Synthesis, Crystal Structure And Thermoelectric Transports Of Mn-doped PbTe And PbSe Nano-Composites

As a green type of energy-conversion technology, thermoelectric materials can generate electricity by utilizing the Seebeck effect and refrigerate by applying Peltier effect. Thermoelectrics have many potential applications, especially in waste heat recovery and electronic device cooling. However, the energy-conversion efficiency of thermoelectric devices is too low to achieve large-scale commercial applications. The lead chalcogenide Pb Te, exhibits excellent thermoelectric performance in the medium temperatures, and has been commonly used in radioisotope thermoelectric generators for space exploration missions. It is also one of the most promising thermoelectric materials for waste heat recovery. Successful strategies have been developed to improve the thermoelectric performance of Pb Te, including band convergence, resonant states, and nanostructures. The highest figure of merit ZT of Pb Te-based bulk nanocomposites has reached 2.2. Pb Se also belongs to the lead chalcogenides, which has a similar crystal structure and band structure with Pb Te. But Pb Se possesses a higher melting point and Se has a more abundant reserve than Te, making it attractive for hightemperature thermoelectric applications. Theoretical and experimental results all suggest that Mn-doping in lead chalcogenides can lower the temperature for band degeneracy, which is beneficial to Seebeck coefficient. The power factor and figure of merit are thus enhanced unless intervalley scattering is strong. Besides optimizing the transport properties, increasing the mechanical strength of thermoelectric materials is also very important in space exploration and automotive industries, where the working conditions of the brittle materials can be critical, including vibrations and thermal stresses. In this work, mechanical and thermoelectric properties of Mn-doped Pb Te and Pb Se are systematically studied.A series of Pb1-x Mnx Te:2%Na(0≤x≤0.36) nanocomposites are prepared by the melting-quenching techniques combined with hot-pressing sintering. The eutectic microcosmic characteristics, mechanical, electrical and thermal transport properties in Pb Te with higher contents of Mn are systematically studied. Discontinued particle-like precipitates Mn Te2 are found for x=0.06, and the mechanical hardness can be improved by ~42%. The highest ZT is 1.5, and the average ZT is 1.0, higher than the average ZT 0.8 for x=0, mainly due to the band degeneracy and enhanced phonon scattering by particle-like precipitates Mn Te2. Meanwhile, both the particle-like Mn Te2 and layered Mn Te-rich precipitates are found when x=0.36, and the mechanical hardness can be improved by ~71%. The lattice thermal conductivity is abnormally increased for 0.12≤x≤0.36. The phenomena may be related to the existence of lamellar Mn Te-rich precipitates, which can act as a continuum for phonon transport. However, the average ZT is still between 0.72-0.86, which is quite close to x=0. Thus, the thermoelectric performance is less influenced by these Mn Te-rich precipitates, but the mechanical hardness can be drastically improved.A series of Pb1-x Mnx Se:2%Na(0≤x≤0.12) nanocomposites are also prepared by using melting-quenching and hot-pressing methods. Effects of Mn doping on the microstructures, mechanical and thermoelectric properties of Pb Se are systematically studied. Similar to Mn-doped Pb Te, smaller Mn contents in Pb Se can optimize the figure of merit ZT due to the band convergence and additional phonon scattering by particle-like Mn Se precipitates. As a result, the highest figure of merit ZT is 0.52 for x=0.02. The Seebeck coefficient is saturated and the lattice thermal conductivity is abnormally increased when 0.04≤x≤0.12, which might be attributed to the existence of lamellar Mn Se precipitates. In fact, the anomaly of thermal conductivity has been also found in Pb Te-Mn Te system. The thermoelectric performance for 0.04≤x≤0.12 is still better than x=0. The hardness of pristine Pb Se can be improved by ~16.6% and ~51.6% for x=0.02 and x=0.06. In summary, the thermoelectric performance and mechanical hardness of Pb Se can be both improved by Mn-doping. Through further adjusting the Na content from 2% to 0.7%, the carrier concentration can be optimized, which contributes to higher Seebeck coefficient and power factor. Finally, a figure of merit ZT of 0.65 has been achieved in the Pb Se-Mn Se nanocomposite.