Method of manufacturing a dispenser cathode

Abstract: A method of producing a dispenser cathode 9 provided with a top layer 10, in which tungsten powder is pressed in a pressing mould 1 to form the cathode. The method comprises a plurality of steps; in a first step of the method the powder is pressed to a moulding at a first pressing pressure which is sufficient to preserve the shape of the moulding when the first pressure is let off but is not sufficient to break the powder grains to a significant extent. This moulding is thereafter provided with a top layer 10 of a second powder, whereafter the overall assembly is compressed in a second pressing operation at a second, higher pressing pressure F₂, by which pressure the powder grains are broken to a significant extent.

Description: The invention relates to a method of manufacturing a dispenser cathode comprising a porous cathode body which predominantly consists of a refractory or refractory alloy and is provided with a top layer which differs from the rest of the cathode body, wherein a powder predominantly comprising a refractory metal or refractory alloy is pressed to form a body. Dispenser cathodes of this type are used in electron guns for television picture tubes, picture pick-up tubes, travelling-wave tubes, clystrons, transmitter tubes and such like. A method of the type described in the opening paragraph is disclosed in the United States Patent 4,625,142, in which a method is described wherein the body is pressed from a tungsten powder, on which, prior to pressing, a 0.2 mm thick layer of a mixture of 95% by weight of tungsten powder and 5% by weight of scandium oxide (Sc₂O₃) is provided. After compression and sintering the cathode body consists of an approximately 0.1 mm thick scandium-oxide-containing, porous tungsten layer having a density of approximately 83% on a 0.7 mm thick porous tungsten layer having a density of approximately 75%. Another method of the type described in the opening paragraph is also described in UK patent application GB-A-2 060 246. Dispenser cathodes comprise a stock of emitter material which has for its object to provide an adequately low exit potential for electrons at emitting surface. Dispenser cathodes of the type described in the opening paragraph comprise a porous impregnated body, having a top layer which differs from the rest of the body. It is desirable for the top layer to have properties which are advantageous for the emission of electrons, whilst the rest of the body has properties which are advantageous for the storage of emitter material. To that end, in said dispenser cathodes, the top layer is formed by a layer which has a composition and/or porosity differing from the rest of the body. The porosity of the top layer and of the rest of the body determine the total maximum quantity of emitter material to be stored in the body, the active surface area and the diffusion rate of active elements from the cathode body to the emissive surface. A low porosity of the top layer combined with a high porosity of the rest of the body combines a relatively slow diffusion rate of active elements to the emissive surface with a relatively large storage capacity, which has a favourable effect on the operating life of the cathode. It is alternatively possible to provide a top layer with emission-stimulating material (e.g. Sc₂O₃) as in the above-mentioned Patent. Compared with cathodes of a homogeneous composition, that is to say cathodes which are provided with emission-stimulating material throughout the entire cathode body, this has the advantage that a greater quantity of emitter material can be stored. It is a disadvantage of the prior art method that it is of poor utilization for the manufacture of cathodes whose emitting surface is not flat, but curved. On compressing powder provided with a top layer of a different composition in a press the die of which has a curved surface, it was found experimentally that, after pressing, the top layer was not uniformly distributed over the surface, but was for the major part or partly shifted to the side edges of the mould. This results in a non-uniform by distributed emission of electrons, immediately or after some time in the future. It is an object of the invention to provide a method by means of which it is possible to provide in a simple way cathode, particularly a cathode with a curved emitting surface, with a uniformly distributed top layer. To that end, a method according to the invention as claimed in claim 1 is characterized in that the cathode has a curved emitting surface and in that a first powder is compressed in a first pressing operation at a first pressure to form a coherent moulding, the first pressure being insufficient to break powder grains to a significant extent, the moulding thereafter is coated with a top layer of a second powder, whereafter the whole assembly is compressed at a second, higher pressure, whereby the powder grains are broken to a significant extent. After the first pressing operation, the surface of the moulding comprises coarse powder grains, which enables an appropriate adhesion of the top layer to this surface, so that during the second pressing operation the top layer is not sheared-off and a top layer is produced which is uniformly distributed over the surface of the moulding. A practical embodiment of the method in accordance with the invention, is characterized in that the first pressure is located at least substantially in the range from 1*10⁷Pa to 8*10⁷Pa. This corresponds to pressures between 100 bar and 800 bar. Experiments have shown that a first pressure within these values is satisfactory. Too high a pressure results in the powder grains breaking which has a negative effect on the adhesion of the top layer to the moulding. Too low a