US1925701A - Electron emissive device - Google Patents

Description

Sept. 5, 1933. F. MEYER 'ET AL 1,925,701

ELECTRON EIISSIVE DEVICE Filed March 28, 1929 INVENTOR Daz'edmb/ueyer /Phns Jgfioamm ATTORNEYS Patented Sept. 5, 1933 PATENT, OFFICE ELECTRON mvnssrvr DEVICE Friedrich Meyer and Hans Joachim Spanner,

Berlin Halensee, Germany, assignors to Electrons, Inc., a corporation of Delaware Application March 28. 1929, Serial No. 350,771, and in Germany June 9, 1926 22 Claims.

This application is a continuation in part of our Patent No. 1,817,636, issued August 4, 1931.

As stated therein our present invention relates to new and improved materials for emission of electrons for treating cathodes and filaments of discharge tubes and other purposes requiring useful electron emission.

A particular object of the invention is to provide a more stable emissive material than heretofore available for the conditions of use to which emissive materials are put, and more particularly in the case of so-called gas-filled tubes in which ionic bombardment is severe.

A further object is to provide an emissive material that will have long life under severe conditions of use, and which will permit of a wide range of temperature variations under different conditions of operation without having the disadvantage of ready disintegration.

Another object is to provide an emissive material that does not require an excessively high temperature of operation for effective emission.

Still another object is to provide an emissive material that can be prepared from raw materials that are not sensitive to atmospheric influence and do not require the special care in manufacturing processes heretofore characteristic of other emissive materials.

An additional object is the provision of an emissive material that will advantageously associate with core or supporting materials suitable for filaments and cathodes of tubes, and to avoid. undesirable chemical interference as between emissive materials and core materials.

From the researches of Prof. Wehnelt, the German scientist, made in about 1905, it is known that the oxides, chlorides and fluorides of the alkaline earth metals, such as barium, calcium and strontium, and mixtures thereof, are effective emitters of electrons at the very low temperatures of from about 600 to about 700 0. Subsequent to the work of Wehnelt it was found that all simply composed compounds of the alkaline earth metals also possess these electron emission characteristics. Among all these compounds, however, only the oxides have become of practical importance. When in practice other starting or raw materials such for example as the nitrates (according to the old method of Wehnelt) were used, the oxide that inevitably formed by conversion during the process of fabrication of the tube was, in the last analysis, the effective emitting substance.

Unfortunately these simply composed compounds just mentioned have the objectionable characteristic that they are little resistant to the destructive or disintegrating effect of the ionic bombardment accompanying a glow discharge, as in a gas-filled tube, and furthermore volatilize or otherwise change their emissive capacity with time under the influence of the temperatures normal to usual operating conditions in tubes of all types. These features are particularly characteristic of those compounds which have the most valuable property of very high emission at very low temperature, of which the compounds of barium are outstanding examples. Barium is the most electropositive of the alkaline earth metals, and in this characteristic even closely resembles the alkaline metals, and for this reason its compounds give the highest emission at the lowest temperatures, but on the other hand suffer most" from the undesired effects before mentioned. This accounts for the well known effect in tubes with oxide coated filaments of gradual loss of emissivity with duration of use, commonly recognized by the usual user in amplifying systems by loss of signal strength. This loss of emission with duration of use indicates an actual change of molecular arrangement in the 0 usual form of oxide coating, or at least a change in that part of the coating which was originally emissively activated.

We alsomention that our researches have brought out the fact that this volatilizing or loss of emission effect in the oxides is often influenced to a marked extent by the substratum or core material used with the oxide, there being apparently some derogatory chemical conversion process maintained between some core materials and the simple coating compounds.

Even up to the present time the manufacture of the Wehnelt form of cathode is still beset with considerable difficulty. During the manufacture the oxides change most readily, one example of this being the conversion that takes place following the absorption of carbonic acid gas from theatmosphere when the cathodes are necessarily handled in proceeding to manufacture. It is therefore most difficult to insure in the use of oxides for cathode coatings the uniformity in manufacture that is necessary to commercial products, and it is therefore difficult to avoid reject shrinkage, which shrinkage .greatly increases expense of production under such. a

process.

