METHOD OF PRODUCING A STORAGE CATHODE

Abstract: A method of producing a storage cathode comprising a porous, sintered body of a refractory metal, in which non-­ interlocking powder particles of a refractory metal are compacted into a body and the body is sintered, characterized in that at least a portion of the powder particles are coated, before compacting, with a thin layer of a ductile metal and that compacting is effected at a temperature lower than 600°C. As a result thereof it is possible to compact the refractory metal powder, before sintering, at lower temperatures, in a non-conditioned space and in an air atmosphere.

Description: The invention relates to a method of producing a storage cathode comprising a porous, sintered body of a refractory metal, in which non-interlocking powder particles of a refractory metal are compacted into a body and the body is sintered. Storage cathodes of this type are used in electron guns for electron tubes such as television tubes, picture pick-up tubes, travelling wave tubes, clystrons etc. Tungsten or molybdenum are usually used as the refractory metals. A method of the type defined in the opening paragraph is disclosed in the abstract in the English language of SU-654981-A from Derwent "World Patent Index". This disclosure describes a method in which tungsten powder, consisting of non-interlocking, in this disclosure substantially spherical, particles is compressed in a hydrogen atmosphere at a pressure of 0.1 to 1.0 GPa, at a temperature from 1100-1400°C for 5 to 30 minutes. Thereafter, the compacted tungsten body is sintered in a hydrogen atmosphere at a temperature of 2000°C for 10 minutes, whereafter the tungsten body is impregnated. The method disclosed in the abstract in the English language of SU-654,981 has the drawback that the tungsten powder is to be compacted at elevated temperature and in a hydrogen atmosphere. This requires the use of a high-pressure press in a conditioned space which is at a high temperature and is suitable for this condition. Compacting is effected in a hydrogen atmosphere at elevated temperature; many metals are attacked by hydrogen at such high temperatures, this process is denoted as "hydrogen embrittlement". The high-pressure press appropriate for this process is to be made of a metal which is immune to hydrogen embrittlement. This process is not so suitable for mass production as the energy required for producing a cathode is great and the process takes much time. It is therefore an object of the invention to provide a method by means of which it is possible to compact non-interlocking powder particles into a body prior to sintering said body, at lower temperatures, in a non-conditioned space and in an air atmosphere. According to the invention, this object is accomplished by a method which is characterized in that at least a portion of the powder particles are coated, before compacting, with a thin layer of a ductile metal and that compacting is effected at a temperature lower than 600°C, the ductile metal being a member of the group consisting of aluminium, copper, silver and alloys of these metals. An important feature of the invention is the fact that the powder particles are compacted at lower temperatures, so that compacting need not to take place in a hydrogen atmosphere, This simplifies both the method and the high-pressure press accommodation. In the prior art, compacting is effected in a hydrogen atmosphere to prevent the powder particles from being attacked by oxygen. Compacting in accordance with the invention is done at a temperature at which no attack of the powder particles occurs. Generally, the required compacting pressure is lower according as the temperature is higher, so that less heavy pressures are required. Disadvantages of higher temperatures reside in the fact that heating of the press and of the powder particles requires energy and time. A preferred embodiment is characterized in that compacting is effected at a temperature which is at least substantially equal to ambient temperature. The temperature of the high-pressure press then need not to be increased and controlled relative to the ambient temperature, which is a simplification of the method. Since compacting is effected at ambient temperature, the body is immediately ready for treatment in a sintering furnace, and the press is immediately available for a new body to be compressed. No heating of the powder particles to high temperatures is necessary. A powder partly consisting of powder particles coated with a thin layer of a ductile metal and partly of powder particles not coated with such a layer is suitable for the method of the invention. The required coherence of the compacted powder determines the minimum of that part of the powder that is to be provided with a thin layer of a ductile metal. The coherence of the compressed body may leave much to be desired for when the distribution of the coated particles over the powder is not uniform. Any problems caused thereby can be reduced by coating at least substantially all the particles with a layer of a ductile metal. The powder particles may be of different shapes, for example granular or spherical. It was found that uncoated spherical powder particles were particularly difficult to compact into a coherent body. The method according to the invention is therefore of particular advantage for spherical particles. Of the refractory metal powders particles tungsten powder particles are particularly difficult to compact into a coherent body and the method according to the invention is of particular advantage for tungsten particles. A further embodiment of the method according to the invention is characterized, in that the powder particles are provided with a thin layer of a ductile metal which predominantly