Carbon nanotube material for cold cathode and method for manufacturing the same, electron emission device

Abstract

This application provides a carbon nanotube material for a cold cathode, a method for preparing the same, and an electron emission device. The method for preparing the carbon nanotube material includes: preparing a catalyst using a transition metal; placing the catalyst in a high-temperature reaction chamber and reducing the catalyst at a first temperature; introducing a carbon source gas into the high-temperature reaction chamber and causing the carbon source gas to decompose under the action of the catalyst at a second temperature, thereby growing a first carbon nanotube mixture; increasing the temperature in the high-temperature reaction chamber and subjecting the first carbon nanotube mixture to high-temperature purification at a third temperature; selecting a portion of the obtained product to obtain a second carbon nanotube mixture; and purifying and drying the second carbon nanotube mixture to obtain the carbon nanotube material.

Description

Technical Field

[0001] This application relates to the fields of nanomaterials and radiation technology, and in particular to carbon nanotube materials for cold cathodes, their preparation methods, and electron emission devices. Background Technology

[0002] Carbon nanotubes (CNTs) possess excellent electrical conductivity, good chemical stability, and a large aspect ratio, with their tip surface area approaching the theoretical limit. Therefore, they hold great promise for applications in field emission vacuum electron sources. Current research indicates that carbon nanotubes are among the best-performing field emission materials known. Their tip size is only a few nanometers to tens of nanometers, exhibiting low field emission voltage, high current density transmission, stable current, and long lifespan, making them ideal for electron emission components. Further improving the performance of carbon nanotubes and increasing the emission current of electron emission components has become an important research topic.

[0003] The information disclosed in this section is only used to understand the background of the inventive concept of this application. Therefore, the above information may include information that does not constitute prior art. Summary of the Invention

[0004] To address at least one aspect of the above-mentioned problems, embodiments of this application provide a carbon nanotube material for a cold cathode, a method for preparing the same, and an electron emission device.

[0005] One aspect of this application provides a method for preparing carbon nanotube materials for cold cathodes. The method includes: preparing a catalyst using a transition metal; placing the catalyst in a high-temperature reaction chamber and reducing the catalyst at a first temperature; the catalyst material includes iron oxide, molybdenum oxide, and aluminum oxide, wherein the molar ratio of the iron oxide, molybdenum oxide, and aluminum oxide is in the range of 0.5–2:0.13–0.18:14–20; introducing a carbon source gas into the high-temperature reaction chamber and causing the carbon source gas to decompose under the action of the catalyst at a second temperature, thereby growing a first carbon nanotube mixture; increasing the temperature in the high-temperature reaction chamber and subjecting the first carbon nanotube mixture to high-temperature purification at a third temperature, selecting a portion of the obtained product to obtain a second carbon nanotube mixture; and purifying and drying the second carbon nanotube mixture to obtain the carbon nanotube material.

[0006] According to some exemplary embodiments, the preparation of the catalyst using a transition metal includes: dissolving at least one salt of the transition metal and a salt of a support metal in water to obtain a metal salt solution; adding an alkali to the metal salt solution to co-precipitate the metal ions in the transition metal salt and the support metal salt to obtain a mixed hydroxide; aging the mixed hydroxide, filtering, washing and drying it to obtain a solid powder; and calcining the solid powder at a fourth temperature to obtain the catalyst.

[0007] According to some exemplary embodiments, the fourth temperature ranges from 800 to 1000°C.

[0008] According to some exemplary embodiments, the particle size of the catalyst is in the range of 10-40 nm.

[0009] According to some exemplary embodiments, the first temperature is lower than the second temperature; the second temperature is lower than the third temperature.

[0010] According to some exemplary embodiments, the first temperature ranges from 360 to 450°C; and / or the second temperature ranges from 600 to 1000°C; and / or the third temperature ranges from 1200 to 1600°C.

[0011] According to some exemplary embodiments, introducing carbon source gas into the high-temperature reaction chamber further includes: simultaneously introducing dilution gas into the high-temperature reaction chamber; the proportion of the carbon source gas to the sum of the carbon source gas and the dilution gas is less than or equal to 10%.

[0012] According to some exemplary embodiments, selecting a portion of the obtained product includes selecting a product from the central region of the obtained product, wherein the central region accounts for less than or equal to 25% of the total product region.

[0013] According to some exemplary embodiments, purifying the second carbon nanotube mixture includes: soaking the second carbon nanotube mixture in a dilute acid solution, filtering it, and rinsing it with deionized water to obtain a first preproduct.

[0014] According to some exemplary embodiments, the purification of the second carbon nanotube mixture further includes: placing the first preproduct in deionized water, stirring and ultrasonically cleaning, filtering and rinsing with deionized water to obtain the second preproduct.

[0015] According to some exemplary embodiments, the purification of the second carbon nanotube mixture further includes: placing the second preproduct in an anhydrous alcohol solution, stirring and ultrasonically cleaning, filtering and rinsing with an anhydrous alcohol solution, and drying to obtain the carbon nanotube material.

[0016] In another aspect of the embodiments of this application, a carbon nanotube material for a cold cathode is provided, the carbon nanotube material comprising carbon nanotube material prepared according to the preparation method described above.

[0017] According to some exemplary embodiments, the carbon nanotube material includes carbon nanotubes, each carbon nanotube comprising at least two graphene layers, each graphene layer being a hollow tubular structure, the at least two graphene layers being nested sequentially and arranged according to the diameter of the hollow tubular structure; wherein the number of graphene layers is between 2 and 5.

[0018] According to some exemplary embodiments, the purity of the carbon nanotubes in the carbon nanotube material is greater than or equal to 99.9%.

[0019] According to some exemplary embodiments, the ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the carbon nanotube material is less than or equal to 0.07.

[0020] Another aspect of the embodiments of this application provides an electron emission device including a cold cathode, wherein the cold cathode of the electron emission device comprises a carbon nanotube material prepared according to the above-described preparation method, or the cold cathode of the electron emission device comprises the above-described carbon nanotube material.

[0021] According to some exemplary embodiments, the current emission density of the electron emission device is not less than 7 A / cm². 2 .

[0022] According to some exemplary embodiments, the electron emission device further includes a gate, wherein the control voltage of the gate does not exceed 1200V when the emission current of the cold cathode reaches 100mA.

[0023] According to some exemplary embodiments, the electron emission device further includes a gate, wherein when the number of emission times of the electron emission device reaches 1 million or the electron emission time reaches 1000 seconds, the voltage growth rate between the cold cathode and the gate is less than 3%.

