US2899299A

US2899299A - Method of manufacturing sintered

Info

Publication number: US2899299A
Authority: US
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11

Description

United States Patent 9 METHOD OF MANUFACTURING SINTERED CATHODE Robert T. Lynch, Berkeley Heights, N .J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York N Drawing. Application May 11, 1956 Serial No. 584,180

7 Claims. (Cl. 75-207) novel method of producing cathodes for thermionic tubes comprising the steps of mixing nickel powder with barium and strontium carbonates, molding under pressure and firing. Although the elements therein described showed promise as compared with the more conventional type of sprayed cathode element, thermionic tubes utilizing such cathode elements have not appeared on the market and do not appear to have been used commercially.

There is herein described and claimed an improved process for the manufacture of molded thermionic cathode elements of the type described in the above-designated United States patent. Briefly, the process of this invention comprises mixing nickel powder with a powdered material containing a barium compound such as powdered double coprecipitated barium-strontium carbonates together with an activating agent and a binder, molding the mixture under pressure into the desired shape and heating and cooling in the presence of a succession of atmosphere gases of different compositions which perform varying functions over selected temperature ranges.

Cathode elements so prepared have been in experimental use for some time and have manifested certain properties, notably hardness and smoothness, which are superior to commercially available structures.

Molded cathode elements prepared in accordance with this invention are less susceptible to harmful atmospheres such as air than are their sprayed counterparts. In ex' periments conducted by the inventor, cathode'elements have actually been removed from their envelopes and exposed to air for long periods of time without peeling or loss of coating, and with no appreciable loss of emission. They have withstood the drawing of large 'D.'C.

current of several amperes per square centimeter with,

no fading.

Molded elements operated under pulsed conditionssuch as to create sparking repeatedly have shown no electrical deterioration, of the pulsed or DC. emission characteristics or physical deterioration of the coated surface. Due, in part, to the increased thickness of the emitting surface,

these elements are less susceptible to ion bombardment than are sprayed oxide cathodes.

Molded thermionic cathode elements may be easily. They may be manufactured on a commercial scale. I pressed in any of a large variety of shapes, limited only by die techniques. For example, they may be pressed into; concave gun-type cathodes, they may be formed into composite structures as by pressing together multiple layers of a pure metal powder such as nickel and emitting layers heater structures by embedding and pressing .the heater "ice material into the emitting mixture. Other structures will suggest themselves to those skilled in the art.

, Automation presents no problem. After the prepara: tion of the initial mixture, all steps may be carried out automatically with no undue precautions being taken regarding cleanliness. Subsequent to pressing and heat, treatment, the elements may be easily machined so as to produce any desired configuration.

A general outline of the methods herein together with the ranges of operating parameters will now be given.

An initial emitting mixture is prepared. This mixture contains nickel powder, an emitting material such as coprecipitated barium-strontium carbonate, an activating agent, sometimes referred to as an activating agent, .and a binder material which latter will be removed during the subsequent heating steps. The grade of nickel powder chosen should be as nearly pure as practical so as not to contain any contaminant which may impair the emitting characteristics of the final structure. Carbonyl-nickel powder has been found suitable in this use. Electrolytic nickel powder may be substituted. Although the particle size of the nickel powder is not critical in most uses, a general preference exists for very fine particles. It has been found that 100-mesh material containing particles of a maximum size of 150 microns produces satisfactory results. In the production of cathodes for use in microoscilloscope tubes Where a very fine uniform surface is required, particles as small as 4 microns have been used with an accompanying improvement in characteristics as compared with the coarser material. W

Any of the powdered emitting mixtures well known in the preparation of sprayed thermionic cathodes may be used in the preparation of the moldedcathode. materials usually contain a barium compound, which will, break down on station to yield barium oxide. Since the temperature attained on station is usually about 1000" Q, for the purpose of the process described herein, it is. considered that any barium compound which will thermal-- ly decompose at a temperature of less than 1000 C. to yield barium oxide is suitable. Such materials include the single carbonate material, barium carbonate; the double carbonate material, coprecipitated barium-stumtium carbonate; and the triple carbonate material, coprecipitated barium-strontium-calcium carbonate. In general, it has been found that the double carbonate is to be preferred over the single and that little further advantage is gained by use of the triple carbonate.

The double carbonate most commonly available for this pur-E pose is a coprecipitant of equimolar portions of barium carbonate and strontium carbonate. The particle size.

