US1835118A - Alkali metal tube - Google Patents

Description

Patented Bee. 8, 1931 e y QFFICE JOHN W. MARDEN AND ERNEST A. LEDERER, OF EAST ORANGE, NEW JERSEY, AS-

SIGNORS TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ALKALI METAL runs No Drawing.

This invention relates to a vacuum electric device employing a thermionic active cathode, the thermionic emission of which is imparted thereto by the presence of an alkali metal vapor such as caesium, rubidium or the like, and more particularly to the introduction of alkali metals into such devices to render the cathode thermionically active and to serve as a getter to clean up residual gases and "apors.

It has been found that alkali metals of high vapor pressure, particularly caesium, under proper conditions greatly increase the electronemission of a metallic cathode, such as a tungsten cathode in a thermionic device,

so that high electron emissivity is imparted thereto at a temperature so low that in the absence of such alkali metal vapor no substantial emission would occur. This phenomenon is explained on the theory that a film of the alkali metal forms continuously 'onthe metallic cathode, as for instance a filament, which film resists evaporation at temperatures much in excess of the normal vaporization temperature of tlfealkali metal when the cathode consists of a suitable metal such as tungsten. .The formation of the caesium film is greatly facilitated by the presence of an electro-negative gas layer on the surface of the cathode. Oxygen has proven to be the most satisfactory material for use in this connection and when an oxide or oxygen film of monotomic thickness is produced on the surface of the tungsten or other metallic body it renders this body capable of holding on to or causing the caesium metal atoms to adhere thereto-,more strongly than does the plain metal surface of the cathode.

In the construction of such devices, difliculty has been encountered due to the sensitivity of this active film of alkali metal and to the ease with which the monotomic layer of oxide or oxygen produced on the surface of the cathode is destroyed, as by the presence of hydrocarbons, hydrogen or other reducing materials in the envelope. Heretofore, the caesium which is employed to activate the oxidized cathode has been relied upon for eliminating all residual gases from the envelope. The caesium metal has been introstructure employed in devices of t Application filed June 11, 1928, Serial No. 284,654.

material. Its clean-up action has been ordinarily considered sufiicient to remove all residual and deleterious gases and vapors.

We have found, however, that while caesium is efl'ective to remove most of the harmful impurities from such devices so as to produce therein a substantially negligible gas pressure below that required in ordinary electron discharge devices of the thoriated tungsten cathode type or of the oxide coated type, yet in the case of devices depending upon caesium emission from the cathode, the minute quantities of reducing gases or vapors which remain in the envelope are in some instances sufiiciently active to destroy the monotomic layer of oxygen or oxide on the cathode and thereby reduce or destroy thepower of the oxidized cathode to hold the caesium metal onto the surface thereof.

Upon seasoning devices of this nature employlng caesium as both the thermionically active material and the clean-up agent, all of the gases seem to disappear and a high vac,- uum is obtained, but upon standing unused for some time gas reappears, in some instances, in suificient quantity to afiect the operation of the tube, at least when first placed ositive ions cathodeortothenegativelycharged The 1s nature is such that the major portions of the ions formed between the cathode and the anode" come in contact with the grid and the grid is charged negatively by an outside source of potential in order to prevent or restrict positive ion bombardment of the cathode by such ions. Assumin gas is hydrogen, for instance, this gas, upon seasoning of the tube, becomes posit1vely ionized and-is drawn over against the negatively charged grid where it is held by electrical that the troublesome .tion on the oxidized cathode. overcome this defect we have previously proattraction as lon as the negative bias is maintained on the grid, thus removing the major portion of the gas from the discharge space. However, when the negative grid bias is removed by discontinuing the operation of the the tube it comes in contact with the oxidized cathode and exerts a reduction action on the monotomic oxide or oxygen layer, eventually destroying the same and rendering the tube inoperative.

While hydrogen has been mentioned as an example of the reducing gas which may be present, it is to be understood that other reducing materials, such as hydrocarbons, carbon monoxide, etc., may be and probably a re present in many instances.

The .aflinity of the alkali metals for hydrogen increases as the atomic weight of the alkali metal decreases and, therefore, since caesium has the highest atomic weight of the alkali metals it is less stable with hydrogen than the other alkali metals of lower atomic weight. Therefore, in order to assist in maintaining non-reducing conditions in the device we introduce into the device, in

' addition to the caesium metal, another alkali metal of lower atomic weight, such as potassium, sodium, lithium or an alkaline earth metal. The introduction of these metals must be carried out with extreme care to prevent the introduction of moisture therewith. The alkali metals themselves, due to their high volatility and chemical activity in the atmosphere, cannot be introduced directly into a device in their free state and even if so introduced prior to evacuation of the device it is not possible to maintain them in a nonyolatile state while baking the envelope durmg the exhaust operation.

