Establishing synthesis-composition-property relationships for enhanced and reproducible thermoelectric properties of MgAgSb

Abstract

α-MgAgSb is a promising p-type thermoelectric (TE) with excellent performance from room temperature up to 300 °C with a figure of merit of zTmax = 1.3. This makes MgAgSb a potential Te-free bismuth telluride (Bi2Te3) substitute for cooling and waste heat conversion applications. However, the material is also known for its sensitivity on synthesis conditions as indicated by various reports on the same nominal composition which show greatly differing TE properties and performance. This indicates a fundamental lack of synthesis control and synthesis-composition-property relationship knowledge. In this work, we establish a modified synthesis route with improved control over the effective sample stoichiometry which allows for reproducible high-performance properties (zTmax = 1.34 ± 0.19 at 561 K) and for systematically targeting different thermodynamic states of MgAgSb. This phase boundary mapping reveals that the homogeneity range for MgAgSb is very small (<0.1 at%) and that the TE properties are not governed by different thermodynamic states. Instead, we rationalize that the TE performance of MgAgSb is mainly controlled by the amount and type of secondary phases, mainly affecting the carrier mobility. We conclude that Sb-excess related secondary phases are the least detrimental, leading the way to upscaled synthesis of high-performance material.