[0024] In the embodiments of this application, the carbon nanotube material prepared by the method for preparing carbon nanotube materials for cold cathodes has 2-5 graphene layers, which is beneficial for preparing carbon nanotubes of a single type and avoids the mixing of carbon nanotubes with different properties. Carbon nanotubes with a 2-5 layer structure exhibit excellent mechanical and electrical properties, can better withstand high-current and high-energy electron emission processes, and can maintain high stability, which is beneficial for extending the service life of electron emission devices. Attached Figure Description

[0025] Other objects and advantages of this application will become apparent from the following description of the application with reference to the accompanying drawings, and will help to provide a comprehensive understanding of the application.

[0026] Figure 1 The schematic diagram illustrates a three-dimensional structure of a carbon nanotube according to some exemplary embodiments of this application;

[0027] Figure 2 The schematic diagram illustrates a structural schematic of a graphene layer in a carbon nanotube according to some exemplary embodiments of this application;

[0028] Figure 3 A flowchart illustrating a method for preparing carbon nanotube materials according to some exemplary embodiments of this application is shown schematically.

[0029] Figure 4 The flowchart schematically illustrates a method for preparing a catalyst using a transition metal according to some exemplary embodiments of this application;

[0030] Figure 5 The schematic diagram illustrates a structural schematic of a chemical vapor deposition apparatus according to some exemplary embodiments of this application;

[0031] Figure 6 This schematic diagram illustrates the structure of a product in the central region of a selected product according to some exemplary embodiments of this application;

[0032] Figure 7A The schematic diagram shows scanning electron microscope images of carbon nanotube materials according to some exemplary embodiments of this application;

[0033] Figures 7B-7C The illustration schematically shows transmission electron microscopy (TEM) images of carbon nanotube materials according to some exemplary embodiments of this application;

[0034] Figure 7D The Raman spectra of carbon nanotube materials according to some exemplary embodiments of this application are illustrated schematically.

[0035] Figure 7E Thermogravimetric analysis results of carbon nanotube materials according to some exemplary embodiments of this application are illustrated schematically.

[0036] Figure 7F The diagram illustrates the relationship between field voltage and emission current when carbon nanotube materials are applied in an electron emission device according to some exemplary embodiments of this application.

[0037] Figure 7G The illustration schematically shows cathode lifetime test diagrams of carbon nanotube materials applied to electron emission devices according to some exemplary embodiments of this application;

[0038] Figures 8A-8C The following are schematic scanning electron microscope images of carbon nanotube materials according to some comparative embodiments of this application;

[0039] Figures 9A-9B Transmission electron microscope (TEM) images of carbon nanotube materials according to some comparative embodiments of this application are shown schematically.

[0040] Figure 10 The schematic diagram illustrates the structure of an electron emission device according to some exemplary embodiments of this application.

[0041] It should be noted that, for clarity, the dimensions of layers, structures, or regions in the drawings used to describe embodiments of this application may be enlarged or reduced, i.e., these drawings are not drawn to actual scale. Detailed Implementation

[0042] The technical solution of this application will be further described in detail below through embodiments and in conjunction with the accompanying drawings. In the specification, the same or similar reference numerals indicate the same or similar components. The following description of the embodiments of this application with reference to the accompanying drawings is intended to explain the overall inventive concept of this application and should not be construed as a limitation of this application.

[0043] Furthermore, in the following detailed description, numerous specific details are set forth for ease of explanation to provide a thorough understanding of the embodiments disclosed herein. However, it will be apparent that one or more embodiments may be practiced without these specific details.

[0044] It should be understood that although the terms first, second, etc., may be used herein to describe different elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of the exemplary embodiments, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that when the terms “comprising” and / or “including” are used herein, it indicates the presence of the stated features, integrals, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or combinations thereof.

[0046] Carbon nanotubes are tubular nanomaterials formed by the coiling of carbon atoms into a hexagonal lattice structure. They have diameters of only a few nanometers and lengths typically in the micrometer range, while directionally grown array structures can reach meters in length. They possess both excellent properties and broad application prospects. In terms of mechanical properties, carbon nanotubes have extremely high structural strength, 100 times that of steel, while their density is only 1 / 6 that of steel. Their elastic modulus is close to that of diamond (approximately 1 TPa), hence they are hailed as "super fibers." Regarding electrical properties, carbon nanotubes exhibit outstanding electrical conductivity; some single-walled carbon nanotubes can achieve conductivity up to 10,000 times that of copper. In terms of thermal properties, carbon nanotubes demonstrate exceptional thermal conductivity, with thermal conductivity reaching up to 3000 W / (m·K), nearly an order of magnitude higher than that of copper. In terms of applications, carbon nanotubes can be used as drug carriers, tissue engineering scaffolds, and bioimaging tools in the biomedical field; in the new energy field, they can be used as conductive additives for lithium batteries to improve charge and discharge efficiency, and can also be used as high-efficiency hydrogen storage materials; in the aerospace field, they can be used to prepare lightweight, high-strength composite materials for use in aircraft fuselages and other components to improve structural performance; in the electronics field, carbon nanotubes are expected to replace silicon-based materials in the manufacture of ultra-small transistors, chips, flexible screens, and sensors.

[0047] Carbon nanotubes exist in various morphologies and can be classified into single-walled and multi-walled types based on the number of coiled layers. Different morphologies of carbon nanotubes exhibit significant differences in properties. For example, single-walled carbon nanotubes, due to their unique structure, can exhibit metallic or semiconducting conductivity. Different application areas have varying performance requirements for carbon nanotubes. Therefore, it is necessary to precisely select the morphology of carbon nanotubes according to the specific application scenario to achieve the best application results.

[0048] Carbon nanotubes, as electron emission materials, emit electrons based on the field emission principle. They utilize the unique nanostructure and high aspect ratio of carbon nanotubes to achieve electron emission through the quantum tunneling effect. In X-ray tubes, when carbon nanotubes are used as cold cathodes, although the amount of carbon nanotubes used in each cathode is only in the microgram range and the amount used in a single X-ray tube is extremely small, it has a crucial impact on the operating current, focal spot size, stability, and lifespan of the X-ray tube. In X-ray tube applications, carbon nanotube cathodes are expected to have large emission currents, good stability, and long lifespans; these characteristics are also important conditions for their large-scale mass application. Therefore, the selection of carbon nanotube materials and their preparation methods are particularly important.

[0049] The existing methods for preparing carbon nanotubes have several drawbacks. For example, while chemical vapor deposition (CVD) produces high-purity products with low energy consumption, it is prone to the introduction of amorphous carbon impurities, making it difficult to separate metallic and semiconducting carbon nanotubes. Furthermore, the equipment is complex to operate, and parameter optimization is challenging. Laser evaporation produces high-quality carbon nanotubes, but the equipment is expensive, energy-intensive, has low yield, and purity needs improvement. The process is also complex, and parameter optimization is difficult. Arc evaporation is suitable for preparing single-walled carbon nanotubes, but it suffers from high energy consumption, low purity, and difficulty in large-scale production. Single-walled nanotubes are prone to entanglement, resulting in low yields of multi-walled nanotubes. While biomass catalytic pyrolysis can prepare multi-walled carbon nanotubes on a large scale at low cost, the complex composition of biomass and large fluctuations in the concentration of pyrolysis gas components lead to numerous impurities, defects, and uneven graphitization in the prepared carbon nanotubes. Due to these shortcomings, existing carbon nanotube-based cold cathode ray tubes cannot meet the requirements for commercial applications, especially in achieving high-current emission above 100 mA.

