Nonemissive electrode and method of manufacturing

Feb. 12, 1952 k. e. everett 2,584,994

NONEMISSIVE ELECTRODE AND METHOD OF MANUFACTURING

Filed Aug. 7, 1947

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GRID PRIMARY EMISSlON(#a)

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KENNETH E. EVERETT

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ATTORNEY

Patented Feb. 12, 19S2

2,584,994

UNITED STATES PATENT OFFICE

2,584,994

NONEMISSIVE ELECTRODE AND METHOD OF MANUFACTURING

Kenneth Edward Everett, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware ' Application August 7,1947, Serial No. 766,980.

In Great Britain March 15,1946

Section 1, Public Law 690, August 8,1946 Patent expires March 15,1966

Claims. (

This invention relates to Improved electrode and a method of manufacture thereof for electron discharge devices whereby electron emission from said electrodes is inhibited. The method is of particular application to the grids of high power tubes. In high power vacuum tubes such as transmitting valves it is common for the control grid to dissipate considerable power. Under such conditions difficulties have arisen in the past through electron emission from the surface of the : electrode. Such emission is objectionable in most valves, although the problem is more acute in those of the high power class in which it is intended that grid current shall flow. The electron emission arises from two sources—thermi- 1 onic or primary emission due to the heating of the electrode and secondary emission due to bombardment, ionic or electronic, of the atoms in the surface layer of the electrode.

It has been known for some time that primary ! emission may be reduced by applying to the grid -. coating of carbon, usually in the form of lamp black. More recently, it has been found that a coating of zirconium is very effective in preventing secondary emission, more so than carbon. A ‘ carbon coating however, is more effective in preventing primary emission than one of zirconium alone. Although both types of coating effect a considerable reduction in primary emission, the reduction so obtained is not as great as would 3 be desired. Since thermionic emission is largely a surface phenomenon it might be expected that no advantage would arise in applying superimposed coatings of carbon and zirconium. Applicant has found, however, that this is not the 3 case and that, particularly if a double coating be applied in the manner to be described, a double coating is, out of all proportion, more effective in reducing primary emission. For a better understanding of the present invention reference 4 may be had to the following description taken in connection with the accompanying drawing, in which.

Fig. 1 is a graph showing the emissivity of an electrode under various conditions of coatings 4 including that type by my invention,

Fig. 2 is a perspective view of a portion of an electrode partly in section, made in accordance with this invention.

In Fig. 1 of the attached drawing, curve A shows 5 the measured relationship between the primary emission from a particular grid electrode and the power dissipation therein. Curve B shows the improvement obtained when a similar grid was coated with zirconium and curve C for a third ⁰

1.313—107) similar electrode coated with lamp black, while curve p shows the result obtained with a double coating in accordance with the present invention. in all the cases, B, C and D, the weight of total coating was the same.

It should be noted that it is already known in the art to coat an electrode with carbon and then to paint on a zirconium coating for the purpose of removing residual gases from the evacuated valve by absorption of the gases by the highly oxidizable zirconium. Carbon is here used merely to bind the metallic coating to the electrode structure. For the purpose of reducing primary and secondary emission I have found it not only more effective, but a more convenient process to apply the carbon on top of the zirconium coating.

According to the present invention there is provided a method of manufacturing an electrode for an electron discharge device comprising coating the electrode with zirconium and then coating the coated electrode with carbon.

For application to the grid of high power valves, I have found it preferable to apply a coating of zirconium to the clean metal of the grid by means of electrophoresis, the process being continued until the grid is seen to be just covered uniformly with zirconium. The zirconium-coated grid is then sprayed with lamp black until it is seen that a uniform coating which just covers the previous coating has been obtained. In order to obtain a uniform product there may be selected any suitable air pressure applied to the spraying pistol together with a desired number and speed of passes of the spray over the article under treatment. Finally, the coated grid is fixed in vacuo at a very high temperature—about 1600° C. being suitable. Fig. 2 shows an electrode wire coated with zirconium and then with a stratified coating of carbon. Optimum results are obtained when the weights of zirconium, and carbon coating are in the ratio of about 8 to 1; ratios of this order are obtained by applying the coatings as above.

The action whereby my process enables such a reduction of primary emission to be obtained is not understood but it is thought that during heat treatment the surface is changed so that it is no longer carbon, or else that a semi-conducting 0 layer is formed between the two coatings of zirconium and carbon.