US2421984A - Material for gettering electron discharge devices - Google Patents

Description

June 10, 1947. E. N. BOBROW 2,421,984

MATERIAL FOR GETTERING ELECTRON DISCHARGE DEVICES Filed April 25. 1944 ventor Patented June 10, 1947 MATERIAL FOR GETTERING ELECTRON DISCHARGE DEVICES Edwin N. Bobrow, East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 25, 1944, Serial No. 532,650

4 Claims.

My invention relates to the introduction into evacuated or gas filled containers, such as electron discharge devices, of chemically active metals or clean-up agents, and more particularly to the introduction of alkaline earth metals, especially barium, into such devices and to an improved clean-up agent or getter for this purpose.

Barium is a desirable and useful clean-up agent or getter for electron discharge devices and similar tubes. Frequently a reducing agent is mixed with a barium compound, and when the mixture is heated a reaction occurs and barium metal is liberated as a vapor. One compound which gives a high yield of barium in a. readily controlled well directed pure barium flash is the barium berylliate, disclosed in U. S. Patent 2,173,258, Lederer, mixed with titanium powder as a reducing agent. In very humid weather such as frequently occurs in summer when the relative humidity may be greater than 60% this material is not as stable as desired for factory use as shown by a tendency to pufi up to several times normal size when exposed to moisture and then heated. Investigation had indicated that a high efllciency barium getter necessarily had poor resistance to moisture and that only rather unstable barium compounds would give high barium yields.

. Tests on the comparatively few stable barium compounds of high barium and low gas content, including barium titanate, which contains initially nearly 60% by weight of barium, made with titanium, tantalum, and zirconium as reducing agents showed either low or no yield of barium, or the vapor of some other metal. These tests indicate that none of the compounds of barium which are stable and moisture resistant would give a high barium yield.

The principal purpose of this invention is to provide an active high yield reaction barium getter which is very resistant to moisture and will give a high yield of barium when heated to the temperatures of about 1300 C. at which getters of the barium berylliate type are usually flashed.

In accordance with my invention the chemically active metal, such as barium, is introduced into the device in the form of a compound of the metal which is very stable and resistant to moisture, together with a reducing agent which by a reaction that does not evolve gas liberates the active metal in pure form and free from water vapor. The stable barium compound, preferably barium titanate, is brought into contact with metallic beryllium which serves as a reducing agent. The metallic beryllium may be a mass of the metal, for example a ribbon or strip, but for convenience both the barium titanate .and the metallic beryllium may be finely powdered and thoroughly mixed to form the improved getter material. This getter mixture may be supported in conventional ways in the device in a position where it may be heated at will to a temperature at which reduction of the barium compound occurs.

Beryllium as a reducing agent has given high yields of barium metal from very stable compounds of barium oxide with the acidic oxides of metals from the first sub-groups of Groups IV, V, and VI of the Mendeleeif periodic table, for example barium titanate, zirconate, molybdate, tungstate, vanadate, columbate, and tantalate. 01 these stable barium compounds the one which has been found preferable is barium titanate having a composition lBaO-lTiOz. The getter material with which good results have been obtained i a mixture of finely powdered or comminuted barium titanate lBaO- lTlOz and finely powdered or comminuted beryllium in the ratio of from 1 to 2 moles of beryllium to 1 mole of barium titanate, good results having been obtained in many cases with a mixture having a ratio of about 1 moles of beryllium to 1 mole of barium titanate, and in other cases a ratio of 2 moles of beryllium to 1 mole of the barium titanate, In the getter mixture containing one mole of barium titanate less than 1 mole of beryllium does not give a reduction complete enough for practical purposes, and more than 2 moles of beryllium is apt to result in some vaporization of beryllium at 1300 C. with some impairment of the getter action.

For getter requirements the highest ratio of barium oxide to titanium oxide consistent with stability is desirable. Tests have shown that if a dried powder of barium titanate lBaO-lTiOz is soaked in water, only about 0.1% of moisture is retained; if the compound has the composition 2BaO-1Ti0z, up to about 10% of moisture may be retained. Although either of these compounds may be used without encountering the pulling observed with barium berylliate in a very humid atmosphere, the greate stability of the 1BaO-1Ti02 barium titanate makes it the preferable barium compound.

Barium titanate for use in accordance with the invention may conveniently be prepared by mixing 990 grams of anhydrous barium carbon ate BaCOs preferably of a purity of 99.5% or better with 400 grams of pure anhydrous titanium oxide TiOz and firing the mixture in air at 1250" C. for from 4 to 6 hours. The fired mass is broken up into powder of about 20 mesh, then finely powdered by ball-milling for 6 to 7 hours and sifted through a 200 mesh sieve. The beryllium metal powder may be prepared from beryllium chips, preferably of a purity of 99.5% or better, cleaned and degreased in acetone, dried, and then ball-milled for 20 hours or more to comminute the beryllium chips into afine powder of beryllium particles which will pass through a 200 mesh screen. Good results have been obtained with a getter material prepared by mixing about 226 grams of the barium titanate with very stable barium 3 about 14 grams of the beryllium powder, thus obtaining a getter mixture of the approximate composition 1.6 moles beryllium to 1 mole of barium titanate, and in which the beryllium is about 6% by weight of the mixture, the balance being barium titanate. To this mixture a nitrocellulose binder may be added to facilitate the placing of the getter material in the getter holder.

