npj：二维声子晶体超低热导率——边界散射和键序缺陷散射
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二维硅声子晶体因其可重复的低热导率和优良电性能，在热电应用领域引起了广泛的研究兴趣。工作在高温环境下的热电器件，相干声子的干涉效应受到强烈抑制，因此非相干的声子输运机制对于调控热导率来说就变得尤为重要。然而，现有的非相干声子模型在很大程度上高估了二维硅声子晶体的热导率，因此超低热导率机制尚未得到揭示。
来自中国湖南科技大学、湘潭大学的谢国锋教授以及新加坡高性能计算研究所的张刚教授等研究人员，基于纳米结构表面的键序缺陷，引入声子散射过程为实验数据构建了模型。作者将具有强烈频率依赖性的声子-键序缺陷散射率纳入声子玻尔兹曼输运方程，理论计算重现了多孔硅纳米结构的超低热导率实验值，揭示热导率的显着降低，既源于经典边界散射对低频声子的阻碍，又源于表面键序缺陷散射对高频声子的严重抑制。他们的理论揭示了多孔表面对声子输运的作用机制，预测了声子晶体孔壁粗糙化降低热导率的可能性，同时也提供了纳米结构热导改性的计算工具。该文近期发表于npj Computational Materials 4: 21 (2018); doi: 10.1038/s41524-018-0076-9。英文标题与摘要如下，点击阅读原文可以自由获取论文PDF。
Ultra-low thermal conductivity of two-dimensional phononic crystals in the incoherent regime
Guofeng Xie, Zhifang Ju, Kuikui Zhou, Xiaolin Wei, Zhixin Guo, Yongqing Cai & Gang Zhang
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.
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