. of this emitting mixture is not critical, a preference existing again for fine particles. A commercially availablei coprecipitant containing particles, percent of which.

are smaller thanlO microns, has proved suitable.

Activators which perform the function of producing the emission characteristics of the structure are wellL known in the sprayed cathode art and reference may be six thousand hours of use. Other activators include carbon which is even more rapid than zirconium, and silicon which has some of the characteristics of titanium but which may develop an interface of silicon dioxide which impairs the operating characteristics of the resultant device. Ma-gnesiu-m has been tried and found to be un's'atis- I factory in this use, it being found to be too reactivean d resulting in the production of excessive amounts of magnesium oxide which prevents further activation. Depending on the use to which the final structure is to be put, of the activators mentioned, either zirconium or titanium, is to be preferred. Whichever activator material is used, it should be powdered as finely as is feasible, an average particle size of 15 microns being found satisfactory.

The preceding paragraph is not intended to be an exhaustive discourse on the general subject of activator ma terials. Such materials and their characteristics are wellknown to those skilled in the art. See, for example, Theoretical Study of the Chemistry of Oxide-Coated Cathodes, E. S. Rittner, Phillips Research Reports, vol. 8, page 184, 1953. It is well known that activator materials such as zirconium and titanium are conveniently used in a chemical form which breaks down to give a fine dispersion during processing. It is quite common in the instance of these elements to use the corresponding hydrides. Activator properties indicating the selection of one or another of the materials included in this broad grouping are also well known.

. In addition to the nickel powder, the carbonate, and the activator materials listed above, there may be added to the first mixture a binder material. Binder materials which may perform the second additional function of acting as a lubricant are well known to those skilled in re lated fields such as, for example, ferrite art. It is a general requirement of such materials that they leave little or no residue in the end product after sintering. Common binder materials which will operate satisfactorily here include acetone solutions of either isobutylmethacrylate or stearic acid. For other common binders and associated characteristics, see Treatise on Powder Metallurgy," Goetzel, vol. 2, Interscience Publishers, Inc., New York, 1949. Binders should be added to the mixture in minimum quantities. Where excessive amounts are present, resultant diificulties include porosity and excessive flexibility of final product, possible contamination due to impurities which may be contained in the binder, and difficulty of removal. Where it is undesirable to use a binder as, for example, when the cathode is sintered in vacuum, the die plungers may be lubricated with paraffin.

The following is an outline of the procedure to be followed in producing a cathode element from the above materials.

To the nickel powder there is added from zero to 2 percent by weight of an activator material. For most uses the preferred amount of activator is of the order of 1 percent by weight of the nickel powder. Use of amounts of activator in excess of about 2 percent results in a falling off of the activity of the end product. The mixture of nickel powder and activator is thoroughly dryrnixed as, for example, in a mortar and pestle or in a ball mill. Experience indicates that a mixture of about 20 or 30 grams may be thoroughly mixed in a mortar and pestle in less than 15 minutes. This mixing step is carried out in air at room temperature.

The acetone solution of binder is produced by dissolving from 1 to 2 percent of binder in the acetone in air at room temperature. Although heating will hasten the formation of this solution, it is to be avoided unless proper precautions are taken to prevent fire.

A binder solution in an amount of up to about 2 percent by weight of nickel powder is slowly added to the nickel powder-activator mix in a mortar at such a rate as to maintain a slurry. Mixing is continued with a pestle as the binder solution is added in air at room temperature until the mixture is dry, the acetone evaporating as the binder is added. Again, by reason of the flammability of acetone, this mixing step is carried out in an unheated mortar unless additional precautions are taken.

, This mixture is herein referred to as the nickel mix may be stored until required.

, A second basic mixture is now produced by mixing a portion of the nickel mix above with a portion of single, double or triple carbonate. The amount of carbonate used represents a compromise between pure nickel mix which is best from a mechanical standpoint and pure carbonate which is best for emission. The amount of carbonate is generally in the range of from about 10 percent to about 50 percent by weight of nickel mix, the preferred amount for pressed cathodes having a supporting portion of metallic nickel being about 30 percent by weight. Mixing is carried out in a mortar and pestle or ball mill and is continued until the color is homogeneous. Since the nickel mix is black and the carbonate is an olf-white, this final mixture will be gray. With a total amount of about 30 grams, mixing in a mortar and pestle will take about 15 minutes. This final mixture is herein referred to as the emitting mix.

