Mechanical Alloying of Optimized Mg₂(Si,Sn) Solid Solutions: Understanding Phase Evolution and Tuning Synthesis Parameters for Thermoelectric Applications

Mechanical alloying by high energy ball milling is an attractive solid-state technique for synthesizing a diverse range of stable and metastable materials. We have studied the synthesis of n-type thermoelectric Mg₂Si_0.4Sn_0.6 solid solution, aiming for a fundamental understanding of the mechanisms underlying this synthesis technique. The investigations on powders by XRD and SEM show that milling leads to welding of Mg and Sn but fracturing of Si. This fractured Si diffuses into the ductile matrix on longer milling times resulting in a phase mixture close to the nominal starting composition after 35 h of milling. However, phase pure material was only achievable after sintering; hence, the synthesis of Mg₂(Si,Sn) is a two-step process. Furthermore, a thorough study on the effect of varying synthesis parameters on the thermoelectric properties was performed. This was done by systematically varying the milling and consolidation parameters. No strong influence of milling time on the thermoelectric properties was observed, and just 2 h of milling followed by compaction was sufficient to obtain a pellet with optimal thermoelectric properties. Moreover, increasing sinter temperature/time deteriorated carrier concentration, hence degrading the electronic properties. Thus, optimized thermoelectric properties were obtained for the powder consolidated at 973 K/20 min. Mg₂Si_0.4Sn_0.6 synthesized by mechanical alloying achieved a thermoelectric figure-of-merit zT_max ∼ 1.4.