Scandate cathode

Abstract: For maintaining a monolayer of scandium which is necessary for a satisfactory emission on the surface of a scandate cathode, at least the top layer of the cathode is provided with scandium coated with a scandium oxide film. Even after repeated ion bombardment the emission is found to recover up to approximately 90% of the initial value at a current density of ca. 100 A/cm².

Description: The invention relates to a scandate cathode having a cathode body which comprises a matrix of at least a high-melting point metal and/or alloy with a barium compound at least in the matrix in contact with the matrix material, which compound can supply barium on the emissive surface by a chemical reaction with the matrix material. The invention also relates to methods of manufacturing such a cathode and to an electron beam tube provided with such a cathode. Cathodes of the type mentioned in the opening paragraph are described in the article "Properties and manufacture of top layer scandate cathodes", Applied Surface Science 26 (1986), pp. 173-195, J. Hasker, J. van Esdonk and J.E. Crombeen. In the cathodes described in this article scandium oxide (Sc₂O₃) grains of several microns or tungsten (W) grains which are partially coated with either scandium (Sc) or scandium hydride (ScH₂) are present at least in the top layer of the cathode body. The cathode body is manufactured by means of pressing and sintering whereafter the pores are impregnated with barium-calcium-aluminate. In order to maintain the electron emission the barium-calcium-aluminate supplies barium on the emissive surface by a chemical reaction with the tungsten of the matrix during operation of the cathode. To be able to realise a very high cathode load after mounting in, for example a cathode ray tube and activation of the cathode, it is important that a scandium-containing layer having a thickness of approximately one monolayer has formed on the cathode surface during impregnation by means of a reaction with the impregnating agent. As has been proved in experiments described in the above-mentioned article, the scandium-containing layer may be completely or partly removed by an ion bombardment which may occur in practice, for example during the manufacture of television tubes, which leads to detrimental consequences for the electron emission. Since Sc₂O₃ is not very mobile (oxidation occurs during impregnation in the cathodes manufactured with W which is partly coated with Sc or ScH₂) the said scandium-containing layer cannot be fully regenerated by reactivation of the cathode. The described experiments have also proved that a regeneration which is sufficient for a complete recovery of the emission is not achieved. As compared with an impregnated tungsten cathode coated or not coated with, for example osmium, this may be considered as a drawback. One of the objects of the invention is to provide scandate cathodes which are considerably improved in comparison with the above-mentioned drawback. The invention is based on the recognition that this can be achieved by using diffusion of scandium through scandium oxide. To this end a scandate cathode according to the invention is characterized in that in at least the top layer of the matrix of the cathode body scandium which is coated with a scandium oxide film is present When raising the temperature in vacuo, a monolayer oxide film comprising scandium is deposited on the surface of the top layer because scandium is diffused to the exterior from the said grains through the scandium oxide. The scandate cathode may be of the impregnated type in which the barium compound is introduced into the cathode body by means of impregnation, but alternatively the cathode may be a pressed scandate cathode or an L cathode. A method of manufacturing an impregnated cathode according to the invention is characterized in that a matrix is pressed from scandium powder and a powder of the high-melting point metal (for example, tungsten), whereafter the scandium powder is partly oxidized and the assembly is subsequently sintered and impregnated. The scandium may be obtained by dehydration of scandium hydride. In another method according to the invention before sintering and impregnation, a matrix is pressed from the high-melting point metal, and from scandium coated with a scandium oxide film. The latter is obtained by partial oxidation beforehand of scandium and/or scandium hydride. The increase in weight due to oxidation of the scandium(hydride) is preferably at least 5% and at most 30%. In the case of a smaller increase the oxide film is too thin or incomplete, whereas the oxide film will be too thick for the diffusion process or too much scandium is lost in the case of a larger increase in weight. Similar restrictions apply to the oxidation of the scandium after pressing. In the case of previous oxidation the pressure should not be too high (for example < 1000 N /mm²) so as to prevent the oxide film from breaking, which results in a loss of the above-described effect. In the case of sintering at high temperatures scandium is lost by evaporation. To avoid this as much as possible, the sintering operation is preferably performed in hydrogen (approximately 1 atmosphere) at temperatures up to approximately 1500° C. To limit the effect of unfavourable reactions between impregnating agent and scandium to a maximum possible extent (for