US2154131A

US2154131A - Getter

Info

Publication number: US2154131A
Application number: US171814A
Authority: US
Inventors: Ernest A Lederer
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-10-30
Filing date: 1937-10-30
Publication date: 1939-04-11
Anticipated expiration: 1956-04-11
Also published as: GB520751A

Description

April 11, 1939. E. A.ILEDERER GETTER Filed Oct. 50, 1937 Rm Y OD E TE N m R A c M M T I VW N Patented Apr. 11, 1939 UNITED STA'iE PATENT @FFECE GETTER Delaware Application October 30, 1937, Serial No. 171,814

8 Claims.

My invention relates to vaporization of metals, particularly to liberation and dispersion of predetermined and controlled quantities of active metals in electron discharge devices.

In the manufacture of electron discharge devices and tubes chemically active metals usually of the alkali or alkaline earth groups are introduced into the envelope of the device for various purposes, such as reducing the pressure of residual gases, activating cathodes, or sensitizing light responsive electrodes. One method of introducing these charges of metals into the envelope is to place in the envelope before sealing a quantity of the solid compound of the active metal mixed with a reducing agent, commonly known as a getter and in pellet form, and then heating the getter to reduce the active metal compound and liberate the active metals in the form of vapor. The amount of active metal vapor driven from different pellets by heat is cliificult to control, apparently because the reaction temperatures of the pellets may vary considerably. Further, the amount or degree of reduction of the metal compounds is uncontrol lable apparently because the reaction between the compound and the reducing agent is exothermic and when the getter mixture is heated to a sufficient temperature the reaction proceeds with a sudden flash or explosion and results in an uncontrollable liberation of the active metal and the production on the bulb wall and electrodes of a film of active metal. This film appears to be more or less contaminated with reaction byproducts and other impurities and tests have shown that the active metal thus deposited is not in the most desirable and effective form. Difierent yields of the active metal from dilferent pellets often result in the manufacture of tubes with non-uniform characteristics. For example, the light sensitivity of photo-tubes or iconoscopes varies materially with the quantity andquality of sensitizing metal liberated in the envelope, an excess of the vapor being undesirable because condensation on insulating parts of the tube cause troublesome leakage current. The usual alkali metal compounds on cores of commercial metal are unstable below the reaction temperature between the metal and the compound.

It is an object of my invention to prepare getters of active metals for electron discharge devices which will liberate in said devices predetermined quantities of vapor from the active metals.

Another object of my invention is to provide applied to the core.

an improved getter of the active metal type by means of which a profuse yet easily controllable evolution of the active metal may be obtained.

Another object of my invention is to provide an improved method of gettering, useful in all types of vacuum devices.

Further objects and advantages of my invention will appear from the following specification and detailed description.

In accordance with my invention, the active getter metal is prepared as a compound, such as an oxygen compound of the active metal, and coated on a coherent body or core, such as a metal plate. As distinguished from the usual getter in which the reducing agent is powdered and intimately mixed with the compound of the active metal, the compound, substantially pure and undiluted, is, according to my invention, The reducing agent for the compound, in accordance with one characteristic feature of my invention, is introduced to the compound as a vapor which at elevated temperatures of the compound reacts with the compound and liberates the active metal. The reducing agent in vapor form may conveniently be obtained from an electrically heated element, such as a refractory metal ribbon or wire, coated with the vaporizable reducing agent and placed near the coated core. The wire may then serve to heat the core and its coating by radiant heat, as well as supply easily controlled quantities of vapor of the reducing agent to the coating on the core. The amount and rate of reduction of the compound and the liberation of the active metal depends on the temperature of the heated core, fixed by the distance between the heated wire and the core, and the rate reducing vapor is supplied to the compound on the core. The reduction of the compound on the core may, if desired, be augmented by making the core of a metal containing a reducing constituent. I have found that the liberation of the active metal may at will be started by raising and stopped by reducing the temperature of the core, or by starting or stopping the flow of reducing vapors to the heated compound coating. An oxygen compound of an alkali or alkaline earth metal, such as caesium permanganate, CsMnO4, coated on a plate of nickel or iron may readily be reduced and active caesium vapor liberated by heating the plate to a temperature of about 600 C. and projecting upon the coated surface of the plate a small amount of an alkaline earth metal vapor such as barium vapor preferably from a heated tantalum wire covered with a barium-strontium carbonate, such as specifically described in my copending application, Ser. No. 69,440, filed March 18, 1936, and assigned to the same assignee.

