US2185410A - Metal compositions - Google Patents

Description

Patented Jan. 2, 1940 METAL COMPOSITIONS Ernest A. Lederer, Essex Fells, N. 1., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation 01' Delaware No Drawing. Application September 30, 1938 Serial No. 232,530

My invention relates to metallic compositions containing a reducible alkaline earth compound, particularly to thermionically active metal bodies.

e Barium is considered one of the most thermionically active metals, and is widely used mainly as oxides although the metal is unstable in air, is volatile and is dimcult to handle in manufacture. Small amounts of the pure metal have been alloyed in nickel, and stable compounds have been coated on cores and reduced in part to its pure state in vacuum to produce electron emitters. In all cases, however, the available active metal is limited and the emissivity is uncertain.

An object of my invention is an alloy containing a stable yet reducible compound of an alkaline earth metal which has good electron emission and is easy to manufacture. According to my invention a stable gas-free oxygen compound of an alkaline earth metal is mixed with a powdered or granulated refractory base metal which will readily reduce the compound of the alkalineearth metal and with a second metal having a melting point lower than the melting point of either the compound or base metal. The compound is stable in air, low in gaseous produces of decomposition and has a high melting point. Onesuch compound comso prises barium-beryllium oxide, or barium berylliate BaBeOz, described in detail in my copending application, serial No. 176,825 filed November 27, 1937. One method of preparing the barium berylliate is to mix equal parts of powdered a 35 barium carbonate and beryllium oxide, mix wet in a ball mill for 24 hours, dry, fire in hydrogen for two hours at 1065 C., sinter in air for four hours at 1200" C., and pulverize for use. Other methods of preparation and characteristics are described in my above mentioned application.

The refractory base metal capable of reducing the barium beryllium compound below the melting point of the compound may be tungsten or molybdenum. Metal oi the iron group, preferably nickel, is employed according to my invention to alloy or physically combine with the reiractory base metal to hold the compound and its reducing agent in intimate admixture.

nickel may be replaced inwhole or in part by other metals of the iron group, such as iron or cobalt.

Good results have been obtained by mixing 100 grams of tungsten powder, 30 grams of nickel powder, and 6 grams of barium-berylliate,

and ball milling in'an indifferent organic sus- The pending medium such as diatol for about 48 to 62 hours. The clear diatol is decanted and the metal sludge is, while still moist, mixed with a nitrocellulose binder and formed into a paste and extruded as rods or wire. After drying for about 5 one-half hour in air, a bundle of such extruded wires is placed in a molybdenum boat and fired at 1100 to 1300 C. in hydrogen from 3 to 5 minutes. This removes the binder and sinters together the particles so that the pieces may be 10 conveniently handled. The longitudinal shrinkage of the pieces during this treatment is between 10 and 15%. The wires are then heated for about thirty seconds to one minute in hydrogen by sending an electric current through 15 the wires and increasing the current until a maximum temperature of 1400 to 1500 C. is obtained for a few seconds. Observed longitudinal shrinkage during this treatment is approximately 50% and the finished wire has a shiny metallic 20 appearance, can be bent to sharp right angles and can be flattened by cold rolling. The mixture with its binder may if desired be extruded into hollow seamless tubes, sintered, heat treated at 1400 to 1500 C; and drawn down to required 26 size.

To prepare the alloy in sheets from which tubular cathode sleeves can be made, the metal sludge obtained after ball milling in diatol is dried in air and the powder is pressed at 1200 to 1700 pound pressure per square inch into a flat ingot measuring for example l "x "x12". To increase the adherence of the metal particles a volatile binder, such as camphor, may be added. The finished ingot is then fired in hy- 80 of the alloy, the lower the nickel content the higher is the melting point. The finished heat treated ingot can then be rolled into strip or coil in the usual manner, it-being of advantage to as anneal the ingot between rolling operations.

