WO2003043046A1 - Carbon nanotube coated anode - Google Patents

Carbon nanotube coated anode Download PDF

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Publication number
WO2003043046A1
WO2003043046A1 PCT/US2002/033627 US0233627W WO03043046A1 WO 2003043046 A1 WO2003043046 A1 WO 2003043046A1 US 0233627 W US0233627 W US 0233627W WO 03043046 A1 WO03043046 A1 WO 03043046A1
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WO
WIPO (PCT)
Prior art keywords
anode
coating
nanotubes
comprised
anode coating
Prior art date
Application number
PCT/US2002/033627
Other languages
French (fr)
Inventor
Donald A. Shiffler, Jr.
Michael D. Haworth
Original Assignee
United States Of America As Represented By The Secretary Of The Air Force
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Filing date
Publication date
Application filed by United States Of America As Represented By The Secretary Of The Air Force filed Critical United States Of America As Represented By The Secretary Of The Air Force
Publication of WO2003043046A1 publication Critical patent/WO2003043046A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/36—Solid anodes; Solid auxiliary anodes for maintaining a discharge
    • H01J1/38—Solid anodes; Solid auxiliary anodes for maintaining a discharge characterised by the material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J23/00—Details of transit-time tubes of the types covered by group H01J25/00
    • H01J23/02—Electrodes; Magnetic control means; Screens
    • H01J23/027—Collectors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
    • H01J29/08—Electrodes intimately associated with a screen on or from which an image or pattern is formed, picked-up, converted or stored, e.g. backing-plates for storage tubes or collecting secondary electrons
    • H01J29/085—Anode plates, e.g. for screens of flat panel displays
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/14—Manufacture of electrodes or electrode systems of non-emitting electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00—Nanotechnology
    • Y10S977/70—Nanostructure
    • Y10S977/734—Fullerenes, i.e. graphene-based structures, such as nanohorns, nanococoons, nanoscrolls or fullerene-like structures, e.g. WS2 or MoS2 chalcogenide nanotubes, planar C3N4, etc.
    • Y10S977/742—Carbon nanotubes, CNTs
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00—Nanotechnology
    • Y10S977/84—Manufacture, treatment, or detection of nanostructure
    • Y10S977/842—Manufacture, treatment, or detection of nanostructure for carbon nanotubes or fullerenes
    • Y10S977/847—Surface modifications, e.g. functionalization, coating
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/30—Self-sustaining carbon mass or layer with impregnant or other layer

Definitions

  • the invention is in the field of vacuum tubes, and more particularly relates to a carbon nanotube coated anode for reducing secondary electron production and the concomitant formation of neutral gases and plasma.
  • Every vacuum electronics device ranging from a radio frequency tube to a microwave tube, has a region in which the cathode-emitted electrons impact after participating in the desired interactions.
  • This region is usually an anode or collector fabricated from stainless steel, oxygen-free high-conductivity copper or some other metal.
  • An electrical terminal having a positive polarity is hereinafter referred to as an anode, although collector is another term of art that is sometimes used to denote this element.
  • a metal is generally the optimum material for this purpose due to its relatively high electrical and thermal conductivity as well as superior vacuum performance. Occasionally the metal is coated with an insulating material such as titanium nitride.
  • a major drawback attendant to using these materials is the production of secondary electrons from the impingement thereon of electrons in the primary electron beam.
  • the impingement of a single primary electron can produce from several to hundreds of secondary electrons.
  • These secondary electrons then cause the formation of plasmas and neutral gases from the anode.
  • Neutral gases contribute to raising the pressure in a vacuum tube, thereby reducing the vacuum.
  • Plasmas not only increase the pressure inside the vacuum tube, but can also cause the tube to electrically short, thus limiting the duration of microwave or radio frequency output. Plasmas can also damage other components, e. g., the cathode or other metallic structures.
  • the secondary electrons can easily be re-accelerated back into the anode, causing a cascading process producing more secondary electrons. Accordingly, there is a need in the prior art for an anode coating that can significantly reduce the production of secondary electrons and, concomitantly, the formation of plasma and neutral gases.
