Foil structure with carbon nanotubes, method for producing same, cold cathode and x-ray tube

Abstract

Embodiments of the present application provide a foil structure with carbon nanotubes, a method for manufacturing the foil structure, a cold cathode, and an X-ray tube. The foil structure includes a metal substrate having a first surface and a first side perpendicular to the first surface, the first side being located at an edge of at least one side of the metal substrate; a bonding layer located on the first surface of the metal substrate; and a plurality of carbon nanotubes located on the first surface of the metal substrate, wherein the bonding layer is configured to enhance a bonding strength between the metal substrate and the plurality of carbon nanotubes, the plurality of carbon nanotubes includes a carbon nanotube film located on the first surface and a plurality of discrete carbon nanotubes, at least a portion of the plurality of discrete carbon nanotubes is located on the first surface near the first side, at least another portion of the plurality of discrete carbon nanotubes is located outside the carbon nanotube film and has a projection on the metal substrate located outside the first surface, and the at least another portion of the plurality of discrete carbon nanotubes protrudes in a direction away from the metal substrate relative to the first side.

Description

Foil structures with carbon nanotubes and their preparation methods, cold cathodes and X-ray tubes Technical Field

[0001] This invention relates to the field of radiation scanning technology, and more specifically, to a foil structure with carbon nanotubes and its preparation method, a cold cathode, and an X-ray tube. Background Technology

[0002] Since its discovery by Sumio Iijima in 1991, carbon nanotubes (CNTs) have become a core material in the field of field emission due to their unique structure and properties. They possess excellent electrical conductivity, and their tip surface area is close to the theoretical limit, allowing for the formation of a highly concentrated local electric field that facilitates electron escape through the tunneling effect. Simultaneously, CNTs exhibit extremely low field emission threshold voltages (typically less than 100 volts), enabling extremely high current densities, and demonstrate excellent long-term operational stability. These characteristics make them an ideal choice for cold cathode field emission materials, showing significant application potential in fields such as vacuum electron sources and field emission displays.

[0003] Compared to traditional tungsten filament hot cathode X-ray tubes, carbon nanotube cold cathode X-ray tubes represent a fundamental technological breakthrough. Traditional hot cathodes require high-temperature heating (typically thousands of degrees Celsius) to excite electrons, resulting in drawbacks such as long heating times, high power consumption, easy filament wear, and short lifespan. Furthermore, they are prone to motion artifacts during rotating imaging. In contrast, carbon nanotube cold cathodes, based on the field emission principle, can achieve electron emission at room temperature through grid electric field modulation. They feature rapid start-up, low power consumption, and are compact and easily integrated. More importantly, they can be fabricated into multi-focal-spot array sources for static CT imaging, significantly improving imaging resolution and efficiency. They can also withstand higher electric field forces, producing higher doses of X-rays, and offer superior heat dissipation and operational reliability, making them urgently needed in high-end fields such as medical diagnostics and industrial inspection.

[0004] The basis for electron emission in CNTs is that electron emission occurs under the field emission principle when the CNT tip is subjected to a strong electric field. However, the CNT materials currently prepared are usually lying on the substrate surface and do not form an upright state. Therefore, the large aspect ratio characteristics of the slender CNTs are not utilized, and the CNT tip is difficult to form a field strength enhancement effect similar to that of a "lightning rod", making it difficult to achieve high current emission.

[0005] It should be noted that the information disclosed in this section is only used to understand the background of the inventive concept of the present invention. Therefore, the above information may include information that does not constitute prior art. Summary of the Invention

[0006] This invention provides a foil structure with carbon nanotubes, a method for preparing the same, a cold cathode, and an X-ray tube, which can effectively solve the problem of difficulty in achieving high-current emission due to the carbon nanotubes lying flat on the substrate surface.

[0007] In view of at least one of the above-mentioned technical problems, embodiments of the present invention provide a foil structure having carbon nanotubes, the foil structure comprising: a metal substrate having a first surface and a first side surface, the first surface being perpendicular to the first side surface and the first side surface being located at at least one edge of the metal substrate; a bonding layer located on the first surface of the metal substrate; and a plurality of carbon nanotubes located on the first surface of the metal substrate, wherein the bonding layer is used to enhance the bonding strength between the metal substrate and the plurality of carbon nanotubes, the plurality of carbon nanotubes comprising a carbon nanotube film and a plurality of discrete carbon nanotubes, the carbon nanotube film being located on the first surface and its orthogonal projection on the metal substrate being within the first surface, the plurality of discrete carbon nanotubes being located on the first surface near the first side surface and at least a portion being located within the carbon nanotube film, at least another portion of the plurality of discrete carbon nanotubes being outside the carbon nanotube film and its orthogonal projection on the metal substrate being outside the first surface, the at least another portion of the plurality of discrete carbon nanotubes protruding relative to the first side surface in a direction away from the metal substrate.

[0008] According to some exemplary embodiments, the discrete carbon nanotubes are substantially perpendicular to the first side.

[0009] According to some exemplary embodiments, the first surface is a plane, a cylindrical surface, a helical surface, or a wavy surface.

[0010] According to some exemplary embodiments, the metal substrate includes a metal mesh, the metal mesh including a plurality of metal portions extending intersecting along a first direction and a second direction and a plurality of through holes located between the plurality of metal portions, the aperture of the plurality of through holes being smaller than the length of the discrete carbon nanotubes.

[0011] According to some exemplary embodiments, a portion of the plurality of discrete carbon nanotubes has its orthogonal projection onto the metal substrate located in the plurality of metal portions, and another portion of the plurality of discrete carbon nanotubes has its orthogonal projection onto the metal substrate located in the plurality of through holes.

[0012] According to some exemplary embodiments, a portion of the first side is located in the region where the plurality of metal portions are located, and another portion of the first side is located in the region where the plurality of through holes are located.

[0013] According to some exemplary embodiments, the diameters of the plurality of discrete carbon nanotubes are equal to each other, and the lengths of at least two of the plurality of discrete carbon nanotubes are not equal.

[0014] According to some exemplary embodiments, the bonding layer includes a titanium carbide layer or an adhesive layer; and / or, the material of the metal substrate includes at least one selected from the following materials: copper, molybdenum, stainless steel, titanium, and Kovar alloy; and / or, the thickness of the metal substrate is 10-50 micrometers; and / or, the pore size of the plurality of through holes is 4-10 micrometers; and / or, the porosity of the metal mesh is 20%-45%.

[0015] In another aspect, embodiments of the present invention also provide a cold cathode having carbon nanotubes, the cold cathode comprising: at least one electron emitter, the electron emitter comprising: an emitter substrate; and a foil structure disposed on the emitter substrate, the foil structure being any of the foil structures described above.

[0016] According to some exemplary embodiments, a gate is also included, the gate being located directly above the first side of the foil structure, the discrete carbon nanotubes extending away from the first side of the metal substrate in the extension direction and pointing toward the gate, the discrete carbon nanotubes emitting electrons toward the gate when a positive voltage relative to the emitter substrate is applied to the gate.

