GB472648A - Improvements in or relating to gas or vapour filled electric discharge tubes - Google Patents

Abstract

472,648. Thermionic cathodes. CLEVELAND, F. J. (Oerensofi, N. S.) March 26, 1936, No. 9060. [Class 39 (i)] An electrode of the hot cathode type for a gas or vapour-filled discharge tube has a conductive carrier portion and an emitting coating covering a substantial proportion of its surface, the carrier portion being formed with apertures, extending from the coated to the uncoated surface thereof, from which canal rays are emitted during operation of the discharge tube containing the electrode. The electrode shown comprises a thin strip 14 of tungsten bent into an arc and coated on one side with emissive material 16. Perforations 15 are provided, which in operation are free from emissive material and permit the passage of canal rays. Two such electrodes are mounted at the ends of a discharge tube with the coated concave faces inwards, so that the canal rays may be directed towards the cool ends of the envelope to prevent condensation and to maintain the filling in circulation. The emissive coating is of material which becomes conductive only at a temperature above that at which it becomes emissive. Such a material may comprise, by weight, zirconia 75 per cent, yttria 15 per cent, barium oxide 8À5 per cent, and strontia 1À5 per cent, together with a binder such as potassium waterglass or an easily vaporizable organic or inorganic compound. The binder may be replaced by 50 per cent or less of barium hydroxide. When chemically active gases, such as nitrogen, carbon dioxide, sulphur, selenium or tellurium are employed, the carrier should be of resistant metal such as gold or platinum, or these metals may be coated upon a base metal, such as tungsten, molybdenum, chromium, tantalum or nickel, electrolytically or by decomposing their chlorides. As carbon dioxide attacks alkaline earths, with this filling a mixture may be employed comprising zirconia 75 per cent, erbia 17À5 per cent, thoria 6À5 per cent and ceria 1 per cent. To secure a firm attachment of the emissive coating to carriers of metal or carbon, addition may be made to the emissive mixture of a metal such as zirconium, magnesium or thorium or a substance such as silicon which will alloy or combine chemically with the carrier substance. To a mixture of yttria 75 per cent, thoria 20 per cent, and barium oxide 5 per cent may be added four times its weight of zirconimn. When heated in vacuo or in hydrogen, the zirconium alloys with the tungsten carrier and partially reduces the oxides to form zirconia which is concentrated at the surface. The apertured carrier may be of spiral, helical or other twisted form. Specification 441,250 is referred to.