Crystallographic, Thermoelectric, and Mechanical Properties of Polycrystalline Ba8Al x Si46−x Clathrates

Abstract

The Al content dependence of crystallographic, thermoelectric, and mechanical properties is reported for polycrystalline Ba8AlxSi46−x (nominal x = 15 to 17) clathrates prepared by combining arc melting and spark plasma sintering methods. The elastic constants and the coefficient of thermal expansion (CTE), which are also important properties for designing thermoelectric devices, are presented. Powder x-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy (EDX) indicate that the type I clathrate is the major phase of the samples but impurity phases (mainly BaAl2Si2, Si, and Al) are included in the samples with high Al contents. The actual Al content x determined by EDX ranges from approximately 14 to 15. The absolute value of the Seebeck coefficient increases and the electrical conductivity decreases as the Al content increases. The changes in Seebeck coefficient and electrical conductivity are explained in terms of the dependence of the carrier concentration on the Al content. The elastic constants and the CTE of the samples depend weakly on the Al content. Some of the properties are compared with reported data of single crystals of Ba8Al16Ge30, Ba8Ga16Ge30, Sr8Ga16Ge30, silicon, and germanium as standard references. The effective mass, Hall carrier mobility, and lattice thermal conductivity, which govern the transport properties, are determined to be ~ 2.4m0, ~ 7 cm2 V−1 s−1, and ~ 1.3 W m−1 K−1, respectively, for actual Al content x of about 14.77. The thermoelectric figure of merit ZT is estimated to be about 0.35 at 900 K for actual Al content x of about 14.77.