[0017] According to some exemplary embodiments, the spacing between the first side of the metal substrate and the gate is 100 micrometers to 1 millimeter.

[0018] According to some exemplary embodiments, the cold cathode includes a plurality of said foil structures, which are assembled in an array on the emitter substrate, and a plurality of the first sides of the plurality of said foil structures are arranged in a direction parallel to the gate and at the same distance from the gate.

[0019] According to some exemplary embodiments, the emitter substrate includes a plurality of metal sheets, and the foil structure is sandwiched between the plurality of metal sheets.

[0020] In another aspect, embodiments of the present invention also provide an X-ray tube comprising a cathode assembly including a cold cathode as described in any of the preceding claims.

[0021] According to some exemplary embodiments, the X-ray tube further includes an X-ray tube body and an anode target, wherein both the cathode assembly and the anode target are disposed within the X-ray tube body.

[0022] In another aspect, embodiments of the present invention also provide a method for preparing a foil structure with carbon nanotubes, the method comprising: preparing a carbon nanotube solution: mixing carbon nanotube powder, a dispersant and a solvent to prepare a carbon nanotube solution; preparing a metal substrate; preparing a primary foil structure: placing the metal substrate on a filter element, passing the carbon nanotube solution through the filter element with the metal substrate to form multiple carbon nanotubes on the metal substrate, and obtaining a primary foil structure after a drying process; and preparing a foil structure: subjecting the primary foil structure to high-temperature treatment to improve the bonding force between the metal substrate and the multiple carbon nanotubes, thereby obtaining a foil structure.

[0023] According to some exemplary embodiments, the preparation of the carbon nanotube solution further includes: dispersing the carbon nanotube solution by ultrasonic and centrifugation to form a uniformly dispersed and non-agglomerated carbon nanotube solution.

[0024] According to some exemplary embodiments, between the preparation of the primary foil structure and the preparation of the foil structure, the method further includes: removing the dispersing agent from the surface of the metal substrate and carbon nanotubes of the primary foil structure; washing the surface of the metal substrate and carbon nanotubes of the primary foil structure with deionized water; and densifying the surface of the metal substrate and carbon nanotubes of the primary foil structure with an ethanol solution and a nitric acid solution.

[0025] According to some exemplary embodiments, the preparation of the metal substrate includes: cutting multiple through holes in a metal foil using a laser cutting process to form a metal mesh; and sputtering a titanium metal layer on the surface of the metal mesh using a magnetron sputtering process.

[0026] According to some exemplary embodiments, the high-temperature treatment of the primary foil structure to improve the bonding force between the metal substrate and the plurality of carbon nanotubes includes: performing high-temperature treatment of the primary foil structure in a high-temperature furnace protected by an inert gas to form a titanium carbide thin crystal layer, so as to form intermolecular bonds between the metal substrate and the carbon nanotubes.

[0027] According to some exemplary embodiments, in the preparation of the carbon nanotube solution, the solvent includes anhydrous ethanol or deionized water, the dispersing agent includes sodium dodecyl sulfate, the carbon nanotube powder has a mass percentage content of 0.1%-0.8%, and the sodium dodecyl sulfate has a mass percentage content of 0.2%-0.5%; and / or, a binder is also added in the preparation of the carbon nanotube solution.

[0028] According to some exemplary embodiments, the material of the metal substrate includes at least one selected from the following materials: copper, molybdenum, stainless steel, titanium, and Kovar alloy; and / or, the thickness of the metal substrate is 10-50 micrometers; and / or, the pore size of the plurality of through holes is 4-10 micrometers; and / or, the porosity of the metal mesh is 20%-45%.

[0029] According to some exemplary embodiments, the filtration component includes filter paper or anodized aluminum membrane.

[0030] According to an embodiment of the present invention, by distributing discrete carbon nanotubes protruding relative to a first side of a metal substrate, the portion of the discrete carbon nanotubes protruding from the first side of the metal substrate forms an upright tip structure. This tip structure can generate an electric field enhancement effect, enabling emission at low voltage and facilitating the generation of large current emission. Attached Figure Description

[0031] The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0032] Figure 1 is a surface morphology diagram of a carbon nanotube cathode in a related technology.

[0033] Figure 2 schematically shows a plan view of a foil structure according to an embodiment of the present invention.

[0034] Figure 3 schematically shows a cross-sectional view of a foil structure according to an embodiment of the present invention.

[0035] Figure 4 schematically shows a plan view of the metal substrate in the foil structure according to an embodiment of the present invention.

[0036] Figure 5 schematically shows a partial perspective view of the foil structure according to an embodiment of the present invention at region A1 in Figure 2.

[0037] Figure 6 schematically shows a microscopic image of part of region A1 in Figure 2 taken with an electron microscope.

[0038] Figures 7A-7C schematically illustrate various structural diagrams of the foil structure according to embodiments of the present invention.

[0039] Figures 8A-8D schematically illustrate various three-dimensional structural diagrams of a cold cathode according to embodiments of the present invention.

[0040] Figure 9A schematically shows a partial region structure diagram of a cold cathode according to an embodiment of the present invention.

[0041] Figure 9B schematically illustrates the principle of tip electric field enhancement effect of a cold cathode according to an embodiment of the present invention.

[0042] Figure 10 schematically shows a structural diagram of an X-ray tube according to an embodiment of the present invention.

[0043] Figure 11 schematically illustrates a flowchart of a method for preparing a foil structure according to an embodiment of the present invention.

[0044] Figures 12A-12F schematically illustrate the fabrication process of a foil structure according to an embodiment of the present invention.

[0045] Figure 13 is an emission current-gate voltage curve of a cold cathode according to some exemplary embodiments of the present invention.

[0046] Figure 14 is a bar chart comparing the gate voltages of three groups of cold cathodes under a specific emission current according to some exemplary embodiments of the present invention.

[0047] Figure 15 is a graph of emission current-emission count of a cold cathode according to some exemplary embodiments of the present invention. Detailed Implementation

[0048] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the invention for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0049] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0050] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0051] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0052] Currently, the industry mainly uses four technical approaches for the preparation of carbon nanotube cathodes: array direct growth method, slurry printing method, electrophoretic deposition method, and buckypaper composite process.

[0053] The steps of the direct array growth method include: using silicon wafers, metals, or other substrates as substrates, forming a catalyst layer through sputtering or electron beam deposition, and then growing a super-aligned carbon nanotube array via chemical vapor deposition (CVD) to directly serve as the emitter. To improve current output, the array area is often increased or the catalyst density is optimized.

[0054] The steps of the paste printing method include: mixing carbon nanotubes with terpineol, ethyl cellulose, metal powder, etc. to prepare a paste, coating it onto the surface of a metal substrate using screen printing technology, and then solidifying it through multi-stage heating and sintering to form a cathode layer. Some solutions can enhance conductivity and bonding properties by adding titanium carbide, titanium powder, etc., to meet high current requirements.

