Abstract: The invention relates to a gas sensor for measuring an air-fuel ratio of an air-fuel mixture of an internal combustion engine and to a method of manufacturing the gas sensor. The sensor of the invention comprises: a solid state electrolyte layer made of an oxygen ion conductive metal oxide; a first electrode of a porous thin film having the catalyst function which was coated on one surface of the solid state electrolyte layer; a second electrode of a porous thin film having the catalyst function which was coated on the other surface of the solid state electrolyte layer; an electrode shielding layer made of a sintered material of ultra fine particulates whose average grain diameter is 1 .mu.m or less which covers the surface other than the region of a predetermined area of the second electrode; and a gas diffusion layer made of a porous electric insulative metal oxide which covers the region of the predetermined area of at least the second electrode.

Abstract: This invention relates to substantially full-dense, near-net shape bodies made of dispersion strengthened copper (D.S.C.) powder and products which are cold formed and/or machined from these bodies, such as resistance welding electrodes. The invention includes processes for manufacturing substantially full-dense, near-net shape, substantially equiaxed bodies from D.S.C. powder.

Abstract: An improved ceramic-metal composite comprising a mixture of a ceramic material with a ductile intermetallic alloy, preferably Ni.sub.3 Al.

Abstract: Disclosed is a metal-based composite material comprising aluminum or an aluminum alloy combined with a whisker of aluminum borate represented by the chemical formula of 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3 or 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3. This composite material has excellent mechanical properties such as high tensile strength and high hardness.

Abstract: A chromium additive of the formula: Cr.sub.x C.sub.y O.sub.z where 0.04.ltoreq.y.ltoreq.0.35, and 0.03.ltoreq.z.ltoreq.0.30 for x=1, said additive having an X ray diffraction peak at d=3.32 .ANG. (2.theta.=26.8.degree.).

Abstract: A cured sintered porous metal structure comprising aluminum and aluminum alloys is presented comprising an aluminum oxide durable surface integral to the structure. The surface layer is enhanced in aluminum while the underlying structure is thereby depleted in aluminum. The structure exhibits surface and interfacial durability.

Abstract: A dispersion strengthened copper alloy containing a copper matrix, and dispersion particles dispersed in the copper matrix within a range of 0.5 to 6 vol %. In this alloy, an average diameter of a matrix region where the dispersion particles are not present is 0.3 .mu.m or less, and the total amount of solid solution elements contained in the copper matrix is determined such that, when this amount of the solid solution elements is added to pure copper, the electric conductivity of the matrix is lowered by 5% IACS or less.

Abstract: Composite magnetic compacts having good conductivity and excellent mechanical and magnetic properties and their forming methods. The composite magnetic compacts are basically made by forming mixtures consisting essentially of 1 to 50 percent by weight of a magnetic powder and the remaining percentage of a powder of superplastic Zn-22Al alloy. A drop in the strength of the compacts that occurs when the mixing percentage of the magnetic powder increases is made up for by the impregration of plastic in the compacts or the simpler addition of a plastic powder to the mixture of the powders of magnetic material and superplastic Zn-22Al alloy. The forming methods of the composite magnetic compacts are carried out at different temperatures and under different conditions depending on the composition of the powder mixtures and so on.

Abstract: A composite comprised of a sintered matrix of spinel ferrite and a non-exposed continuous phase of elemental silver or Ag-Pd alloy ranging to 25 atomic % Pd is produced by co-firing a laminated structure of ferrite powder-containing tapes containing non-exposed metallization-forming material. The composite can be formed into a composite product which contains a continuous silver or Ag-Pd alloy phase with two end portions wherein only the end portions are exposed.

Abstract: Subject a heated, non-molten, intimate admixture of a base metal and a superconducting oxide material to pressure sufficient to form a densified article. The densified article is either superconducting as formed or capable of being rendered superconducting by annealing in the presence of oxygen. Depending upon the choice of materials the densified article is stable at temperatures up to 1000.degree. C.

Abstract: The present invention is directed to a method for forming porous oxide dispersion strengthened molten carbonate fuel cell anodes having improved anode creep resistance. The method comprises the steps of forming an alloy powder comprised of a base metal and an alloy metal, forming the alloy into a porous anode structure by sintering, and then placing the porous anode structure under conditions in which the base metal is reduced and the alloy metal is oxidized, thereby internally oxidizing the alloy metal to form oxide particles therein.

