Thermal Transport Properties Of Two-dimensional Silicon Phononic Crystal

Worldwide energy crisis has become one of serious problems to the contemporary people.The emergence of thermoelectric materials which can convert waste heat directly into useful electrical energy provides the possibility of alleviating the energy crisis.The efficiency of the energy conversion for power generation depends on the thermoelectric figure of merit(ZT= S2?T/k).The higher ZT,the better thermoelectric performance.An effective approach to improve ZT is reducing the thermal conductivity.Two-dimensional silicon phononic crystals(2D Si PnCs)have attracted extensive research interest for thermoelectric applications due to their reproducible low thermal conductivity and sufficiently good electrical properties.In this work,we investigate the thermal transport properties of 2D Si PnCs by the phonon Boltzmann transport equation within the relaxation time approximation.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.However,existing incoherent phonon models largely overestimate the thermal conductivity of 2D Si PnCs,hence the ultra-low thermal conductivity is still not well understood.We manage to model the values reported experimentally by incorporating a phonon scattering mechanism,rooted to the bond order imperfection on the surface of the nanostructure.We incorporate this strongly frequency-dependent scattering rate into the phonon Boltzmann transport equation and reproduce the ultra low thermal conductivity of 2D Si PnCs.We reveal that the remarkable reduction of thermal conductivity originates not only from the impediment of low-frequency phonons by classical 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.