pressure results in the cohesion of the moulding to be imperfect after the first pressing operation. Both circumstances may result in shearing off of the top layer. A further embodiment of the method according to the invention, is characterized in that the first powder is shaken before and/or during the first pressing operation. Shaking improves the homogeneity of the first powder and the space between the upper and lower dye of the press mould is appropriately filled. This prevents the occurrence of holes and closed pores in the pressed dispenser cathode, which improves the cohesion of the moulding after the first pressing operation. An improved cohesion reduces the risk of rejects. In addition, the uniform distribution of the top layer is improved, this top layer can be pushed away if the subjacent body does not possess adequate cohesion. It is important that no excessive inhomogeneities occur at the moulding surface. After sintering this also has the advantage that the relative spread in properties of the dispenser cathodes is reduced. A further embodiment is characterized, in that the first powder has an average powder grain size which exceeds the average powder grain size of the second powder. This has an advantageous influence on the adhesion of the top layer to moulding and on the uniform distribution of the top layer. A still further embodiment of the method according to the invention, is characterized in that the average grain size of the first powder is in the range from 20 to 150µm. Experiments have proved that this grain size ensures a sufficient cohesion of the dispenser cathode and of the adhesion of the top layer. An embodiment of the method according to the invention is characterized in that the second powder has an average grain size in the range from 1 to 20µm. Experiments have proved that this grain size provides an appropriate adhesion of the top layer. the method is particularly suitable for manufacturing cathodes having an emissive surface with a characteristic size greater than 1 cm. The above-described disadvantage of the known method is more specifically of importance for cathodes having an emissive surface with a characteristic size exceeding 1 cm. A characteristic size exceeding 1 cm must here be understood to mean, for example, that the diameter of the emissive surface exceeds 1 cm, for a rotational-symmetrical surface, or that a diagonal exceeds 1 cm, for a polygonal surface. Such cathodes are used in particular in travelling-wave tubes, clystrons and transmitter tubes. Some embodiments of the invention will now be described in greater detail by way of example with reference to the accompanying drawing. Therein: Fig. 1 is a schematical cross-sectional view of a press suitable for the method according to the invention; Fig. 2 illustrates an embodiment of the method; Fig. 3 shows a cathode manufactured according to the method of the invention in a cross-sectional view; Fig. 4 shows further embodiments of cathodes manufactured according to the method of the invention; Fig. 5 is a schematical cross-sectional view of an electron gun comprising a cathode manufactured according to the method of the invention, suitable for a clystron. The Figures are schematical and not to scale, corresponding components in the several embodiments usually having been given the same reference numerals. Fig. 1 shows a press suitable for the method. This press 1 is comprised of a holder 2, which includes dies 3 and 4 having curved surfaces 5 and 6. The dies 3 and 4 are freely movable in press 1. Press 1 is supported by supporting member 7. A cathode body 8 is pressed between the dies 3 and 4. Fig. 2 illustrates an embodiment of the invention. In a first step shown in fig. 2a, press 1 is partially filled with tungsten powder 9. In this embodiment the tungsten powder has an average powder grain size of 100µm. This powder is shaken a few times. This causes the powder to be distributed somewhat over a cavity 10 in holder 2. Die 3 is inserted in holder 2. Thereafter the press mould 1 is shaken, which also includes that the press mould 1 is turned upside down a few times. This promotes the homogeneity of the powder and consequently the subsequent homogeneity and cohesion of the pressed moulding, in that the space between the two dies is completely filled. If necessary, this shaking procedure is repeated. Die 3 can then be moved further into holder 2, when the powder 9 has been further compacted. This can be continued until the powder 9 does not become more compact any further. Thereafter, as is shown in Fig. 2b, the tungsten powder 9 is compressed in a first pressing operation by exercising a force F₁ on die 3. In this embodiment, the die 3 is substantially rotational-symmetrical and has a diameter of 22 mm. In this embodiment the force F₁ amounts to 1.5*10⁴N. The pressure exercised during this first pressing operation is sufficient to compress the powder 9 to form a coherent moulding 10, but not high enough to break the powder grains to a significant extent. After the pressing pressure has been let-off, moulding 10 is provided, as is shown in Fig. 2c, with a top layer 11, in this example consisting of a powder having an average powder grain size of 6µm and consisting of 95% by weight of tungsten at 5% by weight of Sc₂O₃. The top layer is applied, for example, using a brush or by showering. Further emission-stimulating materials which can be added to the tungsten powder are, for example, scandium hydride or other scandium compounds or other metals such as osmium, iridium, ruthenium, or rhenium or compounds of these metals. In this embodiment the top layer has a thickness of 100µm As is shown in Fig. 2d, the overall assembly is now compressed to form the body 12 by exercising a force F₂ on die 3. This top layer 11 is