example, formation of too much scandium oxide so that the scandium supply after ion bombardment is detrimentally influenced), the impregnation temperature is chosen to be as low as possible. At a lower temperature the quantity of impregnating agent which is taken up decreases with increasing quantities of scandium or scandium hydride in so-called mixed matrix cathodes in which the scandium coated with scandium oxide is present throughout the matrix. The quantity of scandium or scandium hydride is therefore preferably limited to at most 2.5% by weight in the mixture to be pressed. Another method is characterized in that the cathode is obtained by mixing, pressing and subsequent sintering of powders of a high-melting point metal and/or alloy and scandium, scandium hydride, or scandium coated with a scandium oxide film, together with the powder of a barium compound which can supply barium on the emissive surface by a chemical reaction with the high-melting point metal and/or alloy during operation of the cathode. In this method the sintering temperature is the highest temperature ever acquired by the cathode body. This temperature may be substantially lower than the impregnation temperature which is conventionally used in the method described hereinbefore. The invention will now be described in greater detail with reference to the accompanying drawings in which Fig. 1 shows diagrammatically a cathode according to the invention, Fig. 2, 3 and 4 show the results of measurements on several cathodes. Fig. 1 is a longitudinal section of a scandate cathode according to the invention. The cathode body 11 with an emissive surface 21 and a diameter of, for example 1.8 mm is obtained by pressing a matrix from W powder and a powder of scandium hydride (approximately 0.7% by weight) or scandium, heating for a number of hours in wet argon at approximately 800° C so as to provide the scandium with an oxide film and sintering at 1500° C in, for example a hydrogen atmosphere. The thickness of the matrix is then approximately 0.5 mm. The cathode body which is subsequently impregnated and which may or may not have an envelope 31 is welded onto the cathode shaft 41. A helical cathode filament 51 which may comprise a metal helically wound core 61 with an aluminium oxide insulation layer 71 is present in the shaft 41. The emission of such a cathode, after mounting and activation, is measured in a diode arrangement, at a pulse load and at a cathode temperature of 950° C (brightness temperature). Fig. 2 shows the results of such emission measurements. Curve 1 shows the results measured on a cathode according to the invention for a cathode-anode gap of 0.25 mm. Curve 2 shows the results of emission measurements after the cathode has been subsequently exposed to an argon ion bombardment and reactivation, as described in the article referred to in the opening paragraph. Fig. 3 shows similar measurements on a cathode in which the above-mentioned oxidation step was omitted, while Fig. 4 shows such measurements for a cathode as described in the article referred to in the opening paragraph, in both cases at a cathode-anode gap of 0.3 mm. It appears from the Figures that there is a clear improvement in a cathode according to the invention. Curve 2 in Fig. 2 deviates from curve 1 as from approximately 40 A /cm², while this deviation already begins at approximately 7.5 A /cm² without the oxidation step (see curves 1' and 2' in Fig. 3). The deterioration of a cathode according to the invention is also much less at higher emission values (deviation -8% at 100 A /cm², Fig. 2) than in a cathode (Fig. 3) in which the oxidation step was not used (deviation already approximately -30% at 80 A /cm²). Moreover, the recovery is better than in a cathode with a top layer (Fig. 4) as described in the article referred to in the opening paragraph (deviation -15% at 80 A /cm², curve 2'' already deviates from curve 1'' as from 8.5 A /cm²). As stated in the opening paragraph, the oxidation step may also precede the pressing operation. The pressure used is a critical parameter, which is illustrated in Table I in which the emission recovery after ion bombardment is shown for 2 types of cathodes, as well as the result of Auger measurements as described in the article previously referred to. The cathode body associated with column A was obtained by pressing and subsequent sintering of a mixture of tungsten powder with 0.7% by weight of powder of scandium oxide surrounded by an oxide film (obtained by oxidizing heating of ScH₂ in wet argon). Pressing took place at a pressure of 1840 N /mm² on the surface 21, and sintering took place in a hydrogen atmosphere at 1500° C. The cathode body associated with column B was manufactured in the same manner but at a pressure of 920 N /mm² on the surface 21. Table I shows the variation of the emission after repeated ion bombardment (30 minutes) and reactivation (120 minutes at 950° C, 60 minutes at 1050° C, 1 night at 1050° C). The measurements took place at a cathode temperature of 950° C, at 1000 V and a cathode-anode gap of 0.25 mm. The initial emission (100% level) was 90 A /cm² (A) and 96 A /cm² (B), respectively. Table I shows that the cathode in case A has a poor recovery because a too large pressure is used so that the oxide films are broken and the above-described mechanism (supply by means of diffusion) is no longer active. Table II shows