GB648955A - Improvements in electron-emitting electrodes for electric discharge tubes - Google Patents

Abstract

648,955. Cathode materials and processing. PHILIPS LAMPS, Ltd. April 26, 1946, No. 12657. Convention date, March 21, 1942. [Class 39 (i)] A thermionic cathode consists of or is coated with an electron emissive material comprising at least one ingredient from each of groups A, B and C, group A containing tungsten and molybdenum, group B containing thorium, tantalum, hafnium and zirconium, and group C containing barium, calcium and strontium oxide, and the percentage by weight of the element or elements from group B is above 10 per cent and preferably above 20 per cent. In a modification, a quantity of thorium oxide may be included as part of the percentage allocated to group C. The material may be used to coat a conductive core either of a directly or indirectly heated cathode or may be pressed and sintered to form a cathode body. In the latter case the body is placed on or in a holder in, say, a high pressure mercury vapour discharge tube. The invention is also applicable to other gas- or vapour-filled discharge tubes, e.g. gas discharge lamps, and also to high vacuum discharge tubes. A number of examples are given : (i) a pressed and sintered body of 65 per cent tungsten, 25 per cent thorium, 10 per cent of a mixture of the alkaline earth oxides in group C; (ii) a pressed and sintered body of 10 per cent thoria, 10 per cent barium and calcium oxides, 10 per cent tantalum, 15 per cent hafnium, 5 per cent zirconium, 50 per cent tungsten and/or molybdenum ; (iii) 90 gms. of a mixture of 75 parts tungsten to 25 parts of thorium are mixed with 10 gms. of a mixture of barium, strontium and calcium carbonate. The mixture so formed is made into a paste with a 10 per cent solution of nitro-cellulose in ethyl glycol as binder, and the paste is pressed through a die to give small cylinders 3-4 mm. long and 2 mm. in diameter. The cylinders are mounted on a current supply wire of diameter 500Á, and heated and sintered to convert the carbonates into oxides; (iv) 80 gms. tungsten, 20 gms. of thorium, and 18 gms. of a mixture of barium, strontium and calcium carbonate is mixed with 45 c.cs. of butyl acetate having 1.8 per cent nitro-cellulose dissolved therein, and the mixture is ground in a ball mill for 24 hours. The core consisting of a straight tungsten wire carrying a helix of tungsten is immersed in the resulting paste, dried, and the treatment is repeated two or three times. The cathode thus obtained is then sintered and sealed'into the lamp. Specifications 237,270, 334,460, [both in Class 39 (i)], and 536,221 are referred to. The Specification as open to inspection under Sect. 91 states that alkali metal oxides may be used instead of alkaline earth oxides. Those metallic constituents which have a work function less than that of molybdenum, e.g. thorium, hafnium, tantalum, or iron, are present in the proportion of at least 1 per cent, preferably more than 10 per cent or more than 20 per cent. The tungsten or molybdenum may be replaced partly or wholly by tantalum, nickel, or zirconium. This subject-matter does not appear in the Specification as accepted.