Enhancing the zT Value of Bi-Doped Mg₂Si_0.6Sn_0.4 Materials through Reduction of Bipolar Thermal Conductivity

Solid solutions of Mg 2 Si 0.6 Sn 0.4– x Bi x with 0 ≤ x ≤ 0.03 were prepared by a one-step synthesis and consolidation method, using MgH 2 as a starting material. The thermoelectric properties of these samples were evaluated over the temperature range 300–775 K. Figure of merit, zT, values were determined over this range for all compositions and found to increase with temperature reaching a value of 1.36 at 775 K for samples with x = 0.02. Examination of the components of the total thermal conductivity showed that the bipolar thermal conductivity is suppressed by an increase in the band gap, resulting from solid solution formation, and by low minority carrier mobility. The suppression of the bipolar thermal conductivity is believed to be the consequence of charged grain boundaries.