Hefei Research Institute makes progress in concise characterization of materials thermal conductivity

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[ Instrument Network Instrument R & D ] Recently, Zhang Yongsheng, a researcher at the Institute of Material Physics, Institute of Solid State Physics, Chinese Academy of Sciences, has made new progress in the study of the effect of rattling effect on thermal conductivity in half-Heusler materials. The results were published in Physical Review B under the title of Characterization of rattling in relation to thermal conductivity: Ordered half-Heusler semiconductors.
The conversion efficiency of thermoelectric materials can be measured by the dimensionless ZT value, ZT = S2sT / κ, where S, s, and κ represent the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. In the research of thermoelectric materials, it is very important to understand and analyze the factors that affect the thermal properties of semiconductors, which can provide a theoretical basis for reducing the lattice thermal conductivity of materials or developing materials with low lattice thermal conductivity. The phononic glass-electronic crystal (PGEC) concept states that high-performance thermoelectric materials should have as low thermal conductivity as glass and as high as crystal; this requires materials to have independent electrical and thermal transmission Run the grid.
In general, frame materials containing rattling (near free vibration) atoms will exhibit similar PGEC characteristics as described above. Around this concept, many complex new compounds have been studied, such as cage clathrates and filled skutter materials. In recent years, Half-Heusler (HH) compounds have attracted much attention due to their excellent electrical properties, strong mechanical properties, and good thermodynamic stability. From the perspective of crystal structure, it contains three nested face-centered cubic substructures, which can be regarded as a filled sphalerite crystal structure. If the bond between one atom and its neighbors is weak, it is easy to vibrate in the face-centered cubic lattice, showing rattling behavior, which can effectively scatter phonons and reduce thermal conductivity. In this unique structure, studying the effect of rattling vibration behavior on thermal conductivity is of great significance for the design and search of HH materials with low thermal conductivity.
To this end, based on phonon calculations by density functional theory methods, the research team of Zhang Yongsheng's group studied the effect of rattling vibration behavior on the thermal conductivity in half-Heusler materials. It is found that the thermal conductivity has a positive correlation not only with the average phonon frequency of the material, but also with the average effective spring constant between the atoms. Further, they constructed two parameters that can concisely measure the thermal conductivity: the ratio of the minimum average phonon angular frequency to the maximum phonon angular frequency (ωmin / ωmax) and the minimum effective spring constant and the maximum effective spring constant of the three atoms. Ratio (kmin / kmax). The calculation results show that the smaller the value of ωmin / ωmax, the weaker the bonding between atoms; the smaller the value of kmin / kmax, the lower the rattling frequency. This calculation method can easily and effectively identify materials with low lattice thermal conductivity, and is helpful for understanding the relationship between rattling vibrations and low lattice thermal conductivity in materials, providing a screen for potential low-thermal-conductivity materials. New ideas.
The above research was supported by the National Natural Science Foundation of China and the Hefei Branch of the Chinese Academy of Sciences Supercomputing Center.

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