Synthesis and Characterization of Melt-Spun Metastable Al₆Ge₅

Abstract

The electronic structure and thermoelectric (TE) properties of metastable Al₆Ge₅ were investigated. The crystal structure with nominal composition Al₆Ge₅ is identified as isostructural to the high-performance TE material Zn₄Sb₃. The calculated density of states (DOS) shows that Al₆Ge₅ is a semiconductor with small bandgap of E _g = 0.44 eV. Moreover, as Al₆Ge₅ has sharp edges in the DOS near the Fermi energy, high Seebeck coefficient (S) values are expected. Ribbon samples of Al₆Ge₅ were prepared using a single-roll melt-spinning method. We developed a process to prepare Al₆Ge₅ as the main component using this method under various optimal conditions, mainly cooling rate. Room-temperature values for S and electrical conductivity are −17.5 μV K⁻¹ and 2.64 × 10⁴ Ω⁻¹ m⁻¹, respectively. The κ value of around 0.5 W m⁻¹ K⁻¹ at room temperature is considerably low. Although Al₆Ge₅ has potential to be a good TE material, the S value of the prepared samples is low, like metals, mainly due to impurities existing in the samples. High zT is considered obtainable by preparing high-purity single-phase Al₆Ge₅ with optimized carrier concentration.