Carbon nanotube field emission cathode and method for manufacturing the same

Abstract

The application provides a carbon nanotube field emission cathode and a preparation method thereof, and comprises the following steps: a conductive substrate; a graphene layer arranged on the upper surface of the conductive substrate; carbon nanotubes arranged on the upper surface of the graphene layer; a coating layer arranged on the surface of the carbon nanotubes; and a filling layer arranged between the carbon nanotubes, and the tips of the carbon nanotubes are exposed from the filling layer. The carbon nanotube field emission cathode and the preparation method thereof form a coating layer on the surface of the carbon nanotubes, and the carbon nanotubes are filled with a filling material, thereby constructing a novel composite structure carbon nanotube cathode, which has a low opening electric field, a high emission current density and excellent working stability.

Description

Technical Field

[0001] This application relates to the field of field emission technology, and in particular to a carbon nanotube field emission cathode and its preparation method. Background Technology

[0002] Electron emission sources have important applications in communications, space technology, security detection, and medical imaging, serving as core components in microwave tubes, X-ray tubes, display devices, and microscopic imaging equipment. Currently, electron emission sources still primarily use thermionic cathodes such as tungsten filaments and lanthanum hexaboride. Thermionic cathodes suffer from drawbacks such as large size, high thermal radiation power consumption, long on-time, and material evaporation at high temperatures, limiting the miniaturization and integration of vacuum electronic devices. In recent years, field emission cold cathodes, represented by carbon nanotubes, have received widespread attention and research. Electrons at their nanoscale tips can tunnel under an electric field, generating extremely large currents. Compared to thermionic cathodes, carbon nanotube cold cathodes offer advantages such as room-temperature operation, fast response, low power consumption, and miniaturization capabilities. Their application in vacuum electronic devices can simplify structures and achieve excellent power and frequency characteristics. However, existing carbon nanotube cathodes still suffer from the following problems: First, due to the inherent defects in the carbon nanotubes and the effect of Coulomb forces, electrons scatter and drift during their movement, generating a large amount of Joule heat, causing the carbon nanotubes to evaporate at high temperatures. Second, during field emission, residual gas in the vacuum ionizes under the influence of electrons, and the resulting plasma bombards the carbon nanotubes under the influence of the electric field, destroying their structure. Third, the close packing of carbon nanotubes creates a significant electric field shielding effect between adjacent nanotubes, greatly reducing the local electric field near the tip of the nanotube and suppressing electron emission. These factors mean that carbon nanotube cold cathodes still suffer from key problems such as a large turn-on electric field, low emission current density, and poor operational stability, and cannot fully meet the requirements of high-performance devices. Summary of the Invention

[0003] Therefore, it is necessary to provide a carbon nanotube field emission cathode with low turn-on electric field, high emission current density, and excellent emission stability, and its preparation method, to address the technical shortcomings of existing carbon nanotube field emission cathodes, such as large turn-on electric field, low emission current density, and poor operating stability.

[0004] To solve the above problems, this application adopts the following technical solution:

[0005] One objective of this application is to provide a carbon nanotube field emission cathode, comprising:

[0006] Conductive substrate;

[0007] A graphene layer disposed on the upper surface of the conductive substrate;

[0008] Carbon nanotubes disposed on the upper surface of the graphene layer;

[0009] A coating layer disposed on the surface of the carbon nanotubes;

[0010] A filling layer disposed between the carbon nanotubes.

[0011] In some embodiments, the tips of the carbon nanotubes expose the filling layer.

[0012] In some embodiments, the conductive substrate includes a metal substrate or a highly doped p-type or n-type silicon wafer, wherein the metal substrate includes at least one of titanium, copper, chromium, tungsten, molybdenum, tantalum, and platinum.

[0013] In some embodiments, the graphene layer is horizontally disposed on the upper surface of the conductive substrate.

[0014] In some embodiments, the carbon nanotubes are arranged vertically on the upper surface of the graphene layer.

