MoSi2-type narrow band gap intermetallic compound Al6Re5Si4 as a thermoelectric material

Abstract

The thermoelectric properties of MoSi₂-type intermetallic compound Al₆Re₅Si₄ (investigated compositions: Al_6−xRe_4.7Si_4+x (x = 0–0.9)) related to TiSi₂-type narrow band gap intermetallic compounds were systematically investigated. A first-principles band structure calculation implies that this compound forms a narrow band gap in the electronic density of states that will be suitable for obtaining a large power factor. Indeed, the maximum power factor is 1.65 mW m⁻¹ K⁻² for the sample with x = 0.8. The Al_6−xRe_4.7Si_4+x (x = 0–0.8) samples show degenerate semiconductor behavior as p-type materials; the temperature coefficient of the electrical conductivity is negative and the positive Seebeck coefficient increases with increasing temperature. A negative Seebeck coefficient is observed in the Si-rich sample with x = 0.9, which is understood by shifting the Fermi level to the conduction band. The estimated band gap is 0.53 eV from the Arrhenius plot. Because of a simple crystal structure, a relatively high thermal conductivity ranging from 7.6 to 13.5 W m⁻¹ K⁻¹ is observed at 300 K. The maximum dimensionless figure-of-merit of 0.19 at 973 K is close to the optimal value predicted by a single parabolic band model.