The nickel mix and the emitting mix having been produced, the next step in the process is to press the materials into the desired shape and size. If the final product is to be a composite structure, layers of the nickel mix and the emitting mix may be pressed in one operation. In such a structure, the nickel layer lends mechanical rigidity while the emitting mix layer may be kept relatively shallow so as tokeep to a minimum the time on station. On the other hand, the final structure may consist only of emitting material where, for example, it is to be produced by pressing or otherwise adhering emitting material to an existing structure.

The usual procedure, where the structure is to be composite, is to first insert a layer of nickel mix into a die and after pressing this layer lightly, to then insert a layer of emitting mix into the die. The entirety, consisting of the two layers, is then pressed at a pressure of from 20 tons per square inch to 100 tons per square inch. It has been found that a pressure of about tons per square inch, readily available on commercial hydraulic presses, is suitable in producing a dense mass which may be easily machined. The greater the applied pressure the more dense is the end product and the greater is the initial activity. However, increasing the applied pressure beyond about tons per square inch results in a physical breakdown of the emitting surface.

There are several considerations to be taken into account in determining the optimum shape and size of the pressed body as produced in accordance with the steps outlined above. In general, if the emitting layer, that is the layer produced by firing of the emitting mix portion, is too thick, there is difiiculty in removal of gas which is evolved upon breakdown of the carbonates partially during sintering but primarily on station. In addition, little is gained by increased thickness since in the usual tube configuration all useful emission evolves from the planar surface or other surface facing the anode. Side emission from surfaces of the cathode not facing the anode does not significantly increase the current density of such a structure. In general, emitting layer thicknesses of the order of from about 3 to about 8 mils are to be preferred, this range being sufi'iciently thin to avoid undue difficulty of removal of evolved gases and being sufficiently thick to produce a durable emitting surface which will withstand any expected arcing, bombardment or exposure to harmful atmosphere such as air. If the thickness of the emitting layer is too thin, as below about 3 mils, a discontinuity in the emitting surface may result. Such a discontinuity is undesirable in that the current density of the tube is decreased and other operating characteristics may be impaired. For composite structures as, for example, those utilizing a layer derived from an emitting mix and a layer derived from a nickel mix, the optimum emitting layer thickness may be of the order of 5 mils.

For composite structures the thickness of the non-emit- C ting layer is determined primarily with a view to mechanical considerations. It has been found that with an emitting layerof the .order of about 5 mils in thickness, at

Once the structure has been pressed, whether it consists of a single emitting layer or is a composite structure; it is now subjected to heat treatment. The chief purpose ofthis heat treatment is to sinter the nickel in the emitting mix so as to produce. a mechanically rigid body. This heat treatment step is most critical and precautions must be taken to avoid contamination, to avoid undue oxidation of nickel so as to achieve good sintering and to avoid the reduction of the alkaline earth carbonates to the oxides which will, react with the atmosphere to produce hydroxides. It is considered that the development ofthe' following series of heat treatment steps constitutes the Any inert gas such as. helium or argon may be substituted for the; nitrogen providing its impurity contentis not un-' desirable.

The nitrogen flow is then replaced by a flow of from,

225 to 350 cubic centimeters per second of purified d'r-y hydrogen or prepurified hydrogen (PPH). The exit .hydrogen is burned in a pilot at the exit. end of the furnace.

After the exit hydrogen has been ignited the furnace is put into operation and is heated from room temperature to about 600 C. at a rate of about 100 C. per minute. The purpose of the hydrogen flow. is to prevent any substantial oxidation of the nickel particles in the emittingand to reduce any nickel oxide which may be present- Heating'over this range also has the effect of breaking down a small amountof the carbonate present to oxides a; consequent release of carbon dioxide. If the fur-[. nace-is heated at a substantiallyzgreater rate than 100 C.

per minute, the released gases, the oxygenvfrom the nickeloxide and the carbon dioxide'from the carbonates, may cause eruption and destroy the homogeneity of the pressed body. In general; a slower heating rate during. the hydrogen flow period isnot objectionable, although reducing toa very low rate as, for example, below .the rateof L0.

C. per minutemay result in breakdown of larger arnountsl' of carbonate.

When the temperature of the furnacereaches 600 C. it'is held at that temperature as the gas flow is'changed from hydrogen to nitrogen or otherinert gas as helium orargon until the exit pilot flame becomes extinguished.

