US2661336A - Getter material for electron discharge devices - Google Patents

Description

Dec. 1, 1953 E. A. LEDERER 2,661,336

GETTER MATERIAL FOR ELECTRON DISCHARGE DEVICES Filed Nov. 17, 1948 INVENTOR ERNEST A. LEDERER ATTO EY Patented Dec. 1, 1953 GETTER MATERIAL FOR ELECTRON DISCHARGE DEVICES Ernest A. Lederer, Essex Fells, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 17, 1948, Serial No. 60,525

9 Claims.

. The present invention relates to getters for electron discharge devices and more particularly to a getter material comprisedof a novel chemical compound.

A getter material that has generally been used heretofore is a compound of an alkaline earth metal such as barium, strontium, or calcium oxide with beryllium oxide. If, for example, the compound is barium berylliate its composition may be written as BaO-BeO. A reducing agent such as titanium or tantalum can be made to react with barium berylliate at a temperature of 1000 C. or higher. If th reaction is carried out in an attenuated atmosphere or vacuum or in a noble gas, barium metal will evaporate from the reaction. After the completed reaction the residue contains compounds or mixtures of beryllium oxide, the oxide of the reducing material and some barium oxide.

One of the problems associated with the use of an alkaline earth metal berylliate as a getter material is that it readily becomes hydrolyzed. This makes it mandatory to isolate the material from a humid atmosphere, which adds to the expense and inconvenience of using and storing the material. In an attempt to overcome this problem it has been proposed to use barium titanate, which is relatively free from the objection of hydrolyzation. However, titanium which has been recognized as the most effective metal from the standpoint of its characteristic as a reducing agent, is incapable of reducing barium titanate. In practicing this expedient, therefore, it has been necessary to use the metal beryllium as the reducing agent. The metal beryllium, however, is relatively expensive.

Another respect in which an alkaline earth berylliate is not entirely satisfactory is the fact that the alkaline earth metal flash derived therefrom is always contaminated by some beryllium metal. This contamination reduces the effectiveness of the barium as a gettering substance. Moreover, the upper limit of the flashing temperature of barium berylliate getters is critical and must be kept below the temperature at which evaporation of the beryllium takes place. prevents maximum yield of barium metal.

It is accordingly an object of the present invention to provide an improved getter material. A further object is to provide a stable getter material that is substantially free from deteriorationby humid atmospheric conditions.

Another object is to provide a novel getter compound that is less susceptible to hydrolyzation than contemporary getter material.

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A still further object is to provide a compound including an alkaline earth metal that yields said alkaline earth metal in purer form than heretofore possible to improve the gettering properties of the metal.

Another object is to provide a superior getter material at relatively low cost.

A further object is to provide a getter material that requires less care in its handling and storage without resulting in deterioration of the material.

Another object is to provide a getter material permitting a higher temperature of flashing than heretofore possible to increase the yield of alka-- a line earth metal for improving the gettering action.

Further objects and advantages of the invention will make themselves manifest as the present description continues.

Referring to the accompanying drawing, there is shown in the sole figure thereof by way of illustration only of the invention and not in limitation thereof, a getter structure including the novel getter material of the invention coated upon a strip or core. The getter structure includes a U-shaped strip or core l5) which may be made of refractory metal, supported on a U- shaped support H, which in turn is mounted on support rod l2 fixed to the press l3 of an electron discharge device, not shown.

In one embodiment of the invention the U- shaped strip or core Ill may be made of a refractory metal having reducing properties. In this embodiment the core is filled with my improved compound M. The core l0 may be made of any one of the refractory metals comprising tungsten, molybdenum, tantalum, columbium, tita nium, zirconium, thorium, etc. Some of these metals would ordinarily not be used because of their relatively high cost. For-example, thorium is relatively expensive and would therefore probably not be selected for the core material. I prefer to use tantalum as the core material because of its superior characteristics as a refractory and reducing metal.

In another embodiment of the invention the core 10 need not possess any reducing properties and may be made of a different metal than that specified in the previous embodiment. In' the presently discussed embodiment, the required reducing agent may consist of a mixture of the as augmented by the additional reducing metals comprising aluminum, misch metal, beryllium, etc. In some applications of getters, such as use in a pumping or exhausting system for aiding the evacuating process as by dispersal of an alkaline earth metal, additional reducing agent may be used such as silicon, magnesium, etc.

A further embodiment of the invention may involve a combination of the two preceding embodiments. For example, the core or strip H3 may be made of any one of the metals referred to in the first embodiment, and my novel gettering material may have mixed therewith in powder form one of the reducing metals called for by the second embodiment. In each of these embodiments my new getter compound is in intimate contact with the reducing metal.