contains aluminium. Aluminium is a cheap and relatively inert metal which has a high vapour pressure at a temperature which is relatively low for metals, so that the metal, as was found during experiments, completely disappears from the body during the sintering process, leaving no contaminations behind in the body. Contaminants in the body may negatively influence the emission properties of the storage cathode. It is therefore advantageous if after sintering the ductile metal has completely disappeared from the storage cathode. A still further embodiment is characterized, in that the average thickness of the ductile layer is less than 0.1 µm and less than 1/10th part of the radius of the powder particles and greater than 0.005 µm. Too small a thickness of the thin metal layer negatively affects the compacting properties of the powder, when the thickness exceeds 1/10 part of the radius of the particles of the tungsten powder or when the thickness exceeds 0.1 µm, the sinter properties of the compacted powder may become poorer as then the distance between the tungsten particles is comparatively great. A still further embodiment is characterized in that the average thickness of the thin layer of ductile material is at least substantially located in the range from 0.01 to 0.03 µm. The compacting and sinter properties of the tungsten powder are, as was found from experiments, at least substantially optimal for these thicknesses. It is remarked here that methods of producing a storage cathode are known in which powder particles of a refractory metal are compacted, the powder particles being very irregularly shaped and interlocking. It is possible to compact such powders, due to their interlocking nature, at lower temperatures. During sintering, however, irregularities in the porosity of the sintered body, e.g. closed pores and fully dense-sintered portions, occur, which irregularities result in a loss in intensity and in uniformity of the emission. The present invention concerns methods using non-interlocking particles, wherein such irregularities occur much less or not at all. It is further remarked that from US-A-3.577.277 a method is known for forming a rigid non-ductile tungsten mass which comprises coating tungsten particles with a minor amount of metallic rhenium, whereafter compacting, sintering, shaping and resintering of the mass is performed. Some embodiments of the invention will now be described by way of example with reference to the accompanying drawing. Therein: Figure 1 is a schematical, partly cross-sectional view of a storage cathode produced by means of the method of the invention; Figures 2 and 3 show schematically and in cross-section a vapour deposition arrangement to illustrate the method of the invention; Figure 4 shows an at least substantially spherical particle of tungsten powder provided with an aluminium layer, in cross-section; Figure 5 shows a two-dimensional stack of substantially spherical particles of tungsten powder, coated with a layer of aluminium; Figure 6 shows a two-dimensional stack of two types of substantially spherical particles; Figures 7 and 8 are cross-sectional views of a detail of Figure 5; Figures 9a and 9b are cross-sectional views of a press for compacting a tungsten body. In Figure 1 a schematic, partly cross-sectional view of a storage cathode produced by means of the method of the invention is shown. The cathode shaft 1 which is blackened at its interior side surrounds the heater. The heater 3 consists of a metal core 4 which is provided with a coat 5 which is black at least at its surface. The end face 6 of the cathode shaft is provided with a holder 7. The holder 7 envelops the impregnated tungsten body 8. In Figure 2 a schematical cross-sectional view is shown of a vacuum deposition arrangement to illustrate the method of the invention. In a vacuum space 9 a holder 10 for the tungsten powder 11 is present. The holder 10 is regularly kept in motion so that the powder is regularly shaken. This motion can inter alia be effected by vibration. This promotes a uniform distribution of the vapour-deposited aluminium over the tungsten powder. An aluminium sample 12 is heated in a tungsten coil 13 by resistance heating to a high temperature so that aluminium atoms evaporate from the surface 14 of the aluminium sample 12. These atoms which in the Figure are represented by dots 15, are inter alia deposited on the tungsten powder 11, thus coating the at least substantially spherical particles with a layer of aluminium. The quantity of aluminium deposited can be checked by means of surface thickness gauge 16 during or after the vacuum deposition process. The pumps required for providing a vacuum, and also electric supply wires and any further components arranged in the vacuum space which are customary for such known vacuum deposition arrangements are not shown in this Figure. Figure 3 is a cross-sectional view of a vapour deposition arrangement. The tungsten powder 11 is here contained in a rotating tread mill 17, which is provided with fins 18. The tungsten powder is kept in constant motion so as to obtain as uniform a distribution of the aluminium over the powder and over the surface of the particles as possible. The fins 18 can be of such a large size that the particles make a free fall. The manner of vacuum coating aluminium shown here must not be considered to be the only possible manner. Many other methods are known, such as inter alia different configurations for resistance heating of the sample and methods in which heating of the sample is effected by means of a high-frequency field, by means of a concentrated electron beam or by means of a concentrated ion beam, and also methods in which atoms or sub-microscopic particles are removed from the sample by means of