Carbon nanotube based cold cathode for x-ray generation

Abstract

To provide a formulation and a process for fabricating carbon nanotube-based cathodes in a manner that does not require a support, can withstand high temperatures, and can maintain their physical integrity under mechanical stress while maintaining field emission performance.SOLUTION: A cathode for an electron-emitting device is disclosed, the cathode comprising carbon nanotubes (CNTs), a nano-filler material, and a carbonizable polymer, wherein the cathode exhibits increased hardness, is formed by a high temperature heat treatment, and is free of a substrate. There is further provided a method of forming a cathode of an electron emitting device, the method comprising: a) forming a dispersed mixture comprising carbon nanotubes, a nanofiller material, and a carbonizable polymer in a solvent; b) coating and / or extruding the mixture; c) drying the coated and / or extruded mixture to remove at least a substantial portion of the solvent; and d) subjecting the dried mixture to a high temperature heat treatment, the method resulting in a cathode of an electron emitting device having increased hardness.SELECTED DRAWING: None

Description

[Technical Field]

[0001] Carbon nanotube-based cold cathodes are provided that can be used for x-ray generation. Additionally, multi-walled carbon nanotube-based cold cathodes that exhibit improved hardness and high current density, and methods for their manufacture, are provided. [Background technology]

[0002] The birth of the electronics industry can be attributed to the invention of the vacuum diode. The invention of the vacuum diode in 1904 by Sir John Ambrose Fleming gave birth to the modern electronics industry and had a very simple structure consisting of a glass vacuum tube with two electrodes, an anode and a cathode (see Figure 1). When heated, the cathode emits electrons that are collected by the anode, ensuring the direction of current when connected to an external circuit.

[0003] This basic structure has remained largely unchanged since then, and was used in a wide range of devices, from radio amplifiers to televisions to early computers, until the invention of the solid-state transistor, which allowed for greater miniaturization than was possible with vacuum tubes. Today, the primary uses of vacuum tubes are in x-ray sources for medical and dental applications, and in the development of klystron tubes for microwave heating. However, in recent years there has been a revival of research activity surrounding the use of vacuum tubes, with the aim of reducing the speed of electrons in semiconductors (~5×10 7 The electron velocity in a vacuum (3 × 10 cm / sec) is about three orders of magnitude larger than that 10 It has been determined whether it is possible to exploit the vacuum energy of vacuum nanoelectronics (vacuum nanoparticles) to break through the current limitations of solid-state electronics. (1-5) The results are promising and can be expected to lead to innovative new devices and applications in the rapidly developing field of vacuum nanoelectronics, which are unthinkable with current solid-state semiconductor-based technologies. (6)

[0004] As with all new device technologies, many chemical and material challenges must be overcome to realize the incredible potential in this field. One of these key challenges relates to the cathode, which serves as the electron source in vacuum tubes. In (macro) vacuum tubes, the technique used to generate electrons from the cathode is thermal. The cathode is heated to high temperatures, which can exceed 2000 °C depending on the cathode's composition, allowing it to emit electrons; that is, it relies on thermionic emission of electrons. In the case of vacuum nanoelectronics, such high temperatures adversely affect device operation. Therefore, it is generally understood that alternative approaches are required to generate electrons from the cathode in vacuum nanoelectronics, optionally using electric fields (field emission) (1) or light (photoemission) (2, 5), or a combination of the two. At this stage, field emission appears to be further advanced due to the discovery of the extraordinary field emission properties of carbon nanotubes, early efforts by the display industry to develop devices, and current efforts to utilize these two developments for the miniaturization of x-ray sources. Despite the fact that carbon nanotubes have been demonstrated to have very high field electron emission efficiency at low threshold electric fields, there is a lack of successful carbon nanotube-based field emission devices / products on the market.

