Cathode structure

Feb. 8, 1938.

A. W. HULL ET AL

CATHODE STRUCTURE

Filed Nov. 20, 1934

2.107,945

Sheets-Sheet 1

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Inventors:

AlbertW. Hull, William A.

Jby £ S

Their Attorney

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Feb. 8, 1938.

A. W. HULL ET AL

CATHODE STRUCTURE

Filed Nov. 20, 1934

2,107,945

Sheets-Sheet 2

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CURRENT

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Their Attorney

Patented Feb. 8, 1938

2,107,945

UNITED STATES PATENT OFFICE

2,107,945

CATHODE STRUCTURE ^WU1!^{am A Ku}«’«- Schenectady, N. Y., assignors to General Electric Company, a corporation of New York

Application November 20,1934, Serial No. 753,875

Claims. (CL 250—27.5) ’^plle 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 uselul life than cathodes heretofore used and the operatfen of which in electronic devices is substantially free from the disturbances due to W overload discharges as later will be explained it has been suggested heretofore to provide a thermionie cathode with a reserve supply of activating material, for example, alkaline earth is TL.^theJ^{eserve supply of 511011} material being ^{adjacent th}® heated electron emitting member. In the forms of such cathodes suggested or used the emitting material has been in a position which permits the arc, or other discharge emanating from the cathode, to come 20 into contact with the reserve material, nuring operation, some of the activating material is transferred by volatilization to a heated surface or the cathode, thus replenishing material which has been lost by evaporation. In the operation 25 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 ³⁰ tiv^valueT ^{SPOt tended to} Crease to destrucIn United States Patent 1,244,216, patented to Irving Langmuir on October 24, 1917, are de^^bed ,^tllormionic cathodes containing thorium 3_O material (for example the oxide) which may be intimately admixed with the metal of which such cathodes are made. In the cathodes descnbed in this patent, metallic thorium is capable of slowly diffusing at the operating tem⁴⁰ °^f?^{he cathode} through solid metal fu ^m ϊθ ^mtenor Potions of the cathode to HrfaT ^wh.^er® ^forms a highly emissive surface film of thorium which ordinarily is only of molecular thickness. When a cathode of ⁴⁵ ,^yP! ^{deseribed in this} Patent is subjected to overload, or m 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 m thermionic cathodes, will not behave in the same manner as the oxide of thorium. Apparently any reduced alkaline earth metal which όό may be present in interior parts pf the cathode diffuses to the surface too slowly at normal operating’ temperatures to maintain an emitting Him on the surface of the cathode.

In accordance with our present invention we ^nave Provided thermionic cathodes containing 5 an alkaline earth, or equivalent electron-emitting *2 ^{a location} th which it is shielded »om the discharge, the cathodes further providing pores crevices, joints, or other structure providing migration paths whereby such mate- 10 rial may pass from inaccessible interior regions ^t0J^.^tertor 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 which a monomolecular 15 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 not necessarily both, of whose cross-sectional t^>^enesseit^{tiaUy of} Pore-like rnagni- _2ntude. Cathodes embodying our invention have ²⁰electrical operating characteristics which are similar in many respects to those of the thori^teCribed * „7“ cathodes embodying our present invention ²⁵an emitting material, such as one or more ox°L»^{kallne earth} metals, with or without the addition of a reducing agent is wholly enclosed within a cathode which is constructed to ,_nProride pore-like migration paths whereby the ³⁰alkaline earth metals may pass from the interior portions to and over the exterior surface of the cathode. The emitting material enclosed by said cathode is so completely shielded from the 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 monatomic thickness, and whose electron emission cannot be greatly enhanced by positive ion bombardment. Such cathode hence is immune from undesirable concentration of the discharge Indeed, the monatomic film tends to be removed by the action of an abnormal discharge thereby automatically reducing electron-emissivity and stopping |he abnormal discharge. _r_

Another advantage of our cathode as compared with oxide-coated cathodes which were heretofore known consists in its lessened heat emissivity which is that characteristic of a metal surface and. is only a small percentage of the heat- 55

2,107,945 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 5 cathode at operating temperature.

