US2279821A - Secondary electron emissive electrode - Google Patents

Description

April 14, 1942. c. A. HEDBERG ETAL SECONDARY ELECTRON EMISSIVE ELECTRODE Filed April 17, 1940 FIG.

c A. HEDBERG INVENTORS- OMCNALL},

ATTOBNEV Patented Apr. I4, 1942 SECONDARY ELao'moN EMISSIVE ELECTRODE Carl A. Hedberg, Roselle Park, and James 0. McNally, Maplewood, N. J., assignors to-Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 17, 1940, Serial No. 330,016

5 Claims.

This invention relates to electron discharge devices and more particularly to such devices comprising one or more secondary electron emissive electrodes and to methods of making such electrodes.

The operating characteristics and useful life of electron discharge devices including one or more secondary electron emissive electrodes are dependent primarily upon the character and properties of the emissive surface of such electrodes. In general, such surfaces include a1- kaline or alkaline earth metals in either the free or combined form or both. It has been found that known surfaces are unstable, that the .emission characteristics thereof vary with use of the device and that in a relatively short period the quantity of emission obtainable becomes insufiicient for satisfactory and proper operation of the device.

These undesirable effects are attributable to changes in the character and composition of the emissive surfaces. For example, it has been found that in devices wherein the secondary emissive surface includes alkaline or alkaline earth metal in the form of an oxide, the oxide exists in the form of a relatively thin layer upon a sublayer of free metal. During operation of such a device, some of the free metal diffuses through the oxide layer and appears upon the emissive surface. Inasmuch as, in general, the free or uncombined metal has a lower secondary electron emission factor than the oxide thereof, in devices of the type described, the diffusion of the free metal to the emissive surface results in a marked decrease in the total emission obtainable from the surface.

In other cases wherein the secondary electron emissive element is in the form of a matrix including some free active metal, such for example as in the known caesium silver oxide coatings on electrodes, the emission characteristics of the element deteriorate materially in the presence of even small amounts'of oxygen. If, in devices including such elements, oxygen is liberated from the internal parts during operation, the emissive element is affected deleteriously.

Furthermore, the preparation of secondary electron emissive surfaces heretofore involved manifold difliculties and entailed an exacting procedure due, for example, to the necessity of keeping the surface, once activated, in a high vacuum and to the critical dependence of the characteristics of the surface ultimately produced upon the proportions of the constituent materials of the surface and upon the temperatures employed during the formation process.

General objects of this invention are to improve the operating characteristics and to increase the useful life of electron discharge devices comprising one or more secondary electron emissive electrodes. t More specifically, objects of this invention are Improve the stability and emission characteristics of secondary electron emissive surfaces;

Increase the operating life of secondary electron emissive surfaces;

Facilitate control of the quantity of active ingredient in such surfaces and Simplify and expedite the production or fabrication of secondary electron emissive electrodes.

In accordance with one feature of this invention, a secondary electron emissive electrode comprises a metallic base having thereon a coating of a secondary electron emissive metallic oxide having a secondary electron emission factor of greater than unity, the coating having substantially no free active metal therein. The coating may be, for example, of magnesium oxide, aluminum oxide or beryllium oxide.

In accordance with another feature of this invention, a foundation member of relatively inactive metal is treated to produce thereon a coating of a definite amount of oxide of the metal, a layer of active metal is applied to the oxide coating, the composite thus formed is heated whereby the active metal reduces the oxide coating, and the assembly is heated furtherto vaporize the remaining free active metal.

The invention and the foregoing and other fea-- tures thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. l is a view mainly in section illustrating one form of apparatus which may be employed in producing and testing secondary electron emissive electrodes in accordance with this invention;

Fig. 2 is an enlarged detailview in perspective of a portion of the apparatus shown in Fig. 1; and

Fig. 3 is a view in cross-section of an electron discharge device including a secondary electron emissive electrode constructed in accordance with this invention.

Referring now to the drawing, the apparatus illustrated in Figs. 1 and 2 comprises an enclosing vessel Ill having at one end a stem H which terminates in a press l2 and from which an electrode system is supported. The vessel It! is proone end The electrode system-comprises a cylindrical metallic shield I! having a reduced end portion I I 18 which is closed by a disc I! having a restricted central aperture .lltherein. Amxed to the other end of the Shield" is a cup-shaped cap or closure- I! provided with a plurality of apertures in which insulating bushings 20 are secured. Fixed within the reduced end portion it is a disc or diaphragm 2| having a central aperture 22 with a .tubulati on.l 4 for-pure v poses to be-describedhereinafter.

in alignment with the aperture It. The shield 7 i may be supported in part by a rigid upright 28 afilxed to a rigid wire 24 embedded in the press 12 and having a leading-in conductor connected thereto.

l4".1,- '-Io assist in ontgass ing the target 41 a. current maybe-passed therethrougl by way of the conductors 42; T ongen'is then admitted. the tubulation 14, into the vessel II, for example to a ofjtheorder of 0.2 to 0.8 millimeter of mercury',;anda

producedbetween the target" 15 45 to vaporize the strip or pellet, for example of Mounted within the shield 15 is a cathode 25, 1

for example a tungsten filament, which issupported in alignment with the apertures l8 and 22 by a pair of uprights 26 extending through bushings and carried by wires 21 sealed in the arms of the press I2 and having leadingin conductors connected thereto.

