US1840121A - Vacuum tube - Google Patents

Description

Patented Jan. 5, 1932 LEON HCUULLOCH, F PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA VACUUM TUBE Ho Drawing.

. My-invention relates to electrical discharge devices and more particularly to thermionic ubes and photoelectric tubes.

An object of my invention is to obtain upon a desired surface, a metallic electron emitting film.

Another object is to introduce into electrical discharge devices certain metals which oxi- 'dize easily and without the use of complicated machinery and involved processes.

Still another object is to provide an ellicient getter for the purpose of removing residual gases during vacuum tube manufac ture.

A further object is to decrease the cost of production of such devices as thermionic tubes, photoelectric tubes, and other analogous devices in which are used electronically active metals.

2 It is well known that the alkaline earth metals may be introduced into vacuum tubes by means of distillation. This is described and illustrated in the patent to Kunz No. 1,381,474, June 14, 1921. The process described in the patentinvolves considerable technical skill if a satisfactory product is to be obtained. Another method of introducing a desired metal into the vacuum tube is illustrated and "described in the patent to Langmuir, 1,273,628,=-dated July 23, 1918, in which a filament of the metal is vaporized in the interior of the tube. Both of these methods will result in tubes having photoelectric or electronically active films deposited in desired locations but in their practical application they are accompanied by certain well knowndifiiculties.

Certain metals which are desirable as coatings for tube elements, etc. are so readily oxidizable that it is very difficult to introduce them satisfactorily into tubes. The most notable of such metals is caesium, which is extremely desirable from a thermionic or photoelectric stand-point. It is also diflicult to properly introduce barium in the usual and well known ways. 7

By means of my invention, I am enabled to seal in a tubeany of the alkali and alkali earth metals including, for example, potassium, sodium, rubidium, caesium, barium,

Application filed ma 20,

1926. Serial No. 110,547.

and possibly calcium and strontium Without any special difiiculty.

I make use of chemical reaction involving the decompositionof hydronitric acid salts of the desired metals directly at the point where the metals themselves are desired. In this way I am enabled to perform the whole operation Within the vacuum tube device or directly upon the electrode without exposing the resulting metal to the atmosphere. I am also enabled to deposit within a tube, a metallic getter which will tend to remove certain residual gases.

In practicing my method, the trinitride of the desired metal is placed in a receptacle (5 or vacuum tube or on an electrode thereof and the tube is then baked at a temperature not exceeding 200 C;, at the same time subjecting it to a preliminary exhaust. The temperature of 200 might in some cases be exceeded if the characteristics of the salt used permitted. It should be observed that thc preliminary heating must be well below the point of incipient decomposition of the salt as it is extremely important that this salt be 7 kept from decomposing until such time as the preliminary exhaust is completed.

Having exhausted the tube or other device to a pressure oflO m. m. of mercury, I next bake it at a temperature high enough to de- 0 compose the trinitride into the metal and nitrogen. This second heating Will vary from 200 to 500 C. depending upon the trinitride employed.

The next step is the final exhaust to carry off the nitrogen which, of course, is not desired in the tube. It is obvious that the first baking and exhausting may be performed simultaneously as well as the second corresponding operations, the only precaution necessary being that the decomposition temperature must not be reached until the preliminary exhaust is complete so that there will be no danger of oxidation of the resulting metal. It is thus apparent that, by means of this method, I can either coat the walls of the receptacles or can place a deposit of a thermionic or photoelectric metal on a desired electrode.

It is also believed obvious that the deposit may be first placed on one electrode and then by applying suitable heat by suitable means it ma be driven across to another electrode.

This method of forming a deposit directly at the point at which it is to be utilized without subjecting such deposit to the action of the atmosphere is novel inthis art. I have found that barium, sodium, potassium, rubidium. and caesium trinitrides respond quite readily to this method. Lithium, calcium and strontium trinitrides decompose as readily but appear to break down into lower nitrides.

To lower the temperature of decomposition of caesium trinitride, I have found it best to mix equal parts of barium trinitride and caesium trinitride. When this mixture is heated, the caesium distils out at a temperature below the softening point of soft glass. This mixture is, however, ex losive to a certain extent and I find that y mixing with it a certain proportion of inert material such as zirconium silicate or silicon dioxide (SiO the barium caesium trinitride mixture decomposes easily and quickly and the danger of explosion is entirely obviated. As an alternative method there maybe used a porous mass of quartz or other coherent material through the body of which the barium-caesium trinitride mixture is distributed. This mass may be mechanically held in place'with in the receptacle and the Whole device heated as explained above.

When it is desired to provide a getterl in the process of tube manufacture, thetrini-Q tride of the desired metal is introduced into the tube and the same method is followed, resulting in the metal being deposited where it is desired. This metal, being in a fine state of division, may absorb gases directly, or, upon subsequent heating at a much higher temperature, this metal may be flashed with the resulting absorption of certain residual gases. Among the metals best adapted for this latter purpose are, sodium, barium, strontium, and calcium.

It will thus be seen thatmy invention provides a novel method of introducing an ox-:= idizable metal into a tube or analogous de-L vice where it is to be either utilized for V thermionic emission, for photoelectric purposes, or as a getter.

Among the principal advantages of my invention is the fact that its use greatly lowers the cost of production of electrical discharge devices of the character described.

Although I have described several specific embodiments of my invention, other equivalents will suggest themselves to those skilled in the art and it 'is not intended that I be bound by any limitation-other than that imposed by the prior art and embodied in the following claims. 7 V

I claimas my invention: 1. In a method of providing a thermionically-emissive cathode, the step of heating a mixture of caesium trinitride and barium trinitride.

2. In a. method of providing a thermionically-emissive cathode, the step of heating a mixture of caesium trinitride, barium trinitri'd'e and a chemically inert material.

3. In amethod of providing a thermionically-emissive cathode, the step of heating a mixture of caesium trinitride, barium trinitride' and a granular refractory.

4.'The method of providing a deposit of caesium in a vacuum tube which includes the steps of inserting a mixture of caesium trinitri e and barium trinitride in said tube, evacuating said tube andheating said mixture.

5. The method of providin a deposit-of caesium in a vacuum tube whic includes the steps of inserting a. mixture of caesium trinitride, barium trinitride and a granular refractory in said tube, evacuating said tube, and heating said mixture.

7 In testimony whereof, I have hereunto subscribed my name this llth day of May, 1926.

LEON .McCULLOCH.