US2208920A - Cathode for electron discharge devices - Google Patents

Description

Patented July 23, 1940 UNITE s'r'res CATHOD'E FOR ELECTRON DISCHARGE DEVICES Victor 0. Allen, Madison, N. J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application June 3, 1939,

Serial No. 277,325

8 Claims.

My invention relates to electron discharge devices, more particularly to improvements in thermionic electron emitting cathodes for such devices.

In conventional electron discharge devices with the filament type of thermionic cathode commercial use has been made of pure tungsten filaments, activated thoriated tungsten filaments, and oxide coated nickel filaments. The pure tungsten filal0 ment cathode is a very stable emitter and is efiicient but must be operated at very high temperatures to obtain ample emission. The activated thoriated tungsten cathode, such as described in United States patent to Langmuir 1,244,216, may

be operated at lower temperatures than a pure tungsten filament and is more efficient, but is easily poisoned by oxygen and other gases which adversely affect the emission, is not very stable when operated at high voltages, and requires special processing and seasoning. It has also been found that, when operated at ultra-high frequencies, for example above 15 or 16 megacycles, emission becomes erratic and there is sometimes a total loss of emission. In addition, the activated thori- 35 ated filament is apt to be brittle, and if operated at too high a temperature quickly deactivates and loses its ability to emit a sufiicient number of electrons to be useful. The oxide coated nickel cathode operates at a much lower temperature than either kind of tungsten cathode and is efficient, but cannot be satisfactorily operated in tubes in which very high platevoltages, such as 1000 volts, are used, and requires considerable processing and seasoning.

Thoriated molybdenum has also been thoroughly investigated as a filament cathode emitter, but has proved to be rather unstable, is quite sensitive to poisoning by gas within the tube, and must be operated at fairly high temperatures, be-

tween 1400 and 1600 C., to provide satisfactory emission. For these and other reasons thoriated molybdenum has not come into commercial use as a cathode filament material.

In the conventional high vacuum or gas tube 4.3 operating at high voltages, only filamentary type cathodes have heretofore been used because the oxide coatings of the indirectly heated cathodes quickly disintegrate at high voltages, and without these coatings no material was available which had sumcient emission unless raised to very high temperatures which cannot be practically generated with the conventional indirectly heated cathode construction.

To provide a ductile thermionic cathode which is a stable, efficient and long lived emitter, at desirable operating temperatures, and which can be satisfactorily used at high frequencies and high voltages as well as for indirectly heated cathodes for high voltage gas or vacuum tubes, as disclosed in United States Patent 2,144,250 issued to '5 me and Joseph Johnson on January 17, 1939 and assigned to the Radio Corporation of America, I have successfully used thoriated molybdenum as a base, upon which chromium has been plated. While this is the preferred embodiment of my in- 10 vention, it has also been found that, in place of molybdenum, other equivalent refractory metals, such as tungsten or tantalum, having high melting points, preferably 2400" C. or higher, may be used, as vwell as equivalents of thoria, such as 15 zirconium, uranium, cerium, titanium, vanadium, yttrium, or lanthanum.

However, in some cases under severe operating conditions when operated at high frequencies there is a tendency for the chromium in the cathodes of the above described type to vaporize and condense on the cold parts of the tube, such as the walls, press and spacers, causing leakage between elements at high voltages. This also results in the reduction of emission from the 25 cathode.

Hence it is the principal object of my invention to provide an improved cathode of the type described in which the above noted undesirable features are substantially reduced or eliminated. g

The thoriated molybdenum base may be prepared as a ductile metal in accordance with United States Patent 1,082,933 to W. D. Coolidge, or by squirting the finely divided material admixed with a-binder. For example, if molybde- 3 num is used I may add powdered nitrate of thoria to the powdered oxide of molybdenum before the reduction of the oxide of molybdenum, or I may add thoria to the oxide of molybdenum after reduction but before consolidation of the metal powder by sintering and mechanical working to the solid state as described in the said Coolidge patent. I have found in practice that a content of thoria of about 3% by weight of the molybdenum is satisfactory and that a filament of this thoriated metal may be used although the percentage of the thoria may be varied considerably. While thoria is preferred, equivalents of thoria, for example one of the group of metals consisting of zirconium, uranium, cerium, titanium, va- 50 nadium, yttrium and lanthanum may be used with molybdenum to form a base and the base then chromium plated in a conventional chromium platingbath, the essential constituents of which are'chromic acid and a sulphate. One ex- 5 ample of a satisfactory bath is 33 oz. chromic acid, containing at least 95% chromium oxide (CIOs) and not more than 0.2% S04, and .45 gram chemically pure sulphuric acid (H2804) in a gallon of water. The limits of thickness of the chromium plating may be wide, although a very thin plating appears to be entirely satisfactory. While the current density for plating the chromium on the thoriated metal base may vary between wide limits, for example, from a few hundredths of an ampere to over 1 ampere per square centimeter, for best results a current density of about 1 ampere per square centimeter for a period of one minute seems to produce the most satisfactory results. The resulting chromium plated thoriated wire may then be flashed in ammonia. The method used to flash the filament in ammonia comprises positioning within a bell jar a cathode filament to be treated. Liquid ammonia was allowed to expand into the gas chamber to expell the air within the jar. The filaments were then heated to temperatures of approximately 2000 K. for a period from thirty seconds to one minute. It is also desirable, though not necessary, to insure stability of emission, to season the filament by operating the electron discharge device containing the filament with normal voltages applied for approximately fifteen minutes in an oscillating circuit.

