US3016472A - Dispenser cathode - Google Patents

Description

Jan. 9, 1962 P. P. COPPOLA 3,016,472

DISPENSER CATHODE Filed May 25, 1960 FIG].

F|G.5. FIG.8.

IIIIIII/I/I/l/I/I 'IIIIIIIIIIIIIIIIIIIIIIIIII WIIIIIIIIII/IIIIIIIIIIIIIIIII/A I/I/II/l IIIIIIIIII/II IIIII/Ak INVENTORZ PATRICK COPPOLA,

BY FdWA TORNEY.

United States Patent 3,016,472 DISPENSER CATHODE Patrick P. Coppola, Fayetteville, N.Y., assignor to General Electric Company, a corporation of New York Filed May 25, 1960, Ser. No. 31,761 13 Claims. (Cl. 313-346) The present invention relates to improvements in thermionic dispenser cathodes.

Dispenser cathodes are known in which the emissive surface consists of a thin film or monomolecular layer, hereinafter called monolayer, of activated emission material formed on the surface of a metallic emission substrate member. The monolayer consists of electropositive elements such as barium, bound to the substrate member in adsorption-coupled relation, usually through an oxygen linkage. Suitably continual replenishment of the monolayer occurs by migration of additional activated emission material from. an adjacent reservoir.

An inherent problem with such cathodes, however, is that many of the metals most suitable as emission substrates or constituents thereof from the standpoint of adsorption .aflinity for activated emission material monolayers are also deleteriously reactive with many of the more suitable emission materials in such a manner as to severely deplete the amount of activated emission material available for emission during operational life of the cathode. For example, although tungsten is an excellent monolayer-adsorbing substrate, the most desirable emission material from the standpoint of convenience of use and established emission qualities, such as the oxidizing compounds of electropositive elements such as the rare earths, alkali metals, and alkaline earth metals undesirably react with tungsten to form tungstates. By an oxidizing compound of an electropositive element is meant a compound of that elect-ropositive element containing oxygen in excess of the corresponding electropositive element mono-oxide. Examples of oxidizing compounds are carbonates, nitrates, hydroxides, and peroxides, while aluminates, berylliates, and mono-oxides are, as defined herein, non-oxidizing. The tungstates thus formed from such oxidizing compounds are relatively poorly emissive, and hence all of the alkaline earth metal thus captured by the tungsten is lost to the emission process. Conversely, those metals and their alloys especially suitable from the standpoint ofavoiding deleteri-. ous chemical reduction with the emission material, and from the standpoint of accommodating suitable ingredi cuts for suitably chemically reducing: the emission material incident to cathode activation, such-as nickel, do not promote adequate adsorption-coupled monolayer migration. Certain metals which are both non-depletive of emission materialand have suitable migration promoting qualities, such as platinum and palladium are prohibitive because of their extreme cost. d

In the past one attempt to overcome this problem has beenthe use, for emission material sources, of alkaline earth metal compounds which are of a non-oxidizing nature, i.e. which do not react with tungsten or other adsorption-coupled monolayer migration-promoting substrate metals in a manner entirely depletive of emission material. Examples of such non-oxidizingcompounds are the aluminates and berylliates' of'alkahne earth metals. However, this alternative has not proven entirely satis factory because such non-oxidizingcompounds do not constitute as prolific sources of their" desired oxide constituents as established oxidizing compounds, such as carbonates or nitrates which as alkaline earth compounds readily break down to pure alkaline earth, oxides, so that for a given emission density the non-oxidizing compounds have the disadvantage of requiring cathode operation at increased temperatures.

In accordance with the present invention I have provided a monolayer migration type of dispenser cathode which overcomes the foregoing difliculty heretofore encountered with oxidizing compounds of rare earths, alkali metals, and alkaline earth metals, and enables use of these more established and desirably abundant sources of emission material, such as the carbonates of barium, strontium, and calcium, while substantially avoiding emission material-depleting chemical reaction between the emission material reservoir and the monolayer adsorbing base metal.