Since barium-berylliate, with a barium ratio higher than 25%, slowly hydrolizes with water, the presence and distribution of this material in the alloy can be easily checked by moistening a M fracture of the metal with a dilute solution of alcoholic phenolphtalein and water. A red color indicates the presence of the barium-beryiliate and if the distribution of the berylliate is poor because of insuflicient mixing, red particles in as an indifferent background can be seen. When connected as a cathode in an evacuated diode,

I a green glow indicates barium vapor. The green glow indicating Ba is seen after a hot shot or at sustained high temperature operation of the cathode with plate voltage applied.

A wire or rod of my alloy can be tested for thermionic activity by mounting it adjacent an anode in an exhausted vessel, degassing the parts and flashing a barium getter. During this process the wire need not be heated and after sealing off the wire is operated for a short time with a current to heat it to a temperature of about 1400" C. while an anode voltage is, applied. During this treatment residual gases are removed from the wire, which gases are absorbed by the getter and within a few minutes the emission current increases to its full value. Upon lowering the wire heating current about 10% to its rated value, corresponding to a filament temperature of about 1000 C., the emission current drops at first but increases again slowly and reaches a maximum after 20 or 30 minutes.

By raising the wire temperature to about 1800 C. for a few seconds the electron emission is reduced to a very low value, the electron emission however recuperating rapidly and eventually reaching its former level after a few minutes operation. The proper operating temperature can be easily determined because it is that temperature at which the electron emission builds up Per cent Tungsten '13 Nickel 22 Barium berylliate -5 the proportions may be varied within wide limits without departing from the scope of my invention. Good cathodes may be made from a-composition containing Per cent Tungsten 94 to 44 Nickel 5 to 50 Barium berylliate 1 to 6 The amount of barium berylliate included is determined by the amount of free barium desired in the finished body; the ductility, gas content and workability being practically independent of the barium berylliate content. As the nickel content is increased the melting point is lowered and the finished body becomes softer and more ductile and with the higher nickel content electron emission is slightly reduced. With as much as 50% nickel the body apparently becomes less porous. As the nickel is reduced and the tungsten content increased the metal becomes less workableand rolling or swaging becoming more difiicult. Although the sintering temperature increases with the higher tungsten content, good cathodes may be made without loosing appreciable quantities of the compounds up to a temperature of about 1700 C An excess of BeO in the compound appears to raise the melting point of the compound and the temperature at which the metal body containing the compound may be raised without appreciable loss of the compound. Equal parts of BaO and BeO are preferred for most uses.

One advantage of barium berylliate, in my metal composition, is that it can be melted in a commercial indifferent atmosphere such as hydrogen and heated up to 1700 C. without undergoing chemical change or without reacting with the tungsten, but can be readily reduced by the tungsten in vacuum to liberate metallic barium at 1300 C. Stability of the barium berylliate, in such commercial reducing atmospheres as hydrogen, distinguishes this compound in the metal composition from barium oxide, 330, or barium nitrate, B8.(NO3)2, which readily reacts with the hydrogen to form barium hydroxide, B8.(HO)2. Barium berylliate is so stable that it can be fused in hydrogen in contact with tung sten or molybdenum without decomposing or reacting with the tungsten or molybdenum. For electrodes in gas filled rare gas lamps, one part of barium berylliate, powdered, mixed with three to .ten parts of tungsten or molybdenum, powdered, and pressed with a binding agent in a mold and sintered in hydrogen to just below the melting point of the barium berylliate at 1700" C. produces a metal that can be made into a cathode which will withstand severe ion bombardment and high operating temperatures without losing its barium or emissivity. The presence of the BeO apparently accounts for the high emission of my improved composition, which is approximately milliamperes per square centimeter compared to 10.0 milliamperes per square cen timeter of tungsten-nickel with 32.0 alone, with equal watts input to the filaments. It appears that the Eco residue in the filament body retards recrystallization in a manner similar to T1102 in tungsten, and facilitates emission.

While I am not certain of the exact structure of my new metal body, I believe that the nickel when fired above its melting point flows around the tungsten particles, firmly adhering the par- -ticles in a hard yet ductile mass. It is probable that there is superficial alloying between the tungsten and nickel at the interface of the tungsten particles although the melting temperature of tungsten is above 3000 C. Because of the high melting temperature of the barium berylliate compound, its particles remain chemically unchanged and are evenly disbursed throughout the body. When heated, the oxygen of the compound is taken up by the tungsten leaving free barium and sub-oxides of barium and beryllium.

