Directional Thermoelectric Performance of Ru 2 Si 3
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Directional Thermoelectric Performance of Ru2Si3 Benjamin A. Simkin, Yoshinori Hayashi, Haruyuki Inui, Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 6068501 Japan
ABSTRACT The orthorhombic compound Ru2Si3 is currently of interest as a high-temperature thermoelectric material. In order to clarify the effects of crystal orientation on the thermoelectric properties of Ru2Si3, we have examined the microstructure, Seebeck coefficient, electrical resistivity, and thermal conductivity of Ru2Si3 along the three principal axes, using these measured quantities to describe the relative thermoelectric performance as a property of crystal orientation. Ru2Si3 undergoes a high temperature (HT)→low temperature (LT) phase change and polycrystalline Si platelet precipitation during cooling, both of which are expected to effect the thermoelectric properties. The HT tetragonal→LT orthorhombic phase transformation results in a [010]//[010], [100]//[001] two-domain structure, while polycrystalline Si precipitation occurs on the (100)LT and (001)LT planes. The [010] orientation is found to posses superior thermoelectric properties (with the dimensionless figure of merit, ZT[010]/ZT[100]>4 at 650°C), due principally to the larger Seebeck coefficient along the [010] direction. The effect of the domain structure on the thermoelectric properties is discussed.
INTRODUCTION Ruthenium sesquisilicide, Ru2Si3, has attracted attention over the past decade [1-10] as a potential thermoelectric material for use at high temperatures. This interest has been stimulated by the prediction of greatly enhanced thermoelectric performance of Ru2Si3 over that currently attainable with conventional Si-Ge alloys for use in thermoelectric generators. The physical properties relevant to a thermoelectric material can be determined by the thermoelectric figure of merit, Z, defined by: Z=α2/ρ⋅κ,
(1)
where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity of the material. Because Z has units of 1/K and α, ρ, and κ all tend to vary with temperature, it is often useful to express the thermoelectric performance of a material through the dimensionless figure of merit, ZT, as a function of temperature. All three of the components of Z can be expected to potentially vary depending on crystal orientation, as all three are dependant on either lattice scattering (ρ, κ), or the lattice directional dependence of the band structure (α). Ru2Si3 has two different crystal structures, of which the phase of interest is the orthorhombic low temperature phase. Because of this orthorhombic structure and the dependence of ρ, κ and α on orientation, there is expected to be potential for significant differences in thermoelectric performance depending on Ru2Si3 crystal orientation. Electrical resistivity behavior for single crystal Ru2Si3 has been reported to depend upon orientation [3]. Thermal conductivity for polycrystalline Ru2Si3 has been extensively reported [2,4,5,8], although with signific
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