Thermoelectric Properties of Ba₈Cu_xSi_yGe_46–x–y Clathrates Synthesized by the Czochralski Method

Abstract

A type-I clathrate Ba 8 Cu x Si y Ge 46−x−y ingot consisting of millimeter-sized grains with different crystallographic orientations was synthesized by the Czochralski method. The lower part of the ingot contained more Cu and Ge within the clathrate phase. The electrical resistivities of the top, middle, and bottom of the ingot at 550 °C were 0.78, 1.72, and 1.55 mΩ•cm, respectively. The differences in these values are consistent with the carrier concentration and calculated anisotropic effective mass of the electrons, which are based on the actual chemical composition and crystal orientation, respectively. The resistivity of the top part with few grain boundaries is less than a quarter of that of the hotpressed sample with a similar composition. Despite the different carrier concentrations, the Seebeck coefficient of all parts had similar values of approximately −120 μV/K at 550 °C. The thermal conductivity at 350 °C was approximately 3.0 W/m•K for top part and 2.5 W/m•K for middle and bottom parts, about 1.5−1.8 times larger than those of previous reported hot-pressed samples. The power factor and dimensionless figure of merit ZT value of the top reached 18.8 μW/cm•K 2 and 0.34, respectively. These results indicate that fabrication by the Czochralski method can improve the thermoelectric performance of Ba 8 Cu x Si y Ge 46−x−y clathrates. KEYWORDS: thermoelectric material, Ba 8 Cu x Si y Ge 46−x−y clathrate, Czochralski method, power factor, single crystallization, anisotropic effective mass