Various methods have been proposed for introducing the alkali metals such as vaporization from a side tube or container attached to the device, but the method which we have found preferable is to introduce alkali metals in the form of a stable compound which is subsequently decomposed by means of a simple reducing agent in order to liberate the alkali metal in the device. Some difficulty has been experienced with this last method due to the absorption of moisture by the compounds and the reducing agents prior to their introduction intoan evacuated device. The water vapor liberated at the time the decomposition takes place exerts a reducing ac- In order to posed the use of a class of alkali metal compounds and reducing agents which are extremely non-hygroscopic. In application Serial No. 96,335, of J. W. Mai-den, filed March 20, 1926, entitled Introduction of alkali metals into evacuated containers, there is set forth a number of compounds of this nature. Briefly, these compounds consist of double halide salts, such as caesiumzirconium fluoride or caesium-uranium or zirconium fluoride, or such stable compounds as caesium-permanganate, caesium-chromate or caesium-dichromate. These salts, in powdered form, are mixed with a metallic reducing agent such as magnesium, aluminum, misch metal or a metalloid such as silicon and boron, and pressed into pellets of appropriate size. These pellets may be introduced into the en velope of the electric discharge device in such a manner as to be heated by high frequency induction currents, and when so heated a reaction takes place causing a reduction of the caesium compound and the liberation of the caesium metal.

The compounds which we prefer to employ when caesium alone is introduced are the dichromate or permanganate of caesium mixed with finely divided silicon in the proportions of about three parts of the caesium metal compound to one part silicon. This getter compound is described and claimed in an application, Serial No. 218,570, of J. \V. Mardeu and E. A. Lederer, filed September 9, 1927, and entitled Production of active metals. In accordance with the present invention we propose to add to the getter mixture, set forth in the application just referred to, a small proportion of a stable clean-up agent such as a non-hygroscopic compound of one of the alkali metals of lower atomic weight, preferably potassium dichromate or permanganate. The amount of alkali metal of low atomic weight required for cleaning up residual gases is relatively small and satisfactory results have been obtained using from lto of the compound of alkali metal of lower atomic weight in the getter pellet, such percentage based on the caesium compound content. Lithium forms the most stable hydride of all the alkali metals and should exert the most efi'ective clean-up action, but lithium compounds are more likely to absorb water vapor from the atmosphere than the other alkali metal compounds, as potassium dichromate, and, therefore, in some instances at least, the potassium salts are preferable.

caesium and rubidium dichromates and permanganate are stable and may be dried completely free from water vapor and they do not re-absorb moisture upon standing. This is also true of potassium and sodium permanganates and di'chromates and to a lesser extent of these compounds of lithium. All of these compounds are readily reduced by metalloids such as silicon and boron, which may also be rendered extremely gas and moisture free and which do not re-absorb such deleterious materials.

The metallic reducing agents such as magnesium, calcium, etc., which have been employed heretofore, give ofi' certain gases, such as hydrocarbons, hydrogen or carbon monoxide, in small quantities or in the case of calrial in diluted hydrochloric acid until gas evolution entirely ceases. Various proportions may be used, but we prefer to employ about 50 grams of powdered silicon or boron in 100 cc. of a 1 to 4% solution of hydrochloric acid. After the gas has been completely eliminated in this material, the metalloid is filtered out, washed and dried.

The silicon or boron may be mixed with caesium-dichromate or permanganate and the dlchromate or permanganate of the alkali metal of lower atomic weight, such as potassium, in the proportions of about three parts of the alkali metal compounds to one part silicon. The mixture of alkali metal compounds may consist, as stated above of from 10 to 50% of the compound of the alkali metal of lower atomic weight. This mixture is pressed into pellets and introduced into the envelope in contact with a metallic body capable of being heated by high frequency induction currents or other suitable manner.

Upon the heating of this mixture in the envelope the compound is reduced and the alkali metal liberated without the evolution of any gases or impurities except a small quantity of oxygen. The oxygen, however, is beneficial in that it may serve as the source of oxygen for oxidizing the tungsten cathode or to flush out-the envelope and thus tend to free it from reducing gases. The reaction is such that the oxygen is all liberated subsequent to the liberation of the alkali metal and may be readily removed from the envelope by the pumps before the alkali metals are set free.

The temperature required to start the reae tion between thereducing agent and the alkali permanganates 'or dichromates is estimated at about 900 C., but this temperature may be somewhat reduced by adding to the mixture a small proportion of ametallic re- .ducing agent insulficient'inquantity to give off a detachable quantity of gas, as for instance about 1% of finely powdered aluminum. This additionof aluminum reduces the reaction temperature of the mixture to around 700 C. Commercial powdered a1u- 2000 C. in this oxygen atmosphere, after which the oxygen is removed by pumping.

After the complete removal of the oxygen from the bulb, the caesium is liberated by heating the pellets to decompose the caesium dichromate or permanganate. The caesium forms a deposit on the oxidized tungsten cathode and is held thereto, at least in an atomic layer, by the electro-negative oxygen gas layer and thus serves as the active electron emitting portion of the cathode. At the same time the caesium is liberated, the alkali metal of lower atomic weight, such as potassium, is also liberated and assists the caesium in cleaning-up the residual gases and vapors in the envelope by forming stable com pounds therewith.

