US2173258A - Active metal compound for vacuum tubes - Google Patents

Description

Sept. w, W39 E. A. LEDERER ACTIVE METAL COMPOUND FOR VACUUM TUBES Filed Nov. 27, 1937 INVENTOR. ERNEST ,4. [EDERFR 'Ai IORNEY.

Patented Sept. 19, 1939 PATENT OFFICE ACTIVE METAL CO'MPOUND FOR VACUUM TUBES Ernest A. Lederer, Essex Fells, N. J., assignor, by

direct and mesne as signments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application November 2-1, 1937, Serial No. 176,825

8 Claims.

This invention relates to vacuum tubes, such as electron discharge devices and the like, and more particularly to the introduction into such devices of chemically active vaporizable substances or agents which, either during or after mechanical exhaust, are employed to reduce the pressure of the residual gases or to increase electron emission from cathodes.

It is common practice to use as a gas clean-up w agent, for example, in envelopes an active metal, such as an alkaline earth metal, introduced into the device in the form of a compound that is stable in air and is decomposed with liberation of metal vapor when heated. The commonly em- 5 ployed carbon compounds of the alkaline earth metals, such as barium, unfortunately produce considerable quantities of products of decomposition such as carbonaceous gases including CO and CO2 when the compound is heated. These un- 20 desired gases evolved in the presence of electrodes in the envelope to be exhausted contaminate the surfaces of the electrodes, load the pumps, and are difiicult to remove even with prolonged degassing and pumping. Further, the undesired 25 gases may so contaminate the emitting surface of the cathode, which is usually barium and barium oxide, as to impair electron emission.

Various compounds for gettering residual gases and for activating cathodes have been proposed 30 but none are known which do not produce considerable gas upon being heated.

An object of my invention is to prepare a stable source of alkaline earth metals free of objectionable amounts of gas.

Another object of my invention is to provide an improved getter of the active metal type which is stable in air and may be decomposed to liberate the active metal without producing undesired by-products of decompcsition.

Another object of my invention is to provide an improved method of gettering which is useful in all types of vacuum devices, particularly in vacuum tubes with oxide coated cathodes.

The characteristic features of myinvention 4.5 may be easily understood by considering the specific embodiments of my invention described in; the following specification and shown in the accompanying drawing in which- Figure 1 shows an electron discharge device 50 with a getter made in accordance with my inven tion;

Figure 2 is an enlarged detailed view of a getter made in accordance with my invention; and

Figure 3 shows one means for heating my im- 85 proved getter, and

Figure 4 shows my new compound coated on a filamentary cathode.

In the specific embodiment of my invention illustrated in Figure 1, which shows an application of my invention to getters for metal tubes, I have shown a metal envelope I enclosing an electrode assembly 2 comprising the conventional activated oxide coated cathode surrounded by a grid and an anode. The envelope is closed at the lower end by header 3 having exhaust tube 4!, through which the tube is exhausted mechanically by pumps. For cleaning up residual gases after mechanical evacuation of the envelope I use in one embodiment of my invention a getter 5 comprising a core, such as a U-shaped strip of refractory metal 6 filled, as best shown in Figure 2, with my improved compound '6, and located in any desired position within the envelope. The getter may conveniently be heated with electricity by connecting one end of the strip 6 to electrode lead-in conductor 8 and the other end of the strip to the metal envelope or to a lead-in conductor. To shield the elements in the tube from any active metal which may be thrown off from the getter, the open side of the grooved strip is faced outward toward the bottom or side of the envelope. After the pressure in the envelope has been reduced to a few microns by mechanical pumping, sufiicient current is passed through the strip for a time to drive off the required amount of getter vapor and clean up residual gases in the envelope. The tube may then be sealed and based in the conventional manner. If after aging, gas is found in the tube additional getter vapor may be liberated in the envelope merely by heating the strip with a current byapplying a voltage to the ends of the getter strip. If desired, the getter may be flashed and most of the barium liberated after seal-off.