similar measurements on a cathode which is manufactured in the way as described with reference to Fig. 1 in which the ion bombardment and recovery was performed the first time with a shorter recovery period (2 hours at T = 1050° C instead of one night). The initial emission was 105 A /cm² in this case. It appears from the Auger measurements that the emission recovers after the first ion bombardment by supply of scandium up to 90% of the initial emission. In contrast to known scandate cathodes the emission also recovers after repeated ion bombardment up to approximately 90% of the initial value. Emission Auger measurement pp h (SC)/pp h (W) After activation 100% (105 A /cm²) 5.2 30 min. ion bombardment 0.2 120 min. at T=950°C 75% 1.1 60 min. at T=1050 o C 86% 120 min. at T=1050°C 90% 1.4 30 min. ion bombardment 0.2 120 min. at T=950°C 67% 0.6 60 min. at T=1050°C 77% 1 night at T=1050°C 90% 1.4 30 min. ion bombardment 120 min. at T=950°C 67% 0.6 60 min. at T=1050°C 75% 0.7 1 night at T=1050°C 89% 1.0 In another cathode according to the invention the cathode body 11 with a diameter of 1.8 mm and a thickness of approximately 0.5 mm is obtained by pressing a mixture of tungsten powder, approximately 1% by weight of scandium powder and 7% by weight of barium-calcium-aluminate powder (4BaO-1CaO-1Al₂O₃) and subsequently sintering at 1500° C in hydrogen atmosphere. The cathode body which may or may not have the molybdenum envelope 31 is welded on the cathode shaft 41. The shaft 41 accommodates a helical filament 51 which may consist of a metal helically wound core 61 with an aluminium oxide insulation layer 71. At a cathode temperature of 950° C the measured emission after activation was approximately 100 A /cm² again. An advantage of this cathode is its simple method of manufacturing: impregnation and cleaning is not necessary. Auger measurements have shown that the formation of the scandium grains with an oxide film takes place during sintering via the aluminate. The grains may also be present in the starting material, while scandium hydride may also be chosen as a starting material. The invention is of course not limited to the embodiments shown, but those skilled in the art can conceive several variations within the scope of the invention, notably in the process parameters. The emissive material may be present in a storage chamber under the actual matrix (L cathode), while moreover the design may have many variations. The cathodes according to the invention may be used in electron tubes for television applications and electron microscopy, but also in, for example magnetrons, transmitter tubes etc.

Claims: A scandate cathode having a cathode body (11) which comprises a matrix of at least a high-melting point metal and/or alloy with a barium compound at least in the matrix in contact with the matrix material which compound can supply barium on the emissive surface (21) by a chemical reaction with the matrix material, characterized in that in at least the top layer of the matrix of the cathode body scandium which is coated with a scandium oxide film is present. A scandate cathode as claimed in claim 1, characterized in that the cathode is of the impregnated type the barium compound introduced into the cathode body by means of impregnation. A method of manufacturing a cathode as claimed in Claim 2, characterized in that the matrix is pressed from scandium powder and a powder of the high-melting point metal, whereafter the scandium powder is partly oxidized and the assembly is subsequently sintered and impregnated. A method of manufacturing a cathode as claimed in Claim 2, characterized in that the matrix is pressed from a powder of a high-melting point metal and from scandium coated with a scandium oxide film, whereafter the assembly is sintered and impregnated. A method of manufacturing a cathode as claimed in Claim 4, characterized in that the scandium oxide is obtained by oxidation of scandium or scandium hydride. A method as claimed in any one of Claims 3 to 5, characterized in that the increase in weight of the scandium is 5-30% by weight due to the oxidation. A method as claimed in any one of the preceding Claims, characterized in that the sintering operation is performed in hydrogen at a temperature of at most 1500° C. A method as claimed in any one of the preceding Claims, characterized in that the powder from which the matrix is pressed comprises a maximum quantity of 2.5% by weight of scandium or scandium hydride. A method of manufacturing a scandate cathode as claimed in Claim 1, characterized in that the cathode is obtained by mixing, pressing and subsequent sintering of powders of a high-melting point metal and/or alloy and scandium, scandium hydride or scandium coated with a scandium oxide film, together with the powder of a barium compound which can supply barium on the emissive surface by a chemical reaction with the high-melting point metal and/or alloy during operation of the cathode. An electron beam tube provided with a cathode as claimed in Claim 1 or 2.

Inventors: CROMBEEN JACOBUS EDUARDUS, HASKER JAN, HOKKELING PIETER, VAN DORST PETRUS ADRIANUS MARI, VAN ESDONK JOHANNES, VAN LITH JOSEF JOHANNES, CROMBEEN, JACOBUS EDUARDUS, HASKER, JAN, HOKKELING, PIETER, VAN DORST, PETRUS ADRIANUS MARIA, VAN ESDONK, JOHANNES, VAN LITH, JOSEF JOHANNES
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01J1
IPC codes: H01J1, H01J9