One object of the invention, therefore, is to provide an improved thin film or monolayer-migration type of dispenser cathode having enhanced emission capacity.

Another object is to provide a monolayer-m-igration dispenser cathode in which established and desirably abundant sources of emission material, such as alkaline earth carbonates, may be used without undesired depletion of the emissive constituents thereof during activation.

Another object is to provide a dispenser cathode of the foregoing character, whose emissive surface may be readily provided with any desired size, shape, or degree of smoothness or optical flatness.

Another object is to provide a dispenser cathode of the foregoing character which is capable of operating at reduced temperatures for a given level of emission, in comparison with prior monolayer-migration dispenser cathodes.

Another object is to provide such a dispenser cathode which is simple and inexpensive to fabricate.

These and other objects will be apparent from the following description and accompanying drawing, wherein:

FIG. 1 is a fragmentary sectional view of one form of dispenser cathode constructed according to my invention;

FIG. 2 is a top view of the structure shown in FIG. 1;

FIG. 3 is an alternative form of cathode constructed according to the invention;

FIG. 4 is another -form of cathode according to the invention;

FIG. 5 is a view similar to FIG. 2 of another form of cathode according to the invention;

FIG. 6 is a fragmentary sectional view of a cathodegrid arrangement'according to the present invention;

FIG. 7 is a View similar to FIG. 2 of another form of cathode according to the invention;

FIG. 8 is a fragmentary sectional viewof another form of the invention; and

FIG. 9 is a fragmentary sectional view of sti-ll another form of the invent-ion.

' In the cathode of FIG. 1 an emission material reservoir 2 of annular configuration is formed on a support 4 consisting of apmetallic plate. The emission material reservoir 2 may be comprised entirely of one or more oxidizing compounds of rare earths, alkali metals, and alkaline earth metals, such as a mixture of the carbonates of barium, strontium, and calcium. Alternatively reservoir material may consist of such oxidizing constituents mixed or associated with additional or host constituents, suchas for example nickel powder. The reservoir support 4 is metallic, and the principal metal or metals thereof are preferably substantially non-reactive chem.- ically with the emission material reservoir. Suitable metals for the support are nickel or a metal or metals of the platinum group of the periodic table, namely platinum, palladium, rhodium, ruthenium, osmium and irid:

ium. Preferably the support alsocontains a small per- 0 centag e, e.g. less than 2% by weight, of chemically retitanium, zirconium, hafnium or inter-metallic compounds or alloys thereof. The reservoir material 2 is arranged on the support 4 so as to provide a substantial area of surfacg contact therewith as at the interface 6, to promote chemical reaction between the reservoir material and the reducing constituents of the support during cathode activation.

Also secured to the top of the support plate is an emission substrate member 8. The substrate member 8 is metallic, and includes at least as a major constituent a metal capable of promoting adsorption-coupled monolayer migration of emission material. Suitable migration-promoting constituents for the emission substrate member are tungsten, molybdenum, or rhenium, niobium, or alloys, intermetallic compounds, or chemical compounds such as carbides and silicides thereof. The substrate member may also include other constituents, and if desired may be enriched with additional emission material, for example in pores thereof, of a type such as barium aluminate not depletively reactive with other constituents of the substrate member. The periphery of the substrate member is contiguous to the inner edge of the annular reservoir of emission material '2, as at interface 10, and the surface 12 of the substrate member is shown as substantially coplanar with the surface of the emission material reservoir.

Upon heating in vacuo and subsequent activation of the cathode shown in FIGURE 1, the material of the reservoir 2 is reduced to metal and metal oxide by the chemically reducing constituents of the support 4. Thereafter during cathode operation at moderately elevated temperatures, for example in the range of 925 to 950 C., the electropositive metal and metal oxide of the reservoir migrates in a monolayer onto the adsorption promoting exterior surface 12 of the emission substrate member 8. In the important initial beatings of the cathode, deplctive chemical reaction between the emission material and the substrate member is minimized because physical contact between the reservoir material and the substrate member is limited to the immediate vicinity of their contiguous borders, i.e. at relatively narrow interface 10. Hence a minimum depletion of activated emission material by formation of non-emitting compounds with the material of substrate 8 results, and the bulk of the reservoir emission material is left intact for useful replenishment of the emission surface on substrate 8.