The getter may conveniently be heated in any suitable manner, such as by induced high frequency currents in the closed loop formed by the core or strip H and the U-shaped support H.

Certain alkaline earth metals such as barium, strontium and calcium are effic'ient gas clean-up agents, but are unstable in air. They are therefore included in compounds that are relatively stable. For example, a relatively stable compound may include barium, beryllium and oxygen in a suitable molecular proportion such as characterizes barium berylliate. This compound when associated with a reducing agent such as titanium and heated, liberates barium. As a compound, however, the barium be-rylliate readily becomes hydrolyzed if exposed to humidity. Hydrolyzation of the compound results in deterioration thereof to such an extent that it may become unsuited as a material for providing a gettering action. To avoid such deterioration, therefore, it is essential to observe strict precautions in controlling the humidity of the atmosphere in proximity to the getter material. Furthermore, it has been determined that the reduction of the barium also results in some reduction of the beryllium with the consequence that some beryllium is liberated with the barium. While the amount of such beryllium so liberated is relatively small, being from .1 to .3% in weight of the barium, it is an objectionable contaminant that appreciably interferes with efficient gettering action of the barium.

According to my invention, I provide what I believe to be a cerate compound of an alkaline earth metal such as barium in the molecular proportion BaO-CeOz. Although pertinent literature points out that alkali and alkaline earth metal cerates have not been discovered, it is believed my new compound definitely is such cerate. That my new material is a compound and not a mixture of barium oxide and cerium dioxide is clearly proven by comparing the melting points of my new compound with the melting points of barium oxide and cerium dioxide. Thus, the melting point of my new compound was found to benear 2100 C. while the melting point of barium oxide was 1923 C. and that of cerium dioxide (CeOz) was 1950 C. Furthermore, an X-ray analysis of my new compound shows diffraction lines differing from those of barium oxide and cerium dioxide. While barium oxide has appreciable aflinity to water, it was found that an exposure of my new compound to a condition of relative humidity of 81% at room temperature resulted in a weight gain of only 34%, in 48 hours, which is far less than would have been the case had my material contained free barium oxide. A further test to which I subjected my novel compound was to soak it in water for several hours to determine its reaction. No reaction was noted. Had my compound contained free barium oxide its reaction with the water would have been alkaline. These tests clearly indicate that my new material is a compound having the chemical designation of BaO-Ceoz and not a mixture such as is indicated by the designation BaO-l-CeOz.

My new compound may be prepared by mixing a carbonate of an alkaline earth metal such as 'barium,.strontium, or calcium, and cerium dioxide in appropriate molecular proportions and subjecting the mixture to heat. The mixing may be accomplishedv mechanically as by ball milling, or by a chemical method comprising co-precipitation of barium and cerium as hydroxides from a solution of their salts. The heating can be accomplished in one of several ways. The purpose of the heating is to promote homogeneity and completeness of crystallization, which are essential in order to secure completeness of combination of the barium oxide and the cerium dioxide as distinct from a mixture thereof. It is feasible to employ one heat treatment either in hydrogen, in inert gas, or in air. If the heat treatment is performed in air the temperature to which the mixture is subjected should be between 1300 and 1500 C. However, the single heat treatment is not preferred because of the violent reactions that may result from the application of these high temperatures to the carbonates in the mixture.

The preferred form of the heat treatment comprises several applications of heat to the mixture between which the mixture is thoroughly ground. The first heat treatment is preferably performed in a hydrogen gas at a temperature range from 1000 to 1300 C. The mixture is carried preferably in a boat during the heat treatments. If the boat is made of nickel it is necessary to reduce the temperature range to a maximum of 1200 C. to prevent reaction between the nickel and the mixture. Such reaction is indicated by the formation of a pink compound the composition of which has not been determined. If a ceramic boat is used, however, the temperature range found satisfactory is from 1000 to 1300 C. Although an inert gas or air might be suitable as the medium in which the first heat treatment takes place, hydrogen is preferred since it facilitates decomposition of the carbonate at a lower temperature than in air. For example, if the first heat treatment were performed in air the minimum of the temperature range would be 1350 C. The effect of the first heat treatment is to decompose the barium carbonates and to initiate a combination of the resultant barium oxide with the cerium dioxide.

After thoroughly mixing, such as by grinding, the material at the end of the first heat treatment, it is then subjected to a second heat treatment. This heat treatment may be in hydrogen, an inert gas, or air, but preferably it is performed in air. The carbonate having been previously decomposed during the first heat treatment, the eifect of the second treatment is to further the combination of the oxides. The temperature range found permissible in the second treatment lies between lQ00 and 1400" C., the preferable temperature being about 1300 C. if the boat is made of ceramic. In the second heat treatment a nickel boat cannot be used because of the readiness with which nickel oxidizes in air.