Our invention will be explained with greater particularity in connection with the accompanying drawings in which Fig. 1 is a side elevation and partly in longitudinal section of an electron 10 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 de16 vice 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 20 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 30 of relatively great length with respect to its crosssectional area is connected to the conductors 3, 4. It is surrounded by a housing 7 having an opening for the passage of electrons. One of the main functions of the housing is to shield the 35 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 7. The conductor 3 may 40 be mechanically and electrically connected to the housing 7 by a wire 8. The conductor 4 is insulated electrically from the housing by an insulating bushing 9. The anode 11 in this device is represented by a circular band of metal which 45 is connected to the conductor 5 by welding or otherwise. The anode may assume various forms. The insulator 12 on the anode conductor 5 prevents undesired shortcircuiting of the conductor 5 to the cathode. This insulator 12 may consist 50 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 55 a pressure within a range of about one micron to several millimeters.

The cathode shown in Figs. 2 and 3 comprises an enclosure 14 of a woven or mesh structure which may consist of a multiplicity of bundles 60 of very fine wires of nickel, molybdenum, or tungsten and which, as shown in Fig. 1, 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 65 there is provided a mass 15 (Fig. 3) of oxide of alkaline earth, as for example barium oxide.

The filling 15 of oxide may assume various forms. When the cathode is to be coiled as shown in Fig. 2, I preferably employ coarse granular 70 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 16 (Fig. 3) without showing them in all cases to 75 comprise a number of finer wires. For the sake 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 5 binding the parts together by one or more turns of wire as indicated at 17, 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 10 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 15 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 20 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 25 sufficient 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 30 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 35 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 40 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 45 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 volt- 50 age 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. Graph B illustrates the volt-ampere character- 55 istic 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 27 volts. Further increase 60 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 the-graph, the'current increases so rapidly as to give the discharge a 65 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 18 consists of a tightly woven mesh 19 (see 70 Fig. 5) which is partially surrounded by a slotted metallic shield 20. In the interior of the wire mesh is provided an electric heater 21 which is mounted on a refractory support 22. The heater is connected to electric supply conductors 23 and 75

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M which are sealed into the stem 25. The heater support 22 may consist of alumina, beryllia, magnesia, or a suitable electron-emitting oxide material, such as barium oxide may be used. Packed 5 around the heater and generally filling the space between the heater 21 and the wire mesh 19 is a supply of activating material as shown at 15 in Fig. 3. The ends of the cathode of Fig. 4 are closed by plates 26, which may consist of metal, 10 or of refractory non-conducting material. The envelope 27, which may consist of glass, after evacuation and prior to sealing, 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 31 which passes from an exterior contact plate 32 to an interior contact plate 33 at 20 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 25 which is connected to a contact plate 37.

The space between the conductor 31 and the wire mesh 34 is filled with activating material as indicated. In order to avoid undue conduction of electric current between the conductor 31 and 30 the cathode through the barium oxide or other activating material, this material may be mixed with a sufficient refractory adulterant, such as alumina, to render it non-conducting; or the conductor may be surrounded by an insulating 35 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 40 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 con45 structed by the exterior enclosing envelope 48 which is connected to a contact plate 41. The envelope 40 is provided with end wall members 42 and 43 hermetically sealed as by welding to the cylindrical portion of the envelope. Inter50 Posed between the cathode and the metal envelope 40 is a grid 44 which is joined to an exterior contact plate 45. The various contact plates above described are joined to one another by the cylindrical insulating and sealing 55 members 46α, 46b, and 46c, 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 60 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 65 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, 50'. Enclosed in the hollow structure thus formed is a mass of 70 electron emitting material 5! in which is embedded an electric heater 52, the supply conduc3 tors 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. _r

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 arc or other io 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 15 conduction lamps, such as neon or sodium lamps. Cathodes embodying our invention likewise may be used in vacuum devices.