Microstructure and Thermoelectric Properties of Hot Extruded Sb-Doped Mg₂Si Using MoS₂ Nano-particles as Lubricant

Abstract

Magnesium silicide is a very promising thermoelectric material for applications in the temperature range of 500–800 K, and is of particular interest for large-scale applications because its constituents are non-toxic, inexpensive and very abundant in the Earth’s crust. Although the hot extrusion (HE) method to compact powders has long been considered for thermoelectric applications because it lends itself easily to large-scale industrial applications, advances to obtain Mg₂Si by HE are still difficult to implement. We present the transformations undergone by Mg₂Si powders during the nascent HE as well as the modifications of the structural, thermal and electronic properties of the compacted solid. MoS₂ particles (2 at.%) are added to the starting Mg₂Si:Sb (0.5 at.%) powders which play the role of solid lubricant during this process at 873 K. Samples are extracted from different areas of the die along the extrusion direction and separately characterized, describing the transformations of the material through different stages of the nascent extrusion. X-ray diffraction reveals the expected structure for all samples without any significant texturing. The increase in grain size along the HE direction towards the exit has been determined from analysis of scanning electron microscopy observations. The thermoelectric properties have been characterized using the Harman method between 300 K and 700 K, giving Seebeck coefficients which vary between − 200  μV K⁻¹and − 215 μV K⁻¹ at 700 K. The thermal (λ) and electrical (σ) conductivity decrease as the sample progresses in the extrusion process, and in the case of λ can be accounted for by the increase of sample porosity. The highest figure-of-merit $\left( {\hbox{ZT}} \right)$ is to be found for the sample extracted from the exit of the die. It increases with temperature reaching a maximum value of 0.32 at 700 K, the highest temperature we could attain experimentally.