It is possible that during the interval between the two heat treatments referred to some of the alkaline earth metal may have reverted to its original form as a, carbonate. Therefore, I have found it advantageous to add two additional heat treatments. The first of these additional heat treatments is similar in character and function to the first heat treatment referred to above. Thus, it is preferably performed in hydrogen to convert any carbonate content of the material to the oxide. A fourth heat treatment is then carried out in air under conditions similar to those surrounding the second heat treatment, its purpose being to convert the cerium dioxide and alkaline earth metal oxide to a new compound comprising a cerate of the alkaline earth metal.

While I have found the four heat treatments referred to of advantage in the respect indicated, it is to be understood that the invention may be practiced by employing one heat treatment only as has been pointed out before herein.

The crystal growth characteristic of a cerate of an alkaline earth metal that is produced by the heat treatments referred to, is a function of the temperature used as well as of the time duration such temperature is applied. I have found, for example, in using four heat treatments that the first heat treatment in hydrogen should be at a temperature of about 1100 C. for about one hour. The second heat treatment in air should be at a temperature of about 1300" C. for about two hours. The third heat treatment in hydrogen should be characterized by the same temperature and time duration as the first heat treatment, while the fourth heat treatment should be under circumstances similar to those of the second heat treatment.

If higher temperatures are employed than indicated in the preceding paragraph, a corresponding reduction in the time duration of the heat treatments could be made. However, no matter how long the time duration of a particular heat treatment, optimum results are obtained by not going lower than 1050 C. Going to the other temperature extreme, it is not feasible to use a temperature in excess of 1500'" C. no matter how short the time duration of a heat treatment may be because of limitations of crucible or boat materials such as their tending to react with the cerate at elevated temperatures.

It will be noted that in the illustration in volving four heat treatments, the temperature during the heating in hydrogen is considerably below the temperature used in heating in air. This is for the reason that carbonates that are present in the mixture, particularly prior to the first heat treatment, or which are assumed to be present prior to the third heat treatment referred to above, produce violent reactions at the higher temperatures.

It will be appreciated from the foregoing that the objective sought by the heat treatments is a reaction between two solids both being oxides and having refractory natures. Such reaction is dependent on interdiifusion of the two solids. Since interdiffusion does not take place readily at the relatively low temperatures characterizing the heat treatments referred to, I found it desirable to repeat the heat treatments in air several times, mixing and grinding the material between the treatments. This therefore is an additional reason for resorting to a plurality of heat treatments, rather than depending on one only. 7

Certain appropriate molecular proportions of an alkaline earth metal carbonate and cerium dioxide are required in order to secure a chemical compound of the alkaline earth metal, cerium and oxygen, that is free from objectionable hydrolyzation in accordance with the invention. I have found that molecular proportions of the alkaline earth carbonate and cerium dioxide from the proportion of 1:1 to a proportion that is less than 5:3, respectively, results in the formation of the chemical compound of my invention. While it is possible to use a ratio less than 1:1 and still obtain a chemical compound, and also in part a solid solution, this is not always desirable since less alkaline earth metal would be available from such a ratio. Thus, for practical considerations, a ratio of .1:1 provides the minimum acceptable amount of alkaline earth metal required for a getter. I have found that the employment of the 1:1 ratio results in a compound that will not hydrolyze. I have further found that increasing the ratio to 5:3 results in a product that becomes hydrolyzed with a readiness about equal to that of barium berylliate. By reducing the proportion of barium carbonate to cerium dioxide to a value less than 5:3, I have found that a product results which is less subject to hydrolyzation than barium berylliate. My findings further indicate that there is a gradient in the susceptibility to hy-. drolyzation between the proportion of 1:1 and the proportion of 5:3. Thus, utilizing the proportion of 1:1 or any smaller ratio, I have found that the resultant product is entirely free from hydrolyzation. However, as the ratio is enlarged. there is an increase in the susceptibility to hydrolyzation until at the ratio of 5:3 the hydrolyzation become approximately equal to that of barium berylliate. I prefer to use alkaline earth metal carbonates and cerium dioxide in the molecular proportion of 1:1, since this provides the minimum of alkaline earth metal required for good gettering action, and at the same time results in a material that is easy to handle since it will not hydrolyze.