Abstract: A method for fabricating a ceramic superconductor comprising the following steps: mixing a copper or copper alloy metal, a first metal oxide or carbonate which includes one element selected from group IIA of the Chemical Periodic Table, and a second metal oxide or carbonate which includes one element selected from group IIIB of the Chemical Periodic Table to form a copper/ceramic mixture; heting a copper/ceramic mixture to a temperature in the range between 900.degree.-1100.degree. C.; and cooling the heated copper/ceramic mixture slowly, thereby forming a superconductive material. The copper/ceramic mixture may be extruded through a die prior to the heating step, thereby forming a superconductor of desired shape.

Abstract: Process for shaping and improving the mechanical properties of blanks produced by powder metallurgy from an alloy with increased high-temperature strength by extrusion, and the deformation is successively performed in at least two temperature ranges different from one another or in two phases, in that the blank (2) is first reduced in its cross section at a temperature T.sub.1 and then is either agin reduced in at a lower temperature T.sub.2 or is deformed at a temperature T.sub.3 under counterpressure so that its cross section is further widened. T.sub.3 can be smaller than or equal to T.sub.1.

Abstract: The proposed invention is used for the manufacture from the obtained composite of cutting tools, hard alloy tooling, dies and other products. A method according to the invention involves preparing a mixture, compacting it, placing the mixture into a synthesis zone, igniting the mixture, with subsequent reaction of components of the mixture under combustion conditions. Then cure is carried out during a period ranging from about 0.1 seconds to about 0.5 hours, and the hot combustion products are compacted under pressure at an average pressure rise rate ranging from about 10 to about 2000 kgf/cm.sup.2.s, with subsequent cure of the compacted product under isobaric conditions to complete homogenization of the composite, with subsequent cooling thereof to obtain an end composite.

Abstract: Impact resistant clad composite armor and method for forming such armor. The impact resistant clad composite armor includes a ceramic core, and a layer of metal surrounding the ceramic material and bonded to the ceramic core. The metal layer is formed by cold isostatically pressing powder metal surrounding the ceramic core to a high initial density followed by vacuum sintering. The composite armor may be hot isostatically pressed to densify the powder metal to approximately 99% full density.

Abstract: In a method of producing a sintered aluminium nickel alloy particles of powder are formed with nickel surfaces (either on nickel or on ceramic material). The surface of the nickel is oxidised and these particles are then mixed with aluminium powder and the mixture sintered in known fashion.

Abstract: The method of producing a superconducting product includes: providing a pressed-powder preform consisting essentially of REBa.sub.2 Cu.sub.3 O.sub.x where 6.0<x<7.0; preheating the preform to elevated temperature for a time period between 0 and 10 minutes, within a medium consisting of a mixture of refractory ceramic particles, carbonaceous particles and ultra fine graphitic particles; providing a preheated grain bed and embedding the heated preform in that bed, the bed having the same composition as the medium; and consolidating the preform to at least about 95% of theoretical density by application of pressure to the grain bed, thereby to form the product.

Abstract: The present invention relates to a method of preparing an alloy for use as a cutting tool material comprising hard principles and binder phase by which a uniform distribution of the hard principles in the binder phase is obtained, and the resulting product.

Abstract: A process is provided for the production of a sintered body. The process includes the following consecutive steps: (i) mixing and kneading one or more metal powders and/or one or more alloy powder with a binder into a compound, said metal and alloy powders having an average particle size not greater than 30 .mu.m, (ii) injection-molding the compound into a green body; (iii) debinding the green body to form a debound body; and (iv) subjecting the debound body to first-stage sintering at 1,050.degree.-1,250.degree. C. in a reducing or reduced-pressure atmosphere and then to second-stage sintering at a temperature in a range of 1,100.degree.-1,400.degree. C. which is higher than that of the first-stage sintering. This process can provide sintered Ti bodies and sintered magnetic bodies of the Fe-Si type, which have a density ratio of at least 95%.

Abstract: A process for producing an oxidic superconductor precursor alloy which comprises mechanicallly allowing metallic elemental constituents of the oxidic superconductor in stoichiometric proportions and in the presence of a process control agent non-detrimental to the superconductor to provide a uniform mechanically alloyed product which is compacted and worked to provide a product form such as wire, tape or thin strip. This product form can then be given the configuration required for use (e.g. open coil) and then oxidized to provide the superconductor.