My improved getter when mounted in a glass envelope may conveniently be heated by high frequency induction. The ends of strip 6 are joined to a wire loop 9, through which the induced currents may circulate, and the loop is supported from a stud embedded in the press of the conventional re-entrant stem. l0.

Certain alkaline earth metals, such as barium, have proven to be good electron emitters and efficient gas clean-up agents for electron discharge devices, but being unstable in air and dimcult to handle are usually prepared in a stable compound to protect the agents. I have proposed in my co-pending application, Serial No. 69,440, filed March '18, 1936, to prepare a barium strontium carbonate compound and coat the compound on a refractory reducing core. While the compound described in said co-pending application is readily decomposed and copious and easily controlled quantities of barium are liberated, the decomposition produces carbonaceous gases which require considerable time to be removed by the pumps. According to one embodiment of my invention, an oxide of an alkaline earth metal, such as barium, strontium, calcium and magnesium, is compounded with beryllium oxide to produce a. new compound, free of water of crystallization, which I believe is a berylliate compound of the alkaline earth, such as barium berylliate, which may be indicated in the molecular proportion BaBeOz. I have found that a coating of barium-beryllium compound when decomposed on a" tantalum core in an evacuated envelope produces only 1 to 4% of the amount of gas produced by an equivalent coating of barium strontium carbonate, and that at about 1200-1400 C. free metallic barium is copiously liberated, leaving a hard, stable residue which appears to be beryllium tantalate with low vapor pressure.

While the precise chemical reaction in my im-" proved getter is not known, it is believed that the reducing properties of the core serve to draw the oxygen from the coating compound and liberate the active metal of the coating. Zirconium, titanium, hafnium, vanadium, columbium, tungsten or molybdenum are refractory metals with the necessary reducing properties and may be substituted for tantalum.

The barium-beryllium compound of my invention has been prepared by dry ball-milling 216 grams of commercial barium carbonate and 66 grams of commercial beryllium oxide for two to three hours. The powder mixture is then fired in nickel boats for thirty to sixty minutes in hydrogen at 1065 C.- 20 C. At this temperature the barium carbonate is converted first to barium hydroxide by the action of the hydrogen and the barium hydroxide then reacts with the beryllium oxide forming a barium-oxide-beryllium oxide compound. To sinter the mass and increase its density it is then 'fired in a Denver fireclay crucible for 250 minutes at 1250 to 1300 C. The product is then ball-milled dry or otherwise pulverized to the desired fineness after cooling in air. A coating or spraying solution is then prepared of the powdered compound by mixing it with an organic binder, such as a solution of nitrocellulose in dimethyl phthalate.

The barium-berylliate, also, may conveniently be prepared by adding powdered beryllium oxide to molten barium hydroxide at a temperature of about 500 C. About 25 parts of BeO and about 171 parts of barium hydroxide, free of water of crystallization, are necessary for complete reaction. When solidified the resulting compound may be ground to fine powder in a porcelain ballmill, mixed in a paste with an organic binder, such as nitrocellulose or alcohol, and coated upon the core. Alternatively, my improved compound may be prepared by grinding together barium hydroxide, Ba(OI-I) 2+8H2O, and commercially pure beryllium oxide. Small portions of the mixed powder are dropped into a fireclay crucible at a temperature of about 500 or 600C. The resulting mass, although hard, is pulverized in a porcelain mortar and is then fused at 1400 to 1500 C. in a molybdenum boat in hydrogen. The resulting product has a density of about 4.4, is glass-like and opaque and may be pulverized again, mixed in paste with a binder, and sprayed or painted on the getter core.

My new barium-beryllium compound has several unique physical and chemical characteristics. With an excess of beryllium oxide, X-ray analysis indicates the existence of a definite chemical compound forming, I believe, a solid solution with the beryllium oxide. An excess of beryllium oxide added to the mixture raises the melting point and increases its stability in air. When prepared with more beryllium oxide than is necessary for complete reaction with the barium hydroxide, the compound is amorphous when examined microscopically and is, after melting, glass-like and opaque. When pulverized the powder does not' stick to a glass surface, indicating the compound to be non-hydroscopic, and under the microscope with 700 magnifications, indications of small crystal aggregates are discernible.