uniformly distributed over the moulding 10. In this embodiment F₂ amount to 2.5*10⁵N. The pressure exercised during this second pressing operation is sufficiently high to cause the powder grains to fracture to a significant extent. In this embodiment it is found that after this second pressing operation the average particle size is 2 to 3µm. Fig.3 shows a cathode manufactured according to the method of the invention. This cathode 13, which has a diameter D of 22 mm is provided with top layer 14 on a curved surface 15. Cathodes of this size are used inter alia in travelling-wave tubes, gyratrons, clystrons and transmitter tubes. The method in accordance with the invention renders it possible to provide apparatuses of this type with cathodes having a top layer. This enables a significant improvement of the properties of cathodes suitable for these apparatuses. After the second pressing operation the body is sintered in known manner, for example for two hours at a temperature of 1800°C in a hydrogen atmosphere. Thereafter the body is impregnated in known manner, for example with Ba-Ca-Al compounds. The embodiments of the method of the invention shown here must not be considered as limitative for the method. The method is not limited to the manufacture of a cathode as shown in Fig. 3. Figs. 4a and 4b show some further examples of cathodes manufactured in accordance with the method of the invention. Fig. 4a shows a cathode 16 having a top layer 17 on a ribbled surface 18. Fig. 4b shows a cathode 19 provided with a top layer 20 on a sinusoidal surface 21. It is alternatively possible to impregnate the body, after sintering, with, for example, copper so that it is possible to further work the body, for example on a lathe or by spark erosion. A cathode 22 constituted by the cathode 13 of Fig. 3, is shown in Fig. 4c. This cathode is provided with a cavity 23 in which, for example, a heating element can be positioned. It is not necessary for the cathode to be rotational-symmetrical; square, rectangular or polygonal cathodes can also be manufactured according to the method of the invention. Nor is it necessary for the emissive surface of the cathode to be concave; cathodes having convex emissive surfaces can likewise be manufactured according to the method of the invention. Emitting surfaces such as shown in figures 4b and 4c are, within the framework of the invention, considered to be curved emitting surfaces. Fig. 5 is a schematical, cross-sectional view of an electron gun comprising a cathode manufactured according to the invention and suitable for a clystron. Here the electron gun 24 includes a cathode 25, manufactured according to the invention, provided with a top layer 26. A heating element 28 is applied in cavity 27. This heating element is secured in the cavity 27 by means of electrically insulating material 29. In addition, electron gun 24 includes an anode 30 which has a plurality of apertures 31, and acceleration electrode 32. It is further known from the prior art that the electron gun may be provided with still further acceleration and/or focussing electrodes. Pulsed potential differences between the cathode 25 and the anode 30 and between the anode 30 and focussing electrode 31 generate electron beams 33. By applying an emission-stimulating top layer 26 the electron emission of the surface of cathode 25 is improved. This renders it possible to increase the maximum current or to reduce the temperature of the cathode. This generally has an advantageous effect on the operating life of the cathode. The top layer example shown here must not be considered to be limitative. Other top layers may, for example, only differ from the rest of the body as regards the average particle size. It will be obvious that for a person skilled in the art many variations are possible within the scope of the claims.

Claims: A method of manufacturing a dispenser cathode comprising a porous cathode body which predominantly consists of a refractory metal or refractory alloy and is provided with a top layer which differs from the rest of the cathode body, wherein a powder predominantly comprising a refractory metal or refractory alloy is pressed to form a body, characterized in that the cathode has a curved emitting surface and in that a first powder is compressed in a first pressing operation at a first pressure to form a coherent moulding, the first pressure being insufficient to break powder grains to a significant extent, the moulding thereafter is coated with a top layer of a second powder, whereafter the whole assembly is compressed at a second, higher pressure, whereby the powder grains are broken to a significant extent. A method as claimed in Claim 1, characterized in that the first pressure is located at least substantially in the range from 1*10⁷Pa to 8*10⁷Pa. A method as claimed in Claim 1 or 2, characterized in that the first powder is shaken before and/or during the first pressing operation. A method as claimed in Claim 1, 2 or 3, characterized in that the first powder has an average powder grain size which exceeds the average powder grain size of the second powder. A method as claimed in any one of the preceding Claims, characterized in that the average grain size of the first powder is located at least substantially in the range from 20 to 150 µm. A method as claimed in any one of the preceding Claims, characterized in that the second powder has an average grain size which is located at least substantially in the range from 1 to 20 µm. A method as claimed in any one of the preceding Claims, characterized in that the cathode has a characteristic size exceeding 1 cm.

Inventors: STOFFELS JACOBUS, VAN ESDONK JOHANNES, STOFFELS, JACOBUS, VAN ESDONK, JOHANNES
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J9
IPC codes: H01J9