GB333011A - Improvements in thermionic cathodes of vacuum electric tube devices - Google Patents

Abstract

333,011. Cachemaille, A. S., (Westing- house Lamp Co.). June 24, 1929. Vacuum tubes.-In the construction of indirectly heated cathodes containing a heating element surrounded by an insulator, e.g. quartz, porcelain thoria, or the material known under the registered trade mark " Isolantite," erosion and consequent failure of the heating filament, due to deleterious gases, such as water vapour and oxides of carbon, evolved by the insulator, are prevented by the provision of a protecting material between the filament and the insulator'which absorbs and retains these gases. As protecting material, elements such as chromium, thorium, zirconium, uranium, tantalum, boron, silicon, vanadium and titanium are employed with a heater filament of tungsten, molybdenum or tantalum. Preferably, chromium is the element employed and this may be plated on the filament but, to prevent the wire from being rendered brittle, chromium powder is mixed with a binder and the wire is coated by applying or spraying, the binder being a cellulose derivative dissolved in amyl acetate. As shown in Figs. 1 and 3, a filament 5 is coated with a layer 12 of the chromium powder and binder, bent to hair pin form and inserted in holes 8, 9 in. an insulator 10 which is enclosed in a metal tube 11 coated , with emissive material 11<1>. The cathode is then assembled in the valve which is exhausted and the filament heated, the heated chromium uniting with the gases evolved from the insulator. In some cases the chromium compounds formed are fused to the wall of the insulator thus allowing freedom of movement to the filament. The chromium powder may be in a partially oxidized condition, and the protective material may be applied in the first instance to the wall of the insulator.