Crystal Structure and Thermoelectric Properties of Al-containing Re Silicides
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Crystal Structure and Thermoelectric Properties of Al-containing Re Silicides Eiji Terada1, Min-Wook Oh2, Dang- Moon Wee2 and Haruyuki Inui1 1 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan 2 Department of Materials Science and Engineering, KAIST, Yuseong-gu, Daejon 305-701, Republic of Korea ABSTRACT The microstructure, defect structure and thermoelectric properties of Al-containing ReSi1.75 based silicides have been investigated. All the Al-containing alloys investigated contain four differently oriented domains accompanied by the twinned microstructure, as the binary alloy does. However, thin defect layers containing a kind of shear structure are locally and sporadically formed at_ some of twin boundaries. In the defect layer, shear occurs by the vector of [100] on either ( 1 09) or (107) planes. Binary ReSi1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along [001], although the ZT value along [100] is just moderately high. Al-containing Re silicides considerably increase the ZT value along [100] so that the maximum value of 0.95 is achieved at 150 °C for the ReSi1.75Al0.02 alloy. The temperature dependence of electrical resistivity changes from of semiconductor for the binary alloy to of metal for the Al-added alloys and the value of electrical resistivity is significantly reduced when compared to the binary counterpart. INTRODUCTION Binary rhenium disilicide is of interest owing to its potentials as a promising candidate material for thermoelectric applications [1]. Although the disilicide has been known to be a semiconductor, there is no general agreement on the crystal structure [2]. In our recent research, the crystal structure of the defect disilicide formed with Re (ReSi1.75) has been refined by transmission electron microscopy combined with first-principles calculation [3,4]. The crystal structure is monoclinic with the space group Cm (mc44) due to an ordered arrangement of vacancies on Si sites in the underlying (parent) C11b lattice. The crystal contains four differently oriented domains; two domains related with each other by the 90o-rotation about the c-axis of the underlying C11b lattice and twin domains for each of the two domains. The twin habit plane is (001) of the underlying C11b lattice and the thickness of twins is very thin ranging from 100~300 nm. Although several researchers have reported the electrical transport properties of the disilicide, they are not necessarily consistent with each other. Siegrist et al. [5] reported the value of Seebeck coefficients ranging from -90 to -130 µV/K at 310 K. On the other hand, Neshpor et al. [6, 7] reported the value in fair agreement with that reported by Siegrist et al. [5] but the sign is reversed. Our recent measurements on the electrical transport properties as well as thermal transport properties for single crystals of ReSi1.75 indicated that the value of electrical resistivity along [001]
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