Abstract: In a process of manufacturing diaphragms, a layer of a difficulty flowable metal powder is applied to a support, a wire net is rolled onto the powder layer and the latter is compacted at the same time and the metal powder is fired at 800.degree. to 1500.degree. C. in an oxidizing atmosphere. In order to impart to the diaphragms a constant thickness, strength and density, the metal powder is uniformly distributed and applied as regards its bulk volume to the support and the powder layer is moved under a distributing roller rotating opposite to the direction in which the powder is fed.

Abstract: Substantially dense, void-free ceramic-metal composites are prepared from components characterized by chemical incompatibility and non-wetting behavior. The composites have a final chemistry similar to the starting chemistry and microstructures characterized by ceramic grains similar in size to the starting powder and the presence of metal phase. A method for producing the composites requires forming a homogeneous mixture of ceramic-metal, heating the mixture to a temperature that approximates but is below the temperature at which the metal begins to flow and presssing the mixture at such pressure that compaction and densification of the mixture occurs and an induced temperature spike occurs that exceeds the flowing temperature of the metal such that the mixture is further compacted and densified. The temperature spike and duration thereof remains below that at which significant reaction between metal and ceramic occurs. The method requires pressure of 60-250 kpsi employed at a rate of 5-250 kpsi/second.

Abstract: High density compacts are made by providing a compactable particulate combination of Class 1 metals selected from at least one of Ag, Cu and Al, with material selected from at least one of CdO, SnO, SnO.sub.2, C, Co, Ni, Fe, Cr, Cr.sub.3 C.sub.2, Cr.sub.7 C.sub.3, W, WC, W.sub.2 C, WB, Mo, Mo.sub.2 C, MoB, Mo.sub.2 B, TiC, TiN, TiB.sub.2, Si, SiC, Si.sub.3 N.sub.4, usually by mixing powders of each, step (1); uniaxially pressing the powders to a density of from 60% to 95%, to provide a compact, step (2); hot densifying the compact at a pressure between 352 kg/cm.sup.2 (5,000 psi) and 3,172 kg/cm.sup.2 (45,000 psi) and at a temperature from 50.degree. C. to 100.degree. C. below the melting point or decomposition point of the lower melting component of the compact, to provide densification of the compact to over 97% of theoretical density; step (3); and cooling the compact, step (4).

Abstract: A process for densifying porous articles comprises providing a non-reactive container with a green article and sufficient solid encapsulating agent selected from the group consisting of tin and the tin-magnesium eutectic to encapsulate the green article when the agent is in the molten state. The temperature is raised sufficiently to melt the agent and to encapsulate the article. The encapsulated article is maintained at a pressure below the infiltrating pressure of the green article while heating the encapsulated article sufficiently to create a surface on the article that is essentially free of porosity and thereafter, the pressure on the encapsulated article is increased while maintaining the article at its sintering temperature for a sufficient time to increase the density of said article to at least about 98% of theoretical.

Abstract: A creep-resistant alloy having a tiered structural arrangement of one or several refractory metals Mo, W, Nb, Ta, V, Cr containing certain doping agents, as well as a process for producing the same. The special doping agents are compounds and/or mixed phases of such compounds selected from the group of oxides, nitrides, carbides, borides, silicates or aluminates having a melting point higher than 1500.degree. C. The size of their grains is .ltoreq.1.5 .mu.m, their proportion in the alloy is comprised between 0.005 and 10% by weight. Unlike in the known state of the art, the use of porassium as doping agent is avoided in this alloy. A good reproducible consolidation and in particular high densities during sintering can thus be obtained. Furthermore, this alloy has better ambient temperature, heat and creep resistance properties than known alloys of refractory metal with a tiered structual arrangement.

Abstract: The present invention provides a ceramic/metal composite material comprising cerium/aluminum mixed oxides as the ceramic phase and an alloy or intermetallic compond of cerium and at least one of aluminum, nickel, iron and cobalt as the metal phase. The ceramic phase may comprise interengaged crystal lattices of ceria and alumina. The material may be a self-sustaining body which may be used as substrate for a dimensionally stable anode in molten sale aluminum electrowinning cells, coated with a protective layer of cerium oxyfluoride in situ during electrolysis in molten cryolite containing cerium ions.

