Here, using homogeneous nonequilibrium molecular dynamics (HNEMD) simulations, we report the thermal transport characteristics of thin Si nanowires (NWs) with varying size and isotope doping ratio. It is identified that crossover in the thermal conductivity (κ) of both isotope doping-free and isotope doped Si-NWs appears at critical sizes, below which κ is enlarged with decreasing size because the hydrodynamic phonon flow predominates, above which, due to the dominant phonon boundary scattering, opposite behavior is observed. With increasing isotope doping, however, the critical size in minimizing the κ is moved to small values because the phonon impurity scattering caused by isotope doping is critically involved. Moreover, there is a critical isotope doping (< 50%) in the critical size motion, originating from that, above which, the critical size no longer moves due to the persistence of hydrodynamic phonon flow. This study provides new insights into the thermal transport behaviors of quasi-1D structures.
Recently, a new phase C1’ H2 hydrate was experimentally identified. In this work, the diffusive behaviors of H2 in C1’ phase clathrate hydrate are explored using classic molecular dynamics (MD) simulations. It is revealed that the cage occupancy by H2 molecule negligibly influences the C1’ phase clathrate structure but greatly dictates the diffusion coefficient of H2 molecule. Due to the small cage size and small windows connecting the neighboring cages in C1’ phase clathrate, non-occupancy of the neighboring cages is demanded to enable the diffusion of H2 molecule that is primarily dominated by hopping mechanism. Moreover, the analysis of diffusive free energy landscape reveals lower energy barrier of H2 molecule in C1’ phase clathrate hydrate than that of other gases in conventional clathrate hydrates, and that H2 molecule travels through the windows between neighboring cages with preferential molecular orientation. This study provides critical physical insights into the diffusion behaviors of H2 in the C1’ phase clathrate hydrate, and implies that the C1’ clathrate hydrate is a promising solid structure for next-generation H2 storage.
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