The getter material of the invention may be used in the conventional way, and may be heated to the flashing temperature of about 1300 C. either electrically or by induction heating, as shown in U. S. Patent 2,173,258, Lederer, relating to barium berylliate getters.

In some cases the getter material made in accordance with this invention does not adhere as well to the getter trough or container as some other getters containing barium compounds. If this lack of adherence is objectionable, the incorporation in the barium compound of a small amount, preferably about 2%, of lithium oxide will improve the adherence of the getter material wtihout deleteriously affecting the evolution of the barium and its action as a getter.

Beryllium metal has desirable properties as a reducing agent for stable moisture resistant barium or other alkaline earth compounds. The strong afflnity of beryllium for oxygen does not appear until high temperatures are reached, hence it is stable at room temperature. It is less volatile than barium and the other alkali and alkaline earth metals, and has little or no tendency to volatilize before the stable barium compound is reduced.

Reducing agents, such as tantalum, titanium, zirconium,.chromium, and aluminum, which have been used to liberate barium from its compounds, beyond the second group in the periodic table and which form oxides much more acidic than barium oxide. Apparently the acidic oxides of these reducing agents combine strongly with the basic barium oxide to form compounds which are difficult to reduce and which retain a considerable part of the barium, so that the yield of barium is not as high as wouldbe expected.

The effectiveness of beryllium as a reducing agent for stable barium compounds appears to be due to the beryllium oxide being more basic than the oxides of other reducing agents beyond the second group in the periodic table, such as titanium, zirconium, and tantalum. Apparently beryllium oxide is sufliciently basic to combine readily with the acidic oxide TiOZ'in the barium compound lBaO-lTiOz, thus making more barium available than would be obtained if the oxide formed from the reducing agent did not combine readily with the acidic oxide of the barium compound. It is believed that the acidic oxide of titanium does not hold back barium oxide to any appreciable extent because the beryllium oxide acting as a basic oxide relative to the titanium oxide holds the titanium oxide as berylluim titanates and titanites. Whatever the reaction may be, I have found that when themixture of beryllium and barium titanate is heated to a reaction temperature. usually about 1300 C., a reaction does occur which liberates a larger percentage of the barium than has heretofore been obtainable from these stable compounds of barium and which produces a very stable gas free reaction product consisting essentially of an oxygen compound of beryllium and titanium.

The principles involved in the use of beryllium for reduction of stable barium compounds such as shown in U.

4 may be applied to obtaining a high yield of other alkaline earth metals and of alkali metals from stable compounds such as lithium titanate which is stable and produces an eflective getter reac tion with beryllium. Titanium, silicon, and similar reducing agents are effective and usetul in thermionic cathodes coated with oxides of the alkaline earth metals. The properties of beryllium are such that it may be used to advantage in the same way as these other reducing agents.

The invention may be used as illustrated in the accompanying drawing, in which Fig. 1 is a view of the getter holder or trough containing the getter, and Fig. 2 shows the preferred way of using the getter in a conventional metal radio tube,

S. Patent 2,208,692, Wamsley, July 23, 1940.

Fig. 1 shows the getter I, preferably a finely powdered mixture, contained in a getter holder or trough 2 of a metal such as molybdenum which may be electrically heated and which will retain its shape at 1300" C. or more. The holder and the getter mixture in it are electrically heated at will to a temperature of about 1300 0., whereupon a reaction occurs which liberates barium in the form of vapor and leaves in the trough stable and gas-free reaction products.

Fig. 2 shows the getter holder containing the getter mounted on a pair of leads 3 of the metal radio tube 4. The getter holder and getter may be electrically heated at will by passing current through the getter holder by connecting the leads of voltage. In accordance with conventional practice, the getter holder is preferably 50 mounted that the flash of barium is directed away from the electrode assembly of the tube.

I claim:

1. A getter material consisting of barium titanate mixed with powdered beryllium in a ratio of from about 1 to about 2 moles of beryllium, to 1 mole of barium titanate.

2. A getter material consisting of a mixture of finely powdered barium titanate and finely powdered beryllium in which the beryllium constitutes about 6% by weight of the total mixture.

3. A clean-up agent capable of evolving barium when heated and consisting of a mixture of powdered barium titanate and powdered beryllium containing from 5% to 10% beryllium by weight of the mixture.

4. A getter material consisting of barium titanate having the composition lBaO- lTiOz mixed with beryllium in a ratio of from about 1 to about 2 moles of beryllium to one mole of barium titanate.

EDWIN N. BOBROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,835,117 Marden Dec. 8, 1931 1,873,730 Wiegand Aug. 23, 1932 2,208,692 Wamsley July 23, 1940 2,254,727 Beggs Sept. 2, 1941 1,172,568 Sch-roter Feb. 22, 1916 FOREIGN PATENTS Number Country Date 300,265 Great Britain Nov. 11, 1927

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