  • the present invention addresses the aforementioned need in the prior art by providing a carbon nanotube coating for an anode that reduces the production of secondary electrons caused by the impingement on an anode of primary electrons from a primary electron beam emanating from a cathode. Accordingly, the present invention reduces the neutral gases and plasma otherwise produced by secondary electrons.
  • the anode surface, or boundary is comprised of carbon or a thin film of carbon that lies atop a metal substrate.
  • the anode surface is first coated with a carbonizable resin.
  • a material including carbon nanotubes is then applied, for example, by chemical vapor deposition or by vapor deposition.
  • the nanotubes may also be deposited onto the anode by suspending the nanotubes in the resin and then applying the resin-nanotube mixture.
  • a felt-like fabric woven from carbon nanotubes may be laid atop the resin. Regardless of whether the coating is comprised of nanotubes or the felt-like fabric, the longitudinal axes of a portion of the nanotubes will lie parallel to the anode surface.
  • the carbon nanotube coating is then pyro-bonded to the anode surface.
  • Pyrolysis through chemical vapor deposition is then used to infiltrate carbon into the coating and create a non-porous, smooth, rigid surface. Lastly, residual water is evaporated by heating the anode in a vacuum.
  • Figure 1 illustrates a tubular assembly comprised of a cylindrical anode and a concentric, cylindrical cathode, with the anode having an electron impact surface coated with the carbon nanotube coating of the present invention.
  • Figure 2 is an end view of the tubular anode and cathode assembly of
  • Figure 3 is an enlarged end view of a section of the tubular assembly shown in Figures 1 and 2, showing the results of a laboratory test having a 475 kV voltage potential between the anode and the cathode.
  • Figure 4 is the same enlarged end view of the section of the tubular assembly shown in Figure 3, and having the same 475 kV voltage potential between the anode and the cathode, but with an uncoated anode.
  • tubular assembly 11 is comprised of cylindrical anode
  • FIG. 13 shows an end view of assembly 11.
  • Anode 13 is fabricated from carbon or, alternatively, a metal substrate coated with a film of carbon.
  • Radial, supports 17 hold cathode 15 in static position relative to and concentrically within anode 13.
  • Cathode 15 emits primary electrons that accelerate towards anode 13.
  • the primary electrons impinge anode 13 on electron impact surface 19 with very high kinetic energy and, in the absence of carbon nanotube coating 21 , cause the production of secondary electrons that, in turn, lead to the formation of neutral gases and plasma.
  • electron impact surface 19 is coated with carbon nanotube coating 21 of the present invention, using a method of the present invention.
  • the method of the present invention will be disclosed only in conjunction with coating electron impact surface 19.
  • Carbon nanotubes are microscopic tube-shaped molecules having the structure of a graphite molecule rolled into a tube.
  • the carbon bonds are such that carbon electrons are tightly bound in the p-orbits in the transverse orbital plane and not readily dislodged.
  • a carbon nanotube has high conductivity along its longitudinal axis and low conductivity along its transverse, or radial, axis.
  • 21 is comprised of carbon nanotubes having a portion of their respective longitudinal axes lying parallel to the anode surface. This orientation reduces the number of secondary electrons produced by impinging primary electrons.
  • electron impact surface 19 To coat electron impact surface 19 with nanotubes, electron impact surface
  • a carbonizable resin e. g., phenolic
  • phenolic is any resin that decomposes when sufficiently heated and leaves only a residue of solid-state carbon, generally in the form of a powder.
  • the resin can be applied by painting or spraying, or by dipping anode 13 in a resin bath.
  • the carbon nanotubes comprising coating 21 are then deposited onto electron impact surface 19 by chemical vapor deposition or vapor deposition. Alternatively, the carbon nanotubes may be deposited onto electron impact surface 19 by suspending the nanotubes in the resin and then applying the resin-nanotube mixture.
  • Carbon nanotubes are commercially available in a powder form from Carbon Nanotechnologies, Inc., of Houston, Texas, U. S. A.
