Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ca16Ge30 are reported in the temperature range 5 K less than or equal to T less than or equal to 3QO K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 10(17)-10(18) cm(-3). The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 1/4 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T >700 K, thus exceeding that of most known materials. (C) 1998 American Institute of Physics.