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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]

There are presented the experimental data that determine the spectral emissivity ελ and its temperature dependence for directionally crystallized LaB6−NbB2 eutectic alloy (19.4 mass% NbB2). Using the black body model, the ελ values were calculated from the data of brightness and true temperature while heating the composite in vacuum in the temperature range 1195–1908 K. It was found that the evaporation rate of lanthanum hexaboride in the alloy is significantly exceeds the evaporation rate of LaB6 single crystal, and the exposure of NbB2 fibers occurs. The obtained results allow us to consider the directionally crystallized LaB6−NbB2 composite of eutectic composition as a promising material for both radiation and ablative heat-shielding coatings for supersonic aircraft and spacecraft. [ABSTRACT FROM AUTHOR]