Composition Optimization And Transport Properties Of Mg₂（Si,Sn） Based Thermoelectric Materials

In thermoelectric (TE) materials, the application of a temperature gradient generates a voltage, and vice versa. By exploiting the coupling between thermal and electrical properties, devices can be made that carry heat from a cold side to a hot side (refrigeration) or that generate electricity from heat flows. The devices do not use any moving parts or environmentally harmful fluids. Because of their high reliability and simplicity, thermoelectrics are used extensively in fields such as space power generation and a variety of cooling applications. Mg2BⅣ (BⅣ=Si, Ge, Sn) based medium temperature materials, which are made of widely abundant, cheap, light and moreover non-toxic elements, have been identified as a promising advanced thermoelectric material in the medium temperature range. It is difficult to fabricate homogeneous and stoichiometric Mg2BⅣ solid solutions, due to the large melting point difference between constituent elements, and the high saturated vapor pressure of magnesium. This makes the systematic investigation problematical. In this paper, B2O3flux method and Ta-tube weld melting method were adopted and stoichiometric products were successfully synthesized. The thermoelectric properties of Mg2(Si,Sn) solid solutions were investigated systematically. La3Te4, with low thermal conductivity and controllable carrier concentration, have been studied over several decades as potential high-temperature thermoelectric materials. In this paper, a melting, ball milling and hot-pressing method was performed to study the effect of La site and Te site substitution on the thermoelectric properties of LasTe4materials. The conclusions are listed below:1. B2O3flux method was explored to synthesize undoped Mg2Si1-xASnx solid solutions and the thermoelectric properties were investigated thoroughly. For undoped Mg2Si1-xSnx, the miscibility gap was confirmed as0.2-0.4and the particular Si/Sn ratios with the highest ZTs were located at Si/Sn=0.5/0.5and0.4/0.6. Ta-tube weld melting method was utilized to prepare Mg2Si1-xSnx solid solutions and Sb doped Mg2Ge. The electron mobility of Mg2Ge is much higher than that of Mg2Si and Mg2Sn. In this paper, Mg and Sb were adopted to regulate the carrier concentration of Mg2Ge1-xSbx, and a ZT of0.2was obtained. Besides, Mg2-xCaxSi thermoelectric materials have been prepared by solid state reaction and hot pressing techniques. The influence of Ca substitution on the thermoelectric properties has been investigated in details. Because of the electro negativity and ionic radius difference between Ca and Mg, the substitution of Ca for Mg reduces the thermal conductivity obviously, and improves the thermoelectric properties.2. For Sb-doped Mg2Si1-xSnx, with a preset Sb doping amount, the carrier concentration was roughly fixed. While the Si/Sn ratio barely changes the lattice thermal conductivity, the energy band structure variation was the crux to trigger the alternation of thermoelectric properties. With the increase of Sn content, the reduced Fermi level becomes smaller, and the carrier effective mass becomes bigger. Samples with the compositions of Mg2Si0.5Sn0.5-0.13Sb0.013and Mg2Si0.3Sn0.7-0.13Sb0.013boast the highest ZT of～1.0at770K.3. The Si/Sn=1ratio with the lowest lattice thermal conductivity was chosen as an object of study. The effect of Sb/Bi doping and Zn substitution was investigated. As an effective electron donor, Sb can manipulate the carrier concentration of Mg2.2Sio.5Sn0.5-xSbx successfully. The ZT of the Mg2.2Si0.5Sn0.5-0.013Sb0.013sample reaches～1.0which is comparable with the commercial PbTe.The effect of Zn substitution on the thermoelectric properties of Mg2Si0.5Sn0.5was explored. While only Zn substitution cannot change the properties of Mg2Si0.5Sn0.5much. But Zn and Sb concurrence enhanced the electrical conductivity enormously in Mg2-yZnySi0.5Sn0.5-0.01Sb0.01.With a favorable carrier concentration fixed by Sb doping, enhanced electron mobility by Zn substitution, and a suppressed lattice thermal conductivity by alloy scattering, the maximum ZT of Mg0.18Zn0.02Si0.5Sn0.49Sb0.01reaches-1.0.4. La3Te4compounds were successfully synthesized by the melting, ball milling and hot-pressing method. The effect of different substitution agent was examined. A peculiar phenomenon was observed in La3Te4-xPbx samples. The microstructure and the effect of Pb dopant on thermoelectric properties were systematically investigated. A small amount of Pb substitution for Te in La3Te4reduced the electrical conductivity, but the impurity phase introduced by more Pb significantly enhanced the electrical conductivity. Randomly sapced pores assures low lattice thermal conductivity which is even close to the theoretical minimum value.