I This. gen-- erally takes about from 1 to Z'minutes. The flow rateis.

indicating a removal of residual hydrogen.

not critical, but as in'purging, a. flow rate of about 50.

cubic centimeters per second of nitrogen has been found.

sufficient.

Although they temperature at. which the changeover from i hydrogen to nitrogen is carried out is' generally held at about 600 C., it has been found that this changeover may."

satisfactorily be carried out over the range of from 5503' am 650 C. Changeover below 550 C. results in in.- sulficient reduction-of total oxide intthe nickel eventually. resulting. in imperfect, sintering, while the presence of a. hydrogenatmosphere at a temperature of over 650 C. is undesirable for the reason that too great an amountof the carbonatesare-reduced to the oxides.

Once the exit pilot is extinguished, the temperaturerofthe. furnace is again caused to rise, this time to a tempera- A nitrogen flow rate of the order of about 50 ture of at least 800 C. During this last heatingstepthe nickel powdersi'nters, substantial sintering. taking placeat a temperature of about 800 C. A sintering temperature of about 1000 C. is usually. preferred. Temperatures above about 1200" C. are unsatisfactory in that larger amounts of carbonate breaks down.

When the temperature of the furnace attains thedesired sintering temperature, .the power is turned 0E and the fur nace allowed to cool to about 600 .C. Nitrogen flow: through the furnace is maintained during this cooling steps In general, the coolingrate is not critical providing that therate is not such'as. to produce serious thermal stress and resultant cracking .of the cathode. Maintenance of: the furnace at any temperature in the: range ofover 800. C. results in increased breakdown. of the carbonates and should be keptat a minimum.

When. the furnace has cooledto about 600 C. the? nitrogen flow is stopped and hydrogen is caused to flow through the furnace, the exhaust pilot again being; lighted to prevent formation of a combustible mixture; outside of the furnace. In this connection, it is again important that, in the temperature range below about 550 C., a reducing atmosphere be maintained within the furnace to remove any reducible oxides which may have formed during the sintering procedure in the higher temperature range.

The furnace is now allowed to cool to room tempera*: tur'e. Although the rate of cooling is not important,- it is desirable to cool. rapidly to prevent unnecessary. contaminationof thesintered material. When the furnace is at room temperature the flow of hydrogen. is stopped and the furnace is purged with nitrogen or other. inert gas until: the flame is extinguished. If it is con--- sidered desirable, there is no objection to substituting nitrogen for hydrogen inv the heating or cooling range between room temperature and a temperature in: the range of 300 C. to 400 C.- since the hydrogen haslittle reducing, action below about 400 C. r

With the processv carried out as set forth above, it is possible to produce a sintered product in which no more than about 5 percent of the 'total carbonate is decom-- posed to the. oxide- The .sintered cathode element may now be machined such-is desired, after which it may either. be placed directly in the vacuum tube structure or may be stored; in vacuumuntil required.

All: thatremains in the manufacture of a usable" cathode: is :the conventional breakdown procedure; Since".

this'procedure is'well' known to those skilled in the art;

it will not be described in detail. In brief, a typical": breakdownprocedure consists of sealing the element on a vacuum station which is evacuated to a pressure of the order of'10- millimeters of mercury. The cathode istihen heated at a maximum pressure of 10 millimeters until the carbonates are broken down to oxides. This heating procedure, which may takeof the order of 15-30 minutes for an emitting layer thickness of approximately 5 mils, is terminated when substantially all of the car :xbonates are broken down. The breakdown point is indicated by a sudden drop in pressure within the chain-i her. The cathode is then heated to about 1000 C. and is held at this temperature for about 5 minutes. The maximum'expected operating anode-potential is then'ap plied:.with the structure at 1000 C. and emission current isdrawn for a period of 5-10 minutes The tent-f perature of the structure is then dropped to about 8509" C. where the total current is then measured by DC.

or pulse measurement.