[0015] In some embodiments, the graphene layers are connected to the graphene via van der Waals forces or covalent bonds.

[0016] In some embodiments, the coating layer includes at least one of aluminum nitride, titanium nitride, titanium carbide, chromium carbide, tantalum carbide, vanadium carbide, zirconium carbide, tungsten carbide, tantalum nitride, vanadium nitride, zirconium nitride, tungsten nitride, silicon carbide, titanium dioxide, magnesium oxide, and zinc oxide, and the coating layer thickness is 5-15 nm.

[0017] In some embodiments, the filler layer comprises at least one of the following materials:

[0018] Polyvinylidene fluoride and its copolymers; or a composite of polyvinylidene fluoride and its copolymers with a dielectric ceramic material, wherein the dielectric ceramic material includes at least one of barium titanate, strontium titanate, barium strontium titanate, calcium copper titanate, and strontium tantalate; or a composite of polyvinylidene fluoride and its copolymers with a polymeric material, wherein the polymeric material includes copper phthalocyanine or polyaniline.

[0019] The second objective of this application is to provide a method for preparing a carbon nanotube field emission cathode, comprising the following steps:

[0020] A graphene layer is deposited on the upper surface of a conductive substrate;

[0021] Carbon nanotubes are disposed on the upper surface of the graphene layer;

[0022] A coating layer is formed on the surface of the carbon nanotubes;

[0023] A filling layer is provided between the carbon nanotubes.

[0024] In some embodiments, the step of forming a graphene layer on the upper surface of the conductive substrate specifically includes the following steps:

[0025] Horizontally oriented graphene was grown using thermochemical vapor deposition.

[0026] The graphene is transferred to the conductive substrate.

[0027] In some embodiments, the step of growing a horizontally oriented graphene layer using thermochemical vapor deposition specifically includes the following steps: placing a cleaned copper sheet into a reaction chamber, heating the copper sheet to the growth temperature under an Ar protective atmosphere and holding it at that temperature; then introducing a mixed gas of carbon source gas and hydrogen to grow horizontally oriented graphene, and cooling it to room temperature under an argon protective atmosphere to obtain the graphene.

[0028] In some embodiments, the carbon source gas includes methane, ethylene, or acetylene, the growth temperature is 950–1050°C, and the volumetric flow rate ratio of the carbon source gas to hydrogen is 5–10.

[0029] In some embodiments, the step of transferring the graphene to the conductive substrate specifically includes the following steps:

[0030] PMMA was spin-coated onto a copper sheet on which graphene was grown, and then the copper sheet was immersed in FeCl3 solution, where it was etched and dissolved. Subsequently, the PMMA / graphene was transferred to the conductive substrate, and then the PMMA was dissolved in acetone to obtain a conductive substrate with the graphene layer horizontally coated.

[0031] In some embodiments, the step of forming a graphene layer on the upper surface of the conductive substrate specifically includes the following steps:

[0032] A uniformly dispersed graphene oxide solution is spin-coated onto the conductive substrate, and then the graphene oxide is reduced to graphene by heating in a vacuum furnace at 300-400°C or by microwave treatment, to obtain a conductive substrate with a horizontally coated graphene layer.

[0033] In some embodiments, the step of setting carbon nanotubes on the upper surface of the graphene layer specifically includes the following steps:

[0034] A catalyst is deposited on the surface of graphene, the catalyst comprising Fe, Co, Ni and their alloys;

[0035] A graphene / conductive substrate with the catalyst deposited thereon is placed in a reaction chamber, heated to the reaction temperature and held at that temperature. Under an H2 or H2 / Ar atmosphere, the catalyst is reduced to metal nanoparticles. Subsequently, a carbon source gas is introduced, and vertically oriented carbon nanotubes are grown in a plasma environment. The reaction temperature is 500-800℃, the carbon source gas is methane, ethylene, or acetylene, and the volume flow ratio of H2 to carbon source gas is 5–20.