[0055] The steps of electrophoretic deposition include: dispersing carbon nanotubes with surfactants, inorganic salts, etc. in solvents such as ethanol / water to form a colloidal suspension containing charged particles (called electrophoretic paint or slurry); inserting two electrodes (anode and cathode) and applying a DC electric field; and using the electric field force to drive the charged carbon nanotubes in the colloidal solution to the electrode surface, thereby forming a uniform and dense coating.

[0056] The steps of the buckypaper composite process include: cutting the performance-optimized carbon nanotube buckypaper into a specific shape, and connecting it with metal substrates such as Kovar alloy and stainless steel by welding or conductive adhesive to form a composite cathode assembly, utilizing the high nanotube density of buckypaper to enhance current carrying capacity.

[0057] Figure 1 is a surface morphology diagram of a carbon nanotube cathode in a related technology.

[0058] The inventors discovered that, although the above methods have been extensively studied, the carbon nanotube cathodes prepared by the aforementioned methods such as screen printing, electrophoretic deposition, and buckypaper composite method usually have carbon nanotubes lying down and creeping on the substrate surface, and exhibiting an intertwined state, as shown in Figure 1. They do not form an upright state, thus failing to utilize the high aspect ratio characteristics of slender CNTs. The tip of the CNT is difficult to form the tip field strength enhancement effect of a "lightning rod", making it difficult to achieve high current emission.

[0059] Figure 2 schematically shows a plan view of a foil structure according to an embodiment of the present invention. Figure 3 schematically shows a cross-sectional view of a foil structure according to an embodiment of the present invention. Figure 4 schematically shows a plan view of a metal substrate in a foil structure according to an embodiment of the present invention. Figure 5 schematically shows a partial perspective view of a foil structure according to an embodiment of the present invention at region A1 in Figure 2. Figure 6 schematically shows a microscopic image of a portion of region A1 in Figure 2 taken with an electron microscope.

[0060] Some embodiments of the present invention provide a foil structure 10 having carbon nanotubes 13. Referring to Figures 2, 3, 4, and 5, the foil structure 10 may include a metal substrate 11, a bonding layer 12, and a plurality of carbon nanotubes 13. The metal substrate 11 has a first surface 11a and a first side surface 11b, the first surface 11a being perpendicular to the first side surface 11b, and the first side surface 11b being located at at least one edge of the metal substrate 11. The bonding layer 12 and the plurality of carbon nanotubes 13 are both located on one side of the first surface 11a of the metal substrate 11. The plurality of carbon nanotubes 13 are distributed in the bonding layer 12, which can enhance the bonding strength between the metal substrate 11 and the plurality of carbon nanotubes 13.

[0061] The plurality of carbon nanotubes 13 may include a plurality of discrete carbon nanotubes 131 and a carbon nanotube film 132. The carbon nanotube film 132 is located on the first surface 11a and its orthogonal projection on the metal substrate 11 is located within the first surface 11a. The plurality of discrete carbon nanotubes 131 are located in the region of the first surface 11a near the first side surface 11b and at least a portion of them are located in the carbon nanotube film 132. At least another portion of the plurality of discrete carbon nanotubes 131 is outside the carbon nanotube film 132 and its orthogonal projection on the metal substrate 11 is located outside the first surface 11a. The at least another portion of the plurality of discrete carbon nanotubes 131 protrudes away from the metal substrate 11 relative to the first side surface 11b.

[0062] In the foil structure 10 with carbon nanotubes 13 provided in this embodiment, the plurality of carbon nanotubes 13 may include a plurality of discrete carbon nanotubes 131. By distributing the discrete carbon nanotubes 131 protruding relative to the first side 11b of the metal substrate 11, the portion of the discrete carbon nanotubes 131 protruding from the first side 11b of the metal substrate 11 forms a protruding tip structure. The actual morphology can be seen with reference to FIG6. This tip structure can form an electric field enhancement effect, enabling emission at low voltage and facilitating the formation of large current emission. Furthermore, since the carbon nanotubes 13 are attached to the metal substrate 11, the metal substrate 11 has high electrical and thermal conductivity, and the metal substrate 11 has high strength, which allows the foil structure 10 with carbon nanotubes 13 to have a long service life.

[0063] For example, the bonding layer 12 may include a titanium carbide layer. The titanium carbide layer may be formed by reacting carbon nanotubes 13 and a titanium layer on the surface of the metal substrate 11 at a high temperature. The titanium carbide layer can be tightly connected to the film layers on both sides by chemical bonds, so that the multiple carbon nanotubes 13 and the metal substrate 11 can have good bonding strength.

[0064] For example, the bonding layer 12 may include an adhesive layer. For instance, the adhesive layer can be formed by adding a certain amount of binder when forming the carbon nanotubes 13 on the metal substrate 11, and then curing the binder into an adhesive layer at a high temperature, thereby bonding the carbon nanotubes 13 to the first surface 11a of the metal substrate 11. As another example, the carbon nanotubes 13 can be formed on the metal substrate 11 and then coated with conductive silver paste by spraying or printing.

[0065] For example, the material of the metal substrate 11 includes at least one selected from the following materials: copper, molybdenum, stainless steel, titanium, and Kovar alloy. The thickness of the metal substrate 11 in the direction perpendicular to the first surface 11a can be 10 micrometers to 50 micrometers. The thinness of the metal substrate 11 makes the overall thickness of the foil structure 10 with carbon nanotubes 13 thinner, allowing it to be cut and processed into the desired shape as needed.

[0066] For example, the thickness of the metal substrate 11 can be 10 micrometers to 1 millimeter, such as 20 micrometers to 30 micrometers. Optionally, the thickness of the metal substrate 11 can be 20 micrometers, 21 micrometers, 22 micrometers, 23 micrometers, 24 micrometers, 25 micrometers, 26 micrometers, 27 micrometers, 28 micrometers, 29 micrometers, or 30 micrometers, etc.

[0067] For example, the thickness of the layer formed by multiple carbon nanotubes 13 on the metal substrate 11 in the direction perpendicular to the first surface 11a can be 10 nanometers to 5 micrometers, for example, less than 1 micrometer. A thinner layer of carbon nanotubes 13 allows for the structure of discrete carbon nanotubes 131 protruding from the first side 11b of the metal substrate 11. The thinner the carbon nanotube film, the closer the morphology of the multiple discrete carbon nanotubes formed during cutting is to that of a single-layer carbon nanotube in the thickness direction of the carbon nanotube film. In this direction, the tips of the discrete carbon nanotubes can obtain a greater field strength enhancement effect, making it easier to generate a higher electron beam emission at a lower gate voltage. Therefore, an extremely thin carbon nanotube film is a key technology for achieving high emission current at low gate voltages.

