Glass-ceramic oxidation protection of higher manganese silicide thermoelectrics

A higher manganese silicide (HMS) thermoelectric, with composition MnSi1.74, densified by spark plasma sintering, was successfully coated with a glass-ceramic, in order to be used at temperatures higher than 500 degrees C, Compositional changes in both the HMS substrate and the glass-ceramic coating are reviewed and discussed with respect to the electrical properties of the uncoated and coated HMS before and after thermal cycles from RT to 600 degrees C in air. The formation of a Si-deficient layer (MnSi) on the uncoated HMS surface is due to the reaction between the HMS and oxygen at 600 degrees C, thus contributing to a lower power factor in comparison with the as sintered HMS. Coated HMS samples (after thermal cycles RT-600 degrees C) show a lower electrical resistivity and a significantly higher power factor in comparison with the uncoated ones. The glass-ceramic coating is self-reparable at 600 degrees C, as demonstrated by the complete sealing of an induced scratch on its surface.