Recently, Zhang Yongsheng, a researcher in the Institute of Material Computation, Hefei Institute of Solid Physics, Chinese Academy of Sciences, has made new progress in the study of low thermal conductivity of thermoelectric materials.

Thermoelectric materials can realize the conversion between thermal energy and electric energy. Their conversion efficiency can be measured by dimensionless ZT value. ZT = S2sT/kappa, where S, S and Kappa represent Sebeck coefficient, conductivity and thermal conductivity, respectively. The higher the ZT value is, the higher the thermoelectric conversion efficiency is. At present, the reported conversion efficiency of thermoelectric materials is relatively low. Finding materials with low thermal conductivity is an important method to improve the conversion efficiency of thermoelectric materials. The relationship between thermal conductivity and non-harmonic parameters (Green's Eisen constant gamma) is as follows:

Zhang Yongsheng Team of Chinese Academy of Sciences: Progress has been made in the study of low thermal conductivity of thermoelectric materials.

The higher the gamma, the lower the thermal conductivity. Because of its low thermal conductivity and low price, ore materials have attracted wide attention of researchers. The experimental thermal conductivity values of two isomorphic ore materials, CuBiS2 and CuSbS2, differ greatly. The thermal conductivity of CuBiS2 at room temperature (0.5W/mK) is only one third of that of CuSbS2 (1.5W/mK). Physical mechanism is of great importance to the design and search of new materials.

Accordingly, Zhang Yongsheng's group studied the physical mechanism that CuBiS2 has lower thermal conductivity than CuSbS2 using density functional theory. The results show that Bi and SB atoms in CuBiS2 and CuSbS2 both contain lone pair electrons, which can lead to strong non-harmonic properties of the materials, so both materials have low thermal conductivity. For CuSbS2 materials, the solitary pair electrons at Sb sites are the main reason for the large Green's Eisen constant. In CuBiS2, besides the solitary pair electrons at Bi sites, the near-free vibration behavior of Cu atoms will further increase the Green's Eisen constant of the materials, and the synergistic effect of the solitary pair electrons and atomic vibration will lead to the relativity of CuBiS2. CuSbS2 has lower thermal conductivity. Relevant studies have shown that the synergistic effect of lone pair electrons and atomic vibration has an important influence on phonon nonharmonicity, and also provides a new idea for the search and design of new thermoelectric materials with ultra-low thermal conductivity and high efficiency.

Relevant research results were published in Physical Review B. The research was funded by the National Natural Science Foundation of China and Hefei Branch Center of the Chinese Academy of Sciences Supercomputing Center.