Me'asurementsat this stage on station at .an operating temperature of 850 C. indicated pulse current emis sionintensities of the order of-3 to 5 amperes per: square centimeter. and D.C. emission current intensities ofthe: order-of 1. to. 2 amperes per square centimeter...

states I The following numerical example relates to, the preps-- I arationofamoldedcathodm Example I A 0.2 grarn of. zirconiumhydride powder of an average particle size of microns was added to grams of carbonyl nickel. powder of. an averageparticle size of 1 150 microns andof a chemical purity of 999 percent by weight, disregarding oxygen. The. combined powders were thoroughly mixed in a 4-inch mortar and pestle I Q for 10 minutes. I To this there was slowly added20 cubic I centimeters of acetone in which .was .dissolved 0.4 gram isobutyl methacrylate. asmixingis continued. The rate of addition of the isobutyl-methacrylate solutionto the powder mixture was such as'to' at all times maintain a slurry in the mortar,. Addition time was about. 15 I minutes. I Subsequent to addition,- the powders were mixed until all of the Iacetone'was' evaporated. Mixing time was about. minutes. This material will be referred to as the nickel mixfi I The emitting mix was formed by dry mixing 3 grams I I of alkaline earth double carbonate (coprecipitated barium I strontium carbonate), in a mortar and pestle with 7 grams I of nickel mix prepared as'above'. Mixing time was 15: I minuteaf I I I A three-piece, double-actingdie of circular cross-sec tion havinga 0.116-inch inside diameter holeandtwo sliding plungers was. used to mold the nickel and emitting mix into a pressed composite body as follows: .The lowerlplunger' of the die was inserted in the die body was to leave a space of a depth of 0.1 inch. The

' space was filled with nickel mix, the diewas tapped gently so as to settle the powder and the excesspowder was.

removed. The. nickel mix in the die was thendepressed ,by .useof the upper plunger or spacer; so. as .to leave I a space of a depth of 0.015 inch; The said space was then. filled .with emitting mix material preparedas above, the emitting mixmaterial wasjlevelecl oil at the top of v the die and the top plunger was inserted into thedie,

I body. The plungers Werethen centered inthe die body, the complete assembly was placed in a hydraulic press and a pressure of 80 tons per square inch was .applied between plungers. The plunger and pressed disc were then ejected from the die body. The pressed disc was then placed in a nickel boat and the boat was inserted in a Globar furnace having a one and one-half inch inside diameter. With the furnace at room temperature, it was purged by passing nitrogen gas of a grade known as prepurified nitrogen containing no more than 0.1 percent impurities by volume at a flow rate of cubic centimeters per second for a period of 5 minutes. After the purging period had terminated, the nitrogen gas flow was replaced by a 25 0 cubic centimeter per second flow of pure dry hydrogen of a grade known as high purity containing as impurities no more than 0.3 percent by volume. burned oil? at a pilot at the exit end of the furnace.

The furnace was then switched on and allowed to heat at a rate of 100 C. per minute to a temperature of 600 C. The flow of hydrogen gas was then replaced by a 50 cubic centimeter per second flow of pure dry nitrogen of the grade utilized in initial purging. With nitrogen flowing through, the furnace was maintained-at 600 C. for one minute at which time the hydrogen flame was extinguished indicating substantial purging .of hydrogen from the system. After extinction of the pilot, the furnace was again allowed to heat, this time at a rate of 250 degrees per minute until a temperature of 1000 C. was attained. Nitrogen flow was maintained through the furnace during the entire heating period from 600 C. to 1000 C. The furnace was allowed to reach a momentary peak temperature of 1000 C. after which it was allowed to cool to a temperature of 600 C. while maintaining the nitrogen flow through the furnace as above setforth. Cooling wasaccomplished by turning The hydrogen was ignited andbe the power source to the furnace and took ,I6 n'iiuutes. Thenitr'ogen howwas thenzreplaced by hydrogenflow of the grade and flow rate set forth above as utilized during the initial heating procedure while maintaining j the furnace at 600 C. The exit'fiow of. hydrogen was again ignited at'the exitpilot and the furnace was allowed I to cool to room: temperature. I

to room temperature .took minutes. peraturethe hydrogen withinthe furnace waspurg'e-d by i I a nitrogen flow of the same grade and'flow rate as above "set forth in connectionwith initial purging; When the I exit pilot was extinguished, indicating substantial purging I of the system, the boat was removed, from the furnace. 1 .The'stored'discs were then utilized in the preparation ofthecathode. structurein a traveling wave oscilloscope I tubeas described by I. R. Piercein Electronics of Noe At room temvernber 1949 at pages 97-99, although they could have been stored in evacuated glass envelopes for any desired I period. I

The cathode elements so produced were welded to or 0.500 inch by spot, welding; The cylinder was sup- I I Thecornplete oscilloscope tube was then sealed to a I vacuum system in which a vacuum of from 5 10- to l 3 I I I portedby; a. ceramic insulator and the structure mounted I in a cathode ray tube electron gun assembly as described in the above-cited reference.