[0036] In some embodiments, the step of forming a coating layer on the surface of the carbon nanotubes specifically includes the following steps: forming a coating layer on the surface of the carbon nanotubes by magnetron sputtering or pulsed laser deposition.

[0037] In some embodiments, the step of providing a filling layer between the carbon nanotubes and having the tips of the carbon nanotubes exposed in the filling layer specifically includes the following steps:

[0038] A filling layer is formed between carbon nanotubes using dip coating, drop coating, or spin coating, and then the filling layer is heat-treated to solidify it. The heat treatment temperature is 80–150°C.

[0039] In some embodiments, the following steps are also included:

[0040] Part of the upper filling layer is removed by plasma etching or mechanical polishing, exposing the tip of the carbon nanotube to the filling layer.

[0041] The present application adopts the above technical solution, and its beneficial effects are as follows:

[0042] The carbon nanotube field emission cathode and its fabrication method provided in this application include: a conductive substrate; a graphene layer disposed on the upper surface of the conductive substrate; carbon nanotubes disposed on the upper surface of the graphene layer; a coating layer disposed on the surface of the carbon nanotubes; and a filling layer disposed between the carbon nanotubes, with the tips of the carbon nanotubes exposed in the filling layer. The carbon nanotube field emission cathode and its fabrication method provided in this application form a coating layer on the surface of the carbon nanotubes, and the carbon nanotubes are filled with a filling material, thus constructing a novel composite carbon nanotube cathode with low turn-on electric field, high emission current density, and excellent working stability. Attached Figure Description

[0043] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a schematic diagram of the structure of the carbon nanotube field emission cathode provided in Embodiment 1 of this application.

[0045] Figure 2 This is a flowchart illustrating the steps of the method for preparing the carbon nanotube field emission cathode provided in Example 2 of this application. Detailed Implementation

[0046] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0047] In the description of this application, it should be understood that the terms "upper", "lower", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0048] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0049] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments.

[0050] Example 1

[0051] Please see Figure 1 The schematic diagram of the carbon nanotube field emission cathode provided in this application embodiment includes: a conductive substrate 110; a graphene layer 120 disposed on the upper surface of the conductive substrate 110; carbon nanotubes 130 disposed on the upper surface of the graphene layer 120; a coating layer 140 disposed on the surface of the carbon nanotubes 130; and a filling layer 150 disposed between the carbon nanotubes 140. The specific implementation of each component is described in detail below.

[0052] In this embodiment, the conductive substrate 110 includes a metal substrate or a highly doped p-type or n-type silicon wafer, and the metal substrate includes at least one of titanium, copper, chromium, tungsten, molybdenum, tantalum, and platinum.

[0053] In this embodiment, the graphene layer 120 is horizontally disposed on the upper surface of the conductive substrate 110.

[0054] In this embodiment, the carbon nanotubes 130 are vertically oriented on the upper surface of the graphene layer 120. The graphene layers 120 are connected to each other by van der Waals forces or covalent bonds.

[0055] In this embodiment, the coating layer 140 includes at least one of aluminum nitride, titanium nitride, titanium carbide, chromium carbide, tantalum carbide, vanadium carbide, zirconium carbide, tungsten carbide, tantalum nitride, vanadium nitride, zirconium nitride, tungsten nitride, silicon carbide, titanium dioxide, magnesium oxide, and zinc oxide, and the thickness of the coating layer 140 is 5-15 nm.

[0056] It should be noted that: In this embodiment, a coating layer 140 is formed on the surface of carbon nanotube 130. On the one hand, during field emission, it prevents positively charged ions from bombarding and damaging the structure of carbon nanotube 130, thereby improving the stability of the emission current. On the other hand, the interaction between the atoms of the coating layer 140 and carbon atoms significantly changes the electronic structure of the carbon nanotube, greatly reducing the tunneling width of the field emission electrons and the work function of the carbon nanotube, thus significantly reducing the turn-on electric field.

