Recently, Professor He Jiaqing, Department of Physics, Southern University of Science and Technology, published an academic paper Extraordinary Thermoelect in Advanced Materials (19.791) and Energy & Environmental Science (29.518). Ric Performance Realized in n-type PbTe through Multiphase Nanostructure Engineering and Large Enhancement of Thermoelectric Properties in n-type PbTe via dual-site point defects. The first paper is co-authored by visiting scholar Zhang Jian and assistant professor Wu Di, and the only correspondent is He Jiaqing. The second paper is co-authored by postdoctoral Fu Liangwei and Yin Meijie, and co-authored by assistant professors Huang Li and He Jiaqing in the Department of Physics. The first unit of both papers is Southern University of Science and Technology.

Energy recovery and effective utilization is an important way to deal with the current energy crisis. Thermoelectric materials themselves can realize direct conversion of electricity and heat in solid state. Thermoelectric devices made of thermoelectric materials can be used in thermoelectric power generation, such as automotive exhaust heat recovery, industrial waste heat conversion into electric energy, and thermoelectric refrigeration, such as efficient refrigeration of microchips and precision instruments. The practical application of thermoelectric devices also has the outstanding advantages of pollution-free, noise-free and environment-friendly. P-type and n-type thermoelectric semiconductor materials are the necessary components of thermoelectric devices, and their performances need to be matched before they can be used at the same time. The higher the matching performance of the two, the higher the thermoelectric conversion efficiency of the thermoelectric devices made from them. LeadTe compounds are widely studied as ideal high performance p-type thermoelectric materials. However, due to the lack of corresponding n-type thermoelectric materials, their commercial applications (making thermoelectric devices) are greatly limited. The research team led by Professor He Jiaqing has always regarded the lead Te thermoelectric materials and devices as one of its main research directions. Previous studies have shown that the lead Te material system as a p-type thermoelectric material has excellent performance. It not only shows a high thermoelectric value of ZT=2.3@923K (Energy Environ. Sci., 2015, 8, 2056), but also in the room. The temperature range from 900K to 900K has a higher average thermoelectric merit value ZTave=1.56, so its theoretical power generation efficiency can reach 20.7% (Nat. Commun. 2014, 5, 4515). Starting from different methods and mechanisms, these two papers have made great breakthroughs in the study of n-type PbTe, greatly balancing the inferior performance of n-type PbTe compared with that of p-type material.

In the first paper, the team found that by combining InSb and controlling experimental conditions, the introduction of multiphase nanostructures into the lead Te matrix material could effectively optimize the thermal and electrical transport properties of the material system at the same time. On the one hand, the energy barrier (potential well) between the nanophase and the matrix can enhance the Seebeck coefficient through energy filtering effect; on the other hand, the introduction of multiple nanophases can enhance the phonon scattering at the interface and reduce the lattice thermal conductivity. Finally, in the n-type PbTe-4% InSb composites, a very high thermoelectric optimal value ZT=1.83 (773 K) is obtained, which is the highest value in the current n-type PbTe material system.

(a) Multiple nanophase diagrams in PbTe matrix; (b) Enhancement of power factor by single barrier and multiple barriers calculated based on single parabolic band model; (c) Thermoelectric optimal value ZT of multi-nanophase complex PbTe-x% InSb (x=3, 4, 5 and 6) varies with temperature; (d) Average thermoelectric optimal value ZTav in the temperature range of 300-723K E and theoretical generation efficiency H.

In the second paper, the team introduced metal Sb phase into n-type PbTe material, and used a small part of the second phase of SB nucleated at low temperature to enter the PbTe lattice at high temperature (773K), replacing the characteristics of two adjacent positions of Pb and Te lattices, to form a "dual-site" point defect microstructure. This structure not only adjusts the electron density of states near Fermi level of n-type PbTe material, but also improves the electrical conductivity. At the same time, additional atom-scale phonon scattering sources are introduced to reduce the lattice thermal conductivity of the material, and ultimately the overall thermoelectric performance is greatly improved. At 773K, the thermoelectric optimal value ZT reaches 1.8. The results of the previous paper are the highest records of the current n-type PbTe system.

(a) Temperature-dependent evolution of Sb nanoprecipitates in PbTe matrix. At low temperature, S B exists in the matrix material of PbTe in the form of nano-precipitates. At high temperature, a small part of Sb Nano-phases enter into the lattice of PbTe to form a "dual-site" point defect; (b) The properties of our samples are compared with the literature values of several best n-type PbQ (Q=Te, Se, S) materials. The peak value of ZT and the average value of ZTave in our samples are better than those in the literature.

This series of research improves the low thermoelectric properties of n-type materials in the lead Te material system, and makes the lead Te have great application prospects in the middle temperature region. The development and completion of these research projects have been supported by the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province and the Basic Research of Shenzhen Science and Creation Commission.