[0005] This failure of carbon nanotube-based field emission devices is related to the difficulty of fabricating freestanding carbon nanotube-based cathodes that can withstand high temperatures and maintain their physical integrity under mechanical stress. Carbon nanotube-based field emission cathodes are typically fabricated by physical or chemical vapor deposition of carbon nanotubes onto a well-defined cathode substrate, or by depositing aqueous or non-aqueous formulations of pre-formed carbon nanotube slurries onto a well-defined cathode substrate, followed by specific heat treatments. While the two methods are fundamentally different and produce cathodes with distinct properties, they are united by the need for a cathode substrate to ensure the cathode's physical integrity. While the choice of cathode substrate can include additional functionality, such as thermal and electrical conductivity, its primary function is to ensure the cathode's physical integrity. As a result, the lack of adequate physical integrity of carbon nanotube-based cathodes is often masked by the properties of the cathode substrate under laboratory conditions unless explicitly evaluated. For example, U.S. Patent No. 6,277,999 discloses a method for fabricating carbon nanotube-based cathodes, which uses a graphite adhesive (composed of a graphite filler and a graphite binder) to bond a thin film of a carbon nanotube compound to a cathode substrate. Key factors here are the substrate requirements and bonding of the compound to the cathode substrate. Implicit in these requirements is that the compound may not be physically stable in the absence of a substrate and proper bonding of the compound to the cathode substrate. Consequently, such compounds, and the associated processes, cannot withstand harsh real-world conditions of use without a substrate.It is important to note that the choice of cathode substrate (e.g., as in U.S. Patent No. 5,999,523) is limited by the specific details of the formulation (called a paste in U.S. Patent No. 5,999,523) that is deposited on the cathode substrate; i.e., there is no formulation that is sufficiently universal and can allow bonding to any cathode substrate; the thermal, electrical, physical, and mechanical properties of the final cathode are limited / limited by the thermal, electrical, physical, and mechanical properties of the cathode substrate that can be bonded to the formulation (called a paste in U.S. Patent No. 5,999,523). The amenability of the limited choice of cathode substrate to be bonded to other materials required to fabricate the final device (e.g., x-ray source) and the high vacuum conditions (e.g., 10) of the device are important considerations. -9 The ability of these limited choices of cathode substrates to survive / perform at pressures below 1000 Torr is generally a major obstacle in fabricating practically useful devices. The choice of cathode substrate is primarily dictated by the need to ensure the physical integrity of the cathode, but also by the need to maintain adequate electrical conductivity (or low resistance) between the cathode and the external circuitry, and the need to maintain high vacuum (e.g., 10 -9 In order for a cathode to perform its function as an electron source, it must be electrically connected to an external power source without adding significant resistance to the electrical circuit. As a result, the selection of a cathode substrate that can provide adequate physical integrity for the cathode must be determined by the requirement of high electrical conductivity (or minimal resistance), as well as the ability to combine these properties of the cathode substrate (high electrical conductivity, physical integrity, and high vacuum (e.g., 10 -9 These are limited by the need for adhesives that maintain their ability to function at temperatures below 1000 rpm. As a result, it is very difficult to find adhesive-substrate combinations that can be of practical value, which is one of the main reasons why there are no commercially viable products in this field that take advantage of the enormous potential offered by carbon nanotubes.

[0006] Furthermore, the use of x-ray imaging in dentistry has not changed significantly since William James Morton's first demonstration of 2D imaging at a special meeting of the New York Dental Society in 1896. The cathode used to generate electrons in William Coolidge's 1913 publication has also remained largely unchanged. However, significant efforts have been made to improve x-ray imaging modalities in dentistry, such as 3D or nearest-neighbor 3D imaging (including chairside tomosynthesis). A key requirement for such modalities is a distributed x-ray source. However, the creation of a distributed x-ray source requires alternative cathode technology. A popular cathode technology in the literature on distributed x-ray sources is based on the field-emission array cathode (see Figure 2), demonstrated by Charles Spindt in 1968, which exploits the field emission behavior of certain materials when exposed to high electric fields, as first demonstrated by R.W. Wood in 1897.