  • coating 21 is a felt-like fabric woven from carbon nanotubes.
  • the felt-like material is laid atop the resin.
  • Processes for fabricating fibers and ribbon-like strips from single-walled, carbon nanotubes are known in the art, e. g., Vigolo, Brigitte, et al., "Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes," Science, Vol. 290, pp. 1331-1334, (17 November 2000).
  • a source of this felt-like material is Dr. Otto Chou, Physics and Astronomy Department, University of North Carolina at Chapel Hill.
  • Coating 21 is then pyro-bonded to electron impact surface 19 by heating anode 13 to a temperature of over 700° C in a non-oxidizing atmosphere. The heat decomposes the carbonizable resin and releases its volatile components. A porous carbon "char" residue embedded with carbon nanotubes is left on electron impact surface 19. Pyrolysis through chemical vapor deposition is subsequently used to infiltrate carbon into the porous char, creating a smooth, non-porous, rigid surface. More particularly, anode 13 is heated to over 1000° C while a hydrocarbon gas, e. g., methane, is flowed at low pressure over electron impact surface 19. The hydrocarbon gas thermally decomposes, depositing carbon and releasing hydrogen. The length of the process is proportional to the size of the area to be coated, i. e., the area of electron impact surface 19, and the thickness of carbon nanotube coating 21 ; and inversely proportional to the gas flow rate.
  • a hydrocarbon gas e. g., me
  • the thickness of carbon nanotube coating 21 required to substantially reduce the production of secondary electrons is proportional to the voltage potential between anode 13 and cathode 15. This is because the kinetic energy of the primary electrons impinging impact surface 19 is proportional to the voltage potential, and the pyrocarbon coating thickness necessary to prevent the production of secondary electrons will vary in proportion to the kinetic energy of the impinging primary electrons.
  • the only constraint on the thickness of coating 21 is the gap between anode 13 and cathode 15.
  • anode 13 After completion of pyrolysis through chemical vapor deposition, anode 13 is heated to an elevated temperature, e. g., over 100° C, in a vacuum oven until any residual water in coating 21 has evaporated. Carbon nanotube coating 21 has sufficient electrical conductivity to conduct the incident primary electrons to a circuit electrically connected to anode 13.
  • Figures 3 and 4 are graphical representations of photographs taken during a laboratory test performed on assembly 11, where anode 13 was photographed both with and without carbon nanotube coating 21. More particularly, Figure 3 is an enlargement of section 3 of assembly 11 in Figure 2, where carbon nanotube coating 21 has been applied to electron impact surface 19. The potential difference between the cathode 15 and anode 13 is 475 kV. No plasma formation is evident. For comparison, Figure 4 also shows section 3 of assembly 11 , but with electron impact surface 19 not coated with carbon nanotube coating 21. The potential difference between the cathode 15 and anode 13 remains at 475 kV. Figure 4 shows the formation of plasma 23 adjacent electron impact surface 19 of anode 13. It is to be understood, of course, that the foregoing description relates only to embodiments of the invention, and that modifications to these embodiments may be made without departing from the spirit and scope of the invention as set forth in the claims.
  • the carbon nanotube coating of the present invention has several significant advantages over the metals and coatings of the prior art. It suppresses the production of secondary electrons in a high or low vacuum.
  • the method of application of the carbon nanotube coating readily lends itself to coating a complex range of shapes. Secondary electron production and, accordingly, neutral gas and plasma formation are greatly reduced, permitting microwave and radio frequency vacuum electronics to be run with higher efficiency because the pumping necessary to maintain their operational vacuum is lower. Many devices have been limited in peak power and pulse duration by the creation of plasma and neutral gas by secondary electrons.
  • the carbon nanotube coating of the present invention removes these performance constraints.
  • An anode realizing the advantages attendant to having the carbon nanotube coating of the present invention has applications ranging from cathode ray tubes to microwave tubes included in radar, communications, and cooking devices.
  • the carbon nanotube coating of the present invention can increase the efficiency of depressed collectors used for energy recovery in microwave and RF tubes.

