| Research Abstract |
There is a renewed interest in thermoelectric materials because of the increased attention to environmental safety. The performance of thermoelectric properties is usually evaluated with the so-called dimensionless figure of merit ZT where Z=α^2σ/κ, and, α, σ, κ denote Seebeck coefficient, electrical conductivity respectively and thermal conductivity. The performance is better for the larger ZT value and it is generally believed that the condition of ZT>1 must be satisfied for thermoelectric materials to be practically used. We have recently found that Re-silicide exhibits a large ZT value close to ZT=1. In the present study, we made an effort to further improve the thermoelectric performance of this Re-silicide through ternary alloying. We expect the performance improvement not only through the changes in electronic structure but also through microstructure changes such as the change in the density and arrangement of Si vacancies and the formation of incommensurate structure. The bina
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ry silicide exhibits ZT=0.75 at 800K when measured along [001]. When transition-metals are replaced with Re, incommensurate microstructures are usually formed starting with the shear structure followed by the adaptive structure as the alloying content increases. The alloying amount at which the transition from the shear to adaptive structures occurs depend on each alloying elements. This occurs at a low content for Mo and W while the adaptive structure does not form within the solubility limit for Ru, Fe, Cr and Nb. Generally speaking, the property improvement occurs when the alloy takes the adaptive-type structure, whereas when the alloy takes the shear-type structure, no significant improvement occurs. The ZT value of 0.85 is achieved for 2at.% Mo added Re-silicide, which exhibits an adaptive structure. When Al is added to replace Si, the adaptive structure is locally formed in the close vicinity of twin boundaries. This leads to the significantly decreased thermal conductivity, resulting in the Zt value greater than 1 at 400K. Less
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