US2308389A - Oxide coated cathode - Google Patents

Description

Patentedtlan. 12, 1943 OXIDE COATED CATHODE George F. Reyling, Rutherford, N. 5., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application November 22, 1941,

Serial No. 420,083

Claims.

My invention relates to cathodes, particularly to cathodes of the type having a heated core coated with an alkaline earth compound.

In the manufacture of commercial radio tubes having specified voltage and current ratings for the electrodes, there is the ever present and difficult problem of making the tubes with uniform ratings. Cathode emission, which must have a high and uniform level, is sensitive to slight changes in materials and manufacturing procedure and often, for no apparent reason, will suddenly drop below rated values.

An object of my invention is a cathode which has a high uniform level of electron emission which is stable and comparatively insensitive to manufacturing processes.

According to my invention a cathode core is polished to a high gloss so that its surface has a mirror-like finish. The polished core is coated in the usual way with alkaline earth compounds, is mounted in the usual way in its radio tube, and is activated. cathodes prepared in the factory according to my invention consistently have a uniform high level of electron emission and are not subject to sudden slumps in emission, which are characteristic of poisoned cathodes prepared in the usual way without polishing.

Electromechanical means is preferred for polishing the cathode cores. One electrolyte which will remove the outer layers of nickel or nickel alloy cores and leave a brightly polished surface is a mixed water-free solution of phosphoric and sulfuric acids. With the nickel cathode core connected to the positive terminal of a direct current potential in this solution, the surface of the nickel is removed by electrolytic action and its surface. in a few minutes, is mirror-like in appearance as distinguished from the etched or mat surface produced by ordinary acid etching.

Cathode sleeves for radio tubes having small envelopes, of the type commercially known as the acorn tube, such as No. 954, and shown in the United States patent to Salzberg 2.030,18'7, February 11, 1936, have been made with considerable success according to my invention. The cathode sleeve of the acorn tube is .025" in diameter and has a wall thickness of .002" and is made of commercial Grade A nickel having the following composition- Per cent Nickel (and cobalt) 99.15 Iron .20 Manganese .20 Copper .20 Silicon .05 All other metals .10

The nickel may, of course, contain cobalt in the small amounts usually obtained in refining nickel, less than one percent, or greater amounts of cobalt, up to several percent, usually used for stillening and hardening the nickel. The electrolyte with which particularly good results have been obtained contains from 50 to phosphoric acid, 1

the remainder being sulfuric acid. After attaching a nickel tab to the end of the sleeve it is degreased by boiling for five minutes in each of two baths of commercial cleaner known as Blakosolv." Seventy-five of these sleeves held in a metal clip and immersed in the phosphoric-sulfuric electrolyte are connected to the positive terminal of a voltage source, and a current of 15 amperes at about 15 volts is passed between the sleeves and a negative electrode in the electrolyte for about 2 minutes. The current density for this particular polishing bath and time of polishing is about 5 amperes per square inch, but since, in electrochemical reactions, the amount of deposition or removal of metal at an electrode is a function both of time and current density, these two factors may be varied at will. The sleeves are then rinsed in water containing a wetting agent, such as commercial- Nacconol. By boiling the sleeves in water containing 01% of this wetting agent, all of the acid is removed from within the sleeves and from cracks, scratches or crevices on the sleeves. An ammonia rinse is desirable if traces of copper are found deposited on the surface of the sleeve. They are then preferably boiled for two five-minute periods in distilled water, after which they may be rinsed in methanol and dried. Firing in commercial or line hydrogen for ten minutes at 600 C. is believed necessary to displace undesirable occluded gas in the metal and prepare the cathode sleeve for its coating. The cathode is finally coated to a depth of about .003 inch with a spray mixture of barium-stron-- tium carbonates. B masking the ends of the sleeves the sprayed coating may be accurately confined to the center section of the sleeves. After drying the coating in air at C. for a few 'minutes the cathode may be mounted with grids,

anodes and other electrodes in its envelope and, after the usual exhausting and cathode activating schedule, the envelope may be sealed off.

