Achieving thermal conductivity reduction of phononic crystals through deepening the transport theory of phonon waves

控制半导体和绝缘体（无论是晶体还是非晶体）的热导率的能力对于确定在诸如热电、热障和热绝缘体等广泛应用中使用的材料的功能性和可用性至关重要。申请人已经研究了与降低具有声子结构的材料的热导率的方式相关的相干和非相干传输的基本机制。申请人表明，由于Akhiezer阻尼，相干效应在晶体和非晶声子材料中都不显著，这显著降低了相干声子的弛豫时间。申请人沿着他的合作者表明，局部弹性模量软化不仅可以通过降低声子群速度来显著降低晶体声子材料的热导率，而且还可以通过在纳米颗粒界面处的声子透射率的大限制来在纳米复合材料中实现巨大的热降低方面发挥重要作用。申请人表明，非晶声子材料和超晶格中的边界不仅可以强烈地散射类声子传播子，而且还表现出扩散子的极端散射，使得热导率低于非晶的扩散极限。这些工作加深了对晶体和非晶声子材料中相干和非相干输运的物理理解，并为工业应用提供了操纵热输运的新方法。

The ability to control the thermal conductivity of semiconductors and insulators, either crystal or amorphous, is of crucial importance to determine the functionality and usability of the material used in a wide range of applications such as thermoelectrics, thermal barriers and thermal insulators. The applicant has investigated fundamental mechanisms of coherent and incoherent transport related to the ways of reducing the thermal conductivity of the material with phononic structures. The applicant showed that coherent effect is not significant in both crystal and amorphous phononic materials due to Akhiezer damping, which significantly reduces relaxation time of coherent phonons. The applicant along with his collaborators showed that local elastic modulus softening can not only significantly reduce the thermal conductivity of crystal phononic materials by reducing phonon group velocity, but also play an important role to achieve huge thermal reductions in nanocomposite via large limitation of phonon transmittance at the interfaces of the nanograins. The applicant showed that boundaries in amorphous phononic materials and superlattice can not only strongly scatter phonon-like propagons, but also exhibit extremely scattering of diffusons such that the thermal conductivity is below the diffusive limit of amorphous. These works have deepened the physical understanding of both coherent and incoherent transport in crystal and amorphous phononic materials, and have provided novel approaches for manipulating thermal transport for industrial applications.