The barium diffuses to the surface of the body and is highly active electronically. Tungsten reacts with the BaO constituent contained in the barium berylliate at a temperature below 1300" C., and the BeO probably remains unchanged chemically because only at temperatures near 3000 C. does it react with tungsten perceptibly.

In order to more tenaciously hold the barium coated cathodes,

metal at the surface of the core may also be provided by coating the cathode with an alkaline earth. Any of the alkaline earths crystalline in structure, chemically inert to the free barium with a melting point above the activation temperature of the core, and which will adhere to the core, may be used. Coating materials with the required properties for good emission include calcium oxide, CaO. barium oxide, BaO, strontium oxide, SrO, and magnesium oxide, MgO. Goodresults have been obtained by coating the cathode with dead burned lime, CaO. The dead burned lime is ball milled for a few hours, suspended in a thin nitrocellulose binder and painted or sprayed on the cathode. The coating thus prepared when dried is inert in air, requires no decomposition and does not contaminate the tube electrodes. The crystalline shape and size of a unit tube of CaO is very similar to that of BaO or SrO, the three belonging to the cubic system, and I have found that such crystalline bodies, provided that they are chemically inert toward barium, may serve as matrices. ,1 have found that emission may be increased to about 140 milliamperes per square centimeter at an operating temperature of 850 C.

In order to preserve the barium film on the filament or on and in the matrix deleterious gases must be prevented from reacting chemically with the barium. The best getter for preserv ng the barium on the cathode is barium elsewhere in the tube and I prefer to use a substantially pure barium getter such as the getter described in my above mentioned co-pending application.

In coating the cathode core with a porous matrix, the barium is dispersed and absorbed throughout the greater volume of the matrix and accordingly more barium is necessary. In such cases it may be desirable to increase the barium compound content to as much as 10% by weight of the core.

An advantage of an'electron emitter having a metallic surface is that it is arc resisting "and non-sputtering. It is well known that oxide for example, sputter when operated in a large potential gradient, and that oxide coated cathodes are destroyed easily by arcs when used in/gas discharge devices. The low gas content of the BaBeO2 in my improved alloy makes it a desirable cathode for electron discharge devices such as vacuum radio tubes and the ready source of barium makes it good 1 material for spark plug electrodes. The low gas content is shown by the fact that upon heating 2 mg. of the compound to a temperature of approximately 1250 C. only 1.5 to 2 liter microns of gas are obtained, whereas the same amount of barium carbonate or nitrate produces about 1000 liter microns of gas.

The influence of tantalum, Ta, in the metallic alloy has also been studied and while the Ta is a better reducing agent than tungsten no marked difierence in operation at the ordinary operating temperature has been observed. However filaments containing Ta can be operated at very high temperatures for long times without loosing their activity.

A metal body prepared in accordance with my invention may be extruded as thin strips of metal and used as filamentary cathodes in electron discharge devices, or may be rolled into sheets and made into indirectly heated cathode sleeves in such devices without the usual troublesome coating of emissive oxides on the cathode. My

improved alloy contains litle gas, is stable in air added quantity constituting 5 to 50% parts of electrical activity is imporlightning arrestors and may be used for the metal apparatus in which electron tant, such as the electrodes of and spark plugs.

I claim:

1. A 'metal body containing an oxygenous compound of an alkaline earth metal and ber'yllium dispersed through the body, the body composed essentially of a high melting point metal which reacts in vacuum with the compound with libera tion of the alkaline earth metal at a temperature below the melting point of the body, and a second metal to make the body ductile sintered with said refractory metal.

2. A coherent sintered metal body containing an oxygen compound of barium and beryllium and composed essentially of particles of a refractory metal which reacts with the constituents of the compound to liberate metallic barium and a ductile binding metal sintered with the refractory particles, said binding metal having a melting point higher than the reaction temperature of the compound and present in sufiicient quantity to make the body workable.