The number of heat treatments employed is dependent on the thoroughness with which the carbonate and the cerium dioxide were initially mixed. The ultimate result sought is such a relationship between the molecules of the alkaline earth metal, the cerium and the oxygen that each molecule of one component element is in physical contact with a molecule of the other two components. Therefore, it is possible that if the initial mixing treatment is thorough enough only one heat treatment is necessary to accomplish the desired change in crystal structure required for a chemical combination of cerium, alkaline earth metal and oxygen. However, it is difficult to mix the original components with sufficient exactness to assure the intimacy between the particles mixed that is required if a single heat treatment is to be employed. Therefore, I prefer to subject the mixture to at least the two heat treatments mentioned above to assure a proper chemical combination of the elements dealt with.

There is obtained, as a result of heat treating a mixture of barium carbonate and cerium dioxide in accordance with the method referred to; a yellowish, greenish, hard lump. This hard lump when subjected to the tests referred to before herein, responded in a manner indicating that it was a chemical compound having the composition formula BaO-CeOz.

accuses 7 The reaction'taking place at the first heat treatment referred to is believed to be as follows:

l 7' 138.00; 0602 H2 Heat=Ba(OH.)2 +S/O 1321 oeo, Heat=Ba on o My novel compound has many advantages. It is substantially cheaper than. the only nonlwdrolyzin-g alternative material barium titan-a-te. It is superior to barium berylliate in several ways. For example, my new compound requires less precaution in handling since it is not appreciably 'afiected by conditions of humidity and is believed not to be poisonous. Further, my novel compound provides a getter material that can be reduced to metallic barium by a number of reducing agents such as have been enumerated previously herein and therefore is characterized by better adaptability for certain purposes. Its action as a getter represents an improvement over that of previously used getters due to the absence of contaminants in the alkaline earth metal yield. Thus, it is capable of yielding a purer form of barium than prior getter-s, which improves its efficiency as a getter.

Furthermore, my novel compound permits a higher flashing temperature to be used than is permitted with a barium .berylliate getter. In the use of the latter type of getter, the flashing temperature limited due to evaporation of beryllium metal. Therefore, with the use of my new compound which makes the use of beryllium metal unnecessary, a nearly theoretical alkaline earth metal yield is possible, which is of advantage in that the gettering action or" the getter can be predetermined with more accuracy than heretofore possible. getteri-n'g material can be more accurately coordinated with the expected gas evolution in a tube in which the getter .is used resulting in substantial economies.

Various modifications may be made in my invention without departing from its spirit and scope as pointed out in the appended claims.

What is claimed is:

1. Method of making a composition comprising a chemically compounded cerate of an alkaline earth metal comprising the steps of mixing barium carbonate and cerium dioxide in a molecular proportion of less than 5:3 respectively, and heating the resultant mixture to a temperature from 1050 to 1500 C. to chemically comblue the barium, cerium and part of the oxygen. 1 2. A getter material comprising the reaction product obtained by heating in hydrogen an alkaline earth metal carbonate and cerium dioxide in the molecular proportions of from 1:1 to 5:3 respectively at a temperature from 1000 to 1300 C., grinding the resultant product, and reheating the ground product in air at a temperature from l000 to 14:00 C., the final product being a chemical compound having a crystal structure and comprising said alkaline earth metal, cerium and oxygen, in chemically combined form, said compound being substantially free from hydrolyzation.

3. A getter material comprising the reaction product resulting from heating in air equal molecular parts of barium carbonate and cerium dioxide at a temperature from 1300 C. to

Thus, the amount of 8 1500 C., said reaction product being a chemical compound having a negative reaction with water, and a smaller moisture absorbing characteristic than free barium oxide, whereby said getter material is adapted to be stored without appreciable deterioration.

4. In an electron discharge device processed at relatively high temperatures, a getter comprising an unreduced material consisting of barium cerate having the chemical expression BaO -C'aOz and being the reaction product of barium carbonate and cerium dioxide within the range of one molecular part of barium carbonate and one molecular part of cerium dioxide to five molecular parts of barium carbonate, and

three molecular parts of cerium dioxide; and a reducing metal of the group consisting of tantalum, tungsten, molybdenum, columbium, titanium, aluminum, misch metal and beryllium mixed with said material to provide a mixture in which said reducing metal is in intimate contact with said barium cerate, said material being free from appreciable vaporization at said temperatures, said reducing metal causing evolution of said barium from said material at a temperature higher than said relatively high temperatures, whereby said getter is free from deterioration prior to said evolution of the barium.

