GB929668A - Improvements in or relating to electrodes for electric discharge apparatus - Google Patents

Abstract

929,668. Powder metallurgy. GENERAL ELECTRIC CO. Ltd. Sept. 25, 1961 [Sept. 28, 1960], No. 33305/60. Class 82. [Also in Groups XI and XL (a)] An electrode for use in electric discharge apparatus, e.g. thermionic valves, discharge lamps, and inert gas shielded tungsten arc welding apparatus, comprises an electron emissive part 8, Fig. 2, which is formed from a composite sintered body composed mainly of tungsten and consisting of a case composed of tungsten containing a dispersion of electronemissive material of low work function and, partially or wholly surrounding the core and integral therewith, an outer layer 9 composed wholly or mainly of tungsten and substantially free from such electron emissive material. The outer layer may be composed of pure tungsten with one or more additative materials, e.g. alumina, potassium silicate, for influencing the crystal structure, and the electron emissive material included in the core may comprise one or more of the oxides of thorium, zirconium, yttrium, lanthanim and the rare earth metals in a proportion in the range 10-15% by weight of the total weight of the core. The ratio of the thickness of the outer layer to the diameter of the core is preferably in the range of 7: 1 to 1:7. When the electrode is used in arc welding or spark cutting apparatus, and when used in discharge tubes one end of the electrode may be exposed, thus forming a small electron emissive portion, or the outer layer may be ground away. In a specific example of a method of manufacturing the electrode, a double funnel of the type shown in Fig. 1 is used. Silicated tungsten powder is inserted into the bottom of the mould to a level a and the inner funnel 2 inserted. The inner portion 4 is filled with thoriated tungsten to the level b, silicated tungstan fed into the portion 5 to a level slightly higher than b, and the funnel 2 carefully removed. The composite mass so formed is subjected to a pressure of approximately 15 tons/sq. inch, and the compressed powder removed. Sintering is accomplished at a temperature of 2950‹ C. in an atmosphere of hydrogen by passage of an electric current through the bar, the temperature being kept at this level for 12 minutes. The final bar may be machined and rolled or pulled into a wire for use as a thermionic cathode. Specification 898,847 is referred to.