Abstract: An aluminum based metal matrix composite is produced from a charge containing a rapidly solidified aluminum alloy and particles of a reinforcing material present in an amount ranging from about 0.1 to 50 percent by volume of the charge. The charge is ball milled energetically to enfold metal matrix material around each of the particles while maintaining the charge in a pulverulant state. Upon completion of the ball milling step, the charge is consolidated to provide a powder compact having a formable, substantially void free mass. The compact is especially suited for use in aerospace, automotive, electronic, wear resistance critical components and the like.

Abstract: A process for preparing ceramic-metal composites without melting the metal is disclosed. A compact or green body is made from a ceramic and a metal, and the compact is sealed in a vacuum in a container such as a glass envelope. The compact is then heated to a temperature below the melting point of the metal, but high enough so that the vapor pressure of the metal is significant, and the metal redistributes through the ceramic by evaporation and condensation. The composite thereby forms a body having ceramic particles uniformly coated by the metal. Products formed by the process and fabrication of a B.sub.4 C/Cr composite are also disclosed.

Abstract: A magnesium-based composite material having improved mechanical strength, and in particular an improved modulus of elasticity, and a relatively low density. The material is provided by pressing and sintering a mixture of magnesium or magnesium-based alloy particles or a particulate combination of magnesium particles and particles of one or more additional metals, with a reinforcement additive of boron, or boron-coated B.sub.4 C, Si.sub.3 N.sub.4, SiC, Al.sub.2 O.sub.3 or MgO particles.

Abstract: The sintered sliding material according to the present invention consists of from 0.1 to 10% by of graphite, from 1 to 7% by of alumina, and balance of copper, and occasionally at least one member selected from the group consisting of from 1 to 10% of Sn and from 1 to 30% of Pb, and/or not more than 1% of P, and includes the alumina dispersed in the copper matrix. The alumina is held by the copper matrix so that the particles of alumina are not separated from the material during sliding under a boundary lubricating condition.

Abstract: Metallization compositions utilizing a (1) relatively refractory flake product, e.g. flake-shaped powder of stainless steel, in combination with (2) a conductive metal such as silver which, upon being fired to melting, is held in the flake type matrix providing an excellent and inexpensive conductor. Termination compounds will comprise ceramic-adhesive frits. Body electrodes will usually comprise organic binders readily removable by thermal oxidation.

Abstract: A process for producing a ceramic material which is electrically conductive by reacting titanium dioxide with intercalated graphite under conditions which effect the reduction of the titanium dioxide, said product comprising an electrically conductive, corrosion-resistant, substoichiometric titanium dioxide combined chemically with an intercalant or residue thereof, for example, a metal such as copper or nickel, and the use thereof in thermal, electrical and electro-chemical applications.

Abstract: Production of a sheathed wire or multiple-filament conductor composed of ceramic high-temperature superconductor by mixing Y.sub.2 O.sub.3, CuO and BaO.sub.2 or BaO.sub.2 +BaO, loading the powder mixture (3) into the interior of a metal sheath (1) lined with Ag intermediate layer (2), slowly heating to a maximum permissible reaction/sintering temperature of 950.degree. C. in a period of at least 0.1 h, holding the sintering temperature for at least 1 h, cooling down to 200.degree. C. at at most 10.degree. to 100.degree. C./h to form a conducting core (4) composed of YBa.sub.2 Cu.sub.3 O.sub.6.5-7.5. Variants having a layer composed of CuO, diffusion barrier composed of Ni, Ta, Nb, V or having Ag intermediate layer doped with AgO or BaO.sub.2. Preferably reactive sintering under a pressure of 10 to 10.000 bar as hot isostatic pressing. Variant: reactive annealing of the powder mixture under oxygen pressure of 10 to 3000 bar at 600.degree. to 950.degree. C.

Abstract: A cathode is made from tungsten powder using as an impregnant the product rmed from adding about 1 mole of a member selected from the group consisting of zirconium, zirconium dioxide, hafnium, hafnium dioxide, uranium, uranium dioxide, titanium, and titanium dioxide to about 50 to about 100 moles of a compound selected from the group consisting of Ba.sub.3 Al.sub.2 O.sub.6, Ba.sub.3 WO.sub.6, and Ba.sub.4 Al.sub.2 O.sub.7.

