ES364237A1 - Thermionic Cathode - Google Patents

Abstract

In a thermionic cathode assembly comprising a hollow support on which is provided thermionically emissive material, the support accommodating a heater comprising a filament on which is provided a surface layer of a dark coloured material, the surface layer is separated from the filament by porous insulating material to inhibit ion conduction therein and diffusion of metal from the dark layer into it thereby allowing the cathode to be operated at up to 1200 C. with a potential difference of 500V between the filament and the support. The drawing illustrates a dispenser cathode having emissive material 2 on a support 1. A tungsten filament 4 is helically wound round a molybdenum core (subsequently removed at 5) which is bifilarly wound round a ceramic cylinder 6, and coated with two layers 7 and 8 of Al 2 O 3 BeO, or MgO deposited respectively by cataphoresis and immersion. The porous layer 9 is formed by immersion in a mixture of one of the above oxides and MoO 3 followed by sintering for 5 minutes at 1500 C. in a reducing atmosphere which converts the MoO 3 into Mo. The assembly is then immersed in a suspension of Al 2 O 3 with WO 3 , followed by heating for 2 minutes at 1600 C. in a reducing atmosphere which reduces the WO 3 to metallic tungsten forming the dark layer 10. The molybdenum core of the filament and the molybdenum in layer 9 are dissolved by etching so that the layer 9 is porous and contains 50-75% of cavities by volume. As alternatives, the porous layer may be formed between two solid insulating layers or the solid insulating layers may be omitted. The cathode may be used in a cathode-ray tube and may have a warming-up time of about 7 seconds.