[0068] Extremely thin carbon nanotube films are difficult to shape and position. This invention addresses this by combining a carbon nanotube film with a metal substrate. First, it gives the carbon nanotube film a stable structural shape, such as a plane or cylinder. Second, it allows for regular cutting of the carbon nanotube film through the regular cutting of the metal substrate, creating a smooth edge (the cut is a straight line, and the cut edge consists of multiple discrete carbon nanotubes with a length of 10 micrometers), solving the problem that carbon nanotube films cannot be directly torn in a straight line. Third, the extremely thin carbon nanotube film attached to the metal substrate allows for high-precision installation and positioning on a macroscopic scale, achieving precise assembly. Fourth, by adjusting the geometry of the metal substrate, various shapes and combinations of the carbon nanotube film can be set, thereby achieving the required emission area and obtaining a larger emission current. Fifth, in various structures, shapes, or assembly relationships, the metal substrate can provide excellent electrical conductivity for the carbon nanotube film, facilitating the flow of current from the cathode substrate through the metal substrate to the carbon nanotubes and out through the tips of the carbon nanotubes.

[0069] For example, the thickness of the bonding layer 12 can be 1 micrometer to 50 micrometers, and the thickness of the bonding layer is greater than or equal to the thickness of the layer formed by the plurality of carbon nanotubes 13, so that the plurality of carbon nanotubes 13 can be distributed in the bonding layer 12.

[0070] According to some exemplary embodiments, referring to Figures 5 and 6, the foil structure 10 includes a plurality of discrete carbon nanotubes 131. The diameters of the plurality of discrete carbon nanotubes 131 may be equal to each other, and the lengths of at least two of the plurality of discrete carbon nanotubes 131 may be unequal. Figure 5 illustrates the plurality of discrete carbon nanotubes 131 with equal lengths. Referring to the actual morphology illustrated in Figure 6, it can be seen that different discrete carbon nanotubes 131 may have different lengths.

[0071] According to some exemplary embodiments, referring to Figures 2, 3 and 5, the extension direction of the discrete carbon nanotubes 131 is substantially parallel to the first surface 11a, and the extension direction of the discrete carbon nanotubes 131 is substantially perpendicular to the first side surface 11b. This allows the portion of the discrete carbon nanotubes 131 protruding from the first side surface 11b of the metal substrate 11 to be perpendicular to the first side surface 11b, thereby forming an upright tip structure perpendicular to the first side surface 11b. This is more conducive to forming an electric field enhancement effect to achieve high current emission.

[0072] It should be noted that the extension direction of the discrete carbon nanotube 131 being basically parallel to the first surface 11a should be understood as the angle between the extension direction of the discrete carbon nanotube 131 and the first surface 11a being less than or equal to 15°, and the extension direction of the discrete carbon nanotube 131 being basically perpendicular to the first side surface 11b should be understood as the angle between the extension direction of the discrete carbon nanotube 131 and the first side surface 11b being 90°±15°.

[0073] According to some exemplary embodiments, referring to Figures 3 and 5, the first surface 11a of the metal substrate 11 can be planar, so that the discrete carbon nanotubes 131 protruding from the first side 11b can be arranged in a straight line.

[0074] Figures 7A-7C schematically illustrate various structural diagrams of the foil structure according to embodiments of the present invention.

[0075] According to some exemplary embodiments, the planar foil structure can also be bent into other shapes as needed. Referring to FIG7A, the foil structure 10 can be rolled into a single roll ring, that is, the first surface 11a of the metal substrate 11 is a cylindrical surface. In this way, multiple discrete carbon nanotubes 131 can be arranged in a ring, which can increase the emission area on the one hand and produce a circular focal point on the other hand.

[0076] Referring to Figure 7B, the foil structure 10 can be rolled into a spiral ring shape, that is, the first surface 11a of the metal substrate 11 is a spiral curved surface, so that multiple discrete carbon nanotubes 131 can be arranged in a spiral ring shape. The spiral ring foil structure 10 has multiple layers from the inside to the outside, and the spacing d1 between adjacent layers can be 5 micrometers to 1 millimeter. The spiral ring shape can further increase the emission area and obtain a larger emission current; and the spacing d1 between adjacent layers is greater than or equal to the protrusion height of multiple discrete carbon nanotubes 131, without affecting the field strength enhancement effect at the tip of the discrete carbon nanotubes 131.

[0077] Referring to Figure 7C, the foil structure 10 can be reciprocated in a serpentine shape along a fixed direction, so that multiple discrete carbon nanotubes 131 can be arranged in a serpentine shape, that is, the first surface 11a of the metal substrate 11 is a wavy surface. Along the reciprocating winding direction, the height h of the foil structure 10 can be 0.3 mm to 2 mm, and perpendicular to the reciprocating winding direction, the distribution length L2 of the foil structure 10 can be 1 mm to 20 mm. This reciprocating serpentine structure results in a larger emission area and a greater number of discrete carbon nanotubes 131, significantly increasing the emission current of the cathode. Simultaneously, the reciprocating layers of the foil structure 10 maintain the same distance, ensuring a uniform surface distribution of the emission current. The distance between the reciprocating layers of the foil structure 10 is greater than or equal to the protruding height of the discrete carbon nanotubes 131, thus not affecting the field strength enhancement effect at the tips of the discrete carbon nanotubes 131. The reciprocating winding of the foil structure 10 makes the electron emission region rectangular overall. In an X-ray tube, the linear focusing principle can be used to set the rectangular length as a projected square, resulting in a square projection of the X-ray focus formed on the reflective anode target.

[0078] It should be noted that the above design of foil structure 10 fully utilizes the structural flexibility of carbon nanotube film attached to metal substrate, increases cathode emission area, and does not affect field strength enhancement effect at carbon nanotube tip, which is conducive to achieving high current intensity operation.

[0079] According to some exemplary embodiments, referring to Figures 2, 4, and 5, the metal substrate 11 may include a metal mesh 111. The metal mesh 111 includes a plurality of metal portions 111a extending intersecting along a first direction D1 and a second direction D2. The first direction D1 and the second direction D2 intersect; for example, the first direction D1 may be perpendicular to the second direction D2. A plurality of through holes 111b are provided between the plurality of metal portions 111a extending intersecting along the first direction D1 and the second direction D2, and the plurality of through holes 111b are arranged in an array along the first direction D1 and the second direction D2. By setting the metal substrate 11 as a mesh-like metal mesh 111, carbon nanotube films 132 and discrete carbon nanotubes 131 can be formed on the metal substrate 11 by filtering. The orthogonal projection of a portion of the discrete carbon nanotubes 131 on the metal substrate 11 can be located at a plurality of metal portions 111a, and the orthogonal projection of another portion of the discrete carbon nanotubes 131 on the metal substrate 11 can be located at a plurality of through holes 111b on the first side 11b. The other portion of the discrete carbon nanotubes 131 located at the plurality of through holes 111b protrudes relative to the first side 11b of the metal substrate 11.

