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Journal of Electronic Materials, 2013, Volume 42, Issue 7, pp 2084-2090.
Tsunehiro Takeuchi, Yuichi Terazawa, Yukihiro Furuta, Akio Yamamoto, Masashi Mikami.
EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan &
Precursory Research for Embryonic Science and Technology, Japanese Science and Technology Agency, Tokyo, 102-0075, Japan &
Department of Applied Physics, Nagoya University, Nagoya, 464-8603, Japan &
Department of Crystalline Materials Science, Nagoya University, Nagoya, 464-8603, Japan &
National Institute of Advanced Industrial Science and Technology, Nagoya, 463-8560, Japan.
Abstract
The thermoelectric performance of Fe2VAl-based alloys was improved by using the effects of (a) heavy element substitution and (b) off-stoichiometric (Fe/V ≠ 2) composition. The former method led to a significant reduction of lattice thermal conductivity, whereas the latter to an evolution of the Seebeck coefficient. As a result of sample preparation, we confirmed that the dimensionless figure of merit with n-type behavior was increased up to 0.25 at 420 K for the sample obtained at the optimized composition of Fe1.98V0.97Ta0.05Al0.9Si0.1. Electronic structure calculations revealed that the increase of the Seebeck coefficient observed for Fe-poor samples was caused by a reduction of the density of states near the chemical potential.
Additional Information:
Thermoelectric generators have attracted considerable interest for its ability in recovering energy from waste heat. The relatively low performance in converting energy, however, prevented us from utilizing thermoelectric generators in various applications. The practical thermoelectric generators possessing the highest performance at low temperatures below 500 K are consisting mainly of tellurium that is toxic, rare, and expensive. This fact makes it further difficult for us to use the thermoelectric generators in practical applications. It is, therefor, of great importance to develop a thermoelectric generators possessing high performance using the materials consisting solely of cheap, environmentally friendly, ubiquitous elements. Since a large part of waste heat is released to the environment at low temperatures below 500 K, we need to develop the thermoelectric generators usable in this particular temperature range.
For making such thermoelectric generator, we tried in this study to improve thermoelectric properties of Fe2VAl based Heusler alloys, which are characterized by possession of a relatively large Seebeck coefficient |S| > 120 {micro}VK-1, metallic electrical conduction {Sigma}< 1m {omega} cm, and the consequently obtained large magnitude of power factor S2 {Sigma} > 3×10-3 Wm-1K-2 at around 300K to 400K. These values are comparable with those of practical thermoelectric materials. Unfortunately, however, their large magnitude of lattice thermal conductivity makes the magnitude of dimensionless figure of merit ZT = S2 {Sgima}/k much smaller than that of practical materials. For developing practical thermoelectric materials using Fe2VAl based alloy, one has to effectively reduce their lattice thermal conductivity without seriously affecting the electron transport properties.
For achieving this rather difficult material tuning, we employed heavy element substitution with help of first principles calculations. The heavy elements that produce no serious impurity states near the chemical potential at the substitution were selected from the first principles calculations and used in the present experiments. We also employed, by considering electronic structure variation, the off-stoichiometry composition because it enhances the magnitude of Seebeck coefficient. As a result of this material tuning, we succeeded both in enhancing Seebeck coefficient and in reducing lattice thermal conductivity. Consequently the dimensionless figure of merit increased up to 0.25.
