EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Magnesium silicide and titanium suboxides (TiOx) are considered promising thermoelectric materials to generate electricity from waste-heat thanks to their thermoelectric performance, low-cost, low-toxicity and light-weight. Nevertheless, the stability and oxidation resistance over time at temperatures higher than 500° C for Mg-silicide and 400°C for TiOx respectively, are critical issues. Therefore, the development of protective coatings for TE legs is fundamental in order to prevent the degradation of their thermoelectrical properties at high temperature. Glass-based materials, with low electrical and thermal conductivity, are good candidates as protective coatings against oxidising atmospheres.
In this work, a Sb doped Mg2Si-Mg2Sn based thermoelectric, densified by spark plasma sintering (SPS), was successfully coated with a new silica-based glass, which was specifically designed as an oxidation protective coating for mid-temperature range (up to 500° C) applications. Despite the high coefficient of thermal expansion of Sb doped Mg2(Si,Sn) based materials (17-17.5·10-6 K-1), very good thermo-mechanical compatibility between the substrate and the coating was obtained. Oxidation tests, performed at 500° C for 120 hrs in air, established the effectiveness of the glass coating for the protection of Sb doped Mg2(Si,Sn) thermoelectrics.
Moreover, a new Y2Ti2O7 containing glass-ceramic was specifically produced as a protective coating for the titanium suboxide (TiOx) up to 600 °C. Excellent thermo-mechanical compatibility between the substrate and the coating was obtained. Oxidation tests, performed at 600 °C for 120 hrs in air, demonstrated the effectiveness of the glass-ceramic coating for the protection of TiOx.