[0080] For example, the pore diameters of the plurality of through-holes 111b are all smaller than the lengths of the discrete carbon nanotubes 131. When at least two of the discrete carbon nanotubes 131 have unequal lengths, the pore diameters of the plurality of through-holes 111b are all smaller than the length of the shortest discrete carbon nanotube 131. Setting the pore diameter of the through-holes 111b to be smaller than the length of the discrete carbon nanotubes 131 can prevent the discrete carbon nanotubes 131 from flowing away from the through-holes 111b of the metal substrate 11 when they are formed on the metal substrate 11. At the same time, setting the pore diameter of the through-holes 111b to be smaller ensures that the metal substrate 11 has high strength.

[0081] For example, the pore size of the multiple through holes 111b can be consistent, and the pore size of the multiple through holes 111b can be 4-10 micrometers, that is, the size of the through hole 111b along the first direction D1 can be 4-10 micrometers, and the size of the through hole 111b along the second direction D2 can be 4-10 micrometers. The porosity of the metal mesh 111 can be 20%-45%.

[0082] For example, the aperture of the through hole 111b can be 5-8 micrometers. Optionally, the aperture of the through hole 111b can be 5 micrometers, 6 micrometers, 7 micrometers, or 8 micrometers, etc.

[0083] For example, the porosity of the metal mesh 111 can be 30%-40%. Optionally, the porosity of the metal mesh 111 can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%, etc.

[0084] According to some exemplary embodiments, referring to Figures 3 and 4, a portion of the first side surface 11b is located in the region where multiple metal portions 111a are located, and another portion of the first side surface 11b is located in the region where multiple through holes 111b are located. The metal substrate 11 has both metal portions 111a and through holes 111b structures at the first side surface 11b, and the portion of the discrete carbon nanotube 131 located at the through holes 111b can protrude relative to the first side surface 11b of the metal substrate 11.

[0085] Figures 8A-8D schematically illustrate various three-dimensional structural diagrams of a cold cathode according to embodiments of the present invention.

[0086] Some embodiments of the present invention also provide a cold cathode 20 with carbon nanotubes. Referring to Figures 2 and 8A, the cold cathode 20 includes at least one electron emitter 21, which includes an emitter substrate 211 and a foil structure 10 disposed on the emitter substrate 211. This foil structure 10 is the foil structure 10 provided in the preceding embodiments, that is, the foil structure 10 includes a metal substrate 11 and carbon nanotubes 13 disposed on the metal substrate 11. Exemplarily, the cold cathode 20 may include only one foil structure 10 with carbon nanotubes 13 to fabricate an X-ray tube 30 with a small focal spot, allowing for a wide range of adjustable focal spot sizes.

[0087] Figure 9A schematically shows a partial region structure of a cold cathode according to an embodiment of the present invention. Figure 9B schematically shows the principle of the tip electric field enhancement effect of a cold cathode according to an embodiment of the present invention.

[0088] According to some exemplary embodiments, referring to Figures 2, 3, 5, 8A, 9A and 9B, the electron emitter 21 is used to emit electrons along a predetermined electron emission direction. The discrete carbon nanotubes 131 in the foil structure 10 serve as the electron emission structure. After being subjected to a strong electric field, the ends 131a of the discrete carbon nanotubes 131 away from the metal substrate 11 emit electrons under the field emission principle. The extension direction of the discrete carbon nanotubes 131 in the foil structure 10 is parallel to the electron emission direction, that is, the foil structure 10 is arranged such that the first surface 11a of the metal substrate 11 is parallel to the electron emission direction.

[0089] When viewed from the cross-section of the foil structure 10 of the cold cathode 20, the discrete carbon nanotubes 131 form a single layer of upright state. In the electric field formed between the substrate and the first side of the cold cathode relative to the gate, the discrete carbon nanotubes 131 are like "lightning rods". Their top tips have a huge field strength enhancement effect. The thinner the carbon nanotube film, the smaller it is relative to the tip, the greater the field strength enhancement, and the easier it is to generate current emission.

[0090] According to some exemplary embodiments, referring to Figures 8A, 9A, and 9B, the cold cathode 20 further includes a gate 22 located directly above the first side 11b of the foil structure 10. Discrete carbon nanotubes 131 extend away from the first side 11b of the metal substrate 11 in their extension direction and approach the gate 22. When a positive voltage relative to the emitter substrate 211 is applied to the gate 22, the discrete carbon nanotubes 131 emit electrons towards the gate 22. The discrete carbon nanotubes 131 include an end 131a extending away from the metal substrate 11 in their extension direction. In the electron emission direction, the gate 22 is spaced apart from the ends 131a of the discrete carbon nanotubes 131. For example, the gate 22 can be a metal grid with micropores (pore diameter can be several micrometers to tens of micrometers), and the width of the ribs between the pores can be less than one-third of the pore diameter (pore gap). By adjusting the position of the gate 22 relative to the electron emitter 21 and the applied voltage, the magnitude and uniformity of the field emission current can be precisely controlled, thereby achieving fine control of the electron emission process.

[0091] For example, in the electron emission direction, the spacing between the first side 11b of the metal substrate 11 and the gate 22 can be 100 micrometers to 1 millimeter. For instance, the spacing between the first side 11b of the metal substrate 11 and the gate 22 can be 150 micrometers to 250 micrometers. Optionally, the spacing between the first side 11b of the metal substrate 11 and the gate 22 can be 150 micrometers, 160 micrometers, 170 micrometers, 180 micrometers, 190 micrometers, 200 micrometers, 210 micrometers, 220 micrometers, 230 micrometers, 240 micrometers, or 250 micrometers, etc. A smaller spacing can reduce the control voltage U between the gate and the foil structure. The control voltage U is generated by the gate control device. A smaller control voltage U is easier to implement and helps reduce the cost of the X-ray source.

[0092] For example, in the electron emission direction, the distance by which the discrete carbon nanotube 131 protrudes relative to the first side 11b can be 3 micrometers to 80 micrometers. For instance, the distance by which the discrete carbon nanotube 131 protrudes relative to the first side 11b can be 10 micrometers to 60 micrometers. Optionally, the distance by which the discrete carbon nanotube 131 protrudes relative to the first side 11b can be 10 micrometers, 20 micrometers, 30 micrometers, 40 micrometers, 50 micrometers, or 60 micrometers, etc.

[0093] According to some exemplary embodiments, referring to FIG9B, the spacing S1 between adjacent discrete carbon nanotubes 131 is greater than the protrusion height S2 of the discrete carbon nanotubes 131 relative to the first side 11b. With this arrangement, the field strength enhancement effect at the tip of the discrete carbon nanotubes 131 will not be affected by the neighboring discrete carbon nanotubes 131, thus achieving high-current emission. More broadly, as long as the difference between S1 and S2 is within one order of magnitude, the field strength enhancement effect at the tip of the discrete carbon nanotubes 131 will not be significantly affected, and these are all feasible arrangement schemes.