Also mounted within the shield l5 and located between the cathode 25 and diaphragm or disc 2| is a modulator disc 28 having a central aperture 29 in alignment with the apertures l8 and 22. The modulator disc 28 may be supported by a pair of rigid uprights 30 extending through bushings 20 and carried by a rigid wire ll embedded in the press i2 and having a leading-in conductor connected thereto.

An inverted cup-shaped electrode 32 having a central aperture 33 in alignment with the aper- 5 plane :r-x, and mounted inthe device wherein it ture l8 isseated upon a centrally apertured insulating disc 34 seated in turn upon the closure member II. The electrode, may be supported further by a plurality of uprights 25 embedded in the press l2, one of the uprights having a leading-in conductor connected thereto.

A centrally apertured disc collector electrode 26 is mounted above and in alignment with the electrode 32 and may be supported by aplurality. of metallic uprights 31 carriedby a pair of collars or bands 38 clamped about the stem ll, one of the uprights 37 having a leading-in conductor connected thereto as shown. The uprights 31 support also a cylindrical mesh screen or shield 40 which is mounted on rigid wires 4| secured to the uprights II.

Mounted opposite the collector electrode 36 andrin alignment with the aperture therein is a target 43 which may be supported by a pair of leading-in conductors 42 sealed in the outer end of a tubulation 44 on the vessel III. This target, which may he a strip of metal, serves as the foundation member for the secondary electron emissive surface produced as described hereinafter.

Air auxiliary filament 45, for example, a helix of tungsten, is positioned opposite the base wall of the target 43 and is supported by a leadingin conductor 48 and oneof the conductors 42 as shown. A strlp or pellet of vaporizable material, not shown, is mounted on or within the helical filament for purposes to be described hereinafter.

The target electrode 43 constitutes the base or foundation member of the secondary electron emissive electrode to be produced. In a specific form, it may be a strip of substantially pure silver or of a base metal plated with silver. In the fabricatlonof the secondary electron emissive electrode, the various electrodes inv the vessel It magnesium, thereon. The vaporized material deposits upon the oxidized target electrode and forms a layer of magnesium thereon. The target electrode is heated, as by passing a current there-'- 20 through by way of the conductors 42, whereby the 30 700 C.has been found satisfactory to reduce the silver oxide and to vaporize any free magnesium that may remain after such reduction. The target electrode may then be removed from the vessel Iii, as by cutting the vessel along the will be used as'a secondary electron emissive electrode.

Magnesium oxide has a high secondary electron emission factor. Magnesium itself, on the 40 other hand, has a low secondary emission factor.

It will be noted that secondary electron emitters produced as described above will have uniform emission characteristics over a long period inas- .much as, because of the decrease or substantial absence of free magnesium, no magnesium spots 5; amount of silver oxide.

5. sive electrodes is greatly facilitated.

Although in the specific illustration described above, a magnesium oxide coating is produced upon a silver base. it will be understood that other oxide coatings may be produced and other base or foundation materials may be employed.

For example, coatings of aluminum oxide. barium oxide and beryllium oxide, each of which has good secondary emission properties, may be produced in the manner described above. The base or foundation metal employed in any particular case should be such that the oxidizing agent has a suitable equilibrium pressure. That is to say, the oxidizing agent should be such that it is reduced by the active metal at a temperature below that at which the active element evaporates appreciably from the. composite element. In the specific illustration described, the oxidizing agent, silver oxide, is reduced by magnesium at a temperature materially below the vaporizing temare outgassed and the vessel is evacuated through 76 perature of magnesium. Gold oxide also may be Fig. 1.

target electrode 43 and the cathode to obtain employed as a base on which magnesium is deposited. Silver oxide also may be used as the oxidizing agent for aluminum to produce a secondary electron emissive surface of aluminum oxide. Cupric oxide or nickel oxides may be used as the oxidizing agent for beryllium or barium.

Although oxides have been described specifically'as the oxidizing material, other compounds may .be utilized. In general, the requisites for such compound are that it be capable of giving up oxygen at the proper temperature and that the residue be stable at the operating temperature of the emitting surface produce. An illustrative compound which may be used is copper chromate.

When any secondary electron emitting electrode has been constructed as described above, the emission factor of the oxide coating may be determined readily in the apparatus shown in For this purpose, the electrodes I5, 25, 28 and 2| are operated as an electron gun to produce a concentrated beam of electrons emanating from the cathode 25, which is accelerated by electrodes 2|, l1 and 32, passes through the aperture 33 and impinges upon the target elecnegative transconductance.

