US2030670A - Process of preparing metallic barium for introduction into electric discharge devices - Google Patents

Description

Patented Feb. 11, 1936 UNITED STATES PROCESS or PREPARING mn'mnuc mm UM FOR mraonucrron nv'ro ELECTRIC mscmmcr: DEVICES Erich Wiegand, Berlin-Waidmannslust, Germany, assignor to General Electric Company, a. corporation of New York No Drawing. Application July 21, 1930, Serial No. 469,639. In Germany August 20, 19 29 3 Claims. 01. 148-l3) My invention relates to a process of preparing; and introducing into an electric discharge device a substantially pure alkaline earth metal in the metallic state for the purpose of gettering the residual gases and increasing the electron emission of the cathode. More particularly my invention relates to a process of preparing alkaline earth metals in such a manner thatthey may be allowed to stand for a considerable time in the atmosphere without appreciable oxidation, and to a method of introducing these metals into an electrical discharge device to act as a getter or clean-up agent-for residual gases and as a means for increasing electron emission of the electrodes.

It is well known in the art that the electron emission of thermionic tubes provided with the usual filament of high refractory material may -be materially increased by coating the filament with various materials including compounds of the alkaline earth metals. For example, the carbonates or oxides 'of strontium, barium and calcium have been extensively used either singly or in combination. However, these materials have decided disadvantages in that they are difficult to apply uniformly and tend to crackv and scale oil, thereby considerably reducing the effectiveness of the tube. Furthermore, such compounds,

. while increasing electron emission, do not materially aid in the clean-up of the residual gases.

It has also been proposed to coat the electron emitting cathode with metallic barium obtained by glowing the cathode in barium vaporintroduced into the tube in the form of reduction mixtures such as alumind-thermic mixtures, barium acid or barium azide' (BaNs). These mix-- tures havethe advantage that they function both to increase the electron emission of the device and also to aid in gettering the residual gases. However, the aluniino-thermic mixture has the disadvantage that a very small output of barium is obtained from a relatively large quantity ,of

the mixture, whfle the other mixtures mentioned above .have the disadvantage that considerable quantities of nitrogen are evolved during their decomposition. 1

It has been realized that it would he a decided advantage to. be able to use substantially pure alkalineearth metals in place of the above mentioned coatings or mixtures, but hitherto there has beenideyised no way of preparing these metals in a 'forini'sufliciently pure for this purpose and in a wayj such that they would resist attack and corrosion by the/atmosphere. This susceptibility to attack been duein a large extent body of the metal.

these metals with a purity much above 98%. It

has been discovered that one of the chief factors in the corrosion of these metals is the separation of anyimpurities in the metal into the form of veins or layers. The corrosion begins at these layers and in a short time converts the metal body into a series of laminations which are very quickly corroded due to their great superficial area; Ithas been discovered that the-corrosion of these metals can be considerably retarded if the metal is treated in such a manner as to uniformly distribute the impurities throughout the Objects of the invention are to provide a method of treating alkaline earth material in such a manner that any traces of impurities therein do not give rise to appreciable corrosion of the metal; to provide a protective coating about a body of alkaline earth material which will serve to prevent-surface attack by the atmosphereor other deleterious gas; to provides, simple method of coating alkaline earth metal with a readily vaporizable material so that the metal may be preserved indefinitely for use in electric discharge devices and the coating may be quickly removed when desired. In carrying my invention into effect I provide a process for preparing an alka-.

line earth metal, preferably barium, by heat treatment so as uniformly to-distribute the impurities throughout the mass; coating a piece of such a metal with a readily vaporizable non-oxidizing coating; introducing the body of metal into an electric discharge device, vaporizing said coating and simultaneously exhausting the device; sealing the device and volatilizing the barium to increase the electron emission of the cathode of the device and to facilitate the gettering of any residual gas.