When 63 grams of barium hydroxide containing 8 molecules of water of crystallization and 5 grams of beryllium oxide, which proportions have been found necessary for complete reaction of the compound without an excess of either constituent, is prepared the resulting compound crystallizes from the molten mass in an elaborate pattern of hexagonal needles. This compound is difiicult to pulverize because the crystals are interwoven, the mass resembling in texture felt or asbestos. When pulverized the powder adheres on a humid day to a clean glass surface, indicating slight hygroscopicity. My new barium-beryllium compound is stable in air, little change in weight being observed after exposure for several days. If my barium-beryllium compound is suspended in water it hydrolyzes slowly forming barium and beryllium hydroxide, the beryllium hydroxide often being observed as gel, and the liquid becomes alkaline.

Good results have been obtained with my barium-beryllium compound as a getter by coating about 2.5 milligrams of the compound in a groove on a tantalum strip .0008 inch thick, .03? inch wide, and .65 inch'long. A current of about 2.8 to 3' amperes heats the strip to a temperature of about 1300 C. and liberates more than 50% of the barium present in the compound.

Although my new barium berylliate compound has been specifically described and its use indicated as a getter in which the compound is coated upon a reducing core, my new compound may, if desired, be reduced by mixing a powdered reducing agent, such as tantalum or silicon, with the compound and coating upon a refractory core or molded into pellets to be heated in the conventional manner. My new compound I may, if desired, be coated upon a filamentary cathode, such as shown at H in Figure 4, or on an indirectly heated cathode, and reduced by material in the cathode core or by material mixed in the compound, to produce an electron emissive layer rich in barium. I have found, for example, that barium-berylliate prepared in accordance with my invention and sprayed upon a tantalum or molybdenum fllament can be activated in vacuum at about 1400" C. without any appreciable evolution of gas. With an input of about six watts per square centimeter to a tantalum filament coated with my new compound, I have obtained an electron emission of about 20 milliamperes per square centimeter. The coating has a dark gray appearance and when activated may be exposed to the air and again activated in a vacuum by aging. Because. 01' the ruggedness of this type 76 of cathode it is particularly useful for indirectly heated cathodes in large transmitter tubes.

My improved compound is economical to manufacture, easy to prepare, mold or coat uponits core, is stable in air, and when heated in a vacuum liberates copious quantities of free metallic barium without the evolution of undesired products of decomposition.

I claim:

1. A source capable of evolving barium comprising barium oxide chemically compounded with beryllium oxide.

2. A source of alkaline earth metal, comprising a chemical compound of an alkaline earth metal, oxygen and beryllium.

3. A source of alkaline earth metal consisting of a reducing agent, and a chemical oxygenous compound of the alkaline earth metal and beryllium in intimate contact with said agent.

4. A source of barium free of products of decomposition consisting of oxygen chemically compounded with bariumand beryllium, and a refractory core capable of reducing the compound, the compound being coated upon said core. v

5. An electron emissive source of barium consisting of oxygen, barium and beryllium chemically combined, and a reducing agent in intimate contact with said compound.

6. A source of barium comprising a core of tantalum, and a coating on said core consisting of a reducible oxygen compound stable in air of barium oxide and beryllium oxide, said compound being non-hygroscopic and containing an excess of beryllium oxide.

7. A gettering device comprising a refractory core of a metal of the group consisting of tantalum, tungsten, molybdenum, columbium, titanium, zirconium, hafnium and vanadium coated with an oxygen compound stable in air of barium and beryllium.

8. A source of active metal comprising a reiractory core, a coating of an oxygen compound of barium and beryllium chemically combined on said core, and a reducing agent for said compound in contact with the coating.

ERNEST A. LEDERER.

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