5 10-T millimeter of mercury was maintained and in which the structure} was baked for 14 hours at 420 C5 After bake-out, cathode heater voltage wastapplied to raise the cathode temperature to 750 C. The cathode was-maintained at this temperature for a period of from I 15 to 20 minutesto remove surface gases. 7

structurewas then heated'by a radiofrequency generator I to a. temperature of 800 C. after which the cathode was broughtup slowly to-a temperature of 1000" C. and was maintained at this latter temperature for a period of 20 .minutes. After, this period the pressure in the system,

suddenly dropped to a lower value (from'abou't 5 10" down to about 5 X 10'" millimeter of mercury) indicating substantially complete breakdown of the carbonates.

After breakdown, a potential of 50 volts was applied etc the first anode and a potential of 1800 volts was applied to the second anode. After about 20 minutes with the indicated potentials applied, the cathode drew 3 amperes. According to the specification for this particular tube this current density marked activation so that the tube was then sealed off the station.

'The oscilloscope tube so produced was operated at 850 C. with a first anode potential of 30 volts and a second anode potential of 1000 volts at current densities of the order of 1 ampere per square centimeter.

What is claimed is: I

1. A method of forming a cathode element comprising mixing nickel powder with a powder material selected from the group consisting of barium carbonate, bariumstrontium carbonate and barium-strontium-calcium carbonate together with an activator material, molding the resultant mixture under pressure, heating to a first temperature in the range of from 550 C. to 650 C. in a nonoxidizing atmosphere, the heating in the range of from about 400 C. to the said first temperature being carried out in a reducing atmosphere, further increasing the C. in a non-oxidizing atmosphere.

2. A method of forming a cathode element compris- Co'oling' from 09 I I ing mixing nickel powder with a powder material selected from the group consisting of barium carbonate, bariumstrontium carbonate and barium-strontium-calcium carbonate together with an activator material selected from the group consisting of zirconium, titanium and carbon, molding the resultant mixture under pressure, heating the pressed mixture to a temperature of about 400 C. in a non-oxidizing atmosphere, further heating from the said temperature of about 400 C. to a first temperature in the range of from 550 C. to 650 C. in a reducing atmosphere, further heating from the said first temperature to a temperature of at least 800 C. in an inert atmosphere, cooling to a second temperature in the range of from 650 C. to 550 C. in an inert atmosphere, which second temperature may be the same as the said first temperature, further cooling from the said second temperature to a temperature of about 400 C. in a reducing atmosphere, and finally cooling to a temperature below 400 C. in a non-oxidizing atmosphere.

3. A method of forming a cathode element comprising mixing nickel powder with a powder material selected from the group consisting of barium carbonate, bariumstrontium carbonate and barium-strontium-calcium carbonate together with an activator, material selected from the group consisting of zirconium, titanium and carbon, molding the resultant mixture under pressure, heating to a temperature of about 400 C. in a non-oxidizing atmosphere, further heating to a first temperature in the range of from 550 C. to 650 C. in a reducing atmosphere, further heating from the said first temperature to a second temperature in the range of from about 800 C. to about 1200 C. in an inert atmosphere, cooling from the said second temperature to a third temperature in the range of from 650 C. to 550 C. in an inert atmosphere, which third temperature may be the same as the said first temperature, further cooling from the said third tem perature to a temperature of about 400 C. in a reducing atmosphere and finally cooling to a temperature below 400 C. in a non-oxidizing atmosphere.

4. A method of forming a cathode element comprising mixing nickel powder with a powder material selected from the group consisting of barium carbonate, bariumstrontium carbonate and barium-strontium-calcium carbonate together with an activator material selected from the group consisting of zirconium, titanium and carbon, molding the resultant mixtureunder pressure, heating from room temperature to about 400 C. in an inert atmosphere, further heating from the said temperature of about 400 C. to a first temperature in the range of from 550 C. to 650 C. in hydrogen, further heating from the said first temperature to a second temperature in the range of from about 800 C. to about 1200 C. in an atmosphere of an inert gas selected from the group consisting of nitrogen, helium and argon, cooling from the said second temperature to a third temperature in the range of from 550 C. to 650 C. in an atmosphere of an inert gas selected from the group consisting of nitrogen, helium and argon, which third temperature may be the same as the said first temperature, further cooling from the said third temperature to a temperature of. about 400 C. in hydrogen and finally cooling from the said temperature of about 400 C. to room temperature in an inert atmosphere.

