US2107945A - Cathode structure - Google Patents

Description

Feb. 8, 1938. A. w. HULL ET AL CATHODE STRUCTURE Filed Nov. 20, 1954 2 Sheets-Sheet l Feb. s, 1938.

CURRENT A; W. HULL ET AL CATHODE STRUCTURE Filed Nov. 20, 19 34 2 Sheets-Sheet 2 Fig.6.

Their Attorney Patented Feb. 8, 1938 UNITED STATES PATENT OFFICE CATHODE STRUCTURE Application November 20, 1934, Serial No. 753,875

GClaims.

The present invention relates to electronic discharge devices and is concerned in particular with cathodes for such devices.

As a consequence of our invention we have provided a new type of thermionic cathode which has improved characteristics, has a longer useful life than cathodes heretofore used and the operatibn of which in electronic devices is substantially free from the disturbances due to overload discharges as later will be explained.

It has been suggested heretofore to provide a thermionic cathode with a reserve supply of activating material, for example, alkaline earth oxide, the reserve supply of such material being located adjacent the heated electron-emitting member. In the forms of such cathodes suggested or used the emitting material hasbeen in a position which permits the arc, or other discharge emanating from the cathode, to come into contact with the reserve material. During operation, some of the activating material is transferred by volatilization to a heated surface of the cathode, thus replenishing material which has been lost by evaporation. In the operation of such cathodes concentration of electric discharge emitted by the cathodes occurs adjacent the activating material resulting in cathode hot spots and in the case of an overload of current through the device the current emitted by such a hot spot tended to increase to destructive values.

to Irving Langmuir on October 24, 1917, aredescribed thermionic cathodes containing thorium material (for example the oxide) which may be intimately admixed with the metal of which such cathodes are made. In the cathodes described in this patent, metallic thorium is capable of slowly difiusing at the operating temperature of the cathode through solid metal from'the interior portions of the cathode to the surface where it forms a highly emissive surface film of thorium which ordinarily is only of molecular thickness. When a cathode of the type described in this patent is subjected to overload, or in any way to excessive ion bombardment, the emitting layer of thorium is removed from the cathode and the electron emission drops to a low value.

It has been found that the oxides of alkaline earth metals, when embodied in the same way in thermionic cathodes, will not behave in the same manner as the oxide of thorium. Apparently any reduced alkaline earth metal which may be present in interior parts of the cathode viding pores,

closed within a cathode provide pore-like migration paths whereby the In United States Patent 1,244,216, patented diffuses to the surface too slowly at normal opcrating temperatures to maintain an emitting film on the'surface of the cathode.

In accordance with our present invention, we have provided thermionic cathodes containing an alkaline earth, or equivalent electron-emitting material, in a location in which it is shielded fromthe discharge, the cathodes further procrevices, joints, or other structure providing migration rial may pass from inaccessible interior regions to an exterior electron-emitting surface of the cathode. By the term migration path we mean to designate a surface (which may be that of a pore or crevice) .over film may spread. Such paths are referred to generically herein as -pore-like'by which we intend to designate a passage at least one, though paths wherebysuch matewhich a monomolecular not necessarily both, of whose cross-sectional dimensions is essentially of pore-like magnitude. Cathodes embodying our invention have electrical operating characteristics which are similar in many respects to those of the thoriated type of cathode described in Langmuir Patent 1,244,216.

In cathodes embodying our present invention an emitting material, such as one or more oxides of alkaline earth the addition of a reducing agent is wholly enwhich is constructed to alkaline earth metals may pass from the interior portions to and over the exterior surface of the cathode. The emitting material enclosed by said cathode discharge that concentration of the discharge and the formation of a hot spot, as well as any appreciable escape of such material by volatilization is prevented.

As a consequence of our invention, the thermionic cathode becomes coated with only a very thin layer of emissive material, which is essentially of monatomio thickness, and whose electron emission cannot be greatly enhanced by positive ion Such cathode hence is immune 45 metals, with or without is so completely shielded from the emissivity of an oxide-coated surface. For example, the emission from a clean nickel surface is only one-fifth of that of a barium-oxide surface. Hence, less heat is required to maintain our cathode at operating temperature.

Our invention will be explained with greater particularity in connection with the accompanying drawings in which Fig. l is a side elevation and partly'in longitudinal section of an electron discharge device containing a cathode embodying our invention; Figs. 2 and 3 are detail views of the cathode of Fig. 1 here shown on different enlarged scales; Fig. 4 is a side elevation also partly in longitudinal section of a discharge device containing a modified cathode; Fig. 5 is an enlarged sectional view of the cathode of Fig. 4; Fig. 6 illustrates partly in vertical section a threeelectrode device embodying another modification of our invention; Fig. 7 is an enlarged sectional view of still another form of cathode; and Fig. 8 is a graph showing a volt-ampere characteristic of a device embodying our invention and also the contrasting volt-ampere characteristic of a device containing an oxide-coated cathode.