Abstract

The electron impact surface (19) of an anode (13) is coated with a carbon nanotube coating (21) to reduce the production of secondary electrons and, concomitantly, to suppress the formation of neutral gases and plasma. A carbonizable resin is first applied to the electron impact surface (19), followed by a coating (21) comprised of carbon nanotubes. The coating (21) is pyro-bonded to the surface (19) by heating the anode (13) to over 700° C in a non-oxidizing atmosphere. Next, the anode (13) is heated to over 1000° C while a low-pressure hydrocarbon gas, e. g., methane, is followed over the carbon nanotube coating (21). The gas decomposes and creates a smooth, non-porous, rigid surface on the carbon nanotube coating (21). The anode (13) is then heated in a vacuum to evaporate any residual water in the carbon nanotube coating (21).

Description

CARBON NANOTUBE COATED ANODE
STATEMENT OF AMERICAN GOVERNMENT INTEREST In accordance with paragraph 1(a) of Executive Order 10096, the conditions under which this invention was made entitle the Government of the United States, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights.
TECHNICAL FIELD The invention is in the field of vacuum tubes, and more particularly relates to a carbon nanotube coated anode for reducing secondary electron production and the concomitant formation of neutral gases and plasma.
BACKGROUND ART Every vacuum electronics device, ranging from a radio frequency tube to a microwave tube, has a region in which the cathode-emitted electrons impact after participating in the desired interactions. This region is usually an anode or collector fabricated from stainless steel, oxygen-free high-conductivity copper or some other metal. (An electrical terminal having a positive polarity is hereinafter referred to as an anode, although collector is another term of art that is sometimes used to denote this element.) A metal is generally the optimum material for this purpose due to its relatively high electrical and thermal conductivity as well as superior vacuum performance. Occasionally the metal is coated with an insulating material such as titanium nitride. A major drawback attendant to using these materials is the production of secondary electrons from the impingement thereon of electrons in the primary electron beam. The impingement of a single primary electron can produce from several to hundreds of secondary electrons. These secondary electrons then cause the formation of plasmas and neutral gases from the anode. Neutral gases contribute to raising the pressure in a vacuum tube, thereby reducing the vacuum. Plasmas not only increase the pressure inside the vacuum tube, but can also cause the tube to electrically short, thus limiting the duration of microwave or radio frequency output. Plasmas can also damage other components, e. g., the cathode or other metallic structures. These problems are amplified when the anode is biased to allow energy recovery from the primary electron beam. In this case, the secondary electrons can easily be re-accelerated back into the anode, causing a cascading process producing more secondary electrons. Accordingly, there is a need in the prior art for an anode coating that can significantly reduce the production of secondary electrons and, concomitantly, the formation of plasma and neutral gases.
DISCLOSURE OF THE INVENTION The present invention addresses the aforementioned need in the prior art by providing a carbon nanotube coating for an anode that reduces the production of secondary electrons caused by the impingement on an anode of primary electrons from a primary electron beam emanating from a cathode. Accordingly, the present invention reduces the neutral gases and plasma otherwise produced by secondary electrons.
The anode surface, or boundary, is comprised of carbon or a thin film of carbon that lies atop a metal substrate. The anode surface is first coated with a carbonizable resin. A material including carbon nanotubes is then applied, for example, by chemical vapor deposition or by vapor deposition. The nanotubes may also be deposited onto the anode by suspending the nanotubes in the resin and then applying the resin-nanotube mixture. Alternatively, a felt-like fabric woven from carbon nanotubes may be laid atop the resin. Regardless of whether the coating is comprised of nanotubes or the felt-like fabric, the longitudinal axes of a portion of the nanotubes will lie parallel to the anode surface. The carbon nanotube coating is then pyro-bonded to the anode surface.
Pyrolysis through chemical vapor deposition is then used to infiltrate carbon into the coating and create a non-porous, smooth, rigid surface. Lastly, residual water is evaporated by heating the anode in a vacuum.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates a tubular assembly comprised of a cylindrical anode and a concentric, cylindrical cathode, with the anode having an electron impact surface coated with the carbon nanotube coating of the present invention. Figure 2 is an end view of the tubular anode and cathode assembly of
Figure 1.