In order to obtain an alkaline earth metal with the highest degree of purity possible I have found the best methodto be that of distilling the metal within a high vacuum or an inert gas at a temperature above 1150 degrees C. According to myinvention the distilled metal so obtained is cooled rapidly. By this method it is found that the impurities tend to be uniformly distributed throughout the ,metal. However, it

has been found that the distribution may be more completely effected by subsequently tempering the metal near its melting point-preferably in an atmosphere of inert gas, and subsequently cooling it very rapidly. As will begexplained hereinafter, this tempering may be accomplished either in a small receptacle or within a recess formed in one of the electrodes of the electric discharge device. It has been foundthat the alkaline earth metal, when so treated, is at first corroded very slowly at the surface if allowed to stand for a considerable time in the atmosphere, but that this corrosive coating protects the metal underneath. After the metal has been treated in this way the laminated structure described above does not appear.

In order further to protect the metal as prepared above from the oxidation of the atmosphere it has been found that a coating of a viscous non-oxidizing liquid such as parafiin oil is highly effective. In the preferred form of my invention the bar or ingot of metal obtained from the distillation process is coated with paraifin oil and rolled or pressed into bars or sheets and subsequently cut up into small pieces of a size suitable for introduction into the electric discharge device. It may be found desirable to coat the metal bar or sheet between each of the rolling or pressing operations as well as after the metal' has been cut into its final form. The small pieces of metal socoated may be kept several weeks without any apparent oxidation or deterioration. In some cases it may bemore convenient to reduce the metal to powdered form in which case it may be mixed with paraflin oil and usedas a paste to be coated upon the electrodes or it'may be compressed into small pellets to be used in place of the small bodies of pure metal produced as described above.

The next step in the process is that of introducing the small'bodies of metal into the electric discharge device. This may be done by attaching the metal particles to the electrodes by clips or strips or in any convenient manner; preferably they should be mounted in close proximity to the hot cathode. For many purposes it has proved advantageous, to carry out the tempering or fusing process described, in small receptacles or in the electrodes provided with suitable recesses, as statedhereinbefore. The electrodes may be made of nickel or molybdenum for example, and when filled with the treated barium may be placed within the tube. receptacles. otherthan the electrodes themselves are employed to contain the alkaline material during the preliminary heat treatment, these containers may likewise be positioned within the tube, if desired. The electric discharge device is then exhausted in any suitable manner well known in the art and the metallic body is simultaneously heated to 200-450 0., either by passing current through the cathode or placing the discharge device within a high frequency field, which induces heating currents in the metal body. In either case the heating of the metallic body vaporizes the protecting coating into volatile undecomposed hydrocarbons which are drawn ofi duringthe exhausting of the device. When the increased due to the eflicient gettering qualities When exhausting operation has been completed the device is sealed off according to the usual practice.

The use of an alkaline earth metal within an electric discharge device has been found to be 5 more eifective if the metal is vaporized within the tube. This may be accomplished preferably by high frequency currents as in the vaporizing of the coating. In this case the metallic body should be heated to a suitable temperature above 600 C. During the vaporizing operation the electron emitting electrode is preferably heated to incandescence so that the metallic vapor will alloy with the surface of the electron emitting electrode.

By the use of the above described process it has been found that the electron emission of such a device may be substantially increased and that the life of the device is also substantially 20 of the alkaline earth metals. 1

In accordance with the patent statutes I have described in detail what I consider at present to be the preferred embodiment of my invention, but it is to be understood that the above description is merely illustrative and that I aim to cover in the appended claims all the various changes and modifications within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process of treating metallic barium for introduction into electric discharge devices comprising distilling barium of a commercially obtainable purity, tempering the distilled metal near its melting point and rapidly cooling the same uniformly to distribute any traces of impurities therein, and applying a vaporizable nonoxidizing coating to said tempered metal to preserve the same.

2. The process of treating metallic barium for introduction into an electric discharge device comprising distilling barium of commercially obtainable purity and, forming an ingot thereof,

tempering said ingot near its melting point and 45 rapidly -c0oling the same to obtain a uniform distribution of the impurities therein, dividing said ingot into pieces suitable for introduction into the electric discharge device, and. applying a coating of parafline oil to all exposed surfaces 50 during the dividingoperation.

3. The process of treating metallic barium for introduction into electric discharge devices comprising distilling barium of a commercially ob-' tainable purity, tempering the distilled metal near its melting point and rapidly cooling the same uniformly to distribute any traces of impurity therein, and applying aprotective coating to said tempered metal to preserve the same.

ERICH WIEGAND.