Abstract: A multiple cell thermionic converter having a generally tubular member of electrically conductive refractory metal with an internal cavity and a tubular envelope of electrically conductive refractory metal disposed in surrounding relationship to such generally tubular member. Electrically insulating ceramic material is disposed upon sections of facing surfaces of the envelope and tubular member, which is in the form of a body that has a central zone of plasma-sprayed refractory metal oxide selected from the group consisting of scandia, hafnia, zirconia and alumina, and a pair of flanking outer zones made of mixtures of the refractory oxide and particulate refractory metal. Refractory metal electron emitters and collectors are supported upon the ceramic bodies and juxtaposed in pairs with an annular gap therebetween to form a plurality of thermionic cells.