[0050] To address at least one of the above problems, embodiments of this application provide a carbon nanotube material for a cold cathode and a method for preparing the same, as well as an electron emission device including a cold cathode.

[0051] Figure 1 The schematic diagram illustrates a three-dimensional structure of a carbon nanotube according to some exemplary embodiments of this application; Figure 2 The schematic diagram illustrates a structural schematic of a graphene layer in a carbon nanotube according to some exemplary embodiments of this application; for example, Figure 2 It can be Figure 1 A schematic diagram of the structure of the middle layer of graphene.

[0052] In some embodiments of this application, reference is made to Figure 1 , Figure 2 The carbon nanotube material includes carbon nanotubes 200, which include at least two graphene layers 10. Each graphene layer 10 is a hollow tubular structure, and the at least two graphene layers 10 are nested in sequence according to the diameter of the hollow tubular structure. The number of graphene layers 10 is between 2 and 5.

[0053] Reference Figure 1 For example, the carbon nanotube 200 may include four graphene layers 10, that is, four hollow tubular structures nested in sequence. The innermost first layer 21 is curled into a small hollow tube, the next innermost second layer 22 is curled into a slightly larger hollow tube, and the second layer 22 wraps around the outside of the first layer 21. The next outermost third layer 23 is curled into an even larger hollow tube, and the third layer 23 wraps around the outside of the second layer 22. The outermost fourth layer 24 is curled into the largest hollow tube, and the fourth layer 24 wraps around the outside of the third layer 23.

[0054] Reference Figure 2 Each graphene layer 10 is a two-dimensional honeycomb lattice structure formed by the close arrangement of carbon atoms 1 with sp² hybrid orbitals. Each carbon atom 1 is connected to its neighboring carbon atoms 1 by three covalent bonds, forming a planar hexagonal grid. In carbon nanotubes, the graphene layer 10 is rolled up to form a hollow tubular structure. Each graphene layer 10 is a two-dimensional plane with a single atom thickness, composed of multiple carbon atoms 1 connected to each other in a hexagonal arrangement.

[0055] In carbon nanotube materials, the number of graphene layers 10 and their nested structure jointly determine the material's properties. A graphene layer 10 with 2-5 layers facilitates the fabrication of carbon nanotubes of a single type, avoiding the mixing of carbon nanotubes with different properties. Carbon nanotubes with 2-5 layers exhibit excellent mechanical and electrical properties, better withstand high-current and high-energy electron emission processes, and maintain high stability, thus extending the lifespan of electron emission devices. It should be noted that in this article, "few-walled" refers to a graphene layer 10 with 2-5 layers, while "more-walled" refers to a graphene layer 10 with more than 5 layers.

[0056] In some embodiments of this application, the purity of the carbon nanotubes in the carbon nanotube material is greater than or equal to 99.9%. It should be noted that "purity" refers to the proportion of the mass of carbon nanotubes in the total mass of the carbon nanotube material.

[0057] Due to the high purity of carbon nanotubes, their electron emission performance can be significantly improved, which is beneficial for increasing the emission current and improving the output power and working efficiency of electron emission devices.

[0058] In some embodiments of this application, the ratio of the intensity of the D peak to the intensity of the G peak in the Raman spectrum of the carbon nanotube material is less than or equal to 0.07.

[0059] The defect level of carbon nanotube materials can be characterized by the ID / IG ratio; an ID / IG ratio ≤ 0.07 indicates high purity and low defect density of the carbon nanotube material. In Raman spectroscopy, the D peak (located at approximately 1350 cm⁻¹) is observed. -1 The peak G (located at approximately 1580 cm⁻¹) is typically associated with defects, disordered structures, or edges in the material. -1 The ratio of the intensity of the D peak to the intensity of the G peak (ID / IG) is related to the ordered structure of the graphene layer, representing the sp² hybridization structure of carbon atoms in the carbon nanotube. When the intensity ratio of the D peak to the G peak is small, it indicates that there are fewer defects and disordered structures in the material, and the graphene layer has a high degree of order. At the same time, this ratio also reflects the high purity of the carbon nanotube material. A low D peak intensity indicates that there is almost no amorphous carbon or other impurities in the material.

[0060] In practical applications, this high-purity and low-defect-density carbon nanotube material has significant advantages. For example, in electron emission devices, high-purity and low-defect-density carbon nanotube materials can provide more stable electron emission performance, thereby improving the operating current and stability of the electron emission device and extending its service life.

[0061] In some embodiments of this application, the diameter of the hollow tubular structure ranges from 5 to 20 nm.

[0062] Carbon nanotubes with diameters ranging from 5 to 20 nm have small tips with low radii of curvature, which results in a lower field emission threshold for field emission applications. The smaller tip curvature significantly enhances the electric field, thereby reducing the electric field strength required for electron emission and improving field emission efficiency. Simultaneously, carbon nanotubes in this diameter range can support higher current densities while maintaining stable electron emission performance.

[0063] In some embodiments of this application, reference is made to Figure 1 , Figure 2 At least two graphene layers 10 of carbon nanotubes are nested in sequence, and at least two hollow tubular structures share a central axis.

[0064] The sequential nesting of graphene layers 10 forms carbon nanotubes with few walls (2-5 layers), a structure that enhances the overall stability of the carbon nanotubes. Adjacent graphene layers 10 interact through van der Waals forces, providing additional interaction forces and contributing to the good stability of the carbon nanotube material. The hollow tubular structures share a central axis, meaning these tubular structures are highly symmetrical and closely arranged in space. This arrangement also helps improve the structural stability of the carbon nanotubes, thus ensuring the high stability of the carbon nanotube material.

[0065] In some embodiments of this application, the minimum distance between two adjacent hollow tubular structures ranges from 0.3 to 0.4 nm.

[0066] The distance between adjacent graphene layers 10 is between 0.3 and 0.4 nm. This distance range matches the van der Waals interaction distance between carbon atoms to provide stable interactions between graphene layers 10, which is beneficial to improving the structural stability of carbon nanotubes.

[0067] Figure 3 The flowchart illustrating a method for preparing carbon nanotube materials according to some exemplary embodiments of this application is shown schematically.

[0068] According to some exemplary embodiments, refer to Figure 3 The preparation method of carbon nanotube materials includes the following steps S310 to S350.

