CARBON NANOTUBE BASED COLD CATHODES FOR X-RAY GENERATION

Abstract

A cathode of an electron emitting device is described, where the cathode comprises a carbon nanotube (CNT); a nano-filler material; and a carbonizable polymer; and where the cathode exhibits increased hardness, is formed by high temperature thermal treatment, and is devoid of a substrate. Also described is a method of forming a cathode of an electron emitting device, where the method comprises a) forming a dispersed mixture comprising a carbon nanotube, a nano-filler 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 thermal treatment; where the method results in the cathode of an electron emitting device having increased hardness.

Claims

1. A cathode of an electron emitting device, the cathode comprising a carbon nanotube (CNT); a nano-filler material; and a carbonizable polymer; wherein the cathode exhibits increased hardness, is formed by high temperature thermal treatment, and is devoid of a substrate.

2. The cathode of claim 1, wherein the carbon nanotube is a multi-walled carbon nanotube (MWCNT).

3. The cathode of claim 2, wherein the multi-walled carbon nanotube is a helical multi-walled carbon nanotube.

4. The cathode of claim 1, wherein the nano-filler 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. The cathode of claim 1, wherein the nano-filler material is graphite.

6. The cathode of claim 1, wherein the carbon nanotubes and nano-filler material are present at a ratio of about 1:10 to about 1:100.

7. The cathode of claim 6, wherein the carbon nanotubes and nano-filler material are present at a ratio of about 1:30 to about 1:50.

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

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

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

11. The cathode of claim 1, wherein the increased hardness results in a bulk-indentation of less than 0.2 mm when the cathode is subjected to a force at 90 degrees to a long axis of the cathode, from a conical steel probe moving at a constant velocity of 50 mm/minute until a maximum load of 500 grams is reached.

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

13. The cathode of claim 1, wherein the high temperature thermal treatment comprises forming the cathode in a vacuum or an environment substantially devoid of oxygen, at temperature from about 600 C. to about 1300 C.

14. The cathode of claim 13, wherein the high temperature thermal treatment occurs in the presence of an inert gas.

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

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

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

18. The cathode of claim 13, wherein the high temperature thermal treatment comprises a dwell time at the temperature ranging from about 30 minutes to about 3,000 minutes.

19. 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 thermal treatment.

20. A method of forming a cathode of an electron emitting device, the method comprising a) forming a dispersed mixture comprising a carbon nanotube, a nano-filler 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 thermal treatment; wherein the method results in the cathode of an electron emitting device having increased hardness.

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

22. The method of claim 20, wherein the carbon nanotube is a multi-walled carbon nanotube (MWCNT).

23. The method of claim 22, wherein the multi-walled carbon nanotube is a helical multi-walled carbon nanotube.

24. The method of claim 20, wherein the nano-filler 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 nano-filler material is graphite.

26. The method of claim 20, wherein the carbon nanotubes and nano-filler material are present at a ratio of about 1:10 to about 1:100.

27. The method of claim 26, wherein the carbon nanotubes and nano-filler material are present at a ratio of about 1:30 to about 1:50.

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

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

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

31. The method of claim 20, wherein the increased hardness results in a bulk-indentation of less than 0.2 mm when the cathode is subjected to a force at 90 degrees to a long axis of the cathode, from a conical steel probe moving at a constant velocity of 50 mm/minute until a maximum load of 500 grams is reached.

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

33. The method of claim 20, wherein the high temperature thermal treatment comprises subjecting the dried mixture to a temperature from about 600 C. to about 1300 C. in a vacuum or an environment substantially devoid of oxygen.

34. The method of claim 33, wherein the high temperature thermal treatment occurs in the presence of an inert gas.

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

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

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

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 shows a schematic of a vacuum diode.

[0016] FIG. 2 shows a schematic of cathode technology for distributed sources based on field emission array cathodes.

[0017] FIG. 3 shows a schematic of a bulk indentation assay used to measure of the hardness of a sample.

[0018] FIG. 4 shows maximum displacement data from a bulk indentation assay for a cathode according to one embodiment (right) and a prior art cathode (left).

[0019] FIG. 5 shows field emission performance of a cathode according to one embodiment (bottom; purple) and a prior art cathode (top; green).

