Effect of magnesium and silver doping on the thermoelectric performance of cast Mg2Sn alloys

Highlights: • Studied TE properties as a function of magnesium (Mg) addition in Mg₂Sn alloys. • Inhomogeneous distribution of Sn in Mg phase lead to irreversibility in properties. • Coarse grain size and their orientations lead to inconsistent Seebeck coefficient. High quality undoped and silver (Ag) doped Mg₂Sn cast products with relatively less cracking were obtained using radio-frequency (RF) induction method under argon atmosphere. The thermoelectric properties of cast alloys were studied as a function of magnesium added up to ∼10.8 wt. % in the temperature range of 323 K (50 °C) to 723 K (450 °C). It was observed that the electrical and thermal conductivity at 323 K (50 °C) was increased by approximately 12 and 4 times as the magnesium concentration changed from 2 to 10.8 wt.%. The lattice thermal conductivity of the near stoichiometric alloy decreased by at least 2 times due to silver doping. However, such effect is faded with an increase in magnesium concentration. This was attributed to the probable increase in the electronic contribution due to metallic magnesium phase. Interestingly, the electrical (and thermal) conductivity was irreversible during the first cycle of heating and cooling. The irreversibility in the conductivity was attributed to the inhomogeneous distribution of tin (Sn) in the Mg phase prior to heating. The Seebeck coefficient of the Ag doped alloy with 10.8 wt. % Mg was ∼120 μV/K and it was reduced to 55 μV/K after the first cycle of heating. However, the decrease in the Seebeck coefficient following cooling was not systematic across all the alloys investigated here. The inability to obtain a unique Seebeck coefficient in the cast alloys at ∼323 K (50 °C) was primarily due to the coarse grain size of Mg₂Sn and Mg phases and their respective grain orientations. Consequently, a maximum ZT of 0.055 was obtained at 323 K. However, a ZT of the order of 0.015 can be readily obtained in near stoichiometric silver doped cast Mg₂Sn alloys at high temperatures ∼723 K.