[0007] The 1972 demonstration by Baker et al. that carbon fibers were good candidates for field emission electron sources in vacuum, and the discovery of carbon nanotubes by Iijima several decades later led to vigorous activity in the development of carbon nanotube-based field emission cathodes (also known as cold cathodes). However, a fundamental challenge that has not been overcome is the development of formulations and processes to obtain such cold cathodes, which would exhibit adequate physical stability and the ability to bond in a robust manner to other materials used in the cathode construction. A key challenge in the fabrication of any carbon nanotube-based cold cathode for microscopy devices, such as x-ray generation, is the need to fabricate a composite containing the active material that is physically robust in high-vacuum, high-temperature environments, while also being electrically conductive and stable enough to bond to electrically insulating interfaces and remain adaptable under various thermal conditions.

[0008] The approaches taken so far can be divided into two categories.

[0009] The first approach utilizes vapor deposition to simultaneously synthesize and deposit (multiple types of) carbon nanotubes onto various surfaces / substrates. While this approach produces high-quality carbon nanotube cathodes, it is not suitable for strong bonding to the surface / substrate, making the final device very fragile. The second issue associated with this approach is the challenge in creating thick films. Inherently, vapor deposition is well suited to thin films, but not thicker films. To achieve higher current densities, it is necessary to be able to achieve thicker films.

[0010] The second approach is the so-called "paste" approach, in which pre-formed carbon nanotubes are formulated with various additives in aqueous or non-aqueous media and deposited by screen printing and other methods onto specific substrates to produce thicker films than those produced by evaporation, which are then thermally treated to produce the final cathode. The choice of additive is crucial and determines the physical properties of the paste (such as viscosity), as well as the mechanical properties (e.g., hardness), electrical properties (e.g., conductivity vs. insulation), and thermal properties (e.g., collapse temperature) of the cathode, without the need for a substrate.

[0011] So far, it has proven difficult to fabricate carbon nanotube-based cathodes with all the properties desired for a cold cathode, such as physical, electrical, and thermal properties, without the need for a substrate, using either approach. In both approaches, carbon nanotube-based cathodes have been able to achieve currents of a few microamperes / cm. 2 ~Several amperes / cm 2While carbon nanotube-based x-ray tubes have been fabricated to have current densities in the range of 1000 to 15000 kJ / cm², none of them appear to have the necessary physical and thermal properties without the need for a substrate. The cathodes are either physically fragile or crumble with repeated routine handling, and / or cannot withstand the high vacuum within the x-ray tube, and / or cannot be properly bonded to other necessary components, and / or cannot withstand the high temperature processing requirements of x-ray tube construction and use. In all approaches, the cathodes fail in one or more required performance categories, making it difficult, if not impossible, to build a carbon nanotube-based x-ray tube into a commercially viable product.

[0012] Therefore, there is a need to develop formulations and processes for fabricating carbon nanotube-based cathodes that do not require supports, can withstand high temperatures, and maintain their physical integrity under mechanical stress while maintaining field emission performance. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] U.S. Patent No. 10,049,847 [Patent Document 2] US Patent Application Publication No. 2017 / 137290 [Non-patent literature]

[0014] [Non-Patent Document 1] Francois Magnin, "Solvents Selection for Industrial Coatings," [online], SpecialChem, Internet <https: / / coatings.specialchem.com / selection-guide / select-solvents-for-industrial-coatings> [Non-patent document 2] Sharma, S., "Glassy Carbon: A Promising Material for Micro- and Nanomanufacturing Materials," Multidiciplinary Digital Publishing Institute, September 28, 2018, Vol. 11, No. 10, p. 1857 Summary of the Invention [Means for solving the problem]