Prior to our invention endeavors to use in the manufacture of emitting heated cathodes the salts of the oxyacids of the alkaline earths, which 110 are much less sensible to atmospheric influences and much more lasting than the more or less unstable alkaline earth oxides discussed, have failed of success. These endeavors have been made with the sulphates, silicates, phosphates and carbonates, which have proven not to have suflicient emission at desired low temperatures for practical uses and requirements, a feature unquestionably due to the highly electronegative acid residue of these compounds interfering materially with the issue of electrons. It is not until the salts disassociate that a perceptible emission ensues; that is, when the volatile acid residue is isolated by high temperature to permit of the existence of emissive oxide free of the electronegative influence of acid, which is a possibllity in many cases. Of these compounds the carbonate is the. easiest to disassociate as set forth, and with a careful and complete disassociation, can produce a useful emissive coating for a cathode. Unfortunately the disassociation takes much time and is beset with difiiculties, so that it offers little towards practical cathode construction.

By a searching investigation of these circumstances we have found that there is quitea series of salts of the oxyacids of the alkaline earth metals, and even salts of non-volatile acids, possessed to a very marked degree of the ability to emit electrons even at a very low order of temperature. These salts are the compounds of the oxides of the highly electropositive metals, examples of which are those compounds in which the element is an alkaline metal andin which the radical consists of an oxide of a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid, such as aluminum, zinc, nickel, cobalt, zirconium or chromium.

The primary elements of these compounds are to be found in Groups I and II, Series 4, 6 & 8 of Mendeleefis Periodic System of the Elements as compiled by Alexander Silverman and published by D. VanNostrand Co. New York, copyright 1922, of which the following is an excerpt and incorporated herein in order that the invention as defined in the claims appended hereto may be fully understood.

All axomar'r mom 'MENDELEEFFS PERIODIC SYSTEM OF THE ELEMENTS MOSELEYS ATOMIC NUMBERS When these elements are formed into a com amphoteric compound such as an oxide of nickel, cobalt, aluminum, zirconium, chromium (of va lence 3 or less) or zinc we have found that an emissive compound is formed the emission temperatures of which are but immaterially higher than the corresponding alkaline metal oxides. These compounds possess however a stability with temperature such as is not possessed by the simple compounds. Thus, in contradistinction to barium which, if applied as an oxide to a platinum core or filament, soon volatilizes and loses emission under the bombardment infiuence of a discharge current, our compounds are capable, even after long use, of withstanding much heavier discharges.

For the manufacture'of cathodes and like applications the needed materials for the emitting compounds hereinbefore referred to may be prepared beforehand in accordance with chemical compounding instructions found in a manual of Prof. Gmelin, entitled .Gmelin-Krauts Handbook for Anorganic Chemistry," published at Heidelberg, Germany, Carl Winter, editor, and applied to the cathode core, filament or other desired surface after compounding. It is, however, desirable that the final chemical conversion to form the final compound or compounds takes place only on the cathode or other surface where the emission is to be used. In this way the coating can be started with materials which are absolutely resistant to air, so that filaments or surfaces can be prepared well in advance of making tubes, rather than immediately before, or kept in a vacuum if made in advance, as required by other coatings.

For preparing the aluminates or zirconiates of the alkaline earth metals, for example, the procedure may be adopted of applying to the sur face treated the'alkaline earth carbonates with hydrated or anhydrous aluminum or zirconium 115 oxide, together with a small quantity of an agglutinant or binder, the treated surface being thereafter heated in a vacuum where the carbonic acid. is easily driven off because of the desired emitting salt being formed atthe same time.

It is also possible to form the final emissive material of our invention without difficulty starting with the oxides or hydroxides of the alkaline earth metals or of the alkaline metals, the carbonic acid gas or other undesirable agents derived from exposure to the atmosphere during preparation, and thus incorporated in the material, being easily and rapidly expelled by the action of the acid oxides in the presence of heat, that is, the heating of the cathode or filament in the tube by heating current and/or bombardment during manufacture of the tube.

