The Optimization Studies On Thermoelectric Properties Of Ge_0.8Pb_0.2Te And Sn_0.8Pb_0.2Te By Mn Doping And Pb_0.96Na_0.04Te_0.8Se_0.2 With Minor Sm₂Se₃

Thermoelectric（TE） materials are functional materials, which can provide a direct conversion between thermal and electrical energy. It has a potential application in power generation from waste heat in industrial or automobile engine, and in thermoelectric solid state cooling, which will play an important role for the reducuion of energy in the future. At present, the relative low conversion efficiency prevents its further commercial application. Element doping and introducing a secondary phase are the effective ways to improve the thermoelectric properties. In this paper, the series alloys of（Ge₀.8Pb₀.2）_1-xMn_xTe,（Ge₀.736Pb₀.184Mn₀.08）Te_1-xSex,（Sn₀.8Pb₀.2）_1-xMn_xTe and（Pb₀.96Na₀.04Te₀.8Se₀.2）_1-x（Sm₂Se₃）x were prepared by melting, quenching and spark plasma sintering（SPS） techniques, and the effects of the Mn and Se doping or the Sm₂Se₃ second phase on the phases, microstructure and thermoelectric properties of their parent alloys were investigated. The thermoelectric properties of the alloys were optimized.（1） The investigation for the series of（Ge₀.8Pb₀.2）_1-xMn_xTe alloys shows that all alloys are composites containing minor Na Cl-type structure Pb Te-based and major Ge Te-based phases with the low temperature rhombohedral and the high temperature cubic structures. The layer-like microstructures with Ge Te and Pb Te are formed from the Spinodal decomposition, which benefits to reduce its thermal conductivity. The electrical resistivity and the Seebeck coefficient of the alloy increase while the thermal conductivity decreases significantly as Mn content x increases. In the result, the thermoelectric properties were improved. The maximum ZT of 1.3 was obtained in the sample（Ge₀.8Pb₀.2）₀.9Mn₀.1Te at 723 K, which is much higher than that（0.89） of Ge₀.8Pb₀.2Te alloy.（2） The investigation for the series of Ge₀.736Pb₀.184Mn₀.08Te_1-x Sex alloys shows that all alloys are composites containing major rhombohedral Ge Te-based phase, minor Na Cl-type structure Pb Te-based and the minor Na Cl-type structure Mn Se phases. The substitution of the Se atoms for Te atoms increases the solubility of Pb in Ge Te. With low Se content x（x=0.1）, the electrical resistivity of alloy decreases while Seebeck coefficient and thermal conductivity increase. As the Se content further increases, the electrical resistivity and the Seebeck coefficient of the alloy increase while the thermal conductivity decreases. The maximum ZT of 1.07 was obtained in the sample Ge₀.736Pb₀.184Mn₀.08Te₀.5Se₀.5 at 673 K.（3） The investigation for the series of（Sn₀.8Pb₀.2）_1-x Mnx Te alloys shows that the single Sn Te-based solid solution was formed in the alloys with lower Mn content（x < 0.06）. A small amount of the second phase block-like MnTe₂ presented in the alloys with x3 0.06, while a small amount of the second phase needle-like Mn Te appeared in the alloys with x3 0.12. Mn doping in the alloy decreases its carrier concentration but increases its Seebeck coefficient significantly and thus increases its power factor. The reduction of the carrier concentration and the presence of the second phase MnTe₂ and Mn Te lead to the reduction of thermal conductivity significantly. As the results, the thermoelectric properties were improved. The maximum ZT of 0.651 was obtained in the sample（Sn₀.8Pb₀.2）₀.88Mn₀.12 Te at 723 K, which is much higher than that（0.29） of its parent alloy Sn₀.8Pb₀.2Te.（4） The investigation for the series of（Pb₀.96Na₀.04Te₀.8Se₀.2）_1-x（Sm₂Se₃）x alloys shows that all alloys consist of Pb Te-based solid solution, and minor Sm₂Se₃ and Sm phases. The electrical resistivities and Seebeck coefficients for the samples increase slightly with increasing Sm₂Se₃ content x up to 0.025, whlie increase obviously with Sm₂Se₃ content further increasing. The solute atoms in Pb Te-based solid solution, the minor phases Sm₂Se₃ and Sm and their phase boundaries enhance the phonon scattering effectively and reduce the thermal conductivity significantly, the thermoelectric property of the parent alloy was improved. The maximum ZT of 1.5 was obtained in the sample（Pb₀.96Na₀.04Te₀.8Se₀.2）₀.975（Sm₂Se₃）0.025 at 723 K, which is much higher than 1.07 of the parent alloy Pb₀.96Na₀.04Te₀.8Se₀.2.