US3166836A - Manufacture of electron tube cathodes - Google Patents

Description

Jan. 26, 1965 Original Filed June 21. 1957 F /"INVENTOR. 5 JOHN M .5/GL 5e VZAMJ 4 4 optimum bonding.

layer completely surrounds the base member. 'cause of the different co-eflicients of expansion of the atent 3,166,835 Patented Jan. 26, 1965 fifice 3,166,836 MANUFACTURE F ELECTRGN TUBE CATHODES John M. Bigler, Landisville, Pa., assignor to Radio Corporation of America, a corporation of Delaware Continuation of application Ser. No. 667,249, June 21, 1957. This application May 24, 1962, Ser. No. 193,698 6 Claims. (Cl. 29-4205) This invention relates to the manufacture of electron tube cathodes where it is desired to provide a porous matrix surface on a base metal member for impregnating with an electron emissive material.

This application is a continuation of my application Serial No. 667,240, filed June 21, 1957, now abandoned.

In the manufacture of thermionic electron tube cathodes, it is often desired to provide an electron emissive coating upon a base metal member. In providing this type of cathode emitter for high power tubes, where high tube currents and consequently high electron emission and high cathode temperatures are required, an application of the electron emissive material directly upon the base member has proved to be impractical. If the electron emissive material is applied directly to the base member in a sufiiciently thick coat to give the desired electron emission during an acceptable life span, it will frequently peel off when subjected to cyclic heating and cooling. One answer to this problem is to bond a porous metal matrix layer to the surface of the base metal to provide a multiplicity of minute reservoirs into which the emissive material may be impregnated. In accordance with this practice, the so-called oxide type emitter has been made using nickel or high nickel alloys as a base metal with a porous, sintered, nickel matrix bonded thereto. However, considerable difiiculty has been experienced in bonding the .matrix to the base metal.

According to one known method, the base metal member is placed in a mold which is similar in shape but slightly larger than the base metal member. Nickel powder is then placed around the base metal member in the mold and the assembly fired at a temperature which the mold will withstand but yet which will cause the nickel powder particles to adhere to each other and to the base member. The joined metals are then removed from the mold and refired at the required temperature to obtain This method does not involve the application of substantial contact pressure between the base member and the nickel powder during the firing operation, and does not insure uniform dimension and density of matrix since the powder is placed in the narrow cavity of the mold without positive distribution. Also the requirement of individually molding each emitter limits production.

Where a cylindrical base member is provided with an adherent sintered metal matrix according to known practices, the resulting encircling configuration of the matrix aids in maintaining adherence of the matrix to the base member. However, in certain electron tubes using filamentary strand cathodes and operating with directional beaming of the emitted electrons, it is desired to coat a ribbon strand on only one of its major surfaces. In such applications it is not possible to obtain the benefits of the self-supporting feature inherently present where the matrix And bematrix and the base member, peeling or separation of the matrix from the base member is likely to occur when the cathode strand is subjected to cyclic heating and cooling.

Selection of materials alone to obtain approximately equal co-efiicients of expansion has not proved sufficient to avoid this peeling.

It is therefore an object of my invention to provide an improved method of bonding a sintered metal matrix to a base member of an electron tube cathode.

Briefly, according to a preferred practice of my invention, a porous metal powder matrix sheet is prepared for subsequent bonding to the surface of an electron tube cathode base member by first separately preforming and partially presintering the matrix sheet out of contact with the base'member. The base member is coated with a. metal powder and binder mixture and' fired in a reducing atmosphere to at least partially sinter the coating thereon. The partially presintered matrix sheet is then bonded to the coated surface of the base member by pressing the two together and firing to complete the sintering of the matrix, and also the coating if it has not already been completely sintered. 1

For use as a filamentary cathode a suitable resistive metal could be used as the base member and an electron emissive material, e.g., a barium-strontium-calcium carbonate mixture, could then be applied to, or impregnated into, the porous matrix sheet.

In the drawings:

FIG. 1 is a perspective view partially in section of a shallow mold used in the practice of my invention for preforming and presintering a matrix layer.

FIG. 2 is a perspective view of a number of filamentary cathode strands positioned on a preformed and partiallypresintered matrix sheet preparatory to the pressing and finish sintering.

FIG. 3 is a cross-section view of one of the strands and a portion of the matrix of FIG. 2 taken along line 3-3 of FIG. 2.