The primary electrons emanating from the cathode 5| will-be directed to the target electrode and impinge upon the wings 43b whereby a secondary electron current to the auxiliary electrode 55 is produced. The current supplied to the load or utilization circuit, connected betrode 43 to cause the emission of secondary electrons from the target electrode. These secondary .electrons will be collected by the collector electrode 36 and shield 40, which, as noted heretofore, are electrically connected together. The magnitude of the difference between primary electron current impinging upon the target and secondary current therefrom can be measured directly as by a meter in the target circuit. The secondary current flowing between the target electrode 43 and the collector electrode 36 and shield 40 can be measured directly. The ratio of secondary electron current to primary electron current is a measure of the emission factor of the secondary electron emissive surface produced.

An electron discharge device illustrative of those in which secondary electron emissive electrodes constructed in accordance with this invention may be employed is shown in Fig. 3. This device comprises an enclosing vessel 50 in which there are mounted in coaxial relation a cathode 5|, an oval or elliptical control grid 52, an oval or elliptical screen grid 53 and a generally cylindrical focussing electrode 54 having a cut-away portion in which the target elec trode is positioned. The target electrode may include a central semicircular portion 43a and wings 43b. Opposite the wings 43b of the tar-'- get electrode are a plurality of parallel linear elements 55, for example wires, which are connected together electrically and constitute an auxiliary electrode or anode. The cathode 5| may be of either the filamentary or indirectly heated type and have its surface coated with a material having good electron emission properties. For example, the surface may have thereon a coating including barium oxide.

During operation of the device, the focusslng electrode 54 is connected directly to the cathode 5| or has a small potential applied thereto, the grid 53 andtarget electrode 43 are connected together and maintained at a positive potential with respect to the cathode 5| and the auxiliary electrode 55 is maintained at a high positive potential with respect to the cathode. A suitable load or utilization circuit may be connected between the electrode 55 and the cathode 5| to obtain a positive transconductance or between the tween the target electrode 43 and cathode 5|, will\be the difierence between the primary electron current to the target electrode and the secondary electron current from the target electrode. When the secondary electron current from the target electrode is greater than the primary electron current to it, a negative transconductance is realized inasmuch as the net electron flow is away from'the target electrode and. hence, opposite in direction to that in positive transconductance devices.

It will be noted that inasmuch as free active metal is not present in secondary electron emissive surfaces. constructed as described above, the construction of devices including secondary electron emissive electrodes is greatly facilitated. Electrodes constructed as described above can be exposed in air. without any detrimental effects and no particular precautions are necemary, therefore, in handling such electrodes and in mounting them in devices wherein they are to be used.

Although a specific embodiment of this invention has been shown and described, it will be understood that this embodiment is but illustrative. For example, although-the process of quantity in an evaporating chamber, the necessary heating being accomplished by high frequency induction or in other ways. Also, although in the specific methods described the oxide or other compound of inactive metal'is produced by treatment of the foundation member, such oxide or compound could be applied to a foundation member as, for example, by spraying. Various other modifications may be made in the specific embodiment described without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. The method of manufacturing a secondary electron emissive electrode, which comprises producing on a foundation member a coating of a compound of a relatively inactive metal which compound will liberate oxygen when heated and have a residue stable at the normal operating temperature of said electrode, depositing upon said coating a layer of an active metal an oxide of which has a secondary electron emission factor greater than unity, reducing said compound by heating the composite thus formed with said active metal whereby a layer of the oxide of said active metal is produced on said member, and removing substantially all free active metal from the assembly thus formed.

2. The method of manufacturing a secondary electron emissive electrode, which comprises producing on one surface of a foundation member f active metal whereby slant of the oxide of said active metal is formed on said member. and re- -moving substantially all free active metal from the assembly thus formed.

3. The method of manufacturing a secondary I electron emissive electrode including a foundation member and a surface on said electrode of an active metal oxide having a secondary electron emission factor greater than unity, said method comprising oxidizing a surface of a foundation member of a metal the oxide of which is reducible by the active metal at a temperature below the vaporizing temperature of the active metal, depositing a layer of the active metal upon the oxidized surface, 'reducing substantially' all the oxide of said foundation metal in said-surface by heating the assembly thus formed with.

said active metal layer whereby a coating of the oxide of said active metal is produced on said foundation member, and vaporizing substantially all free active metal therein from said active metal oxide coating by heating the composite thus formed.

4. Themethod of manufacturing a magnesium oxide secondary electron emitter which comprises oxidizing a surface of a foundation member of a metal the oxide of which is reducible by magnesium at a temperature below the vaporizing temperature of magnesium, depositing magnesium upon the oxidized surface, heating the composite element thus formed ,to reduce the oxide of said metal by the magnesium wherebya coating predominantly of magnesium oxide is produced on the foundation member, and heating the composite element'further to vaporize any free magnesium therein.

I foundation member.

CARL A. HEDBERG. JAMES ofMcNALLY.

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