[0015] In one aspect, a cathode for an electron emission device is provided, the cathode comprising carbon nanotubes (CNTs), a nanofiller material, and a carbonizable polymer, wherein the cathode exhibits increased hardness, is formed by high-temperature heat treatment, and is substrate-free. In one embodiment, the carbon nanotubes are multi-walled carbon nanotubes (MWCNTs). In one embodiment, the multi-walled carbon nanotubes are helical multi-walled carbon nanotubes. In one embodiment, the nanofiller material is selected from the group consisting of graphite, silicon carbide, titanium carbide, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, cadmium sulfide, silicon, silver, copper, titanium, nickel, iron, iron oxide, copper oxide, zinc oxide, and combinations thereof. In one embodiment, the carbon nanotubes and the nanofiller material are present in a ratio of about 1:10 to about 1:100, or about 1:30 to about 1:50. In one embodiment, the carbonizable polymer is a non-graphitizable polymer. In one embodiment, the carbonizable polymer is selected from polyfurfuryl alcohol, phenol-formaldehyde-based polymers, epoxy-based photoresists, carbon fiber-forming polymers, and combinations thereof. In one embodiment, monomeric and / or oligomeric forms of the carbonizable polymer are used, which form the carbonizable polymer during high-temperature heat treatment. In various embodiments, the increased hardness results in a bulk indentation of less than 0.2 mm, or a bulk indentation of 0.15 mm or less, when the cathode is subjected to a force at 90° to the long axis of the cathode from a conical steel probe moving at a constant speed of 50 mm / min until a maximum load of 500 grams is reached. In various embodiments, the high-temperature heat treatment includes forming the cathode at a temperature of about 600° C. to about 1300° C., or about 900° C. to about 1000° C., in a vacuum or substantially oxygen-free environment. In one embodiment, the high-temperature heat treatment is carried out in the presence of an inert gas. In one embodiment, the inert gas is argon gas, nitrogen gas, or a combination thereof. In one embodiment, the high-temperature heat treatment comprises heating at a rate of about 0.1°C per minute to about 5°C per minute. In one embodiment, the high-temperature heat treatment comprises a residence time at temperature ranging from about 30 minutes to about 3,000 minutes.

[0016] In another aspect, a method for forming a cathode of an electron emission device is provided, the method comprising: a) forming a dispersion mixture including carbon nanotubes, a nanofiller material, and a carbonizable polymer in a solvent; b) coating and / or extruding the mixture; c) drying the coated and / or extruded mixture to remove at least a substantial portion of the solvent; and d) exposing the dried mixture to a high-temperature heat treatment, the method resulting in an electron emission device cathode with increased hardness. In one embodiment, the carbon nanotubes are multi-walled carbon nanotubes (MWCNTs). In one embodiment, the multi-walled carbon nanotubes are helical multi-walled carbon nanotubes. In one embodiment, the nanofiller material is selected from the group consisting of graphite, silicon carbide, titanium carbide, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, cadmium sulfide, silicon, silver, copper, titanium, nickel, iron, iron oxide, copper oxide, zinc oxide, and combinations thereof. In one embodiment, the carbon nanotubes and nanofiller material are present in a ratio of about 1:10 to about 1:100, or about 1:30 to about 1:50. In one embodiment, the carbonizable polymer is a non-graphitizable polymer. In one embodiment, the carbonizable polymer is selected from polyfurfuryl alcohol, phenol-formaldehyde based polymers, epoxy-based photoresists, carbon fiber forming polymers, and combinations thereof. In one embodiment, monomeric and / or oligomeric forms of the carbonizable polymer are added in step a), and the monomeric and / or oligomeric forms of the carbonizable polymer are polymerized during the heat treatment to form the carbonizable polymer. In various embodiments, the increased hardness results in a bulk indentation of less than 0.2 mm, or a bulk indentation of 0.15 mm or less, when the cathode is subjected to a force at 90° to the longitudinal axis of the cathode from a conical steel probe moving at a constant speed of 50 mm / min until a maximum load of 500 grams is reached. In one embodiment, the high temperature heat treatment comprises exposing the dried mixture to a temperature of about 600° C. to about 1300° C., or about 900° C. to about 1000° C. in a vacuum or substantially oxygen-free environment. In one embodiment, the high temperature heat treatment is carried out in the presence of an inert gas.In one embodiment, the inert gas is argon gas, nitrogen gas, or a combination thereof. In one embodiment, the high-temperature heat treatment comprises heating at a rate of about 0.1°C per minute to about 5°C per minute. In one embodiment, the high-temperature heat treatment comprises a residence time at temperature ranging from about 30 minutes to about 3,000 minutes. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of a vacuum diode. [Figure 2] FIG. 1 is a schematic diagram of cathode technology for a distributed source based on a field emission array cathode. [Figure 3] FIG. 1 is a schematic diagram of the bulk indentation assay used to measure the hardness of a sample. [Figure 4] FIG. 1 shows maximum displacement data from bulk indentation assays of a cathode according to one embodiment (right side) and a prior art cathode (left side). [Figure 5] FIG. 1 shows the field emission performance of a cathode according to one embodiment (bottom, purple) and a prior art cathode (top, green). DETAILED DESCRIPTION OF THE INVENTION

