Ultra-low thermal conductivity of two-dimensional phononic crystals in the incoherent regime

• Published in: npj computational materials Vol. 4; no. 1; pp. 1 - 7
• Main Authors: Xie, Guofeng, Ju, Zhifang, Zhou, Kuikui, Wei, Xiaolin, Guo, Zhixin, Cai, Yongqing, Zhang, Gang
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.04.2018; Nature Publishing Group
• Subjects: 639/301; 639/925; Boltzmann transport equation; Characterization and Evaluation of Materials; Chemistry and Materials Science; Computational Intelligence; Crystals; Defects; Electrical properties; Electrical resistivity; Frequency dependence; Heat conductivity; Heat transfer; High temperature environments; Materials Science; Mathematical and Computational Engineering; Mathematical and Computational Physics; Mathematical Modeling and Industrial Mathematics; Nanostructure; Perturbation theory; Phonons; Scattering; Silicon; Skin; Theoretical; Thermal conductivity; Thermoelectricity
• ISSN: 2057-3960; 2057-3960
• DOI: 10.1038/s41524-018-0076-9

Two-dimensional silicon phononic crystals have attracted extensive research interest for thermoelectric applications due to their reproducible low thermal conductivity and sufficiently good electrical properties. For thermoelectric devices in high-temperature environment, the coherent phonon interference is strongly suppressed; therefore phonon transport in the incoherent regime is critically important for manipulating their thermal conductivity. On the basis of perturbation theory, we present herein a novel phonon scattering process from the perspective of bond order imperfections in the surface skin of nanostructures. We incorporate this strongly frequency-dependent scattering rate into the phonon Boltzmann transport equation and reproduce the ultra low thermal conductivity of holey silicon nanostructures. We reveal that the remarkable reduction of thermal conductivity originates not only from the impediment of low-frequency phonons by normal boundary scattering, but also from the severe suppression of high-frequency phonons by surface bond order imperfections scattering. Our theory not only reveals the role of the holey surface on the phonon transport, but also provide a computation tool for thermal conductivity modification in nanostructures through surface engineering. Phononic crystals: Surface scattering A phonon scattering process on the surface of phononic crystals can explain their ultra-low thermal conductivity. Two-dimensional silicon phononic crystals are promising for thermoelectric applications, as the periodic arrangement of holes allows for significant reduction of their thermal conductivity. A team from Hunan University of Science and Technology and Xiangtan Universities in China, and the Institute of High Performance Computing in Singapore, manage to model the values reported experimentally by incorporating a phonon scattering mechanism, rooted to the bond imperfection on the surface of the nanostructure. As the bonds towards the surface grow shorter and therefore stronger, they perturb the local potential, and suppress the high-frequency phonons. Low-frequency phonons, on the other hand are suppressed by normal boundary scattering. The authors conclude that these two actions lead to the ultra-low thermal conductivity values, and predict that further reduction can be achieved by roughening the hole walls in the phononic crystal.