[0069] In step S310, a catalyst is prepared using a transition metal.

[0070] In step S320, the catalyst is placed in a high-temperature reaction chamber and reduced at a first temperature.

[0071] In step S330, carbon source gas is introduced into the high-temperature reaction chamber, and the carbon source gas is decomposed under the action of a catalyst at a second temperature to grow a first carbon nanotube mixture.

[0072] In step S340, the temperature in the high-temperature reaction chamber is increased, and the first carbon nanotube mixture is purified at a high temperature at a third temperature. A portion of the obtained product is selected to obtain the second carbon nanotube mixture.

[0073] In step S350, the second carbon nanotube mixture is purified and dried to obtain carbon nanotube material.

[0074] Exemplary, the preparation method of this application is based on chemical vapor deposition. By precisely controlling multiple parameters such as catalyst preparation, reduction temperature, carbon source gas pyrolysis temperature, and purification temperature, the growth process of carbon nanotubes is precisely controlled. This multi-parameter comprehensive optimization method ensures the repeatability and stability of the preparation process, making large-scale production of high-quality carbon nanotubes possible. The multi-step purification and refining process further ensures the high purity and low defect density of the final product, resulting in excellent electrical and mechanical properties of the carbon nanotubes.

[0075] Figure 4 The flowchart illustrates a method for preparing a catalyst using transition metals according to some exemplary embodiments of this application.

[0076] According to some exemplary embodiments, refer to Figure 4 The preparation method of catalyst using transition metals includes the following steps S311 to S314.

[0077] In step S311, at least one salt of a transition metal and a salt of a carrier metal are dissolved in water to obtain a metal salt solution.

[0078] In step S312, an alkali is added to the metal salt solution to co-precipitate the metal ions in the transition metal salt and the carrier metal salt, resulting in a mixed hydroxide.

[0079] In step S313, the mixed hydroxide is aged, filtered, washed and dried to obtain a solid powder.

[0080] In step S314, the solid powder is calcined at a fourth temperature to obtain a catalyst.

[0081] A catalyst may include an active component, a support component, and a co-catalyst component. The active component may be a transition metal, such as iron (Fe), cobalt (Co), or nickel (Ni). The support component may be alumina (Al2O3) or silicon dioxide (SiO2). The co-catalyst component may also be a transition metal, such as molybdenum (Mo) or tungsten (W).

[0082] For example, in step S311, a salt of at least one transition metal (such as nitrates or chlorides of Fe, Co, or Ni), a salt of a support metal (such as nitrates or chlorides of Al), and a salt of a co-catalyst (such as nitrates or chlorides of Mo or W) are dissolved in water to obtain a homogeneous metal salt solution. This step ensures that each metal ion is fully dissolved and uniformly distributed in the solution, providing a basis for the subsequent co-precipitation reaction.

[0083] For example, in step S312, an appropriate amount of alkali (such as ammonia or sodium hydroxide) is added to the metal salt solution to cause the aforementioned metal ions to undergo a co-precipitation reaction simultaneously, generating a mixed hydroxide. By controlling the pH value of the solution and the reaction temperature, the precipitation process can be precisely controlled to ensure the homogeneity and purity of the precipitate.

[0084] For example, in step S313, the obtained mixed hydroxide is subjected to an aging treatment, i.e., held at a certain temperature for a period of time to promote the crystallization and stabilization of the precipitate. The aging treatment helps to improve the crystallinity and mechanical stability of the precipitate and reduce agglomeration in subsequent processing. Subsequently, the aged precipitate is filtered and washed to remove residual impurities and reaction byproducts, and finally dried to obtain a solid powder.

[0085] For example, in step S314, the solid powder is calcined at a fourth temperature. The calcination process not only removes organic matter and moisture but also allows the metal oxide to further crystallize, forming a catalyst with high activity and stability. The selection of calcination temperature and time is crucial to the performance of the catalyst; excessively high or low temperatures can affect the catalyst's activity and selectivity.

[0086] The catalyst can be prepared using the co-precipitation method described above. This method has advantages such as simple operation, low cost, and good reproducibility. Through co-precipitation, the proportion and distribution of each component in the catalyst can be precisely controlled, thereby optimizing the catalyst's activity and selectivity. This method is suitable for preparing high-performance, low-walled carbon nanotubes.

[0087] In some embodiments of this application, the fourth temperature ranges from 800 to 1000°C.

[0088] For example, the fourth temperature, within the aforementioned range, can effectively reduce the agglomeration of Fe-Mo particles and significantly improve their dispersibility on the Al2O3 surface. This high-temperature calcination process within this range not only promotes the crystallization of the metal oxide but also, through thermal diffusion, ensures a uniform distribution of Fe-Mo particles on the Al2O3 support surface, reducing particle aggregation and thus optimizing the catalyst's active site density and reaction contact area, further enhancing its catalytic performance.

[0089] In some embodiments of this application, the catalyst materials include iron oxide, molybdenum oxide and aluminum oxide, and the molar ratio of iron oxide, molybdenum oxide and aluminum oxide is in the range of 0.5 to 2: 0.13 to 0.18: 14 to 20.

[0090] By optimizing the composition and proportions of the catalyst, its activity and selectivity can be improved, thereby producing high-performance low-walled carbon nanotubes. Fe, as the main active component, provides efficient catalytic sites, promoting the cracking of carbon source gas and the growth of carbon nanotubes. Mo, as a co-catalyst, can modulate the electronic structure of iron, further enhancing its catalytic activity. Optimizing the ratio of iron to molybdenum can achieve the best synergistic effect. Al₂O₃, as a support, provides a stable supporting structure; adjusting the proportion of the support can optimize the catalyst's dispersibility and stability, thereby improving its performance in high-temperature reactions.

[0091] In some embodiments of this application, the particle size of the catalyst is in the range of 10-40 nm.

[0092] For example, catalysts with particle sizes within the aforementioned range offer significant advantages for the growth of few-walled carbon nanotubes. Smaller catalyst particle sizes facilitate the growth of smaller-diameter few-walled carbon nanotubes, and by precisely controlling the catalyst particle size, the formation of multi-walled carbon nanotubes can be reduced, thereby improving the purity of few-walled carbon nanotubes.

[0093] Catalysts can also be prepared using the sol-gel method. For example, a transition metal salt, a support metal salt, and a co-catalyst salt are dissolved in a solvent to form a homogeneous solution. Then, by controlling the concentration, pH, and temperature of the solution, appropriate amounts of water and catalysts (such as acids or bases) are added to promote hydrolysis and condensation reactions, forming a stable sol. The sol is then allowed to stand under appropriate conditions to gradually gel, forming a three-dimensional network structure. Subsequently, the gel is dried to remove the solvent, yielding a porous solid. Finally, the dried gel is calcined at a certain temperature to remove organic matter and moisture, allowing the metal oxide to further crystallize and form a catalyst with high activity and stability.