Abstract: An improved ceramic-metal composite comprising a mixture of a ceramic material with a ductile intermetallic alloy, preferably Ni.sub.3 Al.

Abstract: A method for producing a single crystal of chromium is disclosed, comprising sintering a chromium molding to thereby apply thereto a thermal strain and heat treating the resulting thermally strained chromium molding. The method achieves high efficiency of crystal growth and produces a single-crystal chromium molding of complicated shape.

Abstract: The invention comprises a process for co-sintering of conductors of copper or copper-based alloy and a ceramic substrate, the substrate being produced from a ceramic powder of cordierite type, having a sintering temperature of lower than 950.degree. C.The process comprises the following steps:(a) shaping of the ceramic substrate from a powder of cordierite type,(b) deposit of copper on the ceramic substrate, and(c) co-sintering heat treatment, during which the organic compounds are removed, the copper is regenerated in a reducing atmosphere, and co-sintering of the conductors and the substrate is carried out in a flow of moist inert gas with a plateau at a temperature of between 1065.degree. and 1080.degree. C.The process according to the invention makes it possible to produce substrates of cordierite with internal and external conductors.

Abstract: A method for fabricating a superconductive film composed of a RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x compound, or a (Bi.Sr.Ca.Cu.O) compound. In a first embodiment, oxides or carbonates of the component materials, namely Y.sub.2 O.sub.3, BaCO.sub.3, and CuO are mixed in atomic ratios of 1:2:3, according to the chemical formula of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x, and sintered to create a rhombic perovskite structure. The sintered mixture is powdered again, with added powdered amounts of Y.sub.2 O.sub.3 and powdered metallic Cu, and sintered. The sintered product is used as the source for an electron beam evaporator. In a second embodiment the (Bi.Sr.Ca.Cu.O) compound is formed into a sintered pellet which is composed of a mixture of one part of BiO, 3-15 parts of SrCO.sub.3, 4-30 parts of CaCO.sub.3, and 2-5 parts of CuO, in atomic ratios of Bi, Sr, Ca and Cu.

Abstract: A coated cemented carbide alloy having jointly a high toughness and high wear resistance is produced by specifying the cooling rate during sintering in efficient manner, which alloy comprises a cemented carbide substrate consisting of a hard phase of at least one member selected from the group consisting of carbides, nitrides and carbonitrides of Group IVa, Va and VIa metals of Periodic Table and a binder phase consisting of at least one member selected from the iron group metals, and a monolayer or multilayer, provided thereon, consisting of at least one member selected from the group consisting of carbides, nitrides, oxides and borides of Group IVa, Va and VIa metals of Periodic Table, solid solutions thereof and aluminum oxide, in which the hardness of the cemented carbide substrate in the range of 2 to 5 .mu.m from the interface between the coating layer and substrate is 800 to 1300 kg/mm.sup.

Abstract: A process of hot pressing of materials to form articles or compacts is characterized by the steps: (A) providing a compactable particulate mixture; (B) uniaxially pressing the particles without heating to provide article or compact (22); (C) placing at least one article or compact (22) in an open pan (31) having an insertable frame (32) with edge surfaces (34) that are not significantly pressure deformable, where the inside side surfaces of the frame are parallel to the central axis B--B of the open pan, and where each article or compact is surrounded by fine particles of a separating material; (D) evacuating air from the container and sealing the articles or compacts inside the container by means of top lid (36); (E) hot pressing the compacts at a pressure from 352.5 kg/cm.sup.2 to 3,172 kg/cm.sup.

Abstract: The present invention relates to a heat insulating separation wall between the combustion chamber of a solid fuel rocket and a chamber containing an electronic control installation of said rocket.

Abstract: Disclosed is a roller for use in a rotary compressor, which roller comprising a sintered body consisting essentially of 0.5-2.0% by weight of C, 1.0-5.0% by weight of Cu, 1.2-3.0% by weight of Mo and a balance of Fe and unavoidable impurities. In the sintered alloy, hard particles of Fe-Mo alloy are dispersed in one of pearlitic and tempering martensitic matrix, and sintered pores of the sintered body is sealed with tri-iron tetroxide. Resultant sintered body has high wear resistance and scuffing resistance capable of being used as an inverter type compressor.

