THERMOELECTRIC PROPERTIES OF MAGNESIUM SILICIDE-STANNIDE SOLID SOLUTION OF VARIOUS PHASE STRUCTURES

Abstract

The thermoelectric properties of Mg2vSnvxSiv1-x solid solution with varying phase structure were investigated. Through the modeling of lattice thermal conductivity poor degree of mixing between Sn and Si with segregated phases as much as 60% of the entire volume was deduced for the samples prepared by non-fully optimized solid state reaction. As per the proposed direction from the modeling, samples with better phase homogeneity were pursued. With the aid of successful sample preparation by high energy ball milling for heavily doped Mgv2Sn and Mgv2Si whose carrier concentrations are over 10^20 cm^-3 systematic studies with Sn:Si ratios from 3:7 to 7:3 were conducted for exploring the thermodynamic evolution of the constituent phases by various duration of annealing from 0 to 50 hours. Although the phase segregation into Sn-rich and Si-rich phases was observed after longer annealing, the lattice thermal conductivity was maintained as low as that of the idealized solid solution structure. Lastly the existence of the convergence between two valleys in conduction band at the certain Sn:Si ratio (~7:3) was demonstrated by controlling the degree of mixing. The maximum power factor was enhanced from 3.4 mW/m-K^2 to 4.3 mW/m-K^2 as the degree of mixing was improved. It was also found that the valley convergence accompanies the effective mass broadening so as to offset the improvement in power factor due to doubled number of valleys. The simulated effective mass enhancement by two-parabolic valley model demonstrated the strictly narrow range for Sn:Si ratio to obtain the effective convergence, suggesting the importance of best degree of mixing and phase homogeneity.