Simultaneous fluoridation strategies for achieving high thermionic emission and low melting point in aluminate impregnants for Ba-W cathodes

Abstract

BaO-CaO-Al2O3 aluminate impregnants, the source for producing Ba for Ba-W cathodes, play an indispensable role in the cathode performance. Possessing the high thermionic emission property and low melting point is a huge challenge for the aluminate impregnants of Ba-W cathodes. Here, a “kill two birds with one stone” strategy is designed to simultaneously reduce the melting points and increase the thermionic emission capability. A novel BaO-CaO-Al2O3-BaF2 quaternary aluminate impregnant was obtained and the addition of BaF2 decreases the viscosity and melting point of the aluminate impregnants. When the content of BaF2 reaches 0.15 mol, the emission current density of Ba-W cathode impregnated with quaternary aluminate impregnant can reach 15.89 A cm−2 at 1100°Cb, which is 1.11 times higher than that of the without BaF2. In contrast, the continuous rise of BaF2 leads to the decreased emission current density due to the formation of a glassy phase. It is found that the introduction of BaF2 led to the formation of a new skeletal structure in the quaternary aluminate impregnant – [AlO3F], which coexists with [AlOn] (n = 4, 5, 6) to reduce the melting point. Theoretical calculations confirm that introducing fluorine triggers the number of point defects and barium compound in impregnant to gain electron convert to the metastable Baδ+ (0 < δ < 2), enhancing the ability to produce active substance Ba. This work provides new inspiration for developing novel impregnants for Ba-W cathodes.