Haworth, Michael D. (14 Calle Lomita, Sandia Park, NM, 87047, US)
Shiffler Jr., Donald A. (6012 Lola Drive, N.E. Albuquerque, NM, 87109, US)
Haworth, Michael D. (14 Calle Lomita, Sandia Park, NM, 87047, US)
| 1. | An anode coating for reducing the production of secondary electrons, characterized by: a layer comprised of pyrocarbon; and the layer forming a coating by being attached to an underlying substrate. |
| 2. | The anode coating of Claim 1 wherein; the substrate is comprised of carbon; and the substrate is part of an anode. |
| 3. | The coating of Claim 2 wherein the coating is comprised of a plurality of the layers, with only one of the layers being attached to the substrate. |
| 4. | The anode coating of claim 2 or 3 wherein: the anode has an electron impact area; and the coating covers only the electron impact area. |
| 5. | The anode coating of Claim 4 wherein : the anode is comprised of a carbon film affixed to a metal anode; and the substrate is comprised of the carbon film. |
| 6. | The anode coating of Claim 4 wherein the anode is comprised of carbon. |
| 7. | A method of coating an electron impact surface of an anode with pyrocarbon, characterized by: coating the electron impact surface with a carbonizable resin; carbonizing the resin to form a char; depositing carbon on the char to form a coating of pyrocarbon; and removing any residual water from the coating. |
| 8. | A coating method as recited in Claim 7 wherein: the carbonizable resin has volatile components; and carbonizing the resin includes heating the anode to a temperature sufficient to decompose the resin and release the volatile components, leaving the char as a porous residue. |
| 9. | A coating method as recited in Claim 7 wherein the carbonizing step includes heating the anode to a temperature of at least 700 °C in a nonoxidizing atmosphere. |
| 10. | A coating method as recited in Claim 9 wherein the heating step includes baking the anode in an oven. |
| 11. | A coating method as recited in Claim 7 or 8 or 9 wherein the carbonizable resin is a phenolic. |
| 12. | A coating method as recited in Claim 7 or 8 or 9 wherein the depositing step includes pyrolysis through chemical vapor deposition. |
| 13. | A coating method as recited in Claim 12 wherein the pyrolysis step includes heating the anode to at least 1000 °C. |
| 14. | A coating method as recited in Claim 7 or 8 or 9 wherein the depositing step includes directing a flow of hydrocarbon gas over the electron impact surface while heating the electron impact surface to at least 1000 °C. |
| 15. | A coating method as recited in Claim 12 wherein the removing water step includes baking the anode in a vacuum oven. |
| 16. | A coating method as recited in Claim 7 or 8 or 9 wherein the removing water step includes heating the anode to at least 100 °C in a vacuum. |
TECHNICAL FIELD The invention is in the field of vacuum tubes, and more particularly relates to a pyrocarbon coating for an anode or collector 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 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 pyrocarbon anode coating 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.
To form the anode coating of the present invention, an anode having a carbon surface is first coated with a carbonizable resin. Next, the anode is baked in a non- oxidizing atmosphere to carbonize the resin and leave a porous"char"residue. Carbon is then deposited on the char by pyrolysis through chemical vapor deposition, creating a non-porous, rigid surface layer of pyrocarbon that is electrically conductive. Lastly, the anode is heated in a vacuum oven to evaporate any residual water from the pyrocarbon.
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 pyrocarbon 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 425 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 425 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, causing 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 pyrocarbon 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 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.
First, electron impact surface 19 is 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, spraying, or dipping anode 13 in a resin bath. Anode 13 is then baked to a temperature of at least 700 °C in a non-oxidizing atmosphere,. decomposing the resin and releasing its volatile components. A porous carbon"char" residue is left on electron impact surface 19.
Next, anode 13 is heated to a temperature of at least 1000 °C while a low-pressure <BR> <BR> hydrocarbon gas, e. g. , methane, is flowed onto and over electron impact surface 19. In a<BR> process known as chemical vapor deposition, or"CVD, "the hydrocarbon gas decomposes and deposits a layer of carbon on surface 19 while releasing hydrogen gas.
The deposited carbon infiltrates into the porous char, creating a non-porous, rigid surface <BR> <BR> layer of pyrocarbon, i. e. , coating 21. The foregoing step is known as pyrolysis through CVD.
The duration of pyrolysis through CVD is proportional to the size of the area to be coated, i. e., the larger area of electron impact surface 19, the greater the necessary time; as well as the thickness of the applied coating. The duration is inversely proportional to the gas flow rate.
The thickness of 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 pyrocarbon coating 21 is the gap between anode 13 and cathode 15.
After the completion of pyrolysis thorough CVD, anode 13 is heated to an elevated <BR> <BR> temperature, e. g. , 100 °C, in a vacuum oven until any residual water in pyrocarbon coating 21 has evaporated. Pyrocarbon coating 21 has sufficient electrical conductivity to conduct the incident primary electrons to a circuit electrically connected to anode 13.
The desired thickness of pyrocarbon coating 21 may be obtained through one sequence of the aforementioned steps, or by accretion through the sequential creation of multiple layers. Where the thickness is composed of a plurality of layers, each layer is produced according to the aforementioned steps, except that the evaporation of residual water by means of heating anode 13 in a vacuum oven may be performed after all of the layers have been formed instead of after the formation of each individual layer.
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 pyrocarbon coating 21. More particularly, Figure 3 is an enlargement of section 3 of assembly 11 in Figure 2, where pyrocarbon coating 21 has been applied to electron impact surface 19. The potential difference between the cathode 15 and anode 13 is 425 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 pyrocarbon coating 21. The potential difference between the cathode 15 and anode 13 remains at 425 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 pyrocarbon 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 pyrocarbon 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 pyrocarbon coating of the present invention removes these performance constraints.
Anodes realizing the advantages attendant to having the pyrocarbon coating of the present invention have applications ranging from cathode ray tubes to microwave tubes included in radar, communications, and cooking devices. In addition, the pyrocarbon coating of the present invention can increase the efficiency of depressed collectors used for energy recovery in microwave and RF tubes.
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