The cathode shown in FIG. 1 is simple to make, and its fabrication does not require any pressing o-r molding. It may be fabricated for example by securing the central emission substrate member 8 to the support 4, which support may be, as shown, the end wall of a sleeve or cylinder 14 suitable for enclosing a heater 16. The entire exposed surface of the support 4 and substrate member is then coated, as by spraying, with a suitablemixture of' material constituting the emission material reservoir, and then shaving off the portion of the resulting coating which covers the emission substrate member. If an extremely smooth or optically flat exterior surface for the substrate member and the emissive monolayer thereon is desired, the surface of the substrate member 8 may be readily polished to such smoothness or flatness before the reservoir material is applied. Alternatively the substrate member 8 may be concave, convex, or otherwise contoured as desired relative to border with reservoir 2. Thereafter the cathode may be activated by suitable firing in vacuo, for example by heating to 1150'C..for about to minutes in a vacuum of about 10 mm. of mercury.

FIG. 3 shows'an alternative form of cathode constructed according to the invention, and particularly suitable for accommodating differential thenmal expansion which may result between the support and. the emission substrate member in the event that the principal metallic constituents thereof, such as nickel for the support 4 and tungsten for the substrate 8, have substantially different thermal coefiicients of expansion. The support 4 of FIG. 3 has a central opening 20 and the substrate member 8 has an integral depending sleeve provided-with an outturned flange 24 beneath the support, the flange having a downturned portion 26 at its marginal edge which is secured, as by welds 28, to the cylindrical wall portion of the support. With such a construction, the depending sleeve 22 is dimensioned to extend through the aperture with a slight clearance as at 30 accommodating differential expansion, yet the reservoir layer 2 on the support 4 is enabled to contact the periphery of the substrate member at interface 10 with the degree of intimacy suitable for monolayer migration onto the substrate member.

The structure shown in FIG. 3 has the particular advantage of avoiding distortion when heated, so that the external faces of the reservoir 2 and substrate member 8 remain coplanar smooth and flat during operation, an important feature particularly in electron beam tubes such as cathode ray tubes where good beam focusing and small beam spot size on a screen or target is desired. For example, in cathode ray tubes employing cathodes of the type shown in FIG. 3, decrease in beam spot sizes of about 20% in comparison with standard sprayed oxide cathodes has been observed.

Another solution to the problem of differential thermal expansion is shown in FIG. 4, wherein a support similar to 4 in FIG. 1 is formed of a suitably thin layer 40 of a metal, such as nickel, non-depletive of emission material, clad onto a base metal 42 such as molybdenum, having a thermal expansion coefiicient approximately equal that of the substrate 8. The layer is made thin enough to yield relative to substrate 8 and base metal 42 during thermal expansion, and hence does not distort the subrate member 8.

FIG. 5 shows a structure similar to- FIG. 3, wherein an increased degree of minimization of contact between reservoir 2 and substrate 8 is obtained, while preserving the desired migration paths at the surface edge of interface 10. The edge of support 4 at opening 20 is feathered as at 46, and bent upwardly so that it terminates just below the surface of member 8. This minimizes the width of interface 10, thus minimizing the contact area between reservoir 2 and member 8.

