Record Low Thermal Conductivity of Polycrystalline Si Nanowire: Breaking the Casimir Limit by Severe Suppression of Propagons

Thermoelectrics offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand. Nanoengineering existing-low-dimensional 'thermoelectric materials pertaining to, realizing fundarrientally low thermal conductivity has emerged as an efficient route to achieve high energy conversion., performance for advanced thermoelectrics. In this paper,.,by performing nonequilibrium and Green Kubo equilibrium molecular dynamics simulations We report that the thermal conductivity of Si nanowires {NWs) in polycrystalline form can reach record low value substantially below, the Casimir limit, a theory of diffusive boundary limit that regards the direction-averaged mean free' path is limited, by the characteristic size of the nanostructures. The astonishingly- low thermal conductivity of polycrystalline Si NW is 260 and 77 times lower-with respect to that of bulk Si and pristine Si NW, respectively, and is even only about one-third of the value of the purely arnorphous Si NW at room temperature. By examining the Mode level phonon behaviors including phonon group velocities, lifetime, and so forth, we identify the mechanism of breaking the Casimir limit as the strong localization of the middle and: high frequency phonon modes, which leads to-a prominent decrease of-effective mean free path of the heat carriers including both propagons and diffusons. The contribution of the propagons to the overall thermal transport is further quantitatively characterized and is found to be dramatically suppressed' in polycrystalline Si NW form a8 compared with bulk Si, perfectr:Si NW, and pure amorphous Si NW. Consequently, the diffusons, which transport the heat through overlap With other vibrations, carry the majority of the heat in polycrystalline Si NVVs. We also proposed approach of introducing disorder in the polycrystalline Si NWs that could eradicate the contribution of propagons to, achieve-,an even lower thermal conductivity than that ever thought possible. Our investigation proVides a deep insight into the thermal transport in polycrystalline' NWs and offers a promising strategy to construct a new kind- of semiconducting thermoelectric NW with high fipre of Merit.