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

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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.