The Research Of Testure And Composites For（Bi,Sb）₂Te₃ And The Electrical Transport Properties For Chalcogenide Thermoelectric Materials

In order to meet the challenge of social production and development,finding a green and renewable resources has become the focus of current scientific research.In recent years,thermoelectrics have attracted attention as new energy capable of converting between thermal power and electrical energy.The non-dimensional parameter ZT=a²sT/(k_e+k_ph)was measured by the thermoelectric conversion efficiency.So the thermoelectric materials complete with the high Seebeck coefficient（a）,the high electrical conductivity（s）and a low lattice Thermal conductivity(k_ph).Bismuth telluride alloys is the only commercially available thermoelectric materials.Due to the poor mechanical properties of zonemelting ingots,the research focus on sintered bulk,but the thermoelectric performance is low.Thermoelectric properties have anisotropy because of the laminar structure for bismuth telluride alloys,and the in-plane thermoelectric properties are higher.Therefore,increasing the in-plane orientation could be worth to hope for improved thermoelectric performance.On the other hand,by combining high-hardness thermoelectric materials,the mechanical properties of the bismuth telluride alloy could be improved,and high thermoelectric properties are retained.In addition,in the research,we found Ge Mn Te₂ and Pb Se-Ag Sb Se₂ chalcogenthermoelectric materials with peculiar electrical transport properties.We focus on the mechanisms for this special electrical transport performance,and exploit them to achieve decoupling in electrical properties.Clarifing the change law between the thermoelectric performance and these mechanisms（spin entropy and spin-orbit coupling）,and providing a new way for achieving high thermoelectric conversion efficiency.The specific work is as follows:（1）Zonemelting ingots of（Bi,Sb）₂Te₃ have poor mechanical properties,sintered bulks with better mechanical properties,but have lower thermoelectric properties.The in-plane and out-of-plane thermoelectric properties of（Bi,Sb）₂Te₃ alloy is anisotropy.Through the improved orientation in the（00l）direction,the carrier mobility increases from 156 cm²V^-1s^-1 to 195 cm²V^-1s^-1.Combined with liquid phase sintering,liquid phase Te has been extruded during hot pressing,forming dense dislocations.And the lattice thermal conductivity decreases from 0.8 Wm^-1K^-1 to 0.61 Wm^-1K^-1.The electrical and thermal transport performance is optimized simultaneously.The final peak ZT achieve 1.17 at 350 K,and the average ZT is 1.0 in the temperature range of 300-500 K,which is higher than the that of the zonemelting ingots（2）（Bi,Sb）₂Te₃ alloys have been widely used in thermoelectric cooling,but their poor mechanical properties limit the module size to reach higher cooling density.Here,we report the thermoelectric and mechanical properties of（Bi,Sb）₂Te₃–Ge_0.5Mn_0.5Te composites.Alloying with Ge_0.5Mn_0.5Te not only reduces the lattice thermal conductivity,but also improves the mechanical properties.Along with carrier concentration regulation,both excellent thermoelectric and mechanical properties have been achieved in（Bi,Sb）₂Te₃–Ge_0.5Mn_0.5Te composites.The Vickers hardness increased from 0.45 Gpa for the pristine（Bi,Sb）₂Te₃ to 0.81 Gpa for the sample with 1.0% Ge_0.5Mn_0.5Te,and a Young’s modulus of 37.06 Gpa was obtained for the one with 1% Ge_0.5Mn_0.5Te.By adjusting the carrier concentration,the final ZT value of（Bi,Sb）₂Te₃ > 1.1 at 350 K.（3）GeMnTe₂ has a high Seebeck coefficient that does not match its energy gap.We found that the magnetic properties and the Seebeck coefficient are inrrelated in the Ge Mn Te₂ system.The Seebeck coefficient fluctuates as the magnetic susceptibility changes.Here,we adjust the spin entropy in Ge Mn Te₂ by doping Se to improve the Seebeck coefficient.The Seebeck coefficient increases from 121.3 μV / K to 188.3 μV / K at 843 K.It is clear that by adjusting the spin entropy in the Ge Mn Te2 system could improve the Seebeck coefficient effectively.Bi doped regulate the carrier concentration to optimize thermoelectric performance.The final material Ge0.94Bi0.06Te1.94Se0.06 has a ZT value of 1.41 at 843 K,which is higher than the ZT value of 0.9 for the singlely doped Bi reported in literature.This work shows that the thermoelectric properties related to magnetism in some thermoelectric materials with magnetism,and they can be used to improve thermoelectric conversion efficiency.（4）Here we report the anomalous semiconductor AgSbSe₂-alloyed Pb Se,in which the spin-orbit coupling（SOC）induced band gap allows the decoupling of electrical conductivity and Seebeck coefficient.In such a SOC semiconductor,the decrease of lattice constant increases the band gap from 0.23 e V up to 0.5 e V,seemingly in contrast to the previously measured negative pressure coefficient for the band gap of Pb Se.Very different thermoelectric behaviors from the pristine Pb Se were observed in Pb SeAg Sb Se₂,and finally a high peak ZT of 1.65 at 858 K and a remarkable average ZT（300-858 K）of 0.86 were obtained.This study reveals the novel relativistic effects in Pb Se-Ag Sb Se₂ system,and its thermoelectric performance is believed to have a large potential to be further improved.