Figure 3 is an enlarged end view of a section of the tubular assembly shown in Figures 1 and 2, showing the results of a laboratory test having a 475 kV voltage potential between the anode and the cathode. Figure 4 is the same enlarged end view of the section of the tubular assembly shown in Figure 3, and having the same 475 kV voltage potential between the anode and the cathode, but with an uncoated anode.
BEST MODE FOR CARRYING OUT THE INVENTION As shown in Figure 1 , tubular assembly 11 is comprised of cylindrical anode
13, cylindrical cathode 15 and connecting radial supports 17. Figure 2 shows an end view of assembly 11. Anode 13 is fabricated from carbon or, alternatively, a metal substrate coated with a film of carbon. Radial, supports 17 hold cathode 15 in static position relative to and concentrically within anode 13.
Cathode 15 emits primary electrons that accelerate towards anode 13. The primary electrons impinge anode 13 on electron impact surface 19 with very high kinetic energy and, in the absence of carbon nanotube coating 21 , cause the production of secondary electrons that, in turn, lead to the formation of neutral gases and plasma.
To reduce these deleterious effects, electron impact surface 19 is coated with carbon nanotube coating 21 of the present invention, using a method of the present invention. For the sake of brevity, the method of the present invention will be disclosed only in conjunction with coating electron impact surface 19. However, it is to be understood that if it is impractical to coat only surface 19 with coating 21 , then the entire anode 13 can be coated in accordance with the method of the present invention.
Carbon nanotubes are microscopic tube-shaped molecules having the structure of a graphite molecule rolled into a tube. The carbon bonds are such that carbon electrons are tightly bound in the p-orbits in the transverse orbital plane and not readily dislodged. As a result, a carbon nanotube has high conductivity along its longitudinal axis and low conductivity along its transverse, or radial, axis. Coating
21 is comprised of carbon nanotubes having a portion of their respective longitudinal axes lying parallel to the anode surface. This orientation reduces the number of secondary electrons produced by impinging primary electrons.
To coat electron impact surface 19 with nanotubes, electron impact surface
19 is first coated with a carbonizable resin. A carbonizable resin, e. g., phenolic, is any resin that decomposes when sufficiently heated and leaves only a residue of solid-state carbon, generally in the form of a powder. The resin can be applied by painting or spraying, or by dipping anode 13 in a resin bath.
The carbon nanotubes comprising coating 21 are then deposited onto electron impact surface 19 by chemical vapor deposition or vapor deposition. Alternatively, the carbon nanotubes may be deposited onto electron impact surface 19 by suspending the nanotubes in the resin and then applying the resin-nanotube mixture. Carbon nanotubes are commercially available in a powder form from Carbon Nanotechnologies, Inc., of Houston, Texas, U. S. A.
In another alternative, coating 21 is a felt-like fabric woven from carbon nanotubes. The felt-like material is laid atop the resin. Processes for fabricating fibers and ribbon-like strips from single-walled, carbon nanotubes are known in the art, e. g., Vigolo, Brigitte, et al., "Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes," Science, Vol. 290, pp. 1331-1334, (17 November 2000). A source of this felt-like material is Dr. Otto Chou, Physics and Astronomy Department, University of North Carolina at Chapel Hill. Coating 21 is then pyro-bonded to electron impact surface 19 by heating anode 13 to a temperature of over 700° C in a non-oxidizing atmosphere. The heat decomposes the carbonizable resin and releases its volatile components. A porous carbon "char" residue embedded with carbon nanotubes is left on electron impact surface 19. Pyrolysis through chemical vapor deposition is subsequently used to infiltrate carbon into the porous char, creating a smooth, non-porous, rigid surface. More particularly, anode 13 is heated to over 1000° C while a hydrocarbon gas, e. g., methane, is flowed at low pressure over electron impact surface 19. The hydrocarbon gas thermally decomposes, depositing carbon and releasing hydrogen. The length of the process is proportional to the size of the area to be coated, i. e., the area of electron impact surface 19, and the thickness of carbon nanotube coating 21 ; and inversely proportional to the gas flow rate.