3. A thermionically active metal body consisting mainly of tungsten interspersed with particles of an oxygen compound of barium and beryllium.

4. A metal body consisting of tungsten alloyed with a ferrous metal, said body being interspersed with particles of barium berylliate, the barium berylliate content by weight of the body being between 1% and 6%.

5. A thermionically active body comprising an constituent consisting of 1 to 6% by weight of the core and comprising pound, stable in air, of lium oxide.

7. A metal body comprising 1 to 6% oxygen compound of barium and beryllium intimately admixed with a base metal selected from the group consisting of tungsten, tantalum and molybdenum, and a metal of lower melting point alloyed with the base metal.

8. A body composed of'an alloy comprising a base selected from the group comprising tungsten, molybdenum and tantalum, and constituting between 44 and 94% by weight of the body, and an added quantity of substance taken from the group comprising nickel, iron and cobalt, the of the body, and barium berylliate, constituting 1 to 6% of body, interspersed throughout the body.

9. A source of electronically active metal comprising a chemical compound of an alkaline earth metal of oxygen and beryllium interspersed in a metal body consisting of a reducing agent for said compound alloyed with nickel.

10. An electron source comprising a refractory metal body composed of tungsten and nickel intimately admixed with an oxygen compound, stable in air, of barium and beryllium.

11. The process of making an electronically active metal body comprising mixing powdered tungsten, nickel and an oxygen compound of barium and beryllium, pressing the mixture into a self-sustaining body and firing in hydrogen at a reducible oxygen combarium oxide and berylof an iii) a temperature below 1700 C. to sinter the body and then working the body into desired shapes. 12. The process 01' making a thermionically active metal body comprising mixing 44 to 94 grams of powdered tungsten, 5 to'50 grams of powdered nickel and 1 to 6 grams of barium berylliate, mixing in an indifl'erent suspending medium, drying and mixing with an organic binder to form a paste, extruding the paste into a wire and drying in air, heating the wire in hydrogen at 1100 to 1300 vC. and then sintering the particles of the constituents by heating a few seconds at a temperature of 1400 to 1500 C.

13. The method of making a thermionically active metal sheet comprising mixing powdered tungsten, nickel and barium berylliate, pressing the mixture with a binder at high pressures into an ingot, firing the ingot in hydrogen at about 1300 C. from 10 to 30 minutes, reflring the ingot in hydrogen to near the melting point of the ingot, and then rolling the ingot into sheets.

14. A cathode comprising a metal core interspersed with an oxygen. compound of barium and beryllium, the core consisting essentially of a metal which reacts with the compound to reduce the compound and liberate barium at a temperature below the melting point of the core, and a matrix coating on the core chemically inert to the barium and stable in air.

15. A cathode comprising a metal core consisting essentially of a metal which reacts with an oxygen compound 01' barium beryllium to reduce the compound and liberate free metallic barium at a temperature below the melting point oi the core, an oxygen compound of barium and beryllium dispersed throughout said core, a matrix capable of holding said metallic barium comprising an adherent alkaline earth coating on the core, the alkaline earth being crystalline in structure, chemically inert to the barium and having a melting point above the activating temperature of the core.

16. A body composed of an alloy comprising a base selected from the group comprising tungsten, molybdenum and tantalum and consisting between 44 and 94% by weight of the body and an added quantity of substance taken from the iron group comprising nickel, iron and cobalt, the added quantity constituting 5- to 50% of the body, an oxygen compound of barium and beryllium reducible by the metal of the first mentioned group and constituting 1 to 6% of the body interspersed throughout the body, and an adherent coating of calcium oxide on said body.

17. A cathode consisting of tungsten particles sintered with a metal of the iron group, said body being interspersed with particles of barium berylliate, the barium berylliate content oi! the body being by weight between 1 and 6% and an adherent alkaline earth coating on said body, the coating being an alkaline earth selected from the group consisting of calcium oxide, barium oxide, strontium oxide and magnesium oxide.

ERNEST A. LEDERER.