[0057] In this embodiment, the filler layer 150 includes at least one of the following materials: polyvinylidene fluoride and its copolymers; or a composite of polyvinylidene fluoride and its copolymers with a dielectric ceramic material, wherein the dielectric ceramic material includes at least one of barium titanate, strontium titanate, barium strontium titanate, calcium copper titanate, and strontium tantalate; or a composite of polyvinylidene fluoride and its copolymers with a polymer material, wherein the polymer material includes copper phthalocyanine or polyaniline.

[0058] It should be noted that the carbon nanotubes 130 are filled with a filling layer 150. On the one hand, this reduces the electric field shielding effect between the carbon nanotubes 130, which significantly enhances the local electric field near the tip of the carbon nanotube, increases the field emission current, and reduces the turn-on electric field. On the other hand, the high dielectric constant filling material effectively reduces the Coulomb repulsion force on electrons in the carbon nanotubes from those in neighboring carbon nanotubes, which reduces the path distance for electrons to move towards the tip of the carbon nanotube, thus reducing electron scattering. As a result, the generated Joule heat is significantly reduced, thereby greatly improving the emission stability of the cathode.

[0059] In this embodiment, the tip of the carbon nanotube 130 is exposed above the filling layer 150.

[0060] It is understandable that during the filling process, the tip of the carbon nanotube 130 is covered by the filling material, which is not conducive to the field emission of the carbon nanotube, thus causing the tip of the carbon nanotube 130 to be exposed by the filling layer 150.

[0061] The carbon nanotube field emission cathode provided in this application has a coating layer 140 formed on the surface of carbon nanotubes 130, and the spaces between the carbon nanotubes 130 are filled with a filling layer 150 material, thus constructing a novel composite carbon nanotube cathode. Theoretical analysis shows that it has a low work function, a high local electric field, and low Joule heat. As a field emission cathode, it has a low turn-on electric field, high emission current and current density, and the emission current remains stable during long-term operation.

[0062] Example 2

[0063] Please see Figure 2 The following is a flowchart of the preparation method of carbon nanotube field emission cathode provided in the embodiments of this application, including the following steps S110 to S140. The implementation of each step is described in detail below.

[0064] Step S110: Deposit a graphene layer on the upper surface of the conductive substrate.

[0065] In this embodiment, the conductive substrate includes a metal substrate or a highly doped p-type or n-type silicon wafer, and the metal substrate includes at least one of titanium, copper, chromium, tungsten, molybdenum, tantalum, and platinum.

[0066] In some embodiments, the step of forming a graphene layer on the upper surface of a conductive substrate specifically includes the following steps:

[0067] Step S111: Horizontally oriented graphene is grown using thermochemical vapor deposition.

[0068] In this embodiment, a cleaned copper sheet is placed in a reaction chamber and heated to the growth temperature under an Ar protective atmosphere and held at that temperature. Then, a mixture of carbon source gas (CxHy) and hydrogen gas (H2) is introduced to grow horizontally oriented graphene. After the reaction is complete, the mixture is cooled to room temperature under argon protection.

[0069] Wherein: the carbon source gas (CxHy) can be methane, ethylene or acetylene, the growth temperature can be 950–1050℃, and the volumetric flow rate (sccm) ratio of the carbon source gas to hydrogen is 5–10.

[0070] Step S112: Transfer the graphene to the conductive substrate.

[0071] In this embodiment, the transfer of graphene to the conductive substrate specifically involves: spin-coating PMMA onto a copper sheet on which graphene is grown, then immersing the copper sheet in a FeCl3 solution, whereby the copper sheet is etched and dissolved; subsequently, transferring PMMA / graphene onto the conductive substrate, and finally dissolving the PMMA using acetone solvent to obtain a substrate with a horizontally coated graphene layer.

[0072] In other embodiments, the graphene layer on the upper surface of the conductive substrate may further include the following steps: spin-coating a uniformly dispersed graphene oxide solution onto the conductive substrate, and then reducing the graphene oxide to graphene by heating in a vacuum furnace at 300-400°C or by microwave treatment to obtain a substrate with a horizontally coated graphene layer.

