Effects of metallic coatings on the performance of skutterudite-based segmented unicouples

Abstract

An analytical model is developed to investigate the effects of applying thin metallic coatings on the legs of skutterudite based thermoelectric unicouples and to calculate the impact on the dimensions and the performance parameters of the unicouple. These coatings suppress the sublimation of antimony from the legs near the hot junction (∼973 K) and, hence, minimize the degradation in the unicouple performance with operation time. Coating materials investigated are: tantalum (Ta), titanium (Ti), molybdenum (Mo) and vanadium (V), which have different thermal conductivities, electrical resistivities, coefficients of thermal expansion (CTE) and vapor pressures. Detailed comparison of these coating materials was necessary since none clearly satisfies both a minimal decrease in the performance of the unicouple and ease of fabrication and long life. The former favors low thermal conductivity and high electrical resistivity, while the latter favors low CTE and low vapor pressure. The lengths of the coatings of both the n- and p-legs, which were found to be different for best unicouple performance, are determined as functions of the antimony vapor pressure off the exposed portions of the legs, which varied from 1.0 to 1000 mPa, and the thickness of the applied metallic coating (1–9 μm). Results indicated that increasing the coating thickness decreases the conversion efficiency but increases the load electrical power of the unicouple. Despite their high CTE, Ti or V coatings are best in terms of the effect on the unicouple performance when a coating thickness >1 μm in needed. They cause the smallest decreases, while Ta or Mo coating with lower CTEs, result in the largest decreases in the conversion efficiency. With thin coatings (⩽1 μm), however, the effects on conversion efficiency and electrical power of the unicouple are significantly small. In this case, a Mo coating would be best since it has the smallest CTE and low vapor pressure.