[0094] In some embodiments of this application, the first temperature is lower than the second temperature; the second temperature is lower than the third temperature.

[0095] For example, the catalyst is reduced at a low first temperature. This low-temperature reduction avoids excessive sintering of the catalyst particles, maintaining their good dispersibility and high specific surface area, thereby improving the catalyst's activity. At a second temperature, the carbon source gas is cracked under the action of the catalyst, growing into a first carbon nanotube mixture. This second temperature is typically higher than the first temperature, causing the carbon source gas (such as methane, acetylene, etc.) to crack into active carbon atoms or carbon clusters. These active carbon atoms or carbon clusters adsorb onto the catalyst surface and grow into carbon nanotubes. The higher second temperature promotes rapid growth of the carbon nanotubes while maintaining their structural uniformity and low defect rate. At a third temperature, the first carbon nanotube mixture is purified at a high temperature. This third temperature is typically even higher. High-temperature purification removes amorphous carbon, impurities, and residual catalyst particles from the carbon nanotube surface, significantly improving the purity of the carbon nanotube material.

[0096] In some embodiments of this application, the first temperature ranges from 360 to 450°C; and / or the second temperature ranges from 600 to 1000°C; and / or the third temperature ranges from 1200 to 1600°C.

[0097] For example, the first temperature within the aforementioned range helps to avoid excessive sintering of catalyst particles, maintaining their good dispersibility and high specific surface area, thereby providing efficient active sites for subsequent carbon nanotube growth. The second temperature within the aforementioned range facilitates the effective decomposition of the carbon source gas, promoting the adsorption and growth of these active carbon species on the catalyst surface, forming high-quality, low-walled carbon nanotubes. The third temperature within the aforementioned range helps to remove amorphous carbon, impurities, and residual catalyst particles from the surface of the carbon nanotubes, improving the purity and crystallinity of the carbon nanotubes. It also helps to further stabilize the graphene layer structure of the carbon nanotubes, reducing structural defects and thus optimizing their electrical properties.

[0098] In some embodiments of this application, introducing carbon source gas into the high-temperature reaction chamber further includes: simultaneously introducing dilution gas into the high-temperature reaction chamber; the proportion of carbon source gas to the sum of carbon source gas and dilution gas is less than or equal to 10%.

[0099] For example, the carbon source gas can be a gaseous hydrocarbon (such as methane or ethylene), stored in a high-pressure cylinder equipped with a pressure-reducing valve to regulate the pressure of the released gas. Methane can be selected as the carbon source gas because its decomposition process results in high carbon utilization and minimizes the generation of excessive amorphous carbon impurities, thus improving the purity of the grown high-carbon nanotubes. Furthermore, using methane as the carbon source gas allows for decomposition at lower temperatures, which can further increase the proportion of few-walled carbon nanotubes in the grown carbon nanotubes.

[0100] To optimize the growth process of carbon nanotubes, a diluent gas, such as hydrogen (H2), is included in the carbon source gas. The role of H2 is to dilute the concentration of the carbon source gas, ensuring orderly and controllable growth of carbon nanotubes, reducing the formation of amorphous carbon, and thus improving the purity of the carbon nanotubes. In practice, the flow ratio of the carbon source gas to the diluent gas is no greater than 1:9, meaning that after the carbon source gas and the protective gas are mixed through the gas path, the concentration of methane is less than or equal to 10%. By setting a lower concentration of the carbon source gas, the amount of carbon atoms deposited on the catalyst surface can be reduced, thereby controlling the formation of carbon nanotubes to be predominantly few-walled carbon nanotubes.

[0101] In some embodiments of this application, selecting a portion of the obtained product includes selecting the product from the central region of the obtained product, wherein the central region accounts for less than or equal to 25% of the total product region.

[0102] The central region is the core area within the product, encompassing the geometric center of the reaction product area. During carbon nanotube growth, this region experiences the most uniform distribution of thermal field and airflow, resulting in carbon nanotubes with higher purity, fewer impurities, and a more uniform structure.

[0103] In some embodiments of this application, the purification of the second carbon nanotube mixture includes: soaking the second carbon nanotube mixture in a dilute acid solution, filtering, and rinsing with deionized water to obtain a first preproduct; placing the first preproduct in deionized water, stirring and ultrasonically cleaning, filtering, and rinsing with deionized water to obtain a second preproduct; and placing the second preproduct in an anhydrous alcohol solution, stirring and ultrasonically cleaning, filtering, rinsing with anhydrous alcohol solution, and drying to obtain the carbon nanotube material.

[0104] First, the second carbon nanotube mixture is immersed in a dilute acid solution (such as dilute hydrochloric acid or dilute sulfuric acid). The dilute acid effectively removes metal catalyst particles, amorphous carbon, and other impurities from the carbon nanotube surface. The acid dissolves these impurities through a chemical reaction, thereby improving the purity of the carbon nanotubes. Next, the first preproduct is placed in deionized water, stirred, and ultrasonically cleaned to thoroughly remove residual hydrochloric acid, soluble salts, and fine impurities from the carbon nanotube surface, while also dispersing the aggregated carbon nanotubes and improving their dispersibility. Finally, the second preproduct is placed in an anhydrous alcohol solution (such as anhydrous ethanol), stirred, and ultrasonically cleaned. The anhydrous alcohol further removes residual water-soluble small molecule impurities from the carbon nanotube surface and reduces agglomeration between carbon nanotubes. This multi-step progressive purification method can significantly improve the purity and quality of carbon nanotubes.

[0105] The present application will be further described below through specific embodiments. The preparation method of the above-mentioned carbon nanotube material is specifically described in the following embodiments. However, the following embodiments are merely illustrative of the present application, and the scope of the present application is not limited thereto.

[0106] Figure 5 The schematic diagram illustrates a structural schematic of a chemical vapor deposition apparatus according to some exemplary embodiments of this application. Figure 6 The schematic diagram illustrates the structure of a product in the central region of a product obtained according to some exemplary embodiments of this application.

[0107] In step S311, at room temperature of 25°C, 40.4 g of hydrated ferric nitrate Fe(NO3)3·9H2O and hydrated ammonium heptamolybdate (NH4)6Mo7O were weighed using a precision balance. 24 3.0 g of 4H2O and 600.2 g of hydrated aluminum nitrate Al(NO3)3·9H2O, i.e., the molar ratio of the three elements Fe : Mo : Al = 1 : 0.17 : 16.

[0108] The above-weighed Fe(NO3)3·9H2O (iron source) and (NH4)6Mo7O 24 A total of 643.6g of solids, including 4H2O (molybdenum source) and Al(NO3)3·9H2O (aluminum source), were dissolved in 1L of deionized water and stirred for 20 minutes until completely dissolved.