In one specific example of a cathode constructed in accordance with the invention the support was a cylinder of nickel containing as reducing impurities less than 2% by weight of magnesium, silicon and carbon. The cylinder was .125" in outside diameter, had'a wall thickness of .005 and the end wall had a thickness of .005". Centrally located in the end wallwas an aperture of .040" diameter. A circular emission substrate member of tungsten having a diameter of .035" and a thickness of .0025" was supported by an integral depending sleeve and flange, as shown in FIG.'3,welded to the'interior of thenickel cylinder sothat the upper face of the substrate member was parallel to and spaced from the exterior face of the support about .0025"; An emission material reservoir was applied to the support by spraying a mixture of barium, strontium, and calcium carbonate in a vehicle of amyl and ethyl acetates plus nitrocellulose binder over both the end faces of the nickel and tungsten members to a thickness of about .003", such thatupon drying the coating and shaving off the portion thereof covering the tungsten the exposed surface of carbonate was made coplanar with the exposed; face of the tungsten. The cathode was thereupon heated in vacuum in-a conventionalmanner to decompose the carbonates, after which is was activated by heating in a vacuum of. about 10 mm. of mercury for aboutfS- lQim'nuts at about 1.150

C. to promote monolayer'migration over the entire examps/cm}, substantially no emission being observed from the reservoir surrounding the tungsten. The work function for this cathode was computed to be about 1.90 electron volts at 950 C.

A particular advantage of cathodes such as shown in FIGURES 1 through 4 is that when used with an apertured grid 50 and arranged with the substrate member 8 opposite the aperture 52 of the grid 50 and of a diameter equal to orgreater than the grid aperture 52, as shown in FIGURE 6, the reservoir portion of the cathode is free from substantial influence by the electron attracting field beyond the grid whch penetrates through the grid aperture. Field flux lines 48 indicate the shape of the electron accelerating field in this case. Thus substantially the entire emission of the cathode takes place from the surface of the substrate member 8, with resultant desired quality of electron beam focus and spot size of being readily available from the easily controlled degree of flatness or other preferred shape of substrate member 8. Also since substantially no emission occurs from reservoir portion 2 under such field conditions, the influence of the electrical properties such as the conductivity of the reservoir portion is minimized. Moreover the emitting surface can be not only optically flat but pore-free if desired with resultant improvement in electron energy distribution for optimum beam focus.

FIG. 7 shows an alternative arrangement of the surface configuration of the reservoir and emission substrate member in a cathode constructed according to the invention. As shown the emission substrate member is in the form of an annulus surrounding a central first reservoir portion and in turn surrounded by an outer second reservoir portion. Thus activated emission material can migrate in a monolayer onto the surface of the substrate member from both its outer periphery and its inner periphery, while physical contact between the emission substrate member and the reservoir portions is, as in the cathode of FIG. 1, limited to their contiguous edgesso as to minimize depletive chemical reaction between the reservoir material and the emission substrate member during activation.

The present invention contemplates that the reservoir portion may be arranged other than coplanarly with the emission substrate member. One such non-coplanar arrangement is shown in FIG. 8 wherein it will be evident that the reservoir 2 is arranged on a support 4 of cylindrical shape, with the substrate member comprising an end wall of the cylinder. The cylinder may contain 7 a heater 16, and depletive chemical reaction between the reservoir material and the substrate-8 is minimized by the virtue of the narrow contact interface 10' at the surface of whichthe desired monolayer migration path is provided.

FIG. 9 shows an alternative arrangement somewhat similar to FIG. 8 except that the substrate member is of conical shape. This arrangement has the advantage that the emission surface of the member 8 is continually replenished by migration from the reservoir 2 and yet emission from the apex region of the cone defines an emission source of extremely small area.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. It is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