[0073] Step S120: Carbon nanotubes are disposed on the upper surface of the graphene layer.

[0074] In this embodiment, the step of setting carbon nanotubes on the upper surface of the graphene layer specifically includes the following steps:

[0075] First, a catalyst is deposited on the graphene surface. The catalyst can be Fe, Co, Ni, or their alloys. The catalyst deposition method can be electron beam evaporation or magnetron sputtering. Then, the graphene / conductive substrate with the deposited catalyst is placed in a reaction chamber, and vertically oriented carbon nanotubes are grown using plasma-enhanced chemical vapor deposition (PECVD).

[0076] Specifically, the graphene / conductive substrate with the deposited catalyst is placed in a reaction chamber, heated to the reaction temperature and held at that temperature. Under an H2 or H2 / Ar atmosphere, the catalyst is reduced to metal nanoparticles. Then, a carbon source gas (CxHy) is introduced, and vertically oriented carbon nanotubes are grown in a plasma environment.

[0077] Wherein: the carbon source gas (CxHy) can be methane, ethylene or acetylene, the volumetric flow rate (sccm) ratio of H2 to CxHy can be 5–20, and the reaction temperature is 500–800℃.

[0078] It is understood that the diameter of the carbon nanotubes can be achieved by controlling the thickness of the catalyst, and its length can be achieved by controlling the growth time.

[0079] Step S130: A coating layer is formed on the surface of the carbon nanotubes.

[0080] In this embodiment, the carbon nanotube surface has a uniform coating layer, which may be aluminum nitride, titanium nitride, titanium carbide, chromium carbide, tantalum carbide, vanadium carbide, zirconium carbide, tungsten carbide, tantalum nitride, vanadium nitride, zirconium nitride, tungsten nitride, silicon carbide, titanium dioxide, magnesium oxide, zinc oxide, etc.

[0081] In this embodiment, the coating layer is formed on the surface of the carbon nanotube by magnetron sputtering or pulsed laser deposition, and the thickness of the coating layer can be 5–15 nm.

[0082] It should be noted that forming a coating layer on the surface of carbon nanotubes has two main effects: firstly, it prevents positively charged ions from bombarding and damaging the structure of carbon nanotubes during field emission, thus improving the stability of the emission current; secondly, the interaction between the 140 atoms in the coating layer and carbon atoms significantly alters the electronic structure of the carbon nanotubes, greatly reducing the tunneling width of field emission electrons and the work function of the carbon nanotubes, thereby significantly reducing the turn-on electric field.

[0083] Step S140: A filling layer is provided between the carbon nanotubes.

[0084] In this embodiment, the filling layer comprises at least one of the following materials: polyvinylidene fluoride and its copolymers; or a composite of polyvinylidene fluoride and its copolymers with a dielectric ceramic material, wherein the dielectric ceramic material comprises at least one of barium titanate, strontium titanate, barium strontium titanate, calcium copper titanate, and strontium tantalate; or a composite of polyvinylidene fluoride and its copolymers with a polymeric material, wherein the polymeric material comprises copper phthalocyanine or polyaniline.

[0085] In this embodiment, the step of setting a filling layer between the carbon nanotubes specifically includes the following steps: applying a filling layer between the carbon nanotubes using a dip coating method, a drop coating method, or a spin coating method, and then heat-treating the filling layer to solidify the filling layer, wherein the heat treatment temperature is 80–150°C.

[0086] It should be noted that the carbon nanotubes are filled with a filling layer. On the one hand, this reduces the electric field shielding effect between the carbon nanotubes, significantly enhancing the local electric field near the tip of the carbon nanotube, increasing the field emission current, and reducing the turn-on electric field. On the other hand, the high dielectric constant filling material effectively reduces the Coulomb repulsion force on electrons in the carbon nanotubes from those in neighboring carbon nanotubes, reducing the path distance for electrons to move towards the tip and decreasing electron scattering. Therefore, the generated Joule heat is significantly reduced, thereby greatly improving the emission stability of the cathode.