The novel characteristics of my invention may be more fully understood by reading the following specification in connection with the accompanying drawing in which:

Figure 1 shows an electron discharge device of the photoelectric type with a glass envelope and a getter made in accordance with my invention, and

Figures 2 and 3 show in detail getter units constructed in accordance with my invention.

The specific electron discharge device of Figure 1 chosen for illustrating one embodiment of my invention is a photo-electric tube comprising a glass envelope I enclosing a photo-cathode 2 and anode 3 mounted in cooperative relation upon a conventional re-entrant stem 4. Stem 4 at the lower end of the envelope carries an exhaust tube 5 through which the tube may be mechanically exhausted by pumps. For cleaning up residual gases after mechanical evacuation and for activation of the photoelectric cathode, I use, according to one embodiment of my invention, a getter assembly 6 comprising a metal box-like housing with an opening 7 in one end, as shown in Figure 2, and enclosing a heater wire 8 electrically connected at one end to the housing and passing centrally through the opening at its other end. On the inner surface of the housing is coated a layer 9 of a stable compound of the desired active metal, and on wire 8 is coated a reducin agent such as a metal which, when vaporized, will react with coating 9 to reduce the compound and liberate the active metal. The wire coating material may be chosen, not only for its reducing properties with compound 9, but for its gettering action of undesired gases in the envelope. If the desired reducing agent coating material is not stable in air, it may conveniently be prepared as a compound which can be deposited on the wire and decomposed with heat. Barium, for example, is a powerful reducing agent and may be prepared as a compound such as a barium or barium-strontium carbonate. Barium beryllcnate, BaBeOz, also has been found to be stable in air, easy to prepare, coat on the wire and decompose to vaporizable barium. The getter assembly is shown in Figure 1 below the electrode assembly with the metal container supported upon a lead-in wire and the end of the wire in the housing opening connected to a second lead-in wire, preferably one of the electrode lead-in conductors. To shield the elements in the tube and prevent deposition of metal upon insulating parts of the stem, the opening of the housing is preferably directed downwardly into the annular space around the re-entrant stem.

After the pressure in the envelope has been reduced to a few microns by mechanical evacuation, sufficient current is passed through the wire 8 to chemically decompose the wire coating and prepare the reducing agent for vaporization. Continued heating of wire 8 will vaporize some of the reducing metal and heat the walls of the metal housing. At the reaction temperature of the coating on the inner surface of the housing, the reducing vapor appears to gradually combine with and reduce the coating and slowly libcrate the active metal. The tube may then be sealed and based in the conventional manner; If, after aging, gas is found in the tube, or the photocathocle is found to be insufficiently sensitized, ad-

ditional active metal vapor may be liberated in the envelope merely by heating the wire with current by applying a voltage to the ends of the wire.

The metal housing, as shown in Figure 2, comprises two rectangular box portions I0 with outwardly projecting flanges which, when brought into registry and joined as by spot-welding, form a box-like housing open only at the end with the cut-out portions at 1. One end of wire 8 may conveniently be welded to and clamped between the two flanges opposite the opening, any one of the flanges being attached to a supporting lead-in conductor. The housing of Figure 2 may conveniently be stamped from a flat piece of metal, such as sheet-iron or nickel, and roughened on the surface exposed to the interior of the finished housing.

The inner surface of the housing is coated with a layer of an oxygen compound of an alkali or an alkaline earth metal, such as caesium permanganate, CsMnO4, or caesium bichromate, CS2CI2O7. Filament 8 may consist of a refractory wire such as tantalum, tunsten, zirconium, or molybdenum and coated with the desired reducing material or its compound.

After degassing, decomposing the wire coating, and mechanically exhausting the envelope, an electric current is passed through the filament, the temperature being gradually raised to heat the metal housing IE3 and its coating 9 and, at the same time, evolve some vapor from. the wire coating. The first portion of the reducing vapor thus evolved may escape through the opening of the housing and, in case the vapor is barium, function in the usual manner to getter residual gases in the envelope. A larger portion of the vapor, however, reacts with the heated surface of the compound layer 9 in the housing to reduce the compound and liberate active metal which, escaping through the open end of the housing, finds its way into envelope space and condenses upon the surfaces to be activated, such as the photo-cathode surface. According to my invention, the liberation of the active metal may, at will, be started by raising and stopped by reducing the temperature of the electrically heated wire. The rate and amount of evolution of the active metal from the coating 9 is determined by the temperature of the coating and by the rate and amount of reducing vapor supplied to the coating from the wire. The temperature of the coated core, fixed by the distance between the heated wire and the core, and the rate reducing vapor is supplied to the compound on the core determines the rate of reduction of the compound and the liberation of the active metal.