Abstract: This invention concerns a heat treatment method for rare earth type permanent magnets which are primarily of the Nd-Fe-B type. With regard to these permanent magnets, which oxidize rather easily in the air, the alloy is crushed, and either compression formed in a magnetic a non-magnetic field, sintered at 900.degree. to 1,200.degree. C., and then machined into the shape desired, and then solution treated in an atmosphere of oxygen and/or nitrogen at a temperature of 900.degree. to 1,200.degree. C., and then aged at 300.degree. to 900.degree. C. in order that an oxide and/or nitride protective layer of 0.001 to 10 .mu. be formed on the surface of the permanent magnet to prevent corrosion and in order to relieve machining strain.

Abstract: A pellet for fabricating a metal matrix composite is made of a mixture of a matrix member of a metal powder and at least one reinforcement selected from whiskers, short fibers and suitable particles, the reinforcement being uniformly distributed in a matrix of the metal powder and said mixture being kept in a shape with a binder, wherein said pellet has a surface layer of dried and rigid portion of said mixture which is rigid enough to keep its shape under an external pressure applied thereto. The pellet is formed from a flat cake of the mixture separated from a slurry consisting of a solution medium and the mixture dispersed therein uniformly. Alternatively, the pellet is formed from the mixture in a dried condition with a granulation binder diluted with a solution medium.

Abstract: A surface structure of A1N substrate comprising:an A1N substrate,an intermediate layer disposed on the A1N substrate, anda metallized layer disposed on said intermediate layer, said intermediate layer comprising at least aluminum, nitrogen and oxygen. The metallized layer has a main component of one of Mo-Mn alloy, Mo and W, and has a thickness of 1-20 .mu.m,This surface structure is produced by coating a surface of A1N substrate with metallizing layer components, heat treating the resultant coated substrate at a temperature of 200.degree.-500.degree. C. under an oxidizing atmosphere, and further heating the heat treated coated substrate at a temperature of 1200.degree.-1400.degree. C. under a nonoxidizing atmosphere having a dew point of -35.degree.to 5.degree. C.

Abstract: A process for making a porous mass of a metal of iron- or titanium-group comprises steps of forming a sinter of solvent-soluble particles, pressing in a molten metal of the iron- or titanium- group into open interstices in the sinter, and eluting the particles from the composite of the sinter and metal. Solvent-soluble magnesia particles whose surface is either covered or not with a film of boric anhydride or solvent-soluble calcia particles whose surface is either covered or not with film of calcium chloride are sintered and machined into the desired shape. The formed sinter is put in a case of a heat-insulating material that can withstand the melting temperature of a metal of iron- or titanium-group. After being heated to a given temperature together with the case, the sinter is immediately put in a metal mold where a molten metal of iron- or titanium-group is pressed into open interstices in the sinter. Then, only the particles are eluted from the sinter-metal composite by a solvent.

Abstract: The invention is directed to a process for producing metal composites from low-cost earth products and articles manufactured thereby. Fly ash from burned coal and oil is bonded with low-melting metals or alloys to produce economical composite materials with modified strength, conductivity and wear resistance.

Abstract: A thin section fibrous, unrecrystallized dispersion strengthened copper body suitable for use in the manufacture of lead frames for integrated circuits and a method for making same.

Abstract: This invention relates to substantially full-dense, near-net shape bodies made of dispersion strengthened copper (D.S.C.) powder and products which are cold formed and/or machined from these bodies, such as resistance welding electrodes. The invention includes processes for manufacturing substantially full-dense, near-net shape, substantially equiaxed bodies from D.S.C. powder.

Abstract: A method of manufacturing a scandate dispenser cathode having a matrix (1) at least the top layer of which consists substantially of a mixture of tungsten with scandium oxide or with a mixed oxide comprising scandium oxide. When sintering of the matrix is carried out at a temperature between 1300.degree. and 1700.degree. C., preferably at approximately 1500.degree. C. and in a hydrogen atmosphere, cathodes are obtained having a better recovery after ion bombardment compared with cathodes sintered at 1900.degree. C. Sintering in hydrogen results in a better reproducibility.