[0094] According to some exemplary embodiments, referring to Figures 8B, 9A, and 9B, the cold cathode 20 may include a plurality of foil structures 10, which may be assembled in an array on the emitter substrate 211. The plurality of first side surfaces 11b of the foil structures 10 are arranged parallel to the gate 22 and at the same distance from the gate 22. That is, the cold cathode 20 may include a plurality of foil structures 10 with carbon nanotubes, which may be arranged in a row or in multiple rows. Figure 8B schematically illustrates the arrangement of multiple foil structures 10 in a row. This array arrangement of multiple foil structures 10 with carbon nanotubes facilitates the fabrication of high-current X-ray tubes with currents greater than 100 mA.

[0095] For example, the area formed by multiple foil structures 10 can be a square, such as a square with a size of 2mm*2mm; another example is that the area formed by multiple foil structures 10 can be a rectangle, and the long side of the rectangle is perpendicular to the thickness direction of the foil structure 10, and the size of the rectangle can be 1mm*10mm; yet another example is that the area formed by multiple foil structures 10 can be a rectangle, and the long side of the rectangle is parallel to the thickness direction of the foil structure 10, and the size of the rectangle can be 1mm*20mm.

[0096] According to some exemplary embodiments, referring to FIG8B, the emitter substrate 211 may include a plurality of metal sheets 2111, and a plurality of foil structures 10 are respectively sandwiched between the plurality of metal sheets 2111. Two adjacent foil structures 10 are spaced apart on both sides of a metal sheet 2111, and the thickness of the metal sheet 2111 can be set according to the required spacing between two adjacent foil structures 10. For example, the metal sheet 2111 may include a 304 stainless steel sheet. The metal sheet 2111 allows for a larger distance between the plurality of discrete carbon nanotubes 131 on adjacent foil structures 10, so that the field strength enhancement effect at the tips of the plurality of carbon nanotubes 131 is not affected, ensuring that a large current emission can be obtained at a lower gate control voltage U.

[0097] According to some exemplary embodiments, referring to Figures 8B and 8C, in the cold cathode 20, multiple foil structures 10 can all be in a single-rolled loop shape, and the diameters of the multiple foil structures 10 are equal to each other, and the multiple single-rolled loop foil structures 10 are arranged in an array. Figure 8C schematically shows the structure arranged in a row, but multiple rows can also be arranged as needed. Arranging multiple rows of foil structures 10 can multiply the emission current. For example, the diameter D of the single-rolled loop foil structure 10 can be 0.3 mm to 2 mm, and the distribution length L1 of the multiple single-rolled loop foil structures 10 along the arrangement direction can be 1 mm to 20 mm.

[0098] According to some exemplary embodiments, referring to Figures 8B and 8D, in the cold cathode 20, multiple foil structures 10 can all be in a single-rolled ring shape, and the diameters of the multiple foil structures 10 are not equal; for example, the diameters of the multiple foil structures 10 are arranged in an arithmetic sequence. Multiple single-rolled ring-shaped foil structures 10 can be nested sequentially to form a multi-rolled ring shape, so that the multiple discrete carbon nanotubes 131 in the multiple foil structures 10 can be arranged in a multi-rolled ring shape. For example, the spacing d2 between two adjacent foil structures 10 can be 5 micrometers to 1 millimeter. This design can well adapt to the X-ray tube design of the emission target structure. The cathode emission area has a small width and a large length (projection direction), and the focal spot size formed by the projection is not large in either direction, thus increasing the emission area to obtain a large current while ensuring the focal spot size.

[0099] Figure 10 schematically shows a structural diagram of an X-ray tube according to an embodiment of the present invention.

[0100] Some embodiments of the present invention provide an X-ray tube 30. Referring to FIG10, the X-ray tube 30 may include a cathode assembly 31, an anode target 32, and a tube body 33. Both the cathode assembly 31 and the anode target 32 ​​are disposed within the tube body 33. The cathode assembly 31 adopts the cold cathode 20 provided in the previous embodiments. Referring to FIG2, FIG5, and FIG7A, the electron emission direction is the direction from the cathode assembly 31 to the anode target 32, which is referred to as the third direction D3. The extension direction of the discrete carbon nanotubes 131 of the foil structure 10 in the cold cathode 20 is parallel to the third direction D3, and the first surface 11a of the metal substrate 11 of the foil structure 10 is parallel to the third direction D3.

[0101] The cathode assembly 31 includes an electron emitter 21 with carbon nanotubes. The X-ray tube 30 can serve as a carbon nanotube X-ray tube, a novel electron source X-ray tube based on the field emission principle. Compared to traditional thermionic cathode X-ray tubes, it offers advantages such as faster response speed, lower energy consumption, and longer lifespan. A grid 22 is spaced a certain distance from the carbon nanotube electron emitter 21. A certain voltage is applied to the grid 22, creating an electric field. When the voltage at the grid 22 generates an electric field threshold greater than the field emission threshold, the carbon nanotube electron emitter 21 emits electrons. These emitted electrons are accelerated under the influence of the anode voltage and bombard the anode target 32, generating X-rays.

[0102] For example, the anode target 32 ​​is typically made of a high-melting-point metal (such as tungsten, molybdenum, etc.) and is located inside the X-ray tube body 33 at the opposite end to the cathode assembly 31. When electrons emitted from the cathode assembly 31 strike the anode target 32, X-rays are generated due to energy conversion.

[0103] For example, the X-ray tube 33 may be in a vacuum state; for instance, the pressure inside the X-ray tube 33 may be 10. -7 Torr ensures that electrons can move freely from the cathode assembly 31 to the anode target 32 ​​without being hindered by gas molecules.

[0104] For example, the X-ray tube 30 may further include a power supply and a control system, which can supply a certain voltage to the cathode assembly 31 and the anode target 32 ​​respectively. For example, a voltage U of 0-3kV can be supplied to the cathode assembly 31, and a voltage of 0-300kV can be supplied to the anode target 32. The voltage difference between the cathode assembly 31 and the anode target 32 ​​can drive electrons to accelerate from the cathode assembly 31 towards the anode target 32.

[0105] Figure 11 schematically illustrates a flowchart of a method for preparing a foil structure according to an embodiment of the present invention. Figures 12A-12F schematically illustrate a process diagram for preparing a foil structure according to an embodiment of the present invention.

[0106] Some embodiments of the present invention also provide a method for preparing a foil structure with carbon nanotubes. Referring to FIG11, the preparation method may include the following steps S10-S40.

[0107] In step S10, a carbon nanotube solution is prepared by mixing carbon nanotube powder, dispersant and solvent to form a carbon nanotube solution.

[0108] In step S20, a metal substrate is prepared.

[0109] In step S30, a primary foil structure is prepared: a metal substrate is placed on a filter element, and a carbon nanotube solution is passed through the filter element with the metal substrate to form multiple carbon nanotubes on the metal substrate. After a drying process, a primary foil structure is obtained.

[0110] In step S40, a foil structure is prepared: the primary foil structure is subjected to high-temperature treatment to improve the bonding force between the metal substrate and multiple carbon nanotubes, thereby obtaining the foil structure.

[0111] The foil structure preparation method provided by this invention is simple to manufacture, can be mass-produced, has good repeatability and consistency, and has low production cost.