The device shown in Fig. 1 comprises an envelope I, consisting of glass, or other suitable material, through the stem 2 of which are sealed cathode conductors 3, 4, and an anode conductor 5. A helical cathode 6 formed into convolutions of relatively great length with respect to its crosssectional area is connected to the conductors 3, 4. It is surrounded by a housing I having an opening for the passage of electrons. One of the main functions of the housing is to shield the cathode from contamination by water vapor or other gases emanating from the envelope. It also functions incidentally as a heat conserver. The conductor 3 passes through an opening in the bottom of the housing I. The conductor 3 may be mechanically and electrically connected to the housing I by a wire 8. The conductor 4 is insulated electrically from the housing by an insulating bushing 9. The anode II in this device is represented by a circular band of metal which is connected to the conductor 5 by welding or otherwise. The anode may assume various forms. The insulator I 2 on the anode conductor 5 prevents undesired shortcircuiting of the conductor 5 to the cathode. This insulator l2 may consist of magnesia, alumina, or other suitable refractory material. The envelope after thorough evacuation is provided with a filling of gas, or vapor, as for example argon, neon, mercury sodium or caesium, or a mixture of these gases and vapors at a pressure within a range of about one micron to several millimeters.

The cathode shown in Figs. 2 and 3 comprises an enclosure ll of a woven or mesh structure which may consist of a multiplicity of bundles of very fine wires of nickel, molybdenum, 01'- tungsten and which, as shown in Fig. l, is capable of serving as a resistance path for heating current, whereby the cathode is brought to emitting temperature. In the interior of the enclosure there is provided a mass l5 (Fig. 3) of oxide of alkaline earth, as for example barium oxide.

The filling l5 of oxide may assume various forms. When the cathode is to be coiled as shown inFig. 2, I preferably employ coarse granular material which permits of bending, twisting, or other deformation. In other cases a coherent pressed material may be used. The strands of the woven structure have been indicated at l6 (Fig. 3) without showing them in all cases to comprise a number of finer wires. For the 58.12?

of avoiding confusion in the'drawing, these fine wires making up the individual strands have been indicated only in a few spots. An electrical and mechanical connection between the woven envelope and a terminal conductor may be made by binding the parts together by one or more turns of wire as indicated at IT, Fig. 3. Such woven or braided material permits not only of deformation and enlargement to give the cathode any desired form, but by the close contact of its individual members to one another provides a substantially impervious fabric which precludes the rapid volatilization of oxide from the interior to the exterior. The interstices between the juxtaposed contacting elements of the woven material being of pore-like nature as previously defined are too small to permit of the discharge entering the interior of the cathode. The egress of emitting material, on the other hand, is by this structure limited to surface migration over the contiguous surfaces of individual wires.

The oxide may be admixed with a material which will slowly reduce the oxide during the life of the cathode, as for example metallic nickel or tungsten, but for many purposes it is quite sufiicient to employ the alkaline earth compound without the addition of any reducing agent. Even when the emitting compound is unassociated with a reducing agent the active metal which is liberated in small amounts either by dissociation or reduction migrates along the bounding surfaces of the members of the cathode to the exterior and forms upon the exterior surface of the cathode a monomolecular film of high electron emissivity.

The active electron-emitting film formed on a cathode embodying our invention is unchanged by a slight overload-but is, removed by a high voltage drop, and the emission of the cathode is reduced to a low value.

This characteristic is illustrated in Fig. 8 in which graph A illustrates the volt-ampere characteristic of a discharge device containing a cathode constructed in accordance with our invention. As the voltage drop at the cathode increases from about 8 volts to 13 volts the current rises to a saturation value at which this saturation remains until the voltage drop has increased to about 23 volts. The particular value of the current, of course, depends on the rise and other characteristics of the cathode. Increases of voltage drop above the disintegration voltage result in a decrease of current .as clearly shown by the graph. 'This is due to the removal of the electron emitting film by positive ion bombardment. GraphB illustrates the volt-ampere characteristic of a cathode in which alkaline earth oxide, or equivalent activating material, is exposed to the discharge. In such a cathode the current increases slowly as the voltage rises from about 8 volts to approximately 2'? volts. Further increase of voltage drop produces an abrupt increase of current due to positive ion bombardment and the resultant heating of the oxide portion of the cathode. As indicated by thegraph, the current increases so rapidly as to give the discharge a so-called runaway characteristic.