-While the present invention has been described with particular refernce to the use of alkali metal dichromates or permanganates, it is to be understood that any other stable non-hygroscopic compounds of caesium and other alkali metals of lower atomic weight or other clean-up agents may be emploved and we do not desire to be restrictedother alkali metal of lower atomic weight with a reducing agent and heating the m1xture to liberate the alkali metals in the device.

2. The method of activating a metallic cathode for electron emission purposes and cleaning up residual gases and vapors in an electron discharge device of the caesiated cathode type, comprising heating to reaction temperatures in the device a mixture of a compound of caesium and a compound of potassium with a reducing agent to liberate these metals in the device.

3. The steps in the method of producing a thermionic discharge device employing a caesiated cathode which comprises liberatingin the' device from chemical compounds of which they are elements a quantity of forms more stable compounds with hydrogen than does the caesium.

4. The steps in the method of producing a thermionic discharge device employing a cacsiated cathode which comprises liberating in the device from chemical compounds of which they are elements a quantity of caesium and potassium.

5. The steps in the method of producing a thermionic discharge device employing a czesiated cathode which comprises introducing into the device a non-hygroscopic compound of caesium and a compound of another alkali metal of lower atomic weight and reducing said compounds to liberate the caesium metal and other alkali metal of lower atomic weight.

6. The steps in the method of producing a thermionic discharge device employing a cacsiated cathode which comprises introducing into the device a mixture of caesium dichromate and potassium dichromate with a reducing agent, and heating the mixture to cause a reduction of the caesium and potassium dichromates and liberation of thecaesium and potassium metals.

7. The steps in the method of producing a thermionic discharge device employing a czrsiated cathode which comprises introducing into the device a mixture of caesium dichromate and potassium dichromate with silicon. and heating the mixture to cause a reduction of the caesium and potassium dichromates and liberation of the caesium and potassium metals.

8. A getter for electron discharge devices employing caesium therein, comprising a mixture of a compound of caesium and a compound of another alkali metal of lower atomic weight mixed with a reducing agent.

9. A getter for electron discharge devices employing caesium therein, comprising a mixture of a compound of caesium and a compound of potassium mixed with a reducing agent.

10. A getter for electron discharge devices employing caesium therein, comprising a mixture of a nonhygroscopic compound of caesium and a reducing agent and a small percentage of a compound of another alkali metal of lower atomic weight.

11. A getter for electron discharge devices employing caesium therein, comprising a mixture of caesium dichromate, potassium dichromate and a reducing agent.

12. A getter for electron discharge devices employing caesium therein, comprising a mixture of caesium dichromate, potassium dichromate and silicon.

13. A getter for introducing alkali metals 5 into evacuated devices comprising approximately three parts of a mixture of the dichromates of caesium .izid potassium mixed the potassium dichromate comprising about 10 to 50% of the dichromate mixture.

14. A getter for electron discharge devices employing a caesiated cathode, comprising a mixture of a nonhygroscopic compound of caesium and a reducing agent, said admixture containing from 10 to 50% of acompound of another alkali metal of lower atomic Weight, such percentage being based on the total alkali compound content.

15. A material for generating alkali metals in an evacuated device comprising a mixture of a non-hygroscopic compound of caesium, a non-hygroscopic compound of potassium and a reducing agent.

In testimony whereof, we have hereunto subscribed our names this 8th day of June,

JOHN W. MARDEN. ERNEST A. LEDERER.

DBSCLARMER 1,835,118.Joim W. Marden and Ernest A. Lederer, East/ Orange, N. J. ALKALI METAL TUBE. Patent dated December 8, 1931. Disclaimer filed November 4, 1933, by the assignee, Westinghouse Lamp Company.

Therefore, enters this disclaimer to that part of said specification and claims which are in the following words, to wit:

1. The steps in the method of producing a thermionic discharge device employin a caesiated cathode which comprises introducing into the device a mixture of a re ucible compound of caesium and a compound of another alkali metal of lower atomic weight with a reducing agent and heating the mixture to liberate the alkali metals in the device.

2. The method of activating a metallic cathode for electron emission purposes and cleaning up residual gases and vapors in an electron discharge device of the caesiated cathode type, comprising heating to reaction temperatures in the device a mixture of a compound of caesium and a compound of potassium with a reducing agent to liberate these metals in the device. I

3. The steps in the method of producing a thermionic discharge device employing a caesiated cathode which comprises liberating in the device from chemical compounds of which they are elements a quantity of caesium and another alkali metal which. forms more stable compounds with hydrogen than does the caesium.

4. The steps in the method of producing a thermionic discharge device employing a caesiated cathode which comprises liberating the device from chemical compounds of which they are elements a quantity of caesium and potassium.

8. A getter for electron discharge devices employing caesium therein, comprising a mixture of a compound of caesium and a compound of another metal of lower atomic weight mixed witha reducing agent.

9. A getter for electron discharge devices employing caesium therein, comprising a mixture of a compound of cae 'um and a. compound of potass1um,mixed with a reducing agent. r

' [Oflic'ial Gazette December 5, 1.983.]