[0018] The above problems have been addressed by developing specific formulations and processes for fabricating carbon nanotube-based cathodes that do not require supports, can withstand high temperatures, and maintain their physical integrity under mechanical stress while maintaining field emission performance. We have discovered that an approach to fabricating practically useful cold cathodes must be fundamentally different from approaches previously employed, as further described below.

[0019] The "paste" approach, which incorporates pre-formed carbon nanotubes in an aqueous or non-aqueous medium, addresses the problem of being able to produce thicker carbon nanotube deposits than those produced by direct vapor deposition and also provides additives that can act as "binders" that can preserve and improve the physical integrity of the cathode beyond that of cathodes produced by vapor deposition. However, all paste approaches suffer from a fundamental problem: they utilize organic and polymeric additives used in the general colloid science literature that either do not fulfill their intended purpose, e.g., physical integrity / stability, or decompose upon thermal treatment into materials that do not fulfill their intended purpose, e.g., physical integrity / stability, or become impediments to their intended purpose, e.g., physical integrity / stability. This therefore necessitates that different classes of materials must be used as additives when achieving their intended purpose. These additives must belong to either a class of materials that preserves their properties, e.g., provides the physical integrity / stability of the cathode, or improves their initial properties, e.g., provides improved physical integrity / stability of the cathode upon thermal treatment.

[0020] The second problem with the paste approach that needed to be addressed was heat treatment: after identifying and selecting a particular class of materials that would retain or improve the intended properties upon heat treatment, it was necessary to find the appropriate heat treatment conditions to promote this behavior.

[0021] A third, and equally important, issue is reliably bonding the carbon nanotube cathode to other conductive, semiconductive, and insulating materials required for the fabrication of the final device, such as an x-ray tube. The carbon nanotube composite, produced by appropriate selection of materials and heat treatment conditions that are compatible with the cold cathode, must also be capable of bonding to conductive or semiconductive materials so that it can be connected to an external power source without failure during use, and to insulating materials so that the cold cathode is electrically isolated from the anode and other components of the x-ray tube.

[0022] Our approach described herein overcomes all three of these problems by using additives that significantly enhance the physical and electrical properties of carbon nanotube-based cathodes under specific heat treatment conditions, and are amenable to specific additives and processes for bonding to conductive, semiconductive, and insulating materials.

[0023] Nanotubes are members of the fullerene structural family. Their name derives from their long, hollow structure, with walls formed from single-atom-thick carbon sheets called graphene. These sheets are rolled at specific, discrete ("chiral") angles, and the combination of rolling angle and radius determines the nanotube's properties, such as whether individual nanotube shells are metallic or semiconducting. Nanotubes are classified as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). MWNTs consist of multiple rolled layers (concentric tubes) of graphene. Individual nanotubes spontaneously align into "ropes" held together by van der Waals forces, more specifically, π-stacking. In one embodiment, the carbon nanotubes (CNTs) used to form the described cold cathodes are multi-walled carbon nanotubes (MWCNTs). In one embodiment, the carbon nanotubes (CNTs) used to form the described cold cathodes are helical multi-walled carbon nanotubes (MWCNTs). In another embodiment, the carbon nanotubes (CNTs) used to form the described cold cathodes are carbon nanotube filaments or fibers, which are aggregates of carbon nanotubes (CNTs) produced by any fiber / filament extrusion process.

