GB1033448A - Improvements in the manufacture of indirectly-heated cathodes - Google Patents

Abstract

1,033,448. Indirectly - heated cathodes. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. April 30, 1965 [May 5, 1964], No. 18290/65. Heading HID. An indirectly-heated cathode comprises a heating element 1 embedded in an insulating mixture 2 of silicon oxide and/or metal oxide which is brought into contact with the carrier 3 for the emissive layer and subsequently fused to both the heating element and the carrier, e.g. by heating for 2 mins. at a temperature 50‹ above its melting point. The insulating mixture may comprise, by weight, 70 to 20% CaO, 70 to 25% SiO 2 , and 0 to 40% MgO, to which up to 60% Al 2 O 3 and/or SrO may be added, for cathode operating temperatures up to 1100‹ C. This mixture has a coefficient of expansion matched to that of a W or Mo heating element, but if the cathode comprises an oxide coating 5 on a nickel base 4, the insulating mixture is fused to a W or Mo carrier layer 3 to which an Ni layer is secured by sintering or rolling. Alternatively, the cathode carrier may comprise a porous W body impregnated with, or having a cavity containing, emissive material. If the cathode operating temperature is below 900‹ C. the insulating mixture may comprise, by weight, 20 to 80% MnO, 20 to 60% SiO 2 and 0 to 35% Al 2 O 3 , to which up to 15% of other metal oxides may be added, and the intermediate W or Mo layer 3 may be omitted. Ferrochromium or nickel-iron supply leads 6, 7 are connected to the cathode-heater. The cathode may be of very small size or may be tubular. Specific examples and dimensions are given.