An oxide cathode comprising a base 1 which consists substantially of titanium and a heater element 5 coated with aluminium oxide 6 to heat the base 1, which bears a porous alkaline earth metal oxide emissive layer 4. When the surface of the titanium base 1 which is opposed to the heater element 5 bears a metal layer 7 consisting at least of one of the metals Pt, Mo, Ta and W, a solution is obtained to the problem occurring in titanium cathodes in that titanium in contact with aluminium oxide is not chemically stable. The metal layer preferably consisits of pure tungsten and is formed by chemical vapour desposition (CVD). The metal layer is preferably from 1 to 10 .mu.m thick.
A potted heater assembly and a method of forming such a potted heater assembly comprises the steps of introducing into a mold having a filament disposed therein a slurry containing a liquid vehicle and particles of a refractory ceramic, said particles having a cylindrical shape and a diameter of generally less than a micron. The method further includes the step of removing the liquid vehicle from the slurry to leave behind a consolidated ceramic embedding the filament, and heating the ceramic having the embedded filament to further consolidate the ceramic to provide said ceramic having a density of at least 85% of theoretical density for the particular ceramic. The potted heating assembly having a ceramic potting embedding a filament heater is provided having higher post-sinter density than prior assemblies and has substantially higher strength.