[0024] Carbon nanotubes can be functionalized to achieve desirable properties that can be used in a wide variety of applications. The two main methods for functionalizing carbon nanotubes are covalent and non-covalent modification. Because carbon nanotubes are hydrophobic, they tend to aggregate, preventing their dispersion in solvents or viscous polymer melts. The resulting nanotube bundles or aggregates reduce the mechanical performance of the final composite. Therefore, solvent selection can be important. Solvents that can dissolve and / or disperse carbon nanotubes with adequate colloidal stability and that can be easily removed by thermal evaporation, such as ethanol, methanol, acetone, methyl ethyl ketone, and ethyl acetate, commonly used in industrial coating processes, can be used. The following link provides a detailed summary of solvents useful for industrial coatings: (https: / / coatings.specialchem.com / selection-guide / select-solvents-for-industrial-coatings). In one embodiment, methylene chloride is used as the solvent.

[0025] The mechanical, thermal, and electronic properties of CNT compositions can be improved by adding nanofillers to fill the voids. Filler materials can be any inorganic, conductive, and / or semiconductive particles that allow for the creation of coating formulations with appropriate viscosities. In one embodiment, suitable viscosities can range from 5,000 to 50,000 cps. Exemplary filler materials include silicon carbide, titanium carbide, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, cadmium sulfide, silicon, silver, copper, titanium, nickel, iron, iron oxide, copper oxide, zinc oxide, and the like. In one embodiment, graphite nanoparticles are used as fillers. In one embodiment, the carbon nanotubes and filler are combined in a ratio of about 1:10 to about 1:100. In one embodiment, the carbon nanotubes and filler are combined in a ratio of about 1:30 to about 1:50.

[0026] In addition to the fillers described above, carbonizable polymers, or monomeric and / or oligomeric versions thereof, can also be used to provide the composition with adequate colloidal stability to allow for coating and / or extrusion of physical structures and ultimate structural integrity to the resulting solid during thermal processing. In this regard, the carbonizable polymers, and / or monomeric and / or oligomeric versions thereof, can be considered precursor materials for the final components of the processed solid. In one embodiment, the precursor polymer is formed from furfuryl alcohol under appropriate conditions. Other useful carbonizable polymers include non-graphitizable polymers, such as phenol-formaldehyde-based polymers, which are synthetic polymers obtained by reacting phenol or substituted phenols with formaldehyde, epoxy-based photoresists (https: / / en.wikipedia.org / wiki / SU-8_photoresist), and carbon fiber-forming polymers, such as polyacrylonitrile and pitch (petroleum-based, coal-based, naphthalene-based, and / or synthetic). (For a description of carbonizable and graphitizable polymers, see Sharma, S. (2018) Glassy Carbon: A Promising Material for Micro- and Nanomanufacturing. Materials, Vol. 11(10), p. 1857. A description of polymer-derived carbon is provided in the above-mentioned reference (Chapter 4 of the reference, "Graphitizable carbon is a polymer-derived carbon that can be converted to polycrystalline graphite by thermal treatment" and "High-purity non-graphitizable carbon that undergoes at least some coking during pyrolysis is known as glassy carbon"). In one embodiment, furfuryl alcohol is used and thermally polymerized with or without an additional catalyst to produce polyfurfuryl alcohol, or prepolymerized furfuryl alcohol can be used. Monomeric or oligomeric or polymeric versions of the carbonizable polymer are mixed, coated, and / or extruded with CNTs and filler(s) to form one-dimensional (fiber / filament) or two-dimensional (sheet) materials.In either case, the coated and / or extruded material may be formed on a solid support, which is subsequently removed by evaporation of the solvent used in the formulation, followed by heat treatment.