Another practical procedure for arriving at the final emissive compounds of our invention consists of oxidizing metals or alloys of them, or mix- 135 tures of metal powders or alloys of them, which metals or alloys may be the cathode cores or filament cores, and then treating with alkaline or alkaline earth metal oxides or carbonates as before mentioned, the final conversion with the oxide acid on the core taking place when heating is had during the manufacture of the tube as before mentioned. For example, nickel, cobalt, aluminum, zirconium, chromium and zinc are 14.5 suitable elements capable of forming amphoteric compounds which are less acid than titanic acid and the hydroxide of any of which is an amphoteric compound the resultant anhydrous radical pound the radical of which is an anhydrid of an .013 which when combined with one of the elements of Groups I and II Series 4, 6 and 8 of the Periodic Table forms an emissive compound so that the cores or filaments may be formed of any one of such metals, or alloys of them, or of pressed powdered mixtures of these metals or alloys of them, which, when oxidized, serve as the desired oxide acid residue for our new compound of emissive material as previously explained, and act with the alkaline earth metals or alkaline metals to form the final compounds as previously explained.

As shown in the drawing which diagrammatically illustrates the application of the filament to a tube a cathode of the type we propose is one having a core 1a the surface of which is a metal capable of forming an amphoteric compound less acid than titanic acid, such as the metals previously mentioned; that is. the core 1a may be composed of one of said metals or may have one of said metals as an outer layer or plating on one of the usual filament metals. Upon the surface of the core la we incorporate a coating 2a which consists of a compound or compounds as mixtures the element of said compounds consisting of an alkaline metal, that is one of the elements of Groups I and II of the Periodic Table and the radical of which consists of an oxide of a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid. In operative relation with the cathode there is an anode 3a.

For the sake of brevity and definiteness, the term alkaline metal used herein and in the appended claims is to be construed as generic to the alkali metals, i. e. those elements of Group 1 of the Periodic Table, and alkaline earth metals, i. e. those elements of Group 2 of the Periodic Table. As an example of the effectiveness of our compounds as emissive coating materials in practical uses of severe character, we have constructed a gas-filled multiple wave alternating current rectifier tube with gas pressures of about one millimeter of mercury, having a filament core as hereinbefore mentioned and coated with a mixture of the compounds hereinbefore described and operating with a red glow temperature of about 800 C. of about the size of filaments usual in radio receiver amplifier tubes for rectifying voltages of the order of several hundred to deliver currents of the order of one thousand milliamperes, which tubes have shown in use long life without loss of effectiveness of the filaments in spite of the severe ionic bombardment that necessarily exists under such conditions, and which would destroy an ordinary Wehnelt coating of simple compounds in a time so short as to make the tube of no practical value.

While we have described our invention more particularly in connection with cathodes or filaments for discharge tubes, yet its application is general to all requirements for electron emission, and no limitations are intended by reason of our limited description merely for explanatory purposes.

Having thus described our invention what we claim is:-

1. An electron discharge tube containing a gaseous medium and an incandescent electron emissive cathode, said cathode consisting of a core, the surface portion of which is a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid, and a compound on the core exposed to ionic bombardment, the element of which compound is a highly electropositive metal and the radical of which is an oxide of zinc.

2. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode consisting of a core of one of the metals in the group nickel, cobalt, aluminum, zirconium, chromium and zinc, and a compound on said core exposed to ionic bombardment, the element of which is an element of Group II, Series 4, 6 and 8 of the Periodic Table and the radical of which is an oxide of one of the first-mentioned group of metals.

3. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode consisting of a core at least the surface portion of which is one of the metals of the group nickel, cobalt and zinc, and a compound on said core exposed to ionic bombardment, the element of which is barium and the radical of which contains a metal of the said group.

4. An electric discharge tube containing a gaseous medium and an electron emissive cathode exposed to ionic bombardment, said cathode comprising a core the surface portion of which is metallic nickel, said core being coated with an electron emissive substance at least at its surface consisting of a compound the element of whichis one of the alkaline metals and the radical of which is an oxide of zinc.