[0112] According to some exemplary embodiments, in preparing the carbon nanotube solution, the solvent may include anhydrous ethanol or deionized water, the dispersing agent may include sodium dodecyl sulfate, the mass percentage of carbon nanotube powder may be 0.1%-0.8%, and the mass percentage of sodium dodecyl sulfate may be 0.2%-0.5%.

[0113] For example, the mass percentage of carbon nanotube powder can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, or 0.8%, etc.

[0114] For example, the mass percentage of sodium dodecyl sulfate can be 0.2%, 0.3%, 0.4%, or 0.5%, etc.

[0115] For example, a binder may be added during the preparation of the carbon nanotube solution to enhance the bonding strength between the carbon nanotubes and the metal substrate. The carbon nanotube solution may also include other additives as needed.

[0116] According to some exemplary embodiments, the preparation of the carbon nanotube solution further includes dispersing the carbon nanotube solution. The dispersion step may include sonicating and centrifuging the carbon nanotube solution to form a uniformly dispersed and non-agglomerated carbon nanotube solution.

[0117] According to some exemplary embodiments, referring to Figures 12A and 12B, the step of preparing the metal substrate 11 may include: cutting a plurality of through holes 111b in the metal foil 11f using a laser cutting process to form a metal mesh 111, the metal mesh 111 including a plurality of metal portions 111a extending intersecting along a first direction D1 and a second direction D2.

[0118] For example, the material of the metal substrate 11 may include at least one selected from the following materials: copper, molybdenum, stainless steel, titanium, and Kovar alloy. The thickness of the metal substrate 11 may be 10-50 micrometers, the pore size of the plurality of through holes 111b may be 4-10 micrometers, and the porosity of the metal mesh 111 may be 20%-45%.

[0119] According to some exemplary embodiments, referring to FIG12C, the step of preparing the metal substrate 11 may further include: sputtering a titanium metal layer 121 on the surface of the metal mesh 111 using a magnetron sputtering process. For example, the thickness of the titanium metal layer 121 may be 1-10 nanometers.

[0120] According to some exemplary embodiments, referring to FIG12D, the filtration component used in the step of preparing the primary foil structure includes filter paper or anodized aluminum membrane. Filtration allows carbon nanotubes 13 in the carbon nanotube solution to remain on the metal substrate 11, i.e., forming a layer comprising carbon nanotubes 13 on the metal substrate 11.

[0121] According to some exemplary embodiments, between preparing the primary foil structure and preparing the foil structure, the steps further include: removing the dispersing agent from the surface of the metal substrate and carbon nanotubes of the primary foil structure; washing the surface of the metal substrate and carbon nanotubes of the primary foil structure with deionized water; and densifying the surface of the metal substrate and carbon nanotubes of the primary foil structure with an ethanol solution and a nitric acid solution. For example, the dispersing agent on the surface of the metal substrate and carbon nanotubes of the primary foil structure can be removed by isopropanol.

[0122] According to some exemplary embodiments, a primary foil structure is subjected to high-temperature treatment to improve the bonding force between the metal substrate and multiple carbon nanotubes. This includes: subjecting the primary foil structure to high-temperature treatment in a high-temperature furnace protected by an inert gas to form a titanium carbide thin crystalline layer, thereby enabling intermolecular bonding between the metal substrate and the carbon nanotubes. For example, the high-temperature treatment may specifically be a high-temperature treatment at 400-450°C for 15-60 minutes, and the inert gas may include argon.

[0123] According to some exemplary embodiments, in conjunction with Figures 12E, 12F and 5, the preparation of the foil structure 10 may further include: cutting the foil structure 10 into the desired shape by means of laser cutting or mechanical cutting.

[0124] For example, during cutting, the cutting can be performed along the middle of the through hole 111b of the metal substrate 11. Due to the high toughness of carbon nanotubes, they are difficult to be completely cut along the cutting path. Thus, at the cut side, more carbon nanotubes protruding from the first side 11b of the metal substrate 11 can be obtained, i.e., discrete carbon nanotubes 131. At the cut side, the metal portion 111a is cut into a protruding metal protrusion 111c. The protrusion distance of the metal protrusion 111c relative to the metal portion 111a that intersects it can be half the diameter of the through hole 111b. This is beneficial for obtaining more discrete carbon nanotubes 131 at the cutting edge.

[0125] For example, referring to FIG12F, the dimensions of the cut foil structure 10 can be set according to actual needs. For example, the length of the foil structure 10 can be 2-5 cm and the width can be 1-3 cm.

[0126] According to some exemplary embodiments, the foil structure 10 obtained by cutting can be further processed into other shapes as needed. For example, it can be processed into the shapes shown in Figures 8A-8C. For details, please refer to the above description, which will not be repeated here.

[0127] Figure 13 is an emission current-gate voltage curve of a cold cathode according to some exemplary embodiments of the present invention. Figure 14 is a bar chart comparing the gate voltages of three groups of cold cathodes at a specific emission current according to some exemplary embodiments of the present invention. Figure 15 is an emission current-emission count curve of a cold cathode according to some exemplary embodiments of the present invention.

[0128] To verify the emission performance of the foil structure containing carbon nanotubes and the cold cathode provided by the present invention, three sets of cold cathodes (numbered as cold cathode 1, cold cathode 2, and cold cathode 3) were fabricated, and the following verifications were performed on these three sets of cold cathodes.

[0129] The emission current of three sets of cold cathodes was measured under different gate control voltages, as shown in Figure 13. When the gate control voltage reached 1400V, the emission current of all three sets of cold cathodes reached 200mA, demonstrating the excellent emission performance of the cold cathode. A bar chart of the gate control voltage of the three sets of cold cathodes at an emission current of 200mA is shown in Figure 14. The difference in gate control voltage among the three sets of cold cathodes at an emission current of 200mA is within 50V, indicating that the cold cathode composed of the foil structure prepared by the method provided in this embodiment of the invention has good consistency.

[0130] Lifetime test of cold cathode 1: The change in emission current after 100,000 field emission cycles was measured. The emission current change curve is shown in Figure 15. After 100,000 field emission cycles, the change in emission current is only 1%, which shows that the cold cathode has a long service life.

[0131] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.

Claims

1. A foil structure with carbon nanotubes, characterized in that, The foil structure includes: a metal substrate having a first surface and a first side surface, the first side surface being perpendicular to the first surface and located at at least one edge of the metal substrate; a bonding layer located on the first surface of the metal substrate; and a plurality of carbon nanotubes located on the first surface of the metal substrate, wherein the bonding layer is used to enhance the bonding strength between the metal substrate and the plurality of carbon nanotubes, the plurality of carbon nanotubes comprising a carbon nanotube film and a plurality of discrete carbon nanotubes, the carbon nanotube film being located on the first surface and its orthogonal projection onto the metal substrate being within the first surface, the plurality of discrete carbon nanotubes being located on the first surface near the first side surface and at least a portion being located within the carbon nanotube film, at least another portion of the plurality of discrete carbon nanotubes being outside the carbon nanotube film and its orthogonal projection onto the metal substrate being outside the first surface, the at least another portion of the plurality of discrete carbon nanotubes protruding away from the metal substrate relative to the first side surface.