[0109] In step S312, at room temperature (25°C), a precipitant is slowly added to the solution obtained in step S311. Granular NaOH is slowly added to the solution while stirring vigorously. Throughout the process, the pH value of the solution is observed. When the pH value reaches approximately 7.5, the addition of NaOH is stopped. It is important to note that the pH value of the solution must be strictly controlled between 7 and 8 to ensure Fe...3+ Mo 6+ Al 3+ They can co-precipitate as hydroxides.

[0110] In step S313, the precipitate obtained in step S312 is aged at 60-80°C, for example, 70°C, for 1-2 hours, for example, 1.5 hours, to promote the growth of precipitate particles and improve their crystallinity. Then, the precipitate is repeatedly washed with deionized water at room temperature (25°C) to remove residual NO3. - NH4 + Remove impurities, using approximately 2L of deionized water. Then dry the precipitate at 100-120℃ for 12-24 hours to remove moisture, for example, at 110℃ for 18 hours.

[0111] In step S314, the solid powder obtained in step S313 is calcined in air at 800-1000°C for 2-3 hours, for example, at 900°C for 2.5 hours, so that the hydroxide decomposes into oxides (Fe2O3, MoO3, Al2O3) and forms a stable catalyst structure.

[0112] After the catalyst is prepared, the next step is chemical vapor deposition (CVD). The structure of the CVD equipment is as follows: Figure 5 As shown. The chemical vapor deposition equipment mainly includes an inlet 54, a quartz boat 55, a high-temperature furnace 56, an outlet 57, multiple gas flow meters A1-A3, multiple gas valves B1-B4, and gas pipelines 51-53, etc. Among them, the first gas pipeline 51 is used to provide dilution gas (e.g., hydrogen), the second gas pipeline 52 is used to provide carbon source gas (e.g., methane, ethylene), and the third gas pipeline 53 is used to provide protective gas (e.g., argon, nitrogen).

[0113] In step S320, the catalyst obtained in step S310 is placed in a quartz boat 55, with the mass ratio of iron, molybdenum, and aluminum in the catalyst being 5.6:1.63:43.2. The high-temperature furnace 56 serves as the high-temperature reaction chamber. First, at room temperature (25°C), gas valves B3 and B4 are opened to introduce Ar at a flow rate of 10 L / min, continuously flushing the gas pipeline and furnace for 10 minutes to ensure complete replacement of air within the furnace 56, providing an inert environment for subsequent reactions. Then, gas valve B1 is opened to introduce H2, gradually adjusting the flow rate to 10 L / min, while simultaneously closing gas valve B3. The temperature of the high-temperature furnace 56 is set to 400°C and gradually increased to this temperature over 15 minutes. At 400°C, maintaining an H2 flow rate of 10 L / min, the catalyst is reduced for 2 hours, reducing Fe2O3 and MoO3 to their metallic states (Fe and Mo), thereby activating their catalytic activity.

[0114] In step S330, gas valve B2 is opened to introduce carbon source gas. The flow ratio of carbon source gas to dilution gas is controlled to be no greater than 1:9. The temperature in the high-temperature furnace 56 is increased, and the stable operating range is set to 600-1000℃, for example, 900℃. The volume of the constant temperature zone is no less than 3L, and the temperature control accuracy of the constant temperature zone is no worse than 10℃. This stable operating range setting must avoid two problems simultaneously: first, preventing excessively high temperatures from causing excessive growth of carbon nanotubes, resulting in an excessively high proportion of multi-walled carbon nanotubes; second, preventing excessively low temperatures from causing incomplete decomposition of the carbon source gas.

[0115] For example, the heating time is set to 15 minutes. During the heating process, only dilution gas is supplied to the high-temperature furnace 56, gradually raising the temperature of the high-temperature furnace 56 to its stable operating range. The reaction time is set to 30-60 minutes, for example, 40 minutes. During the reaction time, the supply of carbon source gas is increased while maintaining the flow rate of dilution gas, ensuring that the flow ratio of carbon source gas to dilution gas is not greater than 1:9, with CH4 flow rate at 1 L / min and H2 flow rate at 10 L / min. The cooling time is set to 180 minutes. During the cooling process, only dilution gas is supplied to the high-temperature furnace 56, gradually lowering the temperature of the high-temperature furnace 56 to room temperature.

[0116] In step S340, a high-temperature purification step continues in the high-temperature furnace 56. The purpose of this step is to remove amorphous carbon and metallic impurities, and further improve the structural integrity of the carbon nanotubes. For example, the reaction is carried out at 900°C for 40 minutes to ensure the initial growth of carbon nanotubes. After the reaction is complete, gas valve B2 is closed. While keeping the temperature of the high-temperature furnace 56 constant, gas valve B3 is gradually opened, and the argon flow rate is gradually adjusted to 10 L / min, while gas valve B1 is closed. The temperature of the high-temperature furnace 56 is raised to 1400°C within 15 minutes and held at 1400°C for 2 hours. This high-temperature purification removes amorphous carbon and metallic impurities and improves structural integrity. After the holding period, the argon flow rate is maintained at 10 L / min, and the temperature of the high-temperature furnace is gradually reduced to room temperature (25°C) over a cooling time of 280 minutes.

[0117] Reference Figure 6 After the furnace is opened, the grown carbon nanotubes are removed, and only the carbon nanotubes in the central region (61%) with the best CVD growth environment are collected. The central region accounts for less than or equal to 25% of the total product area. This method specifically targets the part of the growth area with the highest purity and the most concentrated type of low-walled carbon nanotubes for further processing. Selecting carbon nanotubes from the central region can ensure the high quality and high performance of the final product.

[0118] In step S350, the purification steps sequentially include acid washing purification, water washing purification, and alcohol washing purification. First, the collected few-walled carbon nanotubes are soaked in dilute hydrochloric acid (concentration 0.1-4 mol / L) with slow stirring during soaking. Then, the dilute hydrochloric acid solution containing the few-walled carbon nanotubes is vacuum filtered and washed with deionized water until the pH is neutral. The few-walled carbon nanotubes, washed to neutral with deionized water, are further dissolved in deionized water, stirred thoroughly, and sonicated. Vacuum filtration is performed again, followed by washing with deionized water. Finally, the few-walled carbon nanotubes washed with deionized water are further dissolved in anhydrous ethanol, stirred thoroughly, and sonicated. Vacuum filtration is performed again, followed by washing with anhydrous ethanol to obtain high-purity few-walled carbon nanotube material.

[0119] The drying process involves placing the high-purity, low-walled carbon nanotubes, purified by alcohol washing, into a drying oven at 100°C for 12 hours. After drying, the carbon nanotubes are weighed and stored in sealed glass bottles, then placed in an electronic desiccant box to ensure their performance stability and purity in subsequent use.