5. An electric discharge tube containing a gaseous medium having an incandescent electron emissive cathode, said cathode consisting of a core the surface portion of which is metallic zinc, said core being coated with an electron emissive substance at least at its surface consisting of a compound, the element of which is one of the alkaline metals and the radical of which is an oxide of zinc.

6. A cathode for a gaseous discharge tube having a core the surface portion of which is one of the group of metals nickel, cobalt, aluminum, zinc, zirconium, and chromium, said cathode being rendered electron emissive at moderate temperatures by a coating on said core of barium zincate.

7. A discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a core,., consisting, at least in its surface portion, of metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid, said cathode being rendered electron emissive at moderate temperatures by a double-metal compound on said core, the components of said compound including barium and the core metal.

8. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a core at least the surface portion of which is metal capable of forming by chemical conversion an amphoteric compound less a cid than titanic acid, and a compound on the core exposed to ionic bombardment, said compound having as an element an alkaline metal, and as a radical an oxide of a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid.

9. An electron discharge tube. as defined in claim 8 in which the cathode comprises a mixture of the said compounds exposed to ionic bombardgroup of metals nickel, cobalt, aluminum and zinc, and a compound on the core exposed to ionic bombardment, said compound having as an element an alkaline metal and as a radical an oxide of a metal in the said group.

11. An electron discharge tube as defined in claim 10 in which the cathode comprises a mixture of compounds on the core each of which has as an element an alkaline metal and as a radical an oxide of a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid'.

12. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination acore having a surface of metallic nickel and an electron emissive substance on the core exposed to ionic bombardment, said substance at least at its surface consisting of a compound, the element of which is one of the alkaline metals and the radical of which is an oxide of nickel.

13. An electron discharge tube containing a gaseous medium and an incandescent electron emissive cathode, said cathode consisting of a core of a metal capable of forming an amphoteric compound less acid than titanic acid and acompound on the core exposed to ionic bombardment, the element of which compound is a highly electro-positive metal and the radical of which is an oxide of nickel.

14. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a nickel core and a compound on the core exposed to ionic bombardment and consisting essentially of a nickelate of barium.

15. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a core of a metal in the group nickel and cobalt and a compound. on the core exposed to ionic bombardment and consisting essentially of a cobaltate of barium.

16. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination a core of a metal in the group nickel, cobalt, and zinc, and a compound on the core exposed to ionic bombardment and consisting essentially of a zincate .of barium.

17. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a nickel core and a compound on the core exposed to ionic bombardment, the element of which is an alkaline metal and the radical of which is a metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid.

weaver 18. An electron discharge tube containing a gaseous medium and an electron emissive oathode, said cathode comprising in combination, a cobalt, core and a compound on the core exposed to ionic bombardment, the element of which is an alkaline metal and the radical ofwhich is a metal capable of forming by chemical conversion andamphoteric compound less acid than titanic aci 19. A cathode for a gaseous discharge tube comprising in combination, a core-consisting, at

least in its surface portion, of metal capable of forming by chemical conversion an amphoteric compound less acid than titanic acid, and an emissive substance exposed to ionic bombardment thereon having distributed throughout its mass a salt of a highly electropositive metal, said salt having as a component a compound capable ofbination of a highly electropositive metal oxide and a metal oxide which is capable of forming by chemical conversion an amphoteric compound less acid than titanic acid.

21. An electron discharge tube containing a gaseous medium and an electron emissive cathode, said cathode comprising in combination, a core at least the surface portion of which is one of the metals in the group aluminum, chromium, zirconium and zinc, and an electron emissive substance on said-core exposed to ionic bombardment and comprising, at least at the surface of said substance, the combination of a highly electropositive metal oxide and an oxide of one of the metals in the said group.

22. A cathode for "a gaseous discharge tube having a core the surface portion of which is one of the group of metals nickel, cobalt, aluminum, zinc, zirconium and chromium, said cathode being rendered electron emissive at moderate temperatures by a double-metal compound on said core, the components of said compound including an alkaline metal and at least one of the metals in the said group.

FRIEDRICH MEYER. HANS JOACHIM SPANNER.

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