An alternative construction of my improved getter assembly is shown in Figure 3. A cylindrical housing H, enclosing a ring-shaped piece of perforated metal I2, is pinched closed at one end over the end of filament 8. A closed circuit including the wire heater may be completed through a supporting loop, as shown, and heated inductively by high frequency current. The perforated metal ring 52 may conveniently be dipped in a solution of the desired active metal compound and slipped with a snug fit into the metal tube. The perforated ring 52 may, if desired, be of a ferrous metal, such as iron which, it has been found, aids in the reduction. of the compound, supplementing the reducing action of the vapor from the wire. It has been found that by raising the temperature of a perforated iron ring coated with caesium permanganate to a' relatively high temperature, the iron functions to reduce the caesium permanganate and liberate caesium in copious quantities.

Good results have been obtained by applying a coating of caesium permanganate, mixed in powdered form with a binding agent of nitrocellulose, diethyl carbonate commercially known, when mixed with ten percent alcohol, as diatol, and diethyl oxylate, to opposite faces of a nickel housing, formed as shown in Figure 2, about .5 cm. on each side, and with .02 cm. tantalum heater wire wound into a coil with an inside diameter of .03 inch and 25 turns covered with 8 to 10 milligrams of barium berryllonate or barium-stronti um carbonate. A current of 3 to 5 amperes through the filament breaks down the barium compound and starts the evolution of barium which, in a phototube commercially known as the RCA 917, getters the residual gases, and a current of about 3 amperes starts a reaction between the barium vapor and the caesium compound. Clean caesium vapor flows from the end of the housing while the current is on, and stops almost immediately after the current through the wire is interrupted.

The getter prepared in accordance with my invention liberates predetermined and easily controlled quantities of active metal vapors. My improved getter is easy to make, inexpensive to manufacture, and is adapted for gettering residual gases, activating cathodes, and sensitizing light responsive electrodes in electron discharge devices with glass or metal envelopes.

I claim:

1. A getter comprising a refractory core, a coating on said core of a compound of a metal of the group consisting of the alkalis and the alkaline earths, a heating element adjacent the coated core, material on said element which, when vaporized, combines with said compound at reaction temperature to liberate said metal.

2. A device for liberating active metal in an envelope, comprising a refractory core, a heating element spaced from said core to heat the core, a reducing agent on said heating element vaporiz able by heat, a layer on the core of a compound of an active metal stable at temperatures below its reaction temperature with vapors of said reducing agent.

3. A device for liberating active metals, comprising a metal housing with an opening in its wall, a heating element or refractory reducing metal in the housing and spaced from the sides of the housing, a compound of a vaporizable metal reducible by said element coated on said heating element, and a compound of an active metal reducible by said vaporizable metal on said walls of the housing.

4. A device for liberating active metals comprising a cylinder, a heating element centrally held in said cylinder, a compound of a vaporizable metal coated on said heating element, and a compound of an alkali metal reducible by said vaporizable metal coated on the inner surface of said cylinder.

5. In combination a refractory core coated with a layer of caesium permanganate, a heating element adjacent the coated surface of said core, and an oxygen compound of barium on said heating element, said heating element being a refractory metal which reduces said compound at elevated temperatures to liberate barium.

6. In combination an iron core coated with a compound of caesium permanganate, means for heating the coated surface of said core to reduce said compound and liberate as vapor free metallic caesium.

'7. A getter comprising a refractory core, a coating of caesium-bichromate on said core, a refractory heating element adjacent the bichromate coating, and a compound of an alkaline earth metal on said heating element, said heating element being a material which reacts with said compound to liberate said metal at elevated temperatures.

8. A getter comprising a refractory core, a coating of a caesium compound on said core, a heating element of tantalum for heating said core, and a coating of a barium compound on said heating element.

ERNEST A. LEDERER.