Enhanced thermoelectric figure of merit by composite effects and low thermal conductivity in distrontium silicide (Sr2Si)

The thermoelectric properties of Sr2Si, Sr5Si3, and their composites are reported. Sr2Si-Sr5Si3 composites were synthesized by partial decomposition of Sr2Si to Sr5Si3 during spark plasma sintering. Semi-conducting Sr2Si grains were surrounded by boundary regions composed of smaller grains of Sr2Si, metallic Sr5Si3, and semiconducting Sr3SiO. Addition of the Sr5Si3 phase reduced the electrical resistivity from 1.69 x 10(-3) Omega m to 6.45 x 10(-5) Omega m at room temperature, which corresponded to the samples having no and the maximum amount of the Sr5Si3 phase, respectively. The electrical resistivity decreased; however, the Seebeck coefficient slightly increased to 157 mu V/K at 773 K, and the thermal conductivity was approximately constant. Therefore, the power factor and ZT monotonically increased with an in-crease in the amount of the Sr(5)Si(3 )phase in the composite samples, which confirmed almost no effect resulted from the Sr3SiO phase. A low thermal conductivity was demonstrated by Sr2Si (ca.1 W/mK at 776 K), which resulted from weaker atomic bonds, heavier atomic mass, greater atomic volume, and a greater number of atoms in the unit cell. A further enhancement of ZT can be achieved for higher amounts of Sr5Si3 and at higher temperatures. (C) 2018 Elsevier B.V. All rights reserved.