Referring to FIG. 1, a shallow mold 12 is provided which is suitable for preforming and presintering metal powder into a matrix sheet. This mold may simply comprise a metallic block having a shallow uniform recess 14 with side walls 16. The mold is filled with a metallic powder, slightly compacted to prevent heterogeneities or holes, and the excess is removed by leveling with a scraper bar. The metallic powder is then fired in a reducing atmosphere at a temperature somewhat below the sintering temperature of the powdered metal so that the powder is only partially sintered. The time duration and temperature of this partial presintering will, of course, depend upon the kind of powdered metal used, and should be only great enough to permit subsequent handling of the matrix sheet without breaking it.

The base member onto which the matrix is to be bonded is provided with a thin, eg one particle thickness, sintered surface coating by applying a metallic powder and binder and then firing in a reducing atmosphere e.g. hydrogen, to sinter it. Here, although this sintering step may be a partial sintering, it is preferably carried to completion to provide a good bond to the base member.

The partially presintered matrix layer is removed from its mold and placed into contact with the sintered surface of the base member. The two are then pressed to gether, e.g. with 23 pounds per square inch pressure, and fired in a reducing atmosphere to complete the sintering of the matrix sheet and to bond it to the base member. Contact pressure is maintained throughout this final firing step.

In applying the method of my invention to the manufacture of filamentary cathode strands, certain special considerations must be given to selection of materials and their processing. These are in addition to certain other considerations to be noted in the general practice of my invention. For example, the metallic powder used to form the matrix sheet should be of a metal which will withstand the operating temperature which the finished cathode strand will be subjected to without softening or gions. creased byv compacting the nickel powder in the mold.

'tungsten, and nickel are examples of suitable metals.

Nickel is preferred because of its low working temperature. As a general consideration, it should also be appreciated that the smaller the particle size range of the metallic powder used for the matrix sheet the greater the porosity of theresulting matrix. The reason for this is that with a variety of particle sizes the small particles tend to fill in the interstices between the larger ones, while the larger particles provide solid non-porous re- In addition, the porosity can be 'adjustably de- However, since for filamentary cathodes a high porosity, or low density, matrix is desired, the powder is simply leveled with only enough compacting to prevent heterogeneites, or holes, therein. Also, since a fine powder tends to compact more tightly than does a coarser powder a minimum powder particle size is preferably indicated.

For nickel powder, which has a sintering tempera ture of about 1300 C., the partial presintering of the 'matrix layer is carried out in a hydrogen atomspherc at about 1100 C. for about 30 minutes. Of course, if a different powdered metal is used, the presintering temperature must necessarily be selected in accordance there- .with. In any event, the temperature is preferably some what below the normal sintering temperature of the selected metal so that the metal will be only partially sintered.

The base member onto which the sintered matrix is to be bonded should be one which: possesses a coefficient of expansion approximately equal to that of the sintered matrix; has a'higher melting point temperature than the sintering temperature of the powdered metal used to make the matrix; has a hot strength sufficient to withstand'the adversecondit-ions present during tube operation; is not readily susceptible to creepage when mounted under tensile stress; and possesses a suitable electrical 1 resistance for use as a directly heated filamentary catl1 ode. A preferred material which meets these requirements is an alloy having a composition of 65% nickel,

. 30% molybdenum, and 5% "iron, commercially available as Hastelloy.

The provision of a sintered metallic coating on the base metal member preparatory to applying the partially sintered matrix layer serves to aid in obtaining a good bond betwen the matrix and the base member. The provision of the sintered metallic coating becomes more important to obtaining a good bond when the base mem her and the matrix are of different materials, e.g. Hastely and nickel, respectively. In providing this sintered metal surface coating the same kind of powdered metal is preferably used as is used for the matrix. Although such a uniformity of choice is not essential to a successful practice of my invention, it is believed to enable optimum bonding between the matrix and the sintered surface coating. On the other hand, since a fine metal powder will diifuse into a base member better upon sintering than will a coarser powder, a much finer powdered metal is used for the sintered metal surface coating than is used for the matrix. This serves to obtain the best possible bond between the base member and the sintered metal surface coating.

Provision of a larger weight and consequently a greater contact pressure during the final firing and bonding acts to compress the matrix and results in a higher density, lower porosity matrix. Conversely, a lower contact pressure permits greater matrix porosity but results in poorer bonding between the matrix layer and the base metal member. In some applications one or the other of these considerations may be of primary importance. However, in the manufacture of filamentary cathodes a preferred contact pressure of about 2.2 pounds per square inch phoretic process.

gives the best balance between the conflicting desirable qualities of high porosity and 'good bonding, both of which bear either directly or indirectly upon cathode life.