While I do not wish to be limited to any particular theory, it is believed that the chromium applied electrolytically when heated with the ammonia gas permitted the chromium to alloy with the molybdenum and tungsten more easily, the result being that the chromium could not be so easily vaporized and deposited on the walls and other. parts of the tube to cause the difficulties enumerated above.

A cathode made in accordance with my invention has a normal operating temperature several hundred degrees K. below the normal operating temperature of the conventional activated thoriated' tungsten cathode, and in the conventional type of tube operated at the customary plate voltage the same plate current can be obtained from our improved cathode at its normal operating temperature with only about one-half the energy per unit area that must be used to obtain the same plate current from the conventional activated thoriated tungsten cathode at its normal operating temperature.

When the cathodes are to be used in low'voltage tubes, for example under 1000 volts, to increase the electron emissivity any of my cathodes above described may be coated with the usual electron emitting coating of alkaline earth metal oxides, such as barium oxide and strontium oxide, which may be applied in a conventional manner by applying barium and strontium carbonates and converting them. into the oxides.

A cathode made in accordance with my invention is a. very efficient emitter. It will operate at a comparatively low temperature (1200 C.) and very high voltages up to approximately 3000 volts can be used on the plate of a tube using my cathode without the cathode suffering any ill effects due to ion bombardment. These last two characteristics make my invention particularly suitable for indirectly heated cathodes in high voltage gas or vacuum tubes.

Cathodes made according to my invention are very satisfactory in tubes used for generating very high frequencies at high voltages and while seasoning improves the initial stabiilty and oper ating characteristics it is not essential in order to provide a cathode which is a good emitter and operates satisfactorily. The resulting cathode is a very stable emitter and is not easily poisoned by gas. The cathode readily recovers its emitting characteristics even if accidentally operated temporarily at too high temperatures. Low voltage ionization will not destroy .the tube due to ion bombardment, and the cathode will recover its emission if temporarily impaired by being subjected to high voltages.

While I do not wish to be limited to any particular theory it is believed that the chromium acts as a catalyst to facilitate the rapid diiiusion of the electrom emitting metal, such as thorium, to the surface of the filament for producing copious emission.

While I have indicated the preferred embodiment of my invention of which I am now aware and have also indicated certain specific applications for which my invention may be employed, it will be apparent that my invention isby no means limited to the exact forms or uses described, but that many variations. may be made in the particular structure used, the purpose for which it is employed, and the treatment given to it, without departing from the scope of my invention as set forth in the appended claims.

What I claim. as new is:

1. The method of treating a cathode body comprising a metal of high melting point, thoria and electrolytically applied chromium, which consists of heating said body to a high temperature in ammonia gas.

2. The method of treating a cathode body comprising one of the group of the metals consisting of tungsten and molybdenum, and one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium and lanthanum, and electrolytically applied chromium, which consists of heating the body to a high temperature in ammonia gas.

3. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, and one of the group of metals consting of thorium, zirconium, uranium, cerium,titanium, vanadium, yttrium, and lanthanum, and electrolytically applied chromium, which consists of heating said body to substantially 2000 K. in ammonia gas.

4. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, and one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium, and lanthanum, and electrolytically applied chromium, which consists of heating said body to substantially 2000 K. in ammonia gas for thirty seconds.

5. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, and one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium, and lanthanum and electroyltically applied chromium, which consists of heating said body to substantially 2000 K. in ammonia gas for a period of time between thirty seconds and one minute.

6. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium, and lanthanum, and electrolytically applied chromium which comprises placing the body in a chamber,

admitting ammonia, gas to said chamber to expel air from said chamber and heating said body to a high temperature in the ammonia gas in said chamber.

7. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium, and lanthanum, and electrolytically applied chromium which comprises placing the body in a chamber, admitting ammonia gas to said chamber to expel air from said chamber, and heating said cathode body to substantially 2000 K. in

the ammonia gas for not less than thirty seconds.

8. The method of treating a cathode body comprising one of the group of metals consisting of tungsten and molybdenum, one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium, and lanthanum, and electrolytically applied chromium which comprises placing the body in a chamber, admitting ammonia gas to said chamber to expel air from said chamber, and heating 10 said cathode body in the ammonia gas to substantially 2000 K. for a period of time between thirty seconds and one minute.

VICTOR O. ALLEN.