High temperature surface composition of hexaboride thermionic electron emitters

Abstract

The high temperature surface composition and the evaporation properties of a series of single crystal hexaboride MB6 (where M = Ba, La, Ce, Pr, Nd, Sm, Eu) are examined by Auger electron spectroscopy and mass spectroscopy. The evaporation studies are also carried out on some (La, M′)B6 mixed hexaboride crystals (where M′ = Ce, Pr, Sm).The BM ratio in the surface layer of heated hexaborides (1400–1700°C) increases in the order LaB6<CeB6<PrB6≈NdB6<SmB6<EuB6. The evaporation activation energies of M and B from the heated MB6 crystal become nearly equal after prolonged heating and these values decrease in the order LaB6>CeB6>PrB6≈NdB6>SmB6⪆BaB6≈EuB6. When a mixed hexaboride is heated, the metal with higher vapour pressure evaporates preferentially from the surface, thus causing surface enrichment of the metal with lower vapour pressure.