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

1. A dispenser cathode comprising an emission material reservoir including a first ingredient consisting of at least one oxidizing compound of an electropositive element selected from the group consisting of rare earths, alkaline earth metals and alkali metals, a reducing ingredient arranged in chemically reactable relation with at least a portion of said first ingredient whereby at least a portion of said first ingredient is reducible to the electropositive element and electropositive element oxide constituents thereof, an emission substrate member including a constituent promoting adsoprtion-coupled migration on an exposed surface thereof of a monolayer of said electropositive elements and electropositive element oxides of said reservoir, said exposed surface of said emission substrate member defining an emission surface, said migration-promoting constituent of said substrate member be ing chemically reactive with the first ingredient of said reservoir contacted by said migration-promoting constituent in a manner substantially entirely depletive of the electropositive elements and electropositive oxides in the contacted portion, said reservoir being disposed in contiguous relation to said substrate member with said first ingredient contacting said substrate member only at a relatively narrow interface across the edge of which adsorption-coupled migration of said monolayer onto said emission surface may take place, whereby contact between said migration promoting constituent of said substrate member and said first ingredient of said reservoir is minimized to the vicinity of said interface to thereby avoid depletion of said electropositive elements and electropositive element oxides during reduction of said first ingredient.

2. A dispenser cathode as defined in claim 1 wherein said first ingredient comprises a carbonate from the group consisting of barium, strontium, and calcium.

3. A dispenser cathode as defined in claim 1 wherein said migration promoting constituent includes a material from the group consisting of tungsten, molybdenum, rhenium and niobium, and alloys and compounds thereof.

4. A dispenser cathode as defined in claim 1 wherein said first ingredient comprises a carbonate from the group consisting of barium, strontium and calcium, and said migration promoting constituent includes a metal from the group consisting of tungsten, molybdenum, rhenium, niobium, and alloys and compounds thereof.

5. A dispenser cathode as defined in claim 1 wherein said emission substrate member and said reservoir are disposed in coplanar concentric relation.

6. A dispenser cathode as defined in claim 1 wherein said reservoir is disposed on an annular first support defining a central opening, said emission substrate member is mounted in generally coplanar relation with said reservoir on a second support extending through said central opening of said first support, and the marginal portion of said first support extending peripherally about said opening overlaps the adjacent marginal edge of said substrate member, whereby the Width. of said interface is restricted to the non-overlapped portion of said marginal edge of said substrate member.

7. A dispenser cathode as defined in claim 1 wherein said reservoir is disposed on a first supporting surface, said emission substrate member is mounted in generally coplanar relation with said reservoir on a second supporting surface, and the marginal portion of said first supporting surface overlaps the adjacent peripheral face of said substrate member whereby the width of said interface is restricted to the non-overlapped portion of said peripheral face of said substrate member.

8. A dispenser cathode comprising a support of nickel containing reducing impurities from the class including magnesium, silicon, carbon, titanium, zirconium, hafnium, or alloys or intermetallic compounds thereof; an emission material reservoir of oxidizing compounds of' elements from the group consisting of alkali and alkaline earth metals contacting the support and convertible thereby upon application of heat to the metallic oxide and metallic forms of said elements, and a substrate member of an adsorption coupled monolayer migratiompromoting metal of the class consisting of tungsten, molybdenum .and rhenium carried by said support with one peripheral r 7 edge substantially coplanar with and continguous to a portion of said emission material reservoir.

9. A dispenser cathode as defined in claim 8 wherein said nickel support consists of a relatively thin cladding onan underlayer of a metallic material having a thermal coeflicient of expansion substantially equal to that of the substrate member,

10. A dispenser cathode comprising an emission material reservoir including a first ingredient consisting of at least one oxidizing compound of electropositive elements selected from the group consisting of rare earths, alkaline earth metals and alkali metals, a reducing ingreaient arranged in chemicaily reactable relation with at least a portion of said first ingredient whereby at least a portion of said first ingredient is reducible by heating to the electropositive element and electropositive element oxide constituents thereof, an emission substrate member including a constituent promoting adsorptionc-oupled migration on an exposed surface thereof of a monolayer of said electropositive elements and electropositive element oxides of said reservoir, said exposed surface of said emission substrate member defining an emission surface, said migration-promoting constituent of said substrate member being chemically reactive with any portion of the first ingredient of said reservoir contacted by said migration-promoting constituent in a manner substantially entirely depletive of the electropositive elements and electropositive oxides in the contacted por tion, said emission substrate member forming at least a portion of a transverse end wall of a tubular support and said reservoir being disposed on the outer surface of said tubular support in contiguous relation to said substrate member with said first ingredient contacting said substrate member only at an interface of minimal area, whereby adsorption-coupled migration of said monolayer onto said emission surface may take place across the exterior edge of said interface and chemical reaction depletive of said electropositive elements and electropositive element oxides is minimized to the vicinity of said interface.

