Bulk Mg2Si Based n-type Thermoelectric Material Produced by Gas Atomization and Hot Extrusion

Abstract

Bulk thermoelectric (TE) materials for large scale energy harvesting applications need to demonstrate not only high TE performance, but should also be composed from elements which are nontoxic, largely available in nature, and preferably be as light as possible for applications such as automobiles. Magnesium silicide based materials possess a combination of these properties and are among the best candidates for these applications. For the successful implementation of Mg2Si based alloys, all material manufacturing steps have to be compatible with the requirements of mass production. We report two processes to manufacture these alloys which can be easily scaled up: (i) a gas atomization method for powder production, followed by (ii) hot extrusion to obtain bulk specimens. The Mg2Si1-xSnx (0.3 ≤ x≤ 0.7) powders were prepared by gas atomization using a stoichiometric mixture of the commercially available high purity elements. Particle size distribution analysis shows that 90% of particles thus obtained have sizes ranging from 10 to 100μm with a mean value of 46.5μm. Hot extrusion was carried out for Mg2Si0.4Sn0.6 composition at 450-480°C and 500-550°C applying pressures up to 1GPa. Addition of 4 vol% of nanometre sized MoS2 particles in the powder facilitates material densification. Specimens extruded at temperature range of 500-550°C demonstrate 99.7% of the theoretical density. The TE properties were studied by the Harman method between 300 K and 700 K. The Seebeck coefficient varies from -150 to -220μV/K, passing through a maximum value at 450K. The electrical conductivity of 88 S/cm at 300 K increases with temperature showing a typical behavior for under-doped TE material. The ZT value reaches a maximum of 0.08 at 620 K. Our work shows that implementation of a gas atomization process for powder production combined with hot extrusion, as a densification and sintering method, can be successfully implemented for bulk Mg2Si1-xSnx manufacturing. This approach to obtain Mg2Si based alloys, once optimized, has a great potential for large scale waste heat recovery applications.