Our invention is capable of various modifications as illustrated by the embodiments of Figs. 4 and 5. In the device shown in Fig. 4 the cathode l8 consists of a tightly woven mesh H! (see Fig. 5) which is partially surrounded by a slotted metallic shield 20. In the interior of the wire mesh is provided an electric heater 2| which is mounted on a refractory support 22. The heater is connected to electric supply conductors 23 and 24 which are sealed into the stem 25. The heater support 22 may consist oi. alumina, beryllia, magnesia, or a suitable electron-emitting oxide ma terial, such as barium oxide may be used. Packed around the heater and generally filling the space between the heater 2| and the wire mesh I9 is a supply of activating material as shown at IS in Fig. 3. The ends of the cathode of Fig. 4 are closed by plates 26, which may consist of metal, or of refractory non-conducting material. The envelope 2?, which may consist of glass, after evacuation and prior to scaling, is charged with a suitable gas or vapor as already described in connection with Fig. 1.

In the device shown in Fig. 6 the cathode is constructed to be heated by passage of current which enters the cathode by a centrally located conductor 39 which passes from an exterior contact plate 32 to an interior contact plate 33 at the opposite extremity of the cathode. The wire mesh 34 is electrically connected to the contact plate 33 and may be bound thereon by a wire 35. The circuit is completed to the opposite end of the wire mesh by a cylindrical conductor 36 which is connected to a contact plate 31.

The space between the conductor 3i and the wire mesh 34 is filled with activating material as indicated. In order to avoid undue conduction of electric current between the conductor 3| and the cathode through the barium oxide or other activating material, this material may be mixed with a suificient refractory adulterant, such as alumina, to render it non-conducting; or the conductor may be surrounded by an insulating tube 38 of alumina or other refractory insulating material.

Surrounding the cathode are helical members 39, 39' spaced to leave a longitudinal slot for the escape of electrons. These members form a shield to prevent contamination of the cathode by gaseous products from the envelope and also to conserve heat radiated from the cathode and to equalize its temperature.

In the device shown in Fig. 6 the anode is constructed by the exterior enclosing envelope 40 which is connected to a contact plate 4|. The envelope 40 is provided with end wall members 42 and 43 hermetically sealed as by welding to the cylindrical portion of the envelope. Interposed between the cathode and the metal envelope 40 is a grid 44 which is joined to an ex terior contact plate 45. The various contact plates above described are joined to one another by the cylindrical insulating and sealing members 48a, 46b, and 460, consisting of glass or other suitable material which will make a vacuum-tight seal with the metal members.

In Fig. 7 is shown a cathode modification, in which the covering for the electron-emitting material comprises a wall structure composed of directly contacting metallic laminations extending transversely of the wall structure. In the particular arrangement illustrated the wall structure comprises a plurality of annular metallic disks comprising a plurality of washer-shaped rings or helically wound ribbon 48 which may be held together by rods 49, 49', together with insulating refractory end plates 50, 5D. Enclosed in the hollow structure thus formed is a mass of electron emitting material 5! in which is embedded an electric heater 52, the supply conductors 53, 54 of which pass through the end plates of the cathode. The contacting surfaces of the disks 48 constitute migration paths for the passage of barium, or the like, to the external surface.

It will be understood that various forms of porous or compositely structured cathodes may be used in accordance with our invention to provide a reservoir of emitting material which is shielded from direct contact with the are or other discharge being supported by the cathode.

While our invention has been illustrated .in connection with devices useful for rectifying or controlling power currents it is capable of use also in other fields, as for example, in gaseous conduction lamps, such as neon or sodium lamps. Cathodes embodying our invention likewise may be used in vacuum devices.

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

l. A cathode comprising a supply of electronemitting material and a covering for the same, said covering being constituted of a mesh of fine metal wires braided or woven so closely as to form a substantially impervious fabric which affords only surface migration paths for the restricted passage of emitting material through the covering.

2. A cathode comprising a supply of electronemitting material and a covering for the same composed of a multiplicity of bundles of fine wires, said bundles being so interwoven as to form a substantially impervious fabric which permits egress. of the emitting material only by surface migration over the contiguous surfaces of individual wires.

3. A cathode including a supply of electronemitting material and a covering for the same comprising a multiplicity of metallic elements assembled in juxtaposed contacting relationship to form a substantially impervious wall structure, said wall structure permitting egress of emitting material primarily by surface migration over the contacting surfaces of adjacent elements.

4. A cathode comprising a supply of electronemitting material and a closed container for the same, said container comprising a multiplicity of metallic elements assembled in juxtaposed conv tacting relationship to form a substantially impervious wall structure but providing porelike migration paths between the contacting surfaces of adjacent elements.

5. A cathode including a supply of electronemitting material and a covering for the same comprising a wall structure composed of directly contacting metallic laminations extending transversely of the Wall structure, said wall structure permitting egress of emitting material only by surface migration over the contacting surfaces of adjacent laminations.

6. A cathode comprising a supply of electron emitting material and a wall structure composed of a plurality of directly contacting annular metallic disks so assembled as to form a substantially impervious enclosure for the electron-emitting material, said wall structure being substantially impervious except for restricted migration paths provided between the contacting surfaces of adjacent disks.

ALBERT W. HULL. WILLIAM A. RUGGLES

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