[0120] Figure 7A The schematic diagram shows scanning electron microscope images of carbon nanotube materials according to some exemplary embodiments of this application; Figures 7B-7C The illustration schematically shows transmission electron microscopy (TEM) images of carbon nanotube materials according to some exemplary embodiments of this application; Figure 7D The Raman spectra of carbon nanotube materials according to some exemplary embodiments of this application are illustrated schematically. Figure 7E Thermogravimetric analysis results of carbon nanotube materials according to some exemplary embodiments of this application are illustrated schematically. Figure 7F The diagram illustrates the relationship between field voltage and emission current when carbon nanotube materials are applied in an electron emission device according to some exemplary embodiments of this application. Figure 7G The illustration schematically shows cathode lifetime test diagrams when carbon nanotube materials are applied to electron emission devices according to some exemplary embodiments of this application.

[0121] The carbon nanotube materials obtained in the above embodiments were tested, referring to... Figure 7A The surface morphology of carbon nanotubes was observed using a high-resolution scanning electron microscope. The nanotubes exhibited a linear structure, with clear bundle shapes, uniform diameter, and consistent thickness. No amorphous carbon impurities, black flaky impurities, or bright metallic particles were present. Image analysis showed that the length of the carbon nanotubes was greater than 100 micrometers. (Reference) Figure 7B The structure of carbon nanotubes was observed using transmission electron microscopy. The carbon nanotubes exhibited a uniformly distributed, hollow tubular structure with diameters ranging from 5 to 20 nm. (Reference) Figure 7CFurther magnification using transmission electron microscopy revealed that the carbon nanotubes consisted of 2-5 layers with smooth walls, clear interlayer boundaries, and virtually no impurities or structural distortion. (Reference) Figure 7D In Raman spectroscopy, carbon nanotube materials at 1570 cm⁻¹ -1 SP at the location 2 The characteristic peaks are strong and sharp, indicating a high degree of order in the arrangement of carbon atoms, and particularly at 1336 cm⁻¹. -1 The D peak, representing disordered structure, is very weak, and the ID / IG ratio is only 0.07, indicating that the carbon nanotube structure is highly ordered, with extremely low defect density and high purity. (Refer to...) Figure 7E Thermogravimetric analysis (TGA) was used to measure the mass change of carbon nanotubes at different temperatures. The residual mass after decomposition of the carbon nanotube material indicates a very low impurity content. The material only begins to decompose rapidly at 550℃, demonstrating excellent thermal stability, further illustrating the high degree of graphitization and structural integrity of the carbon nanotubes. (Reference) Figure 7F By measuring the emission current under different field voltages, it can be seen that the carbon nanotube material (FWNT) obtained in the embodiments of this application can achieve a maximum emission current of 350 mA at a gate-to-cathode spacing of 215 µm and a gate voltage of 1388 V, corresponding to an emission current density of approximately 7777.8 mA / cm². 2 Furthermore, increasing the gate voltage can further increase the emission current, indicating its excellent field emission performance. Commercially available CNTs have relatively poor field emission performance; when used as cold cathodes in X-ray tubes, their operating current is generally several mA to tens of mA, while the maximum short-time pulse emission current disclosed in existing technology is only 200 mA, yet the gate control voltage is as high as 1700V or more. (Refer to...) Figure 7G Under operating conditions of 200 mA cathode current, 1 ms current pulse width, and 0.3% duty cycle, after long-term electron emission tests exceeding 5 million cycles, it can be seen that the carbon nanotube material obtained in the embodiments of this application only increased its voltage from 1342 V to 1372 V during more than 5 million electron emission cycles, an increase of only 30 V. This leaves a growth potential of 1628 V to reach the allowable gate control voltage of 3000 V. If the gate control voltage maintains the current growth rate in subsequent operations, the number of operating pulses for this cathode can be estimated to reach 2.71 × 10⁻⁶. 8 Each cycle (1628V ÷ 30V × 5 million cycles) results in a total operating time of 271,000 seconds (2.71 × 10⁻⁶ cycles). 8 (Times × 1 millisecond), meeting the 200,000 seconds lifespan requirement for medical CT tube registration certification, indicating that the carbon nanotube cathode of this application has stable emission performance and a long service life, and can meet relevant usage requirements in future practical applications.

[0122] Figures 8A-8C The following are schematic scanning electron microscope images of carbon nanotube materials according to some comparative embodiments of this application; Figures 9A-9B Transmission electron micrographs of carbon nanotube materials according to some comparative embodiments of this application are schematically shown.

[0123] In some comparative embodiments, reference is made to Figure 8A The circled areas represent metallic particle impurities, indicating that carbon nanotube materials prepared using other methods may contain a significant amount of metallic particle impurities. (Refer to...) Figure 8B The circled areas represent metallic particle impurities, the rectangular areas represent black flaky impurities, and the pentagonal areas represent amorphous carbon impurities. This shows that carbon nanotube materials prepared by other methods may also contain significant amounts of amorphous carbon impurities, black flaky impurities, and metallic particle impurities. (Refer to...) Figure 8C The circled areas represent metal particle impurities, while the rectangular areas represent sheet-like impurities. This shows that carbon nanotube materials prepared by other methods may also contain a significant amount of sheet-like and metal particle impurities.

[0124] In some comparative embodiments, reference is made to Figure 9A The circled areas represent impurity particles, demonstrating that carbon nanotube materials prepared using other methods have a large number of impurity particles adhering to the carbon nanotubes. (Refer to...) Figure 9B The part indicated by the arrow is a multi-walled carbon nanotube, which shows that carbon nanotube materials prepared by other methods may be doped with more multi-walled carbon nanotubes.

[0125] Figure 10 The schematic diagram illustrates a structural schematic of an electron emission device including a cold cathode according to some exemplary embodiments of this application.

[0126] Embodiments of this application also provide an electron emission device 1000, which includes carbon nanotube material prepared according to the above-described preparation method.

[0127] Reference Figure 10The electron emission device 1000 includes a carbon nanotube cathode 81 (i.e., a cold cathode) made of the aforementioned carbon nanotube material. The carbon nanotube cathode 81 is located on a substrate 80 and forms a triode electron gun structure with a gate 82 and an anode 83. For example, the distance between the gate 82 and the carbon nanotube cathode 81 is 150–300 μm. A certain voltage (0–3 kV) is applied to the gate 82 to form a first electric field E1, the magnitude of which is 1–20 V / μm. When the voltage at the gate 82 generates an electric field threshold greater than the threshold for field emission, the carbon nanotube cathode 81 emits electrons 811. A certain voltage is applied to the anode 83 to form a second electric field E2. The emitted electrons 811 are accelerated under the action of the second electric field E2 and bombard the anode target 84 to generate X-rays 812. The X-rays 812 pass through a beryllium window 85 and are received by a detector 86. Subsequently, signal processing and analysis are performed by devices such as an amplifier 87, an oscilloscope 88, and a computer 89 to achieve imaging or detection of an object.