11. A dispenser cathode as defined in claim 10 wherein said emission substrate member is of generally conical shape coaxial with said tubular support.

12. The combination in an electron discharge device of an annular electrode having a central opening, and dispenser cathode spaced from'said annular electrode substantially concentric with said opening, said dispenser cathode comprising an annular emission material reservoir including a first ingredient consisting of oxidizing compounds of electro-positive elements selected from the group consisting of rare earths, alkaline earth metals and alkali metals, a reducing ingredient arranged in chemically reactable reaction with at least a portion of said first ingredient whereby at least a portion of said first ingredient is reducible by heating to the electropositive element and electro positive element oxide constituents thereof, an emission substrate member centrally disposed within said annular reservoir, said substrate member including a constituent promoting adsorption-coupled migration onto the exposed surface of said substrate member of a monolayer of said electropositive element and electropositive element oxides of said reservoir, said exposed surface of said emission substrate member defining an emission surface and having a diameter not less than that of the central opening in said annular electrode, said migration-promoting constituent of said substrate member being chemically reactive with the first ingredient of said reservoir contacted by said migration-promoting constituent in a manner substantially entirely depletive of the electropositive elements and electropositive oxides in the contacted portion, said reservoir being disposed in contiguous relation to said substrate with said first ingredient contacting said substrate only at a narrow interface across the exterior edge of which adsorption-coupled migration of said monolayer onto said emission surface may take place.

13. A dispenser cathode as defined in claim 12 wherein said first ingredient comprises a carbonate from the group consisting of barium, strontium, and calcium, and said migration promoting constituent includes a metal from the group consisting of tungsten, molybdenum and rhenium.

No references cited.

Claims (1)

1. A DISPENSER CATHODE COMPRISING AN EMISSION MATERIAL RESERVOIR INCLUDING A FIRST INGREDIENT CONSISTING OF AT LEAST ONE OXIDIZING COMPOUND OF AN ELECTROPOSITIVE ELEMENT SELECTED FROM THE GROUP CONSISTING OF A RARE EARTHS, ALKALINE EARTH METALS AND ALKALI METALS, A REDUCING INGREDIENT ARRANGED IN CHEMICALLY REACTABLE RELATION WITH AT LEAST A PORTION OF SAID FIRST INGREDIENT WHEREBY AT LEAST A PORTION OF SAID FIRST INGREDIENT IS REDUCIBLE TO THE ELECTROPOSITIVE ELEMEMT AND ELECTROPOSITIVE ELEMENT OXIDE CONSTITUENTS THEREOF, AN EMISSION SUBSTRATE MEMBER INCLUDING A CONSTITUENT PROMOTING ADSORPTION-COUPLED MIGRATION ON AN EXPOSED SURFACE THEREOF OF A MONLAYER OF SAID ELECTROPOSITIVE ELEMENTS AND ELECTROPOSITIVE ELEMENT OXIDES OF SAID RESERVOIR, SAID EXPOSED SURFACCE OF SAID EMISSION SUBSTRATE MEMBER DEFINING A EMISSION SURFACE, SAID MIGRATION-PROMOTING CONSTITUENT OF SAID SUBSTRATE MEMBER BEING CHEMICALLY REACTIVE WITH THE FIRST INGREDIENT OF SAID RESERVOIR CONTACTED BY SAID MIGRATION-PROMOTING CONSTITUENT IN A MANNER SUBSTANTIALLY ENTIRELY DEPLETIVE OF THE ELECTROPOSITIVE ELEMENTS AND ELECTROPOSITIVE OXIDES IN THE

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