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A2: Medium temperature materials
Ba8Al16Si30 system is practically important among the semiconducting clathrates from environmental and ecomonical points of view. The thermoelectric and elastic properties were reported for Ba8Al16Si30 system [1, 2]. High thermal stability and durability in air at high temperatures are also required for practical use of the material for thermoelectric applications. From this point, we reported the effect of heat treatment in air on the surface of polycrystalline samples with composition of Ba8Al15Si31 [3], where the Al content is almost the solid solution limit in Ba8Al16Si30 system, to examine the thermal stability in air and oxidation resistance. In this study, we further investigated the effect of heat treatment in air on the thermoelectric properties of polycrystalline Ba8Al15Si31 samples. Samples were prepared by combining arc melting and spark plasma sintering techniques, as previously reported [1-3]. The samples were characterized by X-ray diffraction (XRD) measurements, thermal gravimetric and differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). Heat treatments were performed at 873 K in air for 240 h and 480 h. The Seebeck coefficient, electrical conductivity, and thermal conductivity of samples were measured before and after heat treatments in air. Although the oxidation layer was formed on the surface by thermal diffusion (activation energy: approximately 201 kJ/mol [3]), the inner part of Ba8Al15Si31 was found to be stable. The Seebeck coefficient, the electrical conductivity, and the thermal conductivity were found to be almost unchanged after heat treatment in air at 873 K for 480 h, indicating that Ba8Al15Si31 clathrate looks promising as a thermoelectric material with high oxidation resistance.
[1] H. Anno, et al., J. Mater. Sci., 48, 2846 (2013). DOI: 10.1007/s10853-012-6977-y
[2] H. Anno, et al., J. Electron. Mater., 42, 2326 (2013) DOI: 10.1007/s11664-012-2418-6
[3] H. Anno and R. Shirataki, J. Electron. Mater., DOI: 10.1007/s11664-013-2887-2