Enhanced Thermoelectric Performance in Mg2Si by Functionalized Co-Doping

Mg2Si specimens doped with Bi, Al, and Se are synthesized via induction melting followed by rapid compaction using an induction assisted hot-uniaxial press. Phase formation, dopant solubility, and microstructure are studied using X-Ray diffraction and scanning electron microscopy. Results indicate the presence of the dopants to varying extent in the Mg2Si matrix, which is reflected in the lattice parameter and charge carrier concentration values. Temperature dependent thermoelectric properties are measured between room temperature and 673 K. Se is observed to enhance the density of states (DOS) effective mass (mD*), while Al increased the carrier mobility (mu). Best thermoelectric performance are obtained for co-doped Bi,Se and Bi,Al compositions with approximate to 20% increase in engineering figure of merit, ZTeng (Delta T = 350 K) compared to only Bi doped specimen. This enhancement can be explained from the observed electronic (increased mD* due to Se incorporation) and microstructural (mu increase in Al containing compositions due to lower grain boundary magnesium oxide layer)characteristics of the specimens. Also, grain boundary accumulation (of Bi) is observed in Bi doped samples which results in a lowering of the lattice thermal conductivity, kappa L.