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 <BR> <BR> 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 <BR> <BR> 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 <BR> <BR> 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.