[0128] In some embodiments of this application, based on Figure 10 Actual tests of the cold cathode electron emission device show that its current emission density is not less than 7 A / cm². 2 The electron emission device includes a cold cathode and a grid. Actual tests show that when the emission current of the cold cathode reaches 100mA, the control voltage of the grid does not exceed 1200V. Actual tests also show that when the electron emission device emits 1 million times (or the current emission time reaches 1000 seconds), the voltage growth rate between the cold cathode and the grid is less than 3%.

[0129] In the embodiments of this application, carbon nanotubes possess the characteristics of single type, high purity, and few defects, which significantly enhance the performance of the electron emission device 1000. Exemplarily, the operating current of the electron emission device 1000 is increased by at least an order of magnitude compared to similar technologies, while also exhibiting high stability and long lifespan. These advantages enable the electron emission device 1000 to successfully transition from the laboratory research stage to large-scale commercial applications, particularly demonstrating significant potential and value in the application field of cold cathode X-ray sources.

[0130] The electron emission device 1000 has a wide range of applications. For example, it can be used in medical imaging as a cathode material in X-ray generators for CT scans and radiotherapy. In scientific research equipment, it can serve as an electron source in electron microscopes, providing high-resolution imaging capabilities. In industrial inspection, it can be used in non-destructive testing equipment for inspecting the internal structure of materials. Finally, it can be used in flat panel displays as an electron source in field emission displays, achieving high brightness and high contrast.

[0131] It should be understood that the display device 1000 according to some exemplary embodiments of this application has all the features and advantages of the carbon nanotube 200 described above, which can be referred to above for carbon nanotube 200 and will not be repeated here.

[0132] While some embodiments based on the overall inventive concept of this application have been illustrated and described, those skilled in the art will understand that changes may be made to these embodiments without departing from the principles and spirit of the overall inventive concept of this application, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for preparing carbon nanotube materials for cold cathodes, characterized in that, The preparation method includes: A catalyst was prepared using transition metals; the catalyst materials included iron oxide, molybdenum oxide, and aluminum oxide, and the molar ratio of the iron oxide, molybdenum oxide, and aluminum oxide ranged from 0.5 to 2: 0.13 to 0.18: 14 to 20. The catalyst is placed in a high-temperature reaction chamber and reduced at a first temperature, which is between 360-450°C. A carbon source gas is introduced into the high-temperature reaction chamber, and the carbon source gas is decomposed under the action of the catalyst at a second temperature to grow a first carbon nanotube mixture. The temperature in the high-temperature reaction chamber is increased, and the first carbon nanotube mixture is purified at a high temperature at a third temperature. A portion of the obtained product is selected to obtain a second carbon nanotube mixture. The third temperature is between 1200-1600℃. The product from the central region of the obtained product is selected, and the proportion of the central region to the total product region is less than or equal to 25%. The first temperature is lower than the second temperature. The second temperature is lower than the third temperature. After purifying and drying the second carbon nanotube mixture, carbon nanotube material is obtained.

2. The preparation method according to claim 1, characterized in that, The catalyst prepared using transition metals includes: Dissolve at least one salt of the transition metal and a salt of the carrier metal in water to obtain a metal salt solution; An alkali is added to the metal salt solution to co-precipitate the metal ions in the transition metal salt and the carrier metal salt, resulting in a mixed hydroxide. The mixed hydroxide was subjected to aging treatment, filtered, washed and dried to obtain a solid powder; The solid powder was calcined at a fourth temperature to obtain the catalyst.

3. The preparation method according to claim 2, characterized in that, The fourth temperature range is between 800-1000℃.

4. The preparation method according to claim 1, characterized in that, The particle size of the catalyst is in the range of 10-40 nm.

5. The preparation method according to claim 1, characterized in that, The second temperature range is between 600-1000℃.

6. The preparation method according to claim 1, characterized in that, Introducing carbon source gas into the high-temperature reaction chamber further includes: Simultaneously, dilution gas is introduced into the high-temperature reaction chamber; the proportion of the carbon source gas to the sum of the carbon source gas and the dilution gas is less than or equal to 10%.

7. The preparation method according to claim 1, characterized in that, Purification of the second carbon nanotube mixture includes: The second carbon nanotube mixture was immersed in a dilute acid solution, filtered, and then rinsed with deionized water to obtain the first preproduct.

8. The preparation method according to claim 7, characterized in that, Purification of the second carbon nanotube mixture also includes: The first preproduct was placed in deionized water, stirred and ultrasonically cleaned, filtered and rinsed with deionized water to obtain the second preproduct.

9. The preparation method according to claim 8, characterized in that, Purification of the second carbon nanotube mixture also includes: The second preproduct was placed in an anhydrous alcohol solution, stirred and ultrasonically cleaned, filtered, rinsed with anhydrous alcohol solution, and dried to obtain the carbon nanotube material.

10. A carbon nanotube material for a cold cathode, characterized in that, The carbon nanotube material includes the carbon nanotube material prepared by the preparation method according to any one of claims 1 to 9.

11. The carbon nanotube material according to claim 10, characterized in that, The carbon nanotube material includes carbon nanotubes, each of which includes at least two graphene layers. Each graphene layer is a hollow tubular structure, and the at least two graphene layers are nested sequentially, arranged according to the diameter of the hollow tubular structure. The number of graphene layers is between 2 and 5.

12. The carbon nanotube material according to claim 11, characterized in that, The purity of the carbon nanotubes in the carbon nanotube material is greater than or equal to 99.9%.

13. The carbon nanotube material according to claim 10, characterized in that, The ratio of the D peak intensity to the G peak intensity in the Raman spectrum of the carbon nanotube material is less than or equal to 0.

07.

14. An electron emitting device including a cold cathode, characterized in that, The cold cathode of the electron emission device comprises carbon nanotube material prepared by the preparation method according to any one of claims 1 to 9, or the cold cathode of the electron emission device comprises carbon nanotube material according to any one of claims 10 to 13.

15. The electron emission device according to claim 14, characterized in that, The current emission density of the electron emission device is not less than 7A / cm2.

16. The electron emission device according to claim 14, characterized in that, The electron emission device also includes a gate, and when the emission current of the cold cathode reaches 100mA, the control voltage of the gate does not exceed 1200V.

17. The electron emission device according to claim 14, characterized in that, The electron emission device also includes a gate, and when the number of emission times of the electron emission device reaches 1 million or the electron emission time reaches 1000 seconds, the voltage growth rate between the cold cathode and the gate is less than 3%.

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