In a preferred embodiment as applied to the manufacture of filamentary cathode strands, a sheet matrix mold 12 comprising a block of commercial 302 stainless steel having a- 0.013 incn deep recess is used, and is provided with an oxidized surface in order to prevent the metallic powders sintered therein from sticking to the mold. Nickel, powdered to a particle size of about 40-70 microns, is placed in the mold and gently compacted by patting with a inch wide ceramic bar. This size bar has proved to give the proper amount of compacting in that the powder will tend to puff out from under the bar and provide only a predetermined degree of compacting. The powder is then leveled and partially 'presintered by heating in a hydrogen atmosphere at about 1100" C. (about 200 C. below the sintering temperature of nickel) for 30 minutes to form the matrix sheet 26.

Referring to FIGS. 2 and 3, a base metal having a composition of 65% nickel, 30% molybdenum, and 5% iron is first preformed into the desired ribbon strands 22, each having end sections 24 adapted for engaging suitable sup- 7 ports. These are then painted with a suitable binder, ve.g. nitrocellulose, and sprinkled with nickel powder. A powder having a' particle size of about 0-40 microns has been found to provide good results. The excess powder is shaken off thus leaving an extremely thin layer of powder and binder, which may be no more than a single particle thick; Alternatively, a paintable slurry of nickel powder and binder may be provided and applied to the base metal member. The coated ribbon strands are then fired in a hydrogen at 1300 C. for 30 minutes to provide a sintered nickel coating 25 thereon. The partially presintered matrix sheet 26 is removed from its mold 12 and, as

shown in FIG. 2, placed on a fiat ceramic block 28. A

number of the nickel-sintered-surfaced metal strands 22 are laid upon the nickel matrix sheet 26 with their sintered coatings 25 in contact with the matrix and weighted against the matrix sheet to provide a contact pressure of approxi- J mately 2.2 pounds 'per square inch. sembly of the base metal strands and the contacting matrix The Weighted asand the matrix layer 126 trimmed to the edges thereof.

The matrix of each strand 22 is then impregnated with an electron emissive coating.

The-coating with electron emissive material does not form an essential part of my invention and may be performed in any suitable manner known in the art. In my preferred embodiment, I choose todo this by a cata- I prepare a suspension of'powders of barium, strontium, and calcium carbonates in a binder solution containing a small constituent amount of an ionizable salt. I have found that methyl methacrylate binder dissolved in acetone and dibutyl phthalate solvents to be suitable for this purpose. If sufi'icient impurities in the form of ionizable salts are not present in the carbonate materials, a small amount of calcium nitrate may be added to produce an ionized electrolyte solution. The matrix-covered filamentary strands 22 are then immersed in the suspension and connected to the negative terminal of a DC voltage source. The positive terminal of the voltage source is connected to an anode which is placed in contact with the suspension. By proper control of the time duration and amount of applied voltage the matrices can be impregnated with the carbonates to any desired depth. I have found that in the case of a 10 mil matrix,

a penetration of 7 mils is satisfactory and gives optium results.

Several advantages of applying a matrix to a base member are obtained by the use of my method. For example, the application of pressure between the matrix material and the base material during heating in itself results in a superior bonding of the matrix to the base member. Also, through the application of pressure, the density of the matrix can be adjustably increased, within limits, to any desired amount simply by compressing the matrix. Placement of the matrix over a predetermined area of the base member is simplified by the provision of a separate, previously-processed matrix'sheet which can be handled independently of the base member and accurately placed thereon. Moreover, the use of a coherent matrix sheet handled out of contacat with its mold simplifies the application of pressure since the side walls of the mold do not interfere in those cases where the base member would otherwise extend past the edge of the matrix. Such, of course, is the case in the production of filamentary cathode strands where only a portion of the cathode is to be provided with a matrix layer. The prior partial presintering of the metal powders to form the matrix allows uniformity of the structure of the matrix, ease in handling the powders, and better dimensional control of the matrix. Prior partial presintering also permits visual inspection and weighing of the matrix prior to bonding to the base member. Any matrices having imperfections or unacceptable uniformity can be culled at this point thus preventing a high shrinkage in production of tie finished filamentary cathode strands. And, also in the case of filamentary cathode strands, the application of a solid one piece form of matrix to the base material simplifies the fixtures required for firing and allows greater production.

