Rhenium silicide as a new class of thermoelectric material
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Rhenium silicide as a new class of thermoelectric material Haruyuki Inui Department of Materials Science & Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, JAPAN ABSTRACT The microstructure, defect structure and thermoelectric properties of binary and some ternary Re silicide have been investigated as a new class if thermoelectric material. Binary Re silicide is identified to contain many Si vacancies, which are arranged in an ordered manner in the underlying tetragonal C11b structure so that the silicide is formulated to be ReSi1.75 with a monoclinic unit cell and contains four differently oriented domains accompanied by the twinned microstructure. The density and arrangement of Si vacancies can be controlled by ternary alloying. When the number of valence electrons of a ternary element is smaller than that for Re, the density of Si vacancies decreases with ternary additions, whereas the density of Si vacancies increases with ternary additions when the number of valence electrons of a ternary element is larger than that for Re. For both cases, the variation of the density of Si vacancies upon ternary alloying is accompanied by the introduction of the so-called shear structure.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. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition, by which an incommensurate microstructure is formed as result of extensive shear operation on the nano-scale. INTRODUCTION There is a renewed interest in thermoelectric materials for applications in power generation and refrigeration in environmental safety manners [1]. In recent years, many new thermoelectric materials such as those of Skutterudite-type and clathrate-type have been found to exhibit nice thermoelectric performance [1]. These compounds are believed to have a structure to realize the concept of “phonon glass-electron crystal (PGEC)” [2,3] through the “rattling” motion of guest atoms in oversized cages, which scatters heat-carrying phonons, resulting in low thermal conductivity, while electrical conductivity remains relatively high because electronic conduction mainly takes place through the cage framework. On the other hand, many semiconducting transition-metal silicides such as CrSi2 and FeSi2 have also been regarded as nice thermoelectrics but their properties are generally believed as a subject of further improvements for practical usage [4]. Obviously, the thermoelectric properties of this class of material, semiconducting silicides, cannot be interpreted in terms of the concept of PGEC in view of the crystal structure of these transition-metal silicides [4]. While extensive work has been devoted to some particular transition-metal silicides such as CrSi2 and FeSi2, many other silicides have been awaiting in-depth studies as a thermoelectric material. A silicide formed with R
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