a concentrated electron beam or a concentrated ion beam. These last-mentioned methods are alternatively denoted by the term sputtering. All these methods and similar ones are here deemed to be suitable for depositing a ductile metal layer in accordance with the invention. Further suitable methods are considered methods which utilize chemical vapour deposition and methods in which the metal is deposited on the tungsten particles from a solution of the metal, thus forming a metal layer on the tungsten particles and methods in which the tungsten particles are provided by one of the above methods or a combination thereof with a layer of a metal compound or a metal alloy, the metal compound or metal alloy simultaneously or subsequently being converted into a layer of ductile metal. Figure 4 shows a substantially spherical particle 19 of the tungsten powder coated with an aluminium layer 20. In this Figure the thickness of the aluminium layer is shown, for the sake of clarity, greatly increased relative to the other dimensions. In this example the diameter d of the at least substantially spherical particle is 10 µm, the average thickness of the aluminium layer is 0.02 µm. Generally, diameters in the range from 0.1 to 30 µm are suitable. A spread in the powder particle diameters is possible. In this Figure, the thickness of the coating of aluminium is shown as being of a constant value over the surface of the substantially spherical particles. This is not a constraint. Non-uniformities in the thickness of the aluminium layer may occur. Figure 5 shows a two-dimensional stack of at least substantially spherical particles 19 of tungsten powder coated with an aluminium layer 20 in a cross-sectional view. In this Figure the thickness of the aluminium layer is greatly exaggerated for the sake of clarity. In a compacted tungsten powder the at least substantially spherical particles are compressed in a three-dimensional stack. In a compacted tungsten powder variations in the cross-section of the particles may occur. Figure 6 shows a two-dimensional stack of two types of substantially spherical particles 19 and 21. Compared with Figure 5 it is obvious that the interstices between the particles are reduced, the number of points of contact between the particles, the distribution of the particles over the area and the surface of the particles are increased. This Figure illustrates that a person skilled in the art can influence the properties of the storage cathode by the use of two (or more) types of tungsten powder, i.e. particles of different average diameters. Figure 7 shows a detail of Figure 5, the point of contact of two substantially spherical particles 19 of the tungsten powder 11, coated with an aluminium layer 20 before compression, in a cross-sectional view. The aluminium layers 20 abut. Figure 8 shows the detail illustrated in Figure 7 of Figure 5, after compacting. During compacting a cold compression connection 21 is produced between particles 19. Figures 9a and 9b schematically and in a cross-sectional view show by way of illustration of the method of the invention a press pressing the aluminium-coated tungsten powder before and during pressing. In the press 22, which is comprised of holder 23 and cylinders 24 and 25, tungsten powder 11 is compacted into tungsten body 26 by exerting a force F on cylinder 25. In practice forces of 0.1 to 1.0 GPa appeared to yield satisfactory results. The force applied must be sufficient to produce cold compression connections between the particles. After compacting the tungsten powder is sintered in a manner known per se in a hydrogen atmosphere for, for example, 2 hours at a temperature of 1800°C. Hereafter the tungsten body is impregnated in known manner, for example with Ba-Ca-Al compounds.

Claims: A method of producing a storage cathode comprising a porous, sintered body of a refractory metal, in which non-interlocking powder particles of a refractory metal are compacted into a body and the body is sintered, characterized in that at least a portion of the powder particles is coated, before compacting, with a thin layer of a ductile metal and that compacting is effected at a temperature lower than 600°C, the ductile metal being a member of the group consisting of aluminium, copper, silver and alloys of these metals. A method as claimed in Claim 1, characterized in that compacting is effected at a temperature which is substantially equal to ambient temperature. A method as claimed in Claim 1 or 2, characterized in that substantially all the powder particles are provided with a layer of a ductile metal. A method as claimed in any one of the preceding Claims, characterized in that the powder particles are substantially of a spherical shape. A method as claimed in any one of the preceding Claims, characterized in that the refractory metal is tungsten. A method as claimed in any one of the preceding Claims, characterized in that the powder particles are coated with a layer of a ductile metal which predominantly contains aluminium. A method as claimed in any one of the preceding Claims, characterized in that the average thickness of the layer is less than 0.1 µm and less than 1/10th part of the radius of the powder particles and greater than 0.005 µm. A method as claimed in Claim 7, characterized in that the average thickness of the layer of ductile metal is located at least substantially in the range from 0.01 to 0.03 µm. A method as claimed in any one of the preceding Claims, characterized in that the powder particle size is between 0.1 µm and 30 µm. A method as claimed in any one of the preceding Claims, characterized in that two or more types of powder particles are used.

Inventors: WIJNEN, JAN FRANCIS CORNELIS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J1
IPC codes: H01J9