5. A method of forming a cathode element comprising mixing nickel powder with a powder material selected from the group consisting of barium carbonate, bariumstrontium carbonate and barium-strontium-calcium carbonate together with an activator material selected from the group consisting of zirconium, titanium and carbon and a binder, molding the resultant mixture under a pressure of from to tons per square inch, heating from room temperature to a first temperature in the range of from 550 C. to 650 C. in an atmosphere of hydro gen, further heating from the said first temperature to 'a temperature of about 1000 C. in an atmosphere of nitrogen, cooling from the said temperature of about 1000 C. to a second temperature in the range of from 650 C. to 550 C. in an atmosphere of nitrogen, which second temperature may be the same as the said first temperature, and cooling from the said second temperature to room temperature in an atmosphere of hydrogen.

6. A method of forming a cathodeelement comprising mixing nickel powder with coprecipitated bariumstrontium carbonate together with an activator material se lected from the group consisting of zirconium, titanium and carbon and a binder material, molding the resultant mixture under a pressure of from 80 tons per square inch to 100 tons per square inch, heating from room tempera ture to a first temperature in the range of from 550 C. to 650 C. in an atmosphere of hydrogen, further heating from the said first temperature to a temperature of about 1000 C. in an atmosphere of nitrogen, cooling from the said temperature of about 1000" C. to a second tempera ture in the range of from 650 C. to 550 C. in an atmosphere of nitrogen, which second temperature may be the same as the said first temperature, and cooling from the said second temperature to room temperature in an atmosphere of hydrogen.

7. A method of forming a cathode element comprising mixing nickel powder-with-coprecipitated barium-strontium-calcium carbonate together with an activator material selected from the group consisting of zirconium, titanium and carbon and a binder material, molding the resultant mixture under a pressure of from 80 tons per square inch to 100 tons per square inch, heating from room temperature to a first temperature in the range of from 550 C. to 650 C. in hydrogen, further heating from the said first temperature to a temperature of about 1000 C. in an atmosphere of nitrogen, cooling from the said temperature of about 1000 C. to a second temperature in the range of from 650 C. to 550 C. in an atmosphere of nitrogen, which second temperature may be the same as the said first temperature and cooling from the said second temperature to roomtemperature in an atmosphere of hydrogen.

References Cited in the file of this patent UNITED STATES PATENTS 1,191,552 Aeuer July 18, 1916 1,883,898 Halliwell Oct. 25, 1932 2,326,631 Fischer Aug. 10, 1943 2,543,439 Commes et a1 Feb. 27-, 1951

Claims

1. A METHOD OF FORMING A CATHODE ELEMENT COMPRISING MIXING NICKEL POWDER WITH A POWDER MATERIAL SELECTED FROM THE GROUP CONSISTING OF BARIUM CARBONATE, BARIUMSTRONTIUM CARBONATE AND BARIUM-STRONTIUM-CALCIUM CARBONATE TOGETHER WITH AN ACTIVATOR MATERIAL, MOLDING THE RESULTANT MIXTURE UNDER PRESSURE, HEATING TO A FIRST TEMPERATURE IN THE RANGE OF FROM 550*C. TO 650*C. IN A NONOXIDIZING ATMOSPHERE, THE HEATING IN THE RANGE OF FROM ABOUT 400*C. TO THE SAID FIRST TEMPERATURE BEING CARRIED OUT IN A REDUCING ATMOSPHERE, FURTHER INCREASING THE TEMPERATURE OF THE PRESSED MIXTURE ABOVE 650*C. AND THEN DECREASING TO A SECOND TEMPERATURE IN THE RANGE OF FROM 550*C. TO 650*C., WHICH SECOND TEMPERATURE MAY BE THE SAME AS THE SAID FIRST TEMPERATURE, THE LAST HEATING AND COOLING STEPS BEING CARRIED OUT IN AN ATMOSPHERE OF AN INERT GAS, FOLLOWED BY FURTHER DECREASING THE TEMPERATURE OF THE PRESSED MIXTURE FROM THE SAID SECOND TEMPERATURE TO ABOUT 400*C. IN A REDUCING ATMOSPHERE, AND FINALLY DECREASING THE TEMPERATURE BELOW ABOUT 400* C. IN A NON-OXIDIZING ATMOSPHERE.