5. Method of making a composition comprising a chemically compounded cerate of an alkaline earth metal, comprising the steps of mixing in powder form from 3 to 5 molecular parts of the carbonate of said alkaline earth metal and 3 molecular parts of cerium dioxide to provide a mixture having a homogeneous distribution of said carbonate and dioxide, and heating said mixture for from one to six hours at a tempera-- ture of from 1000 to 1500 C. inform a chemical compound having a crystal structureconsisting of said alkaline earth metal, cerium and oxygen, and having a melting point temperature of about 2100" C.

'6. Method of making a composition comprising a chemically compounded cerat'e of an al lral ine earth metal, comprising the steps of mixing from '3 to -'5 molecular parts of the carbonate of said alkaline earth metal and 3 parts of cerium dioxide to provide a mixture having a substantiallv uniform distribution of said carbonate and dioxide, heating said mixture in a ceramic boat in hydrogen at a temperature from 1000 C. to 1300 C. for about one hour, to form a material consisting at least in part of the reaction product of said carbonate and dioxide, further mix-- ing said material for substantially uniformly distributing any of said carbonate and dioxide contained therein and not combined in said reaction product, and further heating said material in air for about two hours at a temperature of about 1390" C. to complete a chemical combination of said carbonate and dioxide to form said reaction product, said reaction product consisting of the cerate of said alkaline earth metal.

'7. Method of making a composition comprising a chemically compounded cerate of an alkaline earth metal, comprising the steps of mixing from 3 to 5 molecular parts of the carbonate of said alkaline earth metal and 3 molecular parts of cerium dioxide, to provide a substantially uniform mixture wherein molecules of said carbonate are individually adjacent the molecules of said dioxide, said carbonate and dioxide having a characteristic crystal structure, and heating. said mixture to cause the characteristic crystal structure of said carbonate and dioxide to grow 9 to form a crystal structure consisting of said alkaline earth metal, cerium and oxygen.

8. A getter material comprising a chemical compound obtained by reacting from 1 to 1 /3 molecular parts of the carbonate of an alkaline earth metal and one molecular part of cerium dioxide at a temperature of from 1000" C. to 1500 C. for from one to six hours, said chemical compound having a crystal structure consisting of said alkaline earth metal, cerium and oxygen and having a melting point temperature of about 2100 C., whereby said getter material comprising said chemical compound is adapted to be flashed at a relatively high temperature with a relatively large yield of said alkaline earth metal.

9. A getter material comprising the reaction product resulting from heating from 1 to 1% parts of the carbonate of an alkaline earth metal and one molecular part of cerium dioxide at a temperature from 1000 C. to 1500 C. for from one to six hours, said reaction product being a chemical compound having a crystal structure and comprising said alkaline earth metal, cerium and oxygen in chemically combined form,

and having negative reaction with water, and a 2 smaller moisture absorbing characteristic than 5 Naturwissenschaften, vol. 31, 466 (1943).

10 the free oxides of said alkaline earth metal, whereby said getter material comprising said reaction product is adapted to be stored without appreciable deterioration.

ERNEST A. LEDERER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,849,594 Schroter Mar. 15, 1932 1,946,603 Von Wedel Feb. 13, 1934 1,977,278 Judy Oct. 16, 1934 15 2,111,460 Rockstroh Mar. 15, 1938 2,17 ,258 Lederer Sept. 19, 1939 2,173,259 Lederer Sept. 19, 1939 2,208,692 Wamsley July 23, 1940 2,267,292 Wamsley Dec. 23, 1941 20 2,364,436 Frisch Dec. 5, 1944 2,421,984 Bobrow June 10, 1947 2,460,975 Carosella Feb. 8, 1949 OTHER REFERENCES Abstracted in Chem. Abstracts, vol. 38, 2866 (1944).

Claims (1)

1. 4. IN AN ELECTRON DISCHARGE DEVICE PROCESSED AT RELATIVELY HIGH TEMPERATURES, A GETTER COMPRISING AN UNREDUCED MATERIAL CONSISTING OF BARIUM CERATE HAVING THE CHEMICAL EXPRESSION BAO.CAO2 AND BEING THE REACTING PRODUCT OF BARIUM CARBONATE AND CERIUM DIOXIDE WITHIN THE RANGE OF ONE MOLECULAR PART OF BARIUM CARBONATE AND ONE MOLECULAR PART OF CERIUM DIOXIDE TO FIVE MOLECULAR PARTS OF BARIM CARBONATE, AND THREE MOLECULAR PARTS OF CERIUM DIOXIDE; AND A REDUCING METAL OF THE GROUP CONSISTING OF TANTALUM, TUNGSTEN, MOLYBDENUM, COLUMBIUM, TITANIUM, ALUMINUM, MISCH METAL AND BERYLLIUM MIXED WITH SAID MATERIAL TO PROVIDE A MIXTURE IN

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