[0087] In some embodiments, the following steps are also included:

[0088] Part of the upper filling layer is removed by plasma etching or mechanical polishing, exposing the tip of the carbon nanotube to the filling layer.

[0089] It is understandable that during the filling process, the tip of the carbon nanotube is covered by the filling material, which is not conducive to the field emission of the carbon nanotube, thus causing the tip of the carbon nanotube to be exposed by the filling layer.

[0090] The method for preparing a carbon nanotube field emission cathode provided in this application involves forming a coating layer on the surface of carbon nanotubes and filling the spaces between the carbon nanotubes with a filling layer material, thereby constructing a novel composite carbon nanotube cathode. Theoretical analysis shows that it has a low work function, a high local electric field, and low Joule heat. As a field emission cathode, it has a low turn-on electric field, high emission current and current density, and the emission current remains stable during long-term operation.

[0091] It is understood that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above are merely preferred embodiments of this application, and only specifically describe the technical principles of this application. These descriptions are only for explaining the principles of this application and should not be construed as limiting the scope of protection of this application in any way. Based on this explanation, any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, as well as other specific embodiments of this application that can be conceived by those skilled in the art without creative effort, should be included within the scope of protection of this application.

Claims

1. A carbon nanotube field emission cathode, characterized in that, include: Conductive substrate; A graphene layer disposed on the upper surface of the conductive substrate; Carbon nanotubes disposed on the upper surface of the graphene layer; A coating layer disposed on the surface of the carbon nanotubes; A filling layer disposed between the carbon nanotubes; The tip of the carbon nanotube is exposed above the filling layer. The coating layer includes at least one of aluminum nitride, titanium nitride, titanium carbide, chromium carbide, tantalum carbide, vanadium carbide, zirconium carbide, tungsten carbide, tantalum nitride, vanadium nitride, zirconium nitride, tungsten nitride, silicon carbide, titanium dioxide, magnesium oxide, and zinc oxide, and the coating layer thickness is 5-15 nm. The filler layer comprises at least one of the following materials: polyvinylidene fluoride and its copolymers; or a composite of polyvinylidene fluoride and its copolymers with a dielectric ceramic material, wherein the dielectric ceramic material comprises at least one of barium titanate, strontium titanate, barium strontium titanate, calcium copper titanate, and strontium tantalate; or a composite of polyvinylidene fluoride and its copolymers with a polymeric material, wherein the polymeric material comprises copper phthalocyanine or polyaniline.

2. The carbon nanotube field emission cathode as described in claim 1, characterized in that, The conductive substrate includes a metal substrate or a highly doped p-type or n-type silicon wafer, wherein the metal substrate includes at least one of titanium, copper, chromium, tungsten, molybdenum, tantalum, and platinum.

3. The carbon nanotube field emission cathode as described in claim 1, characterized in that, The graphene layer is horizontally disposed on the upper surface of the conductive substrate.

4. The carbon nanotube field emission cathode as described in claim 1, characterized in that, The carbon nanotubes are arranged vertically on the upper surface of the graphene layer.

5. The carbon nanotube field emission cathode as described in claim 1 or 4, characterized in that, The carbon nanotubes are connected to the graphene layer by van der Waals forces or covalent bonds.