The thickness of carbon nanotube coating 21 required to substantially reduce the production of secondary electrons is proportional to the voltage potential between anode 13 and cathode 15. This is because the kinetic energy of the primary electrons impinging impact surface 19 is proportional to the voltage potential, and the pyrocarbon coating thickness necessary to prevent the production of secondary electrons will vary in proportion to the kinetic energy of the impinging primary electrons. The only constraint on the thickness of coating 21 is the gap between anode 13 and cathode 15.
After completion of pyrolysis through chemical vapor deposition, anode 13 is heated to an elevated temperature, e. g., over 100° C, in a vacuum oven until any residual water in coating 21 has evaporated. Carbon nanotube coating 21 has sufficient electrical conductivity to conduct the incident primary electrons to a circuit electrically connected to anode 13.
A significant reduction in secondary electrons was measured for anode 13 having electron impact surface 19 coated with a carbon nanotube coating 21 having as little as five percent of the total length of the longitudinal axes, i. e., the collective longitudinal axis of the nanotubes, lying parallel to the respective local anode surfaces. At the same time, electrons were conducted away parallel to the anode surface. Plasmas that normally would form due to secondary electron emission at
80 keV electron energies did not form until the electron energy exceeded 475 keV.
Figures 3 and 4 are graphical representations of photographs taken during a laboratory test performed on assembly 11, where anode 13 was photographed both with and without carbon nanotube coating 21. More particularly, Figure 3 is an enlargement of section 3 of assembly 11 in Figure 2, where carbon nanotube coating 21 has been applied to electron impact surface 19. The potential difference between the cathode 15 and anode 13 is 475 kV. No plasma formation is evident. For comparison, Figure 4 also shows section 3 of assembly 11 , but with electron impact surface 19 not coated with carbon nanotube coating 21. The potential difference between the cathode 15 and anode 13 remains at 475 kV. Figure 4 shows the formation of plasma 23 adjacent electron impact surface 19 of anode 13. It is to be understood, of course, that the foregoing description relates only to embodiments of the invention, and that modifications to these embodiments may be made without departing from the spirit and scope of the invention as set forth in the claims.
INDUSTRIAL APPLICABILITY
The carbon nanotube coating of the present invention has several significant advantages over the metals and coatings of the prior art. It suppresses the production of secondary electrons in a high or low vacuum. The method of application of the carbon nanotube coating readily lends itself to coating a complex range of shapes. Secondary electron production and, accordingly, neutral gas and plasma formation are greatly reduced, permitting microwave and radio frequency vacuum electronics to be run with higher efficiency because the pumping necessary to maintain their operational vacuum is lower. Many devices have been limited in peak power and pulse duration by the creation of plasma and neutral gas by secondary electrons. The carbon nanotube coating of the present invention removes these performance constraints.
An anode realizing the advantages attendant to having the carbon nanotube coating of the present invention has applications ranging from cathode ray tubes to microwave tubes included in radar, communications, and cooking devices. In addition, the carbon nanotube coating of the present invention can increase the efficiency of depressed collectors used for energy recovery in microwave and RF tubes.

Claims

1. An anode coating for reducing the production of secondary electrons, characterized by: a plurality of carbon nanotubes; and the nanotubes being pyro-bonded to the anode.
2. An anode coating as defined in Claim 1 wherein: the nanotubes are woven into a nanotube fabric; and the fabric is pyro-bonded to the anode.
3. An anode coating as defined in Claim 1 wherein; each of the nanotubes has a longitudinal axis having a length; each of the nanotubes is comprised of sections, and each section lies proximate to a local surface of the anode; and a plurality of sections lie parallel to their respective local surfaces.
4. An anode coating as defined in Claim 3 wherein: the nanotubes have a collective longitudinal axis length equal to a sum of the lengths of the longitudinal axes; and at least 5% of the collective longitudinal axis length is comprised of sections lying parallel to their respective local surfaces.