I claim:

1. The method of making an electron tube cathode comprising a metal base member having a porous surface layer of sintered metal powder bonded thereto, comprising the steps of: y

(a) preforming a thick layer of compacted metal powder having a sintering temperaturev below the melting point temperature of said base member, and partially sintering said layer into a coherent porous sheet;

(b) coating a surface of said metal base member with a thin layer of metal powder having a sintering temperature below the melting point temperature of said base member, and at least partially sintering said coating; and

(c) then pressing said preformed sheet against said coating, and firing, while continuing said pressing, to complete the sintering of said coating and sheet and form said bonded porous surface layer on said base member.

2. The method as in claim 1, wherein said metal powder sheet and said metal powder coating are of nickel powder, and said base member is a different metal having substantially the same coefficient of expansion as nickel.

3. The method of making an electron tube cathode comprising a metal base member having bonded thereto a porous surface layer of a sintered powder of a different metal having substantially the same coeflicient of expansion and having a sintering temperature below the melting point of said base member, comprising the steps of:

(a) preforming a thick layer of compacted nickel powder having a particle size of 40-70 microns, and partially sintering said layer into a coherent porous sheet;

(b) coating a surface of said metal base member with a thin layer of nickel powder having a particle size not greater than microns, and at least partially sintering said coating; and

(c) then pressing said preformed sheet against said coating, and firing, while continuing said pressing, to complete the sintering of said sheet and coating and form said bonded porous surface layer on said base member.

4. The method of making a filamentary electron tube cathode comprising an elongated fiat strand of resistive metal having a porous surface layer of sintered metal powder bonded thereto, comprising the steps of:

(a) preforming a thick layer of compacted metal powder having a sintering temperature below the melting point temperature of said strand, and partially sintering said layer into a coherent porous sheet;

(1)) coating. a surface of said strand with a thin layer of metal powder having a sintering temperature below the melting point temperature of said strand, and firing said coated strand to sinter said powder and bond it to said strand; and

(c) then pressing said preformed sheet against said sintered coating, and firing, while continuing said pressing, to complete the sintering of said sheet and form said bonded porous surface layer on said strand.

5. The method of making a matrix-surfaced filamentary cathode, comprising the steps of:

(a) coating one of the surfaces of a ribbon strand of metal, having a composition of about nickel, 30% molybdenum, and 5% iron, with nickel powder and heating to sinter said nickel powder and bond it to said strand;

([1) preforming and partially presintering into a coherent sheet a mass of nickel powder; and

(c) then pressing said sheet against the sintered surface of said strand with a pressure of approximately 23 pounds per square inch and simultaneously heating to complete the sintering of said sheet and bond it to said strand. I

6. The method of making a matrix-surfaced filamentary cathode, comprising the steps of:

(a) coating one of the major surfaces of a 65% Ni, 30% Mo, 5% Fe composition ribbon strand with a nickel powder and binder and heating said strand in a reducing atmosphere at a temperature of approximately 1300 C. for approximately 30 minutes to sinter said nickel powder;,

(b) preforming a mass of nickel powder into a sheet and heating in a reducing atmosphere at a temperature of approximately 1100 C. for approxi-' References Cited in the file of this patent UNITED STATES PATENTS Calkins et al Nov. 19, 1940 Koehring et al. Aug. 5, 1941 Marvin Feb. 15, 1944 Marvin May 30, 1944

Claims (1)

1. THE METHOD OF MAKING AN ELECTRON TUBE CATHODE COMPRISING A METAL BASE MEMBER HAVING APOROUS SURFACE LAYER OF SINTERED METAL POWDER BONDED THERETO, COMPRISING THE STEPS OF: (A) PREFORMING A THICK LAYER OF COMPACTED METAL POWDER HAVING A SINTERING TEMPERATURE BELOW THE MELTING POINT TEMPERATURE OF SAID BASE MEMBER, AND PARTIALLY SINTERING SAID LAYER INTO A COHERENT PORAOUS SHEET; (B) COATING A SURFACE OF SAID METAL BASE MEMBER WITH A THIN LAYER OF METAL POWDER HAVING A SINTERING TEMPERATURE BELOW THE MELTING POINT TEMPERATURE OF SAID BASE MEMBER, AND AT LEAST PARTIALLY SINTERING SAID COATING; AND (C) THEN PRESSING SAID PREFORMED SHEET AGAINST SAID COATING, AND FIRING, WHILE CONTINUING SAID PRESSING, TO COMPLETE THE SINTERING OF SAID COATING AND SHEET AND FORM SAID BONDED POROUS SURFACE LAYER ON SAID BASE MEMBER.

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