Thermochemistry in the BaO · Sc 2O 3 · WO 3 ternary system

Abstract

It has been established that the addition of scandium to thermionic cathodes, both oxide and dispenser type, results in reduction in work function. Unfortunately, the improved performance is short-lived, as the scandium is removed by ion bombardment and the low work function surface structure does not readily reform. A wide range of experiments have been reported on different scandate cathode configurations, including those incorporating scandium (or its oxide) into the bulk and as a component of a surface layer. In these studies the primary characterization techniques have been limited to one or more surface probe methods in efforts to correlate surface composition with emission behaviour. Surface analysis techniques yield compositional information, but provide no details as to the nature of phase composition of the surface. A study of the thermochemistry manifested in one of the systems representative of scandate cathodes has been undertaken to advance understanding of the mechanism responsible for the improved emission behaviour. Two isothermal sections of the BaO · Sc 2O 3 · WO 3 ternary system are presented detailing the phase equilibria at 1100°C and 950°C. Information regarding compound occurrence and stability, rates of reaction, and stability of products over a range of temperatures is reported. The structure of a previously reported ternary compound, Ba 3Sc 2WO 9, is discussed and a reaction mechanism describing the evolution of this compound, applicable for many scandate cathode configurations, is presented.