[0027] After polymerization or use of a preformed polymer, the composition is then subjected to a heat treatment step. After coating or extruding the formulation and drying it to remove the solvent, the coated / extruded material is subjected to a heat treatment step to carbonize the polymer in the coated / extruded material. The solid support on which the formulation is coated and / or the extruded material is formed is removed immediately after solvent removal or after a specific series of heat treatments, and the free-standing coated / extruded object is then subjected to a further heat treatment. In one embodiment, the heat treatment is carried out in a vacuum or substantially without oxygen. In one embodiment, the heat treatment is carried out in the presence of an inert gas, such as argon or nitrogen gas. In one embodiment, the heat treatment comprises heating the composition to about 600°C to about 1300°C. In one embodiment, the heat treatment comprises heating the composition to about 900°C to about 1000°C. In various embodiments, the heating rate ranges from about 0.1°C per minute to about 5°C per minute. In various embodiments, the residence time at elevated temperature ranges from about 30 minutes to about 3000 minutes.

[0028] A bulk indentation testing procedure was used to evaluate the physical integrity of cathodes produced according to the described formulations and procedures in comparison with those disclosed in the literature, e.g., U.S. Pat. No. 10,049,847 B2 and References 7 and 8. Bulk indentation was used to provide a measure of the "hardness" of each sample type. In these experiments, a 90° conical steel probe was lowered at a constant rate until it made contact with the flat portion of each sample (see Figure 3). Once in contact, the probe continued to press down at a constant rate of 50 mm / min until a maximum load of 500 grams was reached. The probe was then retracted from the sample, and force-displacement data was collected and plotted to determine the displacement at the point of maximum load.

[0029] The formulation and process of the present invention resulted in cathodes that exhibited half the bulk indentation (for a constant 500 gram load) (Figure 4, right bar) compared to those seen for the literature formulation and process (Figure 4, left bar). Under routine handling conditions, the cathodes produced using the literature formulation and process cracked and disintegrated, while the cathodes from the formulation and process of the present invention maintained their physical integrity. The field emission performance of the cathodes from the literature and the present invention was essentially similar when evaluated by Fowler-Nordheim theory (9) (Figure 5). However, as noted above, the cathodes from the literature cracked and disintegrated under routine handling conditions.

Claims

1. 1. A cathode of an electron emission device, comprising: carbon nanotubes (CNTs), nanofilling materials, and a carbonizable polymer; wherein the hardness is increased and the substrate is free of any of the hardness formed by high temperature heat treatment. Cathode.

2. 10. The cathode of claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes (MWCNTs).

3. The cathode of claim 2 , wherein the multi-walled carbon nanotubes are helical multi-walled carbon nanotubes.

4. 10. The cathode of claim 1, wherein the nanofiller material is selected from the group consisting of graphite, silicon carbide, titanium carbide, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, cadmium sulfide, silicon, silver, copper, titanium, nickel, iron, iron oxide, copper oxide, zinc oxide, and combinations thereof.

5. 10. The cathode of claim 1, wherein the nanofiller material is graphite.

6. 10. The cathode of claim 1, wherein the carbon nanotubes and nanofiller material are present in a ratio of about 1:10 to about 1:

100.

7. 7. The cathode of claim 6, wherein the carbon nanotubes and nanofiller material are present in a ratio of about 1:30 to about 1:

50.

8. 10. The cathode of claim 1, wherein the carbonizable polymer is a non-graphitizable polymer.

9. 9. The cathode of claim 8, wherein the carbonizable polymer is selected from polyfurfuryl alcohol, phenol-formaldehyde based polymers, epoxy-based photoresists, carbon fiber forming polymers, and combinations thereof.

10. 10. The cathode of claim 1, wherein the carbonizable polymer is polyfurfuryl alcohol.

11. 10. The cathode of claim 1, wherein the increased hardness produces a bulk indentation of less than 0.2 mm when the cathode is subjected to a force at 90° to a longitudinal axis of the cathode from a conical steel probe moving at a constant speed of 50 mm / min up to a maximum load of 500 grams.

12. 12. The cathode of claim 11, wherein the increased hardness results in a bulk indentation of 0.15 mm or less.

13. 10. The cathode of claim 1, wherein the high temperature heat treatment comprises forming the cathode at a temperature of about 600°C to about 1300°C in a vacuum or substantially oxygen-free environment.