Cathodes prepared according to my invention have been found, in the 954 acorn type tubes, to have an average cathode emission of over 40 milliamperes at rated voltages, as compared to less than 35 milliamperes for a cathode having a core without a polished surface in the same tube when operated at the same voltages. The importance of my polished cathode becomes more significant when the ratio of good to defective tubes made in the factory is considered. The percent of tubes discarded because of emission below rated values with untreated cathodes has often run higher than 50% whereas the percent discarded tubes with cathodes prepared according to my invention is consistently less than 5%.

Cathode sleeves electropolished in a phosphoric-sulfuric acid bath appear under a microscope, having a magnification of about 14 diameters, to be entirely free of surface scratches, cracks, crevices and sharp fissures. While the diffused reflection is negligible and the specular reflection approaches 100%, the gloss cannot be measured in absolute values but may be merely compared to an arbitrary standard selected for one material, as explained in the Bureau of Standards research paper RP 958, Methods of Determining Gloss," January 1937. The surface I prefer and with which I have had good results has a reflectance approaching that of a conventional silvered plate glass mirror and is easily distinguished from the sleeve before treatment which, when viewed under a 14 power microscope, has sharp angular crevices, scratches and grooves. The roughened surface on nickel is produced even by drawing dies that appear to be smooth. Accordingly the term mirror finish" used throughout this specification and the appended claims is understood to mean a finish corresponding to the finish that is produced on the surface of nickel when electropolished in a 70% phosphoric-30% sulfuric acid solution for 2 minutes at 15 volts. There may of course be other solutions that will produce the same degree of high polish as well as mechanical methods, such as rouge or fine abrasives for producing the finish.

It is not apparent why nickel cores polished according to my invention should be more stable in factory production, nor why the electron emission should be uniform and high. Removal of metal from the surface of the core in the polishing bath probably leaves a surface that is free of oxides and all contaminations and leaves no recesses on the core surface which may catch and retain contaminating elements. Further there may be a slight difference in the interaction of the acid upon the nickel and upon the reducing agents, such as silicon and manganese, in the nickel. The molecules or particles of the reducing agents may thus be more effectively exposed at the surface of the core and brought into closer contact with the deposited coating of alkaline earth oxides. This action is evidenced by the fact that the interface between' the core and the coated oxides is of uniform light gray color, is firmly bonded to the core, and does not peel from the core as often as in the case of untreated drawn nickel wire or tubing. Another characteristic of my improved cathode is its lower heat radiating property. Since heat asoaaao will not readily radiate from a polished surface, the core may operate at a higher temperature for a given power input to the core. Further, the wall thickness of the core is estimated to be reduced between 10 and 20% when electropolished as described above. Thinner walls improve heat conductivity through the walls and proportionately reduce heat loss to the end supports.

Cathodes prepared according to my invention have a high uniform level of emission and are comparatively insensitive to ordinary contaminations encountered in handling in radio tube manufacture.

I claim:

1. The method of manufacturing an oxide coated cathode comprising drawing a nickel core, polishing said core to a bright mirror finish that is distinguished from the mat finish of an acid etched nickel surface or a die drawn nickel surface, washing said core and then coating the core with an alkaline earth compound.

2. The method of making an oxide coated cathode comprising drawing a nickel core to the desired size, polishing said core so that the surface of the core has a mirror-like appearance as distinguished from the roughened die marked surface of the core before polishing.

3. The method of making oxide coated cathodes for electron discharge devices comprising immersing a nickel body in an electrolyte containing about '10 to percent phosphoric acid and about 30 to 10 percent sulfuric acid, and passing current to said body through the electrolyte, said current having a density of about 5 amperes per square inch of said body, continuing said current for about two minutes, then rinsing said body and coating said body with an alkaline earth compound, and finally decomposing and activating the coating in vacuum.

4. A cathode comprising a nickel core characterized by a smooth mirror-like surface, when optically magnified 14 diameters, as distinguished from a surface with sharply defined scratches and crevices, and a coating of alkaline earth compounds on said surface.

5. A cathode comprising a nickel core coated with an adherent layer of alkaline earth compounds, the surface of the core having a smooth polished appearance, when optically magnified about 14 diameters, as distinguished from the appearance of the surface of seamless nickel tubing drawn in the usual way.

GEORGE F. REYLING.