Electronic and thermal transport properties of Mg_2Sn crystals containing finely dispersed eutectic structures

摘要

The electronic and thermal transport properties of Mg_2Sn crystals prepared by radio-frequency (RF) induction melting, were investigated in the temperature range 10-700 K to study the effect of adding excess silver to the melt on the microstructure and thermoelectric properties. As the Ag-content increases the amount of silver incorporated at doping sites tends to saturate so that the carrier concentration increases asymptotically to 6.3 × 10~(19) cm~(-3). The excess amount of silver reacts with magnesium to form a fine MgAg + Mg_2Sn eutectic structure. The temperature dependence of the carrier mobility varies from ~T~(-1.5) for the heavily doped samples in the extrinsic range to ~T~(-2.8) for the undoped sample in the intrinsic range. From electrical conductivity and Hall measurements on the undoped samples we find the energy gap to be 0.36-0.38 eV. Doped samples show broad Seebeck peaks of 150-210 μV K(-1) at T= 350-550 K. The thermal conductivity decreases sharply as the addition of Ag is increased, and reaches a minimum value of κ= 3.4 W m~(-1) K~(-1) at 450 - 500 K for the doped sample containing a uniformly distributed eutectic phase, leading to a maximum figure of merit ZT_(max) = 0.30.