GB487699A - Improvements in thermionic cathodes for use in electric discharge tubes - Google Patents

Abstract

487,699. Thermionic cathodes. ABBOTT, S. A. Feb. 5, 1937, Nos. 3493 and 15919. [Class 39 (i)] A cathode for a gas- or vapour-filled discharge tube comprises a body 1 of insulating refractory material such as quartz or silica glass which has part of its surface chemically reduced ; electron emissive material 3 is placed on this surface and by heat is hardened and firmly attached to the body 1. Magnesium or aluminium is used as the reducing agent and the silicon produced in the body acts with the silicon mixed with the emissive material to cause cohesion; the magnesium oxide formed in the reduction process is removed. The material 3 comprises a refractory metal and one or more compounds of the rare earth and /or alkaline earth metals ; tungsten, molybdenum or nickel with barium or thorium oxide or hydroxide may be used with a binder such as water glass. Silicon may be added to the mixture, which is heated in contact with the reduced surface in the presence of oxygen. A tungsten contact wire 2 is passed through the body 3, tube 4 into the mass 3 and may act as an anode ; in a modification a nickel wire may be used as lead to the cathode being riveted over where it projects into the base of the cup in the body 1 ; or a wire may be passed into the mass 3 and the end wound round with finer wire to make contact with the material. In another construction a quartz tube 1a, Fig. 3, has a quartz plug closure in the emissive material 6 being attached to the tube 1a; this may be heated by the discharge or an independent heater 7 may be provided. An independent heater may be provided with bodies as shown in Fig. 1 or a coil heater 24, Fig. 8, may be disposed near the exposed surface of the emissive material 6 ; a double tube construction is shown in this figure. In one method a mixture comprising tungsten 90 per cent, barium oxide 5 per cent and silicon 5 per cent is first pressed against the reduced surface and a mixture is then added of barium oxide 80 per cent and thorium oxide 20 per cent the whole being thereafter heated ; molybdenum nickel or platinum may replace the tungsten. A mixture of tungsten barium, hydroxide, strontium or thorium oxide, silicon and graphite may be used ; proportions being given ; or a pure tungsten and a mixture of tungsten and silicon are first placed in the body 1 and then molten barium hydroxide is added. Boro-silicate glass, porcelain, steatite or zirconia may replace the quartz.