6. A method for preparing a carbon nanotube field emission cathode, characterized in that, Includes the following steps: A graphene layer is deposited on the upper surface of a conductive substrate; Carbon nanotubes are disposed on the upper surface of the graphene layer; A coating layer is formed on the surface of the carbon nanotubes; A filling layer is provided between the carbon nanotubes; The tip of the carbon nanotube is exposed above the filling layer. The coating layer includes at least one of aluminum nitride, titanium nitride, titanium carbide, chromium carbide, tantalum carbide, vanadium carbide, zirconium carbide, tungsten carbide, tantalum nitride, vanadium nitride, zirconium nitride, tungsten nitride, silicon carbide, titanium dioxide, magnesium oxide, and zinc oxide, and the coating layer thickness is 5-15 nm. The filler layer comprises at least one of the following materials: polyvinylidene fluoride and its copolymers; or a composite of polyvinylidene fluoride and its copolymers with a dielectric ceramic material, wherein the dielectric ceramic material comprises at least one of barium titanate, strontium titanate, barium strontium titanate, calcium copper titanate, and strontium tantalate; or a composite of polyvinylidene fluoride and its copolymers with a polymeric material, wherein the polymeric material comprises copper phthalocyanine or polyaniline.

7. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, The step of depositing a graphene layer on the upper surface of a conductive substrate specifically includes the following steps: Horizontally oriented graphene was grown using thermochemical vapor deposition. The graphene is transferred to the conductive substrate.

8. The method for preparing a carbon nanotube field emission cathode as described in claim 7, characterized in that, The steps for growing horizontally oriented graphene layers using thermochemical vapor deposition specifically include the following steps: placing a cleaned copper sheet into a reaction chamber, heating the copper sheet to the growth temperature under an Ar protective atmosphere and holding it at that temperature; then introducing a mixture of carbon source gas and hydrogen to grow horizontally oriented graphene, and cooling it to room temperature under an argon protective atmosphere to obtain the graphene. The carbon source gas includes methane, ethylene, or acetylene, the growth temperature is 950–1050 °C, and the volumetric flow rate ratio of the carbon source gas to hydrogen is 5–10.

9. The method for preparing a carbon nanotube field emission cathode as described in claim 7, characterized in that, The step of transferring the graphene to the conductive substrate specifically includes the following steps: PMMA was spin-coated onto a copper sheet on which graphene was grown, and then the copper sheet was immersed in FeCl3 solution, where it was etched and dissolved. Subsequently, the PMMA / graphene was transferred to the conductive substrate, and then the PMMA was dissolved in acetone to obtain a conductive substrate with the graphene layer horizontally coated.

10. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, The step of depositing a graphene layer on the upper surface of a conductive substrate specifically includes the following steps: A uniformly dispersed graphene oxide solution is spin-coated onto the conductive substrate, and then the graphene oxide is reduced to graphene by heating in a vacuum furnace at 300-400°C or by microwave treatment, to obtain a conductive substrate with a horizontally coated graphene layer.

11. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, The step of depositing carbon nanotubes on the upper surface of the graphene layer specifically includes the following steps: A catalyst is deposited on the surface of graphene, the catalyst comprising Fe, Co, Ni and their alloys; A graphene / conductive substrate with the catalyst deposited thereon is placed in a reaction chamber, heated to the reaction temperature and held at that temperature. Under an H2 or H2 / Ar atmosphere, the catalyst is reduced to metal nanoparticles. Subsequently, a carbon source gas is introduced, and vertically oriented carbon nanotubes are grown in a plasma environment. The reaction temperature is 500-800 °C, the carbon source gas is methane, ethylene, or acetylene, and the volumetric flow rate ratio of H2 to carbon source gas is 5–20.

12. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, The step of forming a coating layer on the surface of the carbon nanotubes specifically includes the following steps: forming a coating layer on the surface of the carbon nanotubes by magnetron sputtering or pulsed laser deposition.

13. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, The step of providing a filling layer between the carbon nanotubes, with the tips of the carbon nanotubes exposed in the filling layer, specifically includes the following steps: A filling layer is formed between the carbon nanotubes using dip coating, drop coating, or spin coating, and then the filling layer is heat-treated to solidify it. The heat treatment temperature is 80–150 °C.

14. The method for preparing a carbon nanotube field emission cathode as described in claim 6, characterized in that, It also includes the following steps: Part of the upper filling layer is removed by plasma etching or mechanical polishing, exposing the tip of the carbon nanotube to the filling layer.

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