Significantly enhanced thermoelectric figure of merit of p-type Mg3Sb2-based Zintl phase compounds via nanostructuring and employing high energy mechanical milling coupled with spark plasma sintering

Abstract

Several nanostructuring methods have been demonstrated to produce a variety of nanostructured materials, and these methods are well recognized as effective paradigms for improving the performance of thermoelectric materials. Among the variety of nanostructured materials, bulk nanostructured materials have been shown to be the most promising because they not only have high ZT, but they can also be fabricated in large quantities, unlike many other nanostructured materials, making them desirable for large scale industrial application. In this study, the nanostructuring paradigm is extended for the first time to the bulk Mg₃Sb₂ and Mg₃Sb_1.8Bi_0.2 Zintl phase compounds, which despite the advantages of price and abundance, so far have been disregarded for thermoelectric research due to low ZT relative to the available state-of-the-art thermoelectric materials. The nanostructuring of bulk Mg₃Sb₂ and Mg₃Sb_1.8Bi_0.2, employing high energy ball milling followed by spark plasma sintering yields a ZT of ∼0.4 and ∼0.94 at 773 K, which are 54% and 56% higher values, respectively, than their respective bulk counterparts. The enhancement in the ZT of these materials is primarily due to the significant reduction in thermal conductivity caused by phonon scattering at numerous grain boundaries of nanostructured materials. The observed decrease in the thermal conductivity of these bulk nanostructured materials is quantified using a simple model that combines the macroscopic effective medium approach (EMA) with the concepts of Kapitza resistance. The microstructural investigation of these nanostructured materials was carried out employing high resolution transmission electron microscopy (HRTEM).