Mass transfer in composites based on BaxSr1-x HfO3 with tungsten at large temperature gradients.

Experimental results on the effect of heating with simultaneous take-off thermionic current on the stability of thermionic emission in composite cathode materials Ba0.25Sr0.75HfO3 with 16% W (by mass) are presented. It is found that cavities with a total absence of barium-strontium hafnate appear inside the samples after 5-hour heating. Due to a decrease in the diffusion flux of Ba and Sr to the emitting surface, the emission activity of the composites drops sharply. The reasons for such cavities formation are clarified: overheating of internal regions of the material by the take-off thermionic current to temperatures comparable to the melting temperatures of the composite individual components, as well as the accelerated diffusion of Ba and Sr in the presence of a large temperature gradient equal to 4.3×106 K/m. The calculated diffusion coefficient of Ba and Sr, equal to 2×10−12 m2/s, exceeds the self-diffusion coefficient of Ba in BaO by two orders of magnitude. [ABSTRACT FROM AUTHOR]