DETAILED DESCRIPTION

[0020] We have addressed the above described issues by developing a specific formulation and process for creating carbon nanotube-based cathodes, which do not require a support, and can survive elevated temperatures and maintain their physical integrity under mechanical stress, while maintaining its field emission performance. We have discovered that the approach to creating a practically useful cold cathodes has to be fundamentally different from the approaches taken thus far, as further described below.

[0021] The paste approach to formulating pre-made carbon nanotubes in aqueous or non-aqueous media addresses the issue of being able to create thicker carbon nanotube deposits, than those created by direct vapor deposition, and also provide additives that may serve as binders that can hold and improve the physical integrity of the cathode, over that of the cathodes generated by vapor deposition. However, all the paste approaches have a fundamental problem, in that they utilize organic and polymer additives that are used in the general colloid science literature, and do not serve their intended purpose, e.g., physical integrity/stability, or disintegrate upon the thermal processing into entities that do not serve their intended purpose, e.g., physical integrity/stability, or are detrimental to their intended purpose, e.g., physical integrity/stability. So, this requires that a different class of materials has to be used as additives if the intended purpose is to be achieved. These additives have to belong to a class of materials that either retain their properties, e.g., provide the physical integrity/stability of the cathode, or improve upon their initial properties, e.g., provide improved physical integrity/stability of the cathode, upon thermal processing.

[0022] The second issue with the paste approach that needed to be addressed was thermal processing. After we identified and chose a special class of materials that either retain or improve upon their intended properties upon thermal processing, it was necessary to discover the appropriate thermal processing conditions, that promotes this behavior.

[0023] The third, and equally important issue, is the secure bonding of the carbon nanotube cathodes to the other conducting, semiconducting and insulating materials, required for the fabrication of the eventual device, such as an x-ray tube. The carbon nanotube composite generated by the appropriate choice of materials and thermal processing conditions to be utilizable as a cold cathode, must also be capable of being bonded to a conducting or semiconducting material, so that it can be connected an external electrical 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.

[0024] Our approach, described herein, overcomes all these three issues, by using additives that result in remarkably enhanced physical and electrical properties of the carbon nanotube based cathodes, upon specific thermal processing conditions, and are receptive to specific additives and processes for bonding to conducting, semi-conducting and insulating materials.

[0025] Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete (chiral) angles, and the combination of the rolling angle and radius decides the nanotube properties, for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Multi-walled nanotubes (MWNTs) consist of multiple rolled layers (concentric tubes) of graphene. Individual nanotubes naturally align themselves into ropes held together by van der Waals forces, more specifically, pi-stacking. In one embodiment, the carbon nanotube (CNT) used on the formation of the described cold cathode is a multi-walled carbon nanotube (MWCNT). In one embodiment, the carbon nanotube (CNT) used on the formation of the described cold cathode is a helical multi-walled carbon nanotube (MWCNT). In another embodiment, the carbon nanotube (CNT) used in the formation of the describe cold cathode is a carbon nanotube filament or fiber, which is an assembly of carbon nanotubes (CNTs) generated by any fiber/filament extrusion process.

[0026] Carbon nanotubes can be functionalized to attain desired properties that can be used in a wide variety of applications. The two main methods of carbon nanotube functionalization are covalent and non-covalent modifications. Because of their hydrophobic nature, carbon nanotubes tend to agglomerate hindering their dispersion in solvents or viscous polymer melts. The resulting nanotube bundles or aggregates reduce the mechanical performance of the final composite. Thus, the choice of solvent can be important. Any solvent in which the carbon nanotubes can be dissolved and/or dispersed with adequate colloidal stability, and can be removed easily by thermal evaporation that are generally used in industrial coating processes, such as ethanol, methanol, acetone, methyl ethyl ketone, ethyl acetate, may be used. The following link provides a detailed summary of solvents that are useful in industrial coatings. (https://coatings.specialchem.com/selection-guide/select-solvents-for-industrial-coatings). In one embodiment, methylene chloride is used as the solvent.

[0027] By addition of a nano-filler to fill voids, the mechanical, thermal, and electronic properties of a CNT composition can be improved. Filler materials can be any inorganic, conductive and/or semi-conductive particles that allow for a coating formulation of adequate viscosity to be generated. In one embodiment, an adequate viscosity can be in the range of 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, etc. In one embodiment, graphite nanoparticles are used as a filler. In one embodiment, the carbon nanotubes and filler are combined at a ratio of about 1:10 to about 1:100. In one embodiment, the carbon nanotubes and filler are combined at a ratio of about 1:30 to about 1:50.