5. , An anode coating as defined in Claim 1 , 2, 3 or 4 wherein: the anode is comprised of a carbon film affixed to a metal anode; and the nanotubes are pyro-bonded to the carbon film.
6. An anode coating as defined in Claim 1 , 2, 3 or 4 wherein the anode is comprised of carbon.
7. An anode coating as defined in Claim 1 , 2, 3 or 4 wherein: the anode includes an electron impact area; and the coating covers only the electron impact area.
8. An anode coating as defined in Claim 1 , 2, 3 or 4 wherein: the anode coating has a thickness; and the anode coating reduces the production of secondary electrons in proportion to the thickness.
9. An anode coating as defined in Claim 3 wherein: each of the nanotubes has a transverse axis; each of the nanotubes has a transverse electrical conductivity along the transverse axis and a longitudinal electrical conductivity along the longitudinal axis; and the longitudinal conductivity is greater than the transverse conductivity, whereby a majority of electrons being conducted by the coating are conducted in parallel with the local surfaces.
10. An anode coating for reducing the production of secondary electrons, characterized by: a felt-like fabric woven from a plurality of carbon nanotubes; and the fabric for being pyro-bonded to the anode.
11. An anode coating as defined in Claim 10 wherein: each of the nanotubes has a longitudinal axis having a length; each of the nanotubes is comprised of sections, and each section lies proximate to a local surface of the fabric; and a plurality of sections lie parallel to their respective local surfaces.
12. An anode coating as defined in Claim 11 wherein: the nanotubes have a collective longitudinal axis length equal to a sum of the lengths of the longitudinal axes; and at least 5% of the collective longitudinal axis length is comprised of sections lying parallel to their respective local surfaces.
13. A method of coating an anode with carbon nanotubes, characterized by: pyro-bonding a coating comprised of carbon nanotubes onto the anode; infiltrating the coating with carbon by pyrolysis through chemical vapor deposition; and removing any residual water from the coating.
14. An anode coating method as defined in Claim 13 further comprising the step of applying a mixture comprised of carbon nanotubes suspended in a carbonizable resin to the anode, before the pyro-bonding step.
15. An anode coating method as defined in Claim 13 further comprising the steps of: applying a carbonizable resin to the anode; and depositing cabon nanotubes onto the carbonizable resin before the pyro- bonding step.
16. An anode coating method as defined in Claim 15 wherein the depositing step is obtained through chemical vapor deposition.
17. An anode coating method as defined in Claim 15 wherein the depositing step is obtained through vapor deposition.
18. An anode coating method as defined in Claim 15 wherein the depositing step is comprised of placing a felt-like fabric woven from a plurality of carbon nanotubes, onto the carbonizable resin.
19. An anode coating method as defined in Claim 13, 14, 15 or 18 wherein the pyro-bonding step is comprised of heating the anode to a temperature of over 700° C in a non-oxidizing atmosphere.
20. An anode coating method as defined in Claim 13, 14, 15 or 18 wherein the pyro-bonding step is comprised of heating the anode in a non-oxidizing atmosphere to a temperature sufficient to decompose the carbonizable resin and release volatile components.
21. An anode coating method as defined in Claim 20 wherein the carbonizable resin is phenolic.
22. An anode coating method as defined in Claim 13, 14, 15 or 18 wherein the pyrolysis through chemical vapor deposition includes directing a flow of low pressure hydrocarbon gas over the coating while heating the anode to a temperature of over 1000° C.
23. An anode coating method as defined in Claim 20 wherein the water removing step includes evaporating the residual water by baking the anode in a vacuum oven.
24. An anode coating method as defined in Claim 22 wherein the water removing step includes heating the anode to at least 100° C in a vacuum.
PCT/US2002/033627 2001-11-13 2002-11-12 Carbon nanotube coated anode WO2003043046A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US09/683,047 US6645628B2 (en) 2001-11-13 2001-11-13 Carbon nanotube coated anode
US09/683,047 2001-11-13

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