US2175696A

US2175696A - Electron emitter

Info

Publication number: US2175696A
Application number: US145050A
Authority: US
Inventors: Ernest A Lederer
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-05-27
Filing date: 1937-05-27
Publication date: 1939-10-10
Anticipated expiration: 1956-10-10

Description

Oct 10, 1939. E, 2,175,696

ELECTRON EMITTER Filed May 27, 1937 INVENTOR ER/VEST ,4. LEDERER w c/m'r ATTORNEY Patented Oct. 10, 1939 UNITED STATES PATENT OFFICE ELECTRON EMITTER Delaware Application May 27, 1937, Serial No. 145,050

3 Claims.

My invention relates to electron emitters, particularly thermionic or secondary electron emitters useful in evacuated or gas filled discharge devices and having long life and low gas content.

5 The conventional thermionic cathode comprises 8. directly or indirectly heated core of nickel coated with barium and strontium compoimds, such as carbonates, which can be decomposed to the oxides by heat. The carbonate coated cathl ode is mounted with other electrodes in an envelope of a discharge device, and while vacuum pumps are exhausting the air the metal parts of the device are heated to free them of gas and decompose the carbonates on the cathode. Gases,

15 including large quantities of gaseous by-products of the carbonate decomposition, are removed, a getter is flashed, the cathode is activated by heating it at a high temperature to free some active metal. Unfortunately, high temperature opera- 20 tion after seal-off liberates considerable occluded gas from the cathode core and its coating and often evolves carbonaceous gases from undecomposed coating material which, when not absorbed by the getter, return to the cathode and 25 re-combine with and poison the coating. Ac-

cordingly, the amount'of active metal that can be produced in the coating and the useful life of the cathode is seriously limited by the gas content of the cathode.

30 An object of this invention is to increase the useful life of an electron emitter.

Another object of this invention is to reduce the gas content of an electron emitter and increase the supply of active electron emissive 35 metal.

The novel electron emitter of my invention comprises a base or core of refractory metal, such as nickel or any of the core metals commonly used in cathodes, on which there is a thin ad- 40 herent layer or coating of refractory metal oxide,

such as magnesium oxide. The oxide coating according to my invention, is impregnated preferably throughout the body of the oxide, and is enriched particularly at its surface, with an active 45 electron emitting metal such as barium. The coating consists of magnesium oxide particles or crystals of such a size and shape that they are capable of retaining large quantities of alkaline earth metals such as metallic barium, which does 50 not readily evaporate from the matrix of magnesium oxide. The active metal of a monotomic layer of barium on the surface of the oxide is supplied with the active metal from within the body and materially extends the useful life of the 55 cathode.

The invention will be more clearly understood from the following detailed description in connection with the accompanying drawing in which Figure 1 illustrates an enlarged sectional view of my improved electron emitter, and Figure 2 illus- 5 trates the adaptation of my improved emitter to an indirectly heated cathode.

Referring to the drawing, a base or core l of a refractory metal, such as nickel, is coated on its surface with a thin adherent layer 2 of a metal oxide, such as magnesium oxide. The magnesium oxide may be deposited on the core metal in a number of ways. Finely ground commercially pure magnesium oxide mixed with water or an organic binder, such as nitrocellulose, may be painted or sprayed upon the core and baked. A mixture of about five grams of powdered magnesium oxide in 100 c. c. of water, which incidentally produces some magnesium hydroxide, sprayed on nickel to a thickness of to 1 mil and dried at room or slightly elevated temperature for 5 minutes has been found to produce a uniform adherent coating of oxide, white in color, and having low heat radiating properties.

The magnesium oxide on the core, according to my invention, is impregnated with a good electron emissive metal 3, preferably barium. The impregnation of the magnesium oxide may conveniently be accomplished before its application to the core by mixing with the oxide coating material a nitrogenous compound of barium, such as barium azide (BaNe). 1'7 grams of BaNs and 5 grams of MgO mixed in 100 c. c. of water may be painted or sprayed upon the core and dried at room or slightly elevated temperature. If the magnesium oxide is first applied to the core the coating in the amorphous state may be soaked in a saturated solution of barium azide. The dried coating is stable in air and is particularly resistant to contamination during handling. Another way of impregnating the magnesium coating with barium is byfirst spraying or coating the core with a very thin, barely visible, coating of barium azide and then coating with a one-half to one mil layer of magnesium oxide. 5

The core shown as the sleeve 4 of a cathode indirectly heated with an insulated filament 5 in Figure 2 may with its magnesium oxide-barium azide coating be mounted with cooperating electrodes in a tube envelope in the usual way. As distinguished from a tube with the conventional barium strontium carbonate coated cathode, in which the carbonates are broken down at a cathode temperature of about 1250 C. and the decomposition gases are removed by the exhaust pumps, a tube containing my improved cathode may be degassed and sealed of! without heating the cathode above the bulb temperature. 6 milligrams of barium strontium carbonate on the conventional cathode used in tubes commercially known as type 6K7 evolves during its decomposition on exhaust about 1200 liter microns of carbon dioxide (CO1) and from. the resulting barium oxide less than 13 micrograms of barium metal may be produced. Even this small conversion of the barium oxide to barium is obtained only by heating the cathode to a temperature of about 1250 C. for a relatively long time. The 6K7 cathode, however, coated according to my invention with a layer of magnesium oxide V2 to 1 mil thick overlying a layer of as little as 40 micrograms of barium azide may be completely activated in a relatively short time at a temperature of 650 to 700 C. This improved cathode yields about 10 liter microns of nitrogen, or only one one hundred twentieth as much gas as the conventional carbonate coated cathode, yet produces 26 micrograms of barium, or more than 2 times as much barium as the carbonate cathode. The small amount of nitrogen gas liberated during activation of my improved cathode is easily absorbed by the conventional getter and materially simplifies the exhaust schedule of the tube.

If desired, the magnesium oxide coating may be impregnated with barium by incorporating the barium in the core metal. A core metal of nickel, for example, alloyed with barium will when heated supply quantities of barium to the oxide coating. It has been found that magnesium oxide will retain metallic barium within its body and on its surface at temperatures above the vaporization temperature of barium.

Barium fluoride (BaFz) may with barium azide (BaNs) be mixed with the magnesium oxide to promote the crystallization of the oxide and increase the conductivity of the coating.

Where the electrode constructed in accordance with my invention is to be used as a secondary electron emitter operating at low temperature, the amount of barium, particularly at the surface of the electrode, may if desired be greater than the amount used on a thermionic electron emitter. The film of barium upon the surface of the magnesium oxide may be thick enough to be visible so that when the electrode is used as a secondary emitter the surface which is bombarded by primary electrons is so rich in barium that large quantities of secondary electrons are emitted.

My improved electron emitter is characterized by the small amount of gas that is liberated during its activation and by the large amount of free metallic barium held in reserve in the body of the oxide coating. Tubes having cathodes coated with barium azide impregnated magnesium oxide and exhausted as described have been found to have a useful life as great as 1500 hours with only a gradual diminution in the emission after this period.

I claim:

1. The method of making an electron emitter with a metal core and an electron emitting layer comprising the steps of coating the core with magnesium oxide and barium azide, and heating the coated core in a non-oxidizing environment to solidify and adhere the magnesium. oxide to the core and to decompose the azide to metallic barium and disperse the barium through the oxide.

2. The method of making an electron emitter with a metal core and an electron emitting layer comprising the steps of coating the core with magnesium oxide and a nitrogenous compound of barium, solidifying the magnesium oxide to adhere it to the core and heating the mixture in an oxygen free environment to decompose the