[0028] In addition to the above described filler, a carbonizable polymer, or its monomeric and/or oligomeric version thereof, is also used, which provides the appropriate colloidal stability to the composition to enable coating and/or extrusion of the physical structure, and the eventual structural integrity to the resulting solid, upon thermal processing. In this regard, the carbonizable polymer, and/or its monomeric and/or its oligomeric version thereof, may be regarded as a precursor material, for the final constituent in the processed solid. In one embodiment, the precursor polymer is formed from furfuryl alcohol under suitable conditions. Others useful carbonizable polymers include non-graphitizable polymers such as the phenol-formaldehyde-based polymers, which are synthetic polymers obtained by the reaction of phenol or substituted phenol 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 description of carbonizable and graphitizable polymers, see Sharma, S. (2018). Glassy Carbon: A Promising Material for Micro- and Nanomanufacturing. Materials, 11(10), 1857; a description of polymer-derived carbon is provided in the above reference (section 4 of the article- . . . graphitizing carbons are those polymers-derived carbons that can potentially be converted into polycrystalline graphite by heat treatment, . . . . . . . . . non-graphitizing carbon with a high purity that has experience at least some coking during pyrolysis is known as glassy carbon.)). In one embodiment, furfuryl alcohol may be used, and thermally polymerized with or without the use of additional catalysts to generate polyfurfuryl alcohol or pre-polymerized furfuryl alcohol may be used. The monomeric or the oligomeric or the polymeric versions of the carbonizable polymer is mixed with the CNT and the filler(s), and coated and/or extruded to form a one dimensional (fiber/filament) or a two dimensional (sheet) entity. In either case, the coated and/or the extruded entity may be formed on a solid support, which is subsequently removed upon the evaporation of the solvent used in the formulation, prior to subsequent thermal treatment.

[0029] After polymerization or use of a pre-formed polymer, the composition is then subjected to a thermal treatment step. After coating or extruding the formulation and drying it to remove the solvent, the coated/extruded material is subjected to a thermal treatment step to carbonize the polymer in the coated/extruded material. The solid support on which the formulation is coated and/or the extruded entity is formed is removed either immediately after the removal of the solvent or after a specific sequence of thermal treatments, after which the free standing coated/extruded entity is subjected to further thermal treatment. In one embodiment, the thermal treatment occurs in a vacuum or substantially devoid of oxygen. In one embodiment, the thermal treatment occurs in the presence of an inert gas, such as argon or nitrogen gas. In one embodiment, the thermal treatment comprises subjecting the composition to heat from about 600 C. to about 1300 C. In one embodiment, the thermal treatment comprises subjecting the composition to heat from about 900 C. to about 1000 C. In various embodiments, the rate of heating ranges from about 0.1 C. per minute to about 5 C. per minute. In various embodiments, the dwell time at the elevated temperature ranges from about 30 minutes to about 3000 minutes.

[0030] To evaluate the physical integrity of the cathodes generated according to the described formulation and procedure, in comparison to what is disclosed in the literature, e.g., U.S. Pat. No. 10,049,847 B2) and references 7, 8, we used a bulk indentation testing procedure. Bulk indentation was used to provide a measure of the hardness of each sample type. In these experiments, a 90 degree conical steel probe is lowered at a fixed velocity until contact is made with a flat piece of each sample (See FIG. 3). Once contact is made, the probe continues to push downward at a constant velocity of 50 mm/minute until a maximum load of 500 grams is reached. The probe is then retracted from the sample, and the force and displacement data is collected and plotted, to determine the displacement at the maximum load point.

[0031] Our inventive formulation and process resulted in a cathode that exhibited half the bulk indentation (for the constant 500 gram load) (FIG. 4, right bar) as compared to what would be seen for a literature formulation and process (FIG. 4, left bar). Under routine handling conditions, the cathode generated using the literature formulation and process cracks and falls apart, while the cathode from the inventive formulation and process, maintains it physical integrity. The field emission performance of the cathodes from the literature and our invention is essentially similar, when evaluated by the Fowler Nordheim theory (9) (FIG. 5). However, as described above, cathodes from the literature cracks and falls apart under routine handling conditions.