation time; a step of drying within a drying time, wherein the concentration of magnetically responsive particles in the extrudate after the drying time is higher than the concentration of magnetically responsive particles in the extrudate before the magnetically consistent arrangement time.

[0118] According to various embodiments, the step of magnetically aligning can include aligning the magnetically responsive particles in the extrudate adjacent to the nozzle, wherein the aligning is responsive to a magnetic field.

[0119] According to various embodiments, the magnetically responsive particles are microparticles.

[0120] According to various embodiments, the step of aggregating can include particles of the magnetically responsive particles having similar orientations packing themselves closer and closer to each other as water is lost from the colloidal gel.

[0121] According to various embodiments, the water may be at least partially lost from the colloidal gel by being drawn away by capillary action of the porous substrate. According to various embodiments, the drying step may include the loss of water from the extrudate away from the nozzle.

[0122] According to various embodiments, the method further comprises a step of sintering the object.

[0123] According to various embodiments, the method further comprises determining a composition of the ink based on a time-based parameter and a viscosity of the ink within a printable and orientable viscosity range. According to various embodiments, the ink is characterized by having a yield stress of less than 45Pa.

[0124] According to various embodiments, the ink includes magnetically responsive particles that can be aligned in either direction in response to a magnetic field.

[0125] According to various embodiments, the magnetically responsive particles include superparamagnetic iron oxide nanoparticles adsorbed on aluminum oxide microplatelets. According to various embodiments, the ink includes Al2O3 microplatelets having 14 vol% superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiO2, and water. According to various embodiments, the ink is characterized by having a viscosity of less than 1000 Pa.s and a yield stress of less than 45 Pa.

[0126] According to various embodiments, the ink further comprises graphite. According to various embodiments, the ink is characterized by comprising 10 wt % of graphite microplatelets with superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt % of SiO2 and water. According to various embodiments, the ink is characterized by having a viscosity of 78 Pa.s and a yield stress of 2.5 Pa.

[0127] According to various embodiments, an ink for a continuous 3D printing method comprises: a plurality of magnetically responsive particles, the magnetically responsive particles comprising: a plurality of microflakes; and a plurality of magnetically responsive nanoparticles adsorbed on the surfaces of each of the plurality of microflakes; water; and 4 wt % of fumed silica, wherein the ink is characterized by having a viscosity of less than 200 Pa.s and a yield stress of less than 4 Pa.

[0128] According to various embodiments, an apparatus for a continuous 3D printing method includes: a 3D printer including a nozzle, the nozzle being displaceable along a nozzle displacement direction; a porous substrate to receive an extrudate from the nozzle; and a magnet coupled to the nozzle, wherein the magnet is simultaneously (i) rotatable relative to the nozzle to provide a rotating magnetic field, and (ii) displaceable together with the nozzle at the same displacement speed and along a nozzle displacement path.

[0129] All examples described herein, whether devices, methods, materials or products, are presented for the purpose of illustration and aiding understanding, and are not intended to be limiting or exhaustive. Various changes and modifications may be made by those skilled in the art without departing from the scope of the claimed invention.

Claims

1. A method for continuous 3D printing to print an object, the method comprising: Displacing the nozzle of the 3D printer along the nozzle displacement direction at the nozzle speed; Simultaneously with the displacement of the nozzle, extruding ink from the nozzle to form an extrudate on a porous substrate, the ink comprising a plurality of magnetically responsive particles and a colloidal gel; and Simultaneously with the displacement of the nozzle, the rotating magnetic field is displaced along the nozzle displacement direction.

2. The method according to claim 1, comprising: rotating a magnet relative to the nozzle to provide the rotating magnetic field, the magnet being disposed adjacent the nozzle; as well as The magnet and the nozzle are simultaneously displaced in the nozzle displacement direction at the nozzle speed.

3. The method of claim 1 or 2, wherein the rotating magnetic field is characterized by a rotation speed greater than 1 Hertz.

4. The method according to any one of claims 1 to 3, wherein the colloidal gel comprises fumed silica nanoparticles in aqueous solution.

5. The method according to any one of claims 1 to 3, wherein the extrudate undergoes the following changes in sequence: (i) a step of magnetically aligning within a magnetically aligning time; (ii) an accumulation step during an accumulation time; and (iii) a drying step within a drying time, wherein the concentration of the magnetically responsive particles in the extrudate after the drying time is higher than the concentration of the magnetically responsive particles in the extrudate before the magnetically aligned time.

6. The method of claim 4, wherein the magnetically aligning step comprises an alignment of the magnetically responsive particles in the extrudate adjacent the nozzle, the alignment being responsive to the magnetic field.

7. The method according to any one of claims 4 to 6, wherein the magnetically responsive particles are micron particles.

8. The method of any one of claims 5 to 7, wherein the accumulating step comprises packing particles of the magnetically responsive particles having similar uniform arrangements more and more closely to each other as water is lost from the colloidal gel.

9. The method of claim 8, wherein water is at least partially lost from the colloidal gel by capillary extraction through the porous substrate.

10. A method according to any one of claims 5 to 9, wherein the drying step comprises the loss of water from the extrudate away from the nozzle.

11. A method according to any one of claims 1 to 10, further comprising the step of sintering the article.

12. The method of any one of claims 1 to 11, further comprising determining a composition of the ink based on a time-based parameter and a viscosity of the ink within a printable and orientable viscosity range.

13. The method of claim 12, wherein the ink is characterized by having a yield stress of less than 45 Pa.

14. The method according to any one of claims 1 to 13, wherein the ink comprises magnetically responsive particles which can be aligned in either direction in response to the magnetic field.

15. The method of any one of claims 1 to 14, wherein the magnetically responsive particles comprise superparamagnetic iron oxide nanoparticles adsorbed on aluminum oxide microplatelets.

16. The method of claim 15, wherein the ink comprises Al2O3 microplatelets having 14 vol% superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% SiO2, and water.

17. The method of claim 16, wherein the ink is characterized by having a viscosity of less than 1000 Pa.s and a yield stress of less than 45 Pa.

18. The method of any one of claims 1 to 14, wherein the ink further comprises graphite.

19. The method of claim 18, wherein the ink is characterized by comprising 10 wt% of graphite flakes having superparamagnetic iron oxide nanoparticles adsorbed thereon, 4 wt% of SiO2 and water.

20. The method of claim 19, wherein the ink is characterized by having a viscosity of 78 Pa.s and a yield stress of 2.5 Pa.

21. An ink for use in the continuous 3D printing method according to claim 1, the ink comprising: A plurality of magnetically responsive particles, the plurality of magnetically responsive particles comprising: a plurality of micro-sheets; and a plurality of magnetically responsive nano-particles adsorbed on the surface of each of the plurality of micro-sheets; Water; and 4 wt% fumed silica, wherein the ink is characterized by a viscosity of less than 200 Pa.s and a yield stress of less than 4 Pa.

22. A device for use in the continuous 3D printing method according to claim 1, the device comprising: A 3D printer, the 3D printer comprising a nozzle, the nozzle being displaceable along a nozzle displacement direction; a porous substrate to receive the extrudate from the nozzle; A magnet is coupled to the nozzle, wherein the magnet is simultaneously: (i) rotatable relative to the nozzle to provide a rotating magnetic field, and (ii) displaceable along the nozzle displacement path in conjunction with the nozzle at a same displacement speed.

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