14. 14. The cathode of claim 13, wherein the high temperature heat treatment is carried out in the presence of an inert gas.

15. 15. The cathode of claim 14, wherein the inert gas is argon gas, nitrogen gas, or a combination thereof.

16. 14. The cathode of claim 13, wherein the temperature is from about 900°C to about 1000°C.

17. 14. The cathode of claim 13, wherein the high temperature heat treatment comprises heating at a rate of about 0.1° C. per minute to about 5° C. per minute.

18. 14. The cathode of claim 13, wherein the high temperature heat treatment comprises a dwell time at temperature ranging from about 30 minutes to about 3000 minutes.

19. 10. The cathode of claim 1, wherein a monomeric and / or oligomeric form of the carbonizable polymer is used, which forms the carbonizable polymer during the high temperature heat treatment.

20. 1. A method of forming a cathode for an electron emission device, comprising: a) forming a dispersion mixture in a solvent comprising carbon nanotubes, a nanofiller material, and a carbonizable polymer; b) coating and / or extruding the mixture; c) drying the coated and / or extruded mixture to remove at least a substantial portion of the solvent; and d) exposing the dried mixture to a high temperature heat treatment. This results in a cathode of the electron emission device with increased hardness. method.

21. 21. The method of claim 20, wherein monomeric and / or oligomeric forms of the carbonizable polymer are added in step a), and wherein the monomeric and / or oligomeric forms of the carbonizable polymer are polymerized during the heat treatment to form the carbonizable polymer.

22. 21. The method of claim 20, wherein the carbon nanotubes are multi-walled carbon nanotubes (MWCNTs).

23. 23. The method of claim 22, wherein the multi-walled carbon nanotubes are helical multi-walled carbon nanotubes.

24. 21. The method of claim 20, wherein the nanofiller material is selected from the group consisting of graphite, silicon carbide, titanium carbide, tungsten carbide, molybdenum carbide, tungsten sulfide, molybdenum sulfide, cadmium sulfide, silicon, silver, copper, titanium, nickel, iron, iron oxide, copper oxide, zinc oxide, and combinations thereof.

25. The method of claim 20, wherein the nanofiller material is graphite.

26. 21. The method of claim 20, wherein the carbon nanotubes and nanofiller material are present in a ratio of about 1:10 to about 1:

100.

27. 27. The method of claim 26, wherein the carbon nanotubes and nanofiller material are present in a ratio of about 1:30 to about 1:

50.

28. 21. The method of claim 20, wherein the carbonizable polymer is a non-graphitizable polymer.

29. 29. The method of claim 28, wherein the carbonizable polymer is selected from polyfurfuryl alcohol, phenol-formaldehyde based polymers, epoxy based photoresists, carbon fiber forming polymers, and combinations thereof.

30. 21. The method of claim 20, wherein the carbonizable polymer is polyfurfuryl alcohol.

31. 21. The method of claim 20, wherein the increased hardness produces a bulk indentation of less than 0.2 mm when the cathode is subjected to a force at 90° to the longitudinal axis of the cathode from a conical steel probe moving at a constant speed of 50 mm / min until a maximum load of 500 grams is reached.

32. 32. The method of claim 31, wherein the increased hardness results in a bulk indentation of 0.15 mm or less.

33. 21. The method of claim 20, wherein the high temperature heat treatment comprises exposing the dried mixture to a temperature of about 600°C to about 1300°C in a vacuum or a substantially oxygen-free environment.

34. 34. The method of claim 33, wherein the high temperature heat treatment is carried out in the presence of an inert gas.

35. 35. The method of claim 34, wherein the inert gas is argon gas, nitrogen gas, or a combination thereof.

36. 34. The method of claim 33, wherein the temperature is from about 900°C to about 1000°C.

37. 34. The method of claim 33, wherein the high temperature heat treatment comprises heating at a rate of about 0.1° C. per minute to about 5° C. per minute.

38. 34. The method of claim 33, wherein the high temperature heat treatment comprises a residence time at temperature ranging from about 30 minutes to about 3,000 minutes.

Images