2. The foil structure according to claim 1, characterized in that, The discrete carbon nanotubes are substantially perpendicular to the first side.

3. The foil structure according to claim 1, characterized in that, The first surface is a plane, a cylindrical surface, a helical surface, or a wavy surface.

4. The foil structure according to claim 3, characterized in that, The metal substrate includes a metal mesh, which includes a plurality of metal portions extending intersecting along a first direction and a second direction, and a plurality of through holes located between the plurality of metal portions, wherein the pore size of the plurality of through holes is smaller than the length of the discrete carbon nanotubes.

5. The foil structure according to claim 4, characterized in that, A portion of the plurality of discrete carbon nanotubes has its orthogonal projection onto the metal substrate located in the plurality of metal portions, and another portion of the plurality of discrete carbon nanotubes has its orthogonal projection onto the metal substrate located in the plurality of through holes.

6. The foil structure according to claim 5, characterized in that, A portion of the first side is located in the area where the plurality of metal parts are located, and another portion of the first side is located in the area where the plurality of through holes are located.

7. The foil structure according to any one of claims 1-6, characterized in that... The plurality of discrete carbon nanotubes have the same diameter as each other, and at least two of the plurality of discrete carbon nanotubes have different lengths.

8. The foil structure according to any one of claims 4-6, characterized in that, The bonding layer includes a titanium carbide layer or an adhesive layer; and / or, the material of the metal substrate includes at least one selected from the following materials: copper, molybdenum, stainless steel, titanium, and Kovar alloy; and / or, the thickness of the metal substrate is 10-50 micrometers; and / or, the pore size of the plurality of through holes is 4-10 micrometers; and / or, the porosity of the metal mesh is 20%-45%.

9. A cold cathode with carbon nanotubes, characterized in that, The cold cathode includes: at least one electron emitter, the electron emitter including: an emitter substrate; and a foil structure disposed on the emitter substrate, the foil structure being the foil structure according to any one of claims 1-8.

10. The cold cathode according to claim 9, characterized in that, It also includes a gate located directly above the first side of the foil structure, wherein the discrete carbon nanotubes extend away from the first side of the metal substrate and toward the gate, and when a positive voltage relative to the emitter substrate is applied to the gate, the discrete carbon nanotubes emit electrons toward the gate.

11. The cold cathode according to claim 10, characterized in that, The distance between the first side of the metal substrate and the gate is 100 micrometers to 1 millimeter.

12. The cold cathode according to claim 10 or 11, characterized in that, The cold cathode includes a plurality of foil structures, which are assembled in an array on the emitter substrate. The plurality of first sides of the plurality of foil structures are arranged in a direction parallel to the gate and at the same distance from the gate.

13. The cold cathode according to claim 12, characterized in that, The emitter substrate comprises multiple metal sheets, and the foil structure is sandwiched between the multiple metal sheets.

14. An X-ray tube, characterized in that, The X-ray tube includes a cathode assembly, which includes a cold cathode according to any one of claims 9-13.

15. The X-ray tube according to claim 14, characterized in that, The X-ray tube also includes an X-ray tube body and an anode target, with both the cathode assembly and the anode target disposed within the X-ray tube body.

16. A method for preparing a foil structure with carbon nanotubes, characterized in that, The method includes: preparing a carbon nanotube solution: mixing carbon nanotube powder, a dispersant, and a solvent to prepare a carbon nanotube solution; preparing a metal substrate; preparing a primary foil structure: placing the metal substrate on a filter element, passing the carbon nanotube solution through the filter element with the metal substrate to form multiple carbon nanotubes on the metal substrate, and obtaining a primary foil structure after a drying process; and preparing a foil structure: subjecting the primary foil structure to high-temperature treatment to improve the bonding force between the metal substrate and the multiple carbon nanotubes, thereby obtaining a foil structure, wherein the metal substrate has a first surface and a first side surface, the first side surface being perpendicular to the first surface. The first side is located at the edge of at least one side of the metal substrate. The plurality of carbon nanotubes include a carbon nanotube film and a plurality of discrete carbon nanotubes. The carbon nanotube film is located on the first surface and its orthographic projection on the metal substrate is within the first surface. The plurality of discrete carbon nanotubes are located on the first surface in a region close to the first side and at least a portion of them are located within the carbon nanotube film. At least another portion of the plurality of discrete carbon nanotubes is outside the carbon nanotube film and its orthographic projection on the metal substrate is outside the first surface. This at least another portion of the plurality of discrete carbon nanotubes protrudes away from the metal substrate relative to the first side.

17. The preparation method according to claim 16, characterized in that, The preparation of the carbon nanotube solution further includes: dispersing the carbon nanotube solution by ultrasonic and centrifugation to form a uniformly dispersed and non-agglomerated carbon nanotube solution.

18. The preparation method according to claim 16 or 17, characterized in that, Between the preparation of the primary foil structure and the preparation of the foil structure, the method further includes: removing the dispersing agent from the surface of the metal substrate and carbon nanotubes of the primary foil structure; cleaning the surface of the metal substrate and carbon nanotubes of the primary foil structure with deionized water; and densifying the surface of the metal substrate and carbon nanotubes of the primary foil structure with ethanol solution and nitric acid solution.

19. The preparation method according to claim 16 or 17, characterized in that, The preparation of the metal substrate includes: cutting multiple through holes in a metal foil using a laser cutting process to form a metal mesh; and sputtering a titanium metal layer onto the surface of the metal mesh using a magnetron sputtering process.

20. The preparation method according to claim 16 or 17, characterized in that, The high-temperature treatment of the primary foil structure to improve the bonding force between the metal substrate and the plurality of carbon nanotubes includes: performing high-temperature treatment on the primary foil structure in a high-temperature furnace protected by an inert gas to form a titanium carbide thin crystal layer, so as to form intermolecular bonds between the metal substrate and the carbon nanotubes.

21. The preparation method according to claim 16 or 17, characterized in that, In the preparation of the carbon nanotube solution, the solvent includes anhydrous ethanol or deionized water, the dispersing agent includes sodium dodecyl sulfate, the carbon nanotube powder has a mass percentage content of 0.1%-0.8%, and the sodium dodecyl sulfate has a mass percentage content of 0.2%-0.5%; and / or, a binder is also added in the preparation of the carbon nanotube solution.

22. The preparation method according to claim 19, characterized in that, The material of the metal substrate includes at least one selected from the following: copper, molybdenum, stainless steel, titanium, and Kovar alloy; and / or, the thickness of the metal substrate is 10-50 micrometers; and / or, the pore size of the plurality of through holes is 4-10 micrometers; and / or, the porosity of the metal mesh is 20%-45%.

23. The preparation method according to claim 16 or 17, characterized in that, The filtration component includes filter paper or anodized aluminum membrane.

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