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Jason Larkin's PhD Projects

Vibrational Mean Free Paths and Thermal Conductivity Accumulation Functions for Amorphous Materials (in progress)

Building on the work and analysis in the project: Predicting Alloy Vibrational Mode Properties using Lattice Dynamics Calculations, Molecular Dynamics Simulations, and the Virtual Crystal Approximation, we studied the thermal conductivity of amorphous silicon (a-Si) and silica (a-SiO2) by quantifying the contributions from propagating ( phonon-like, kpr) and non-propagating (diffuson, kdiff) vibrational modes to predict the total vibrational thermal conductivity, kvib = kpr + kdiff. The propagating contribution kpr is due to phonon-like vibrational modes with long mean free paths (MFPs); energy is transported over long distances before scattering. The motivation for this study is recent experimental measurements by Regner at al. of the thermal conductivity as a function of the vibrational mean free paths.

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Because there is no equivalent virtual crystal (VC) for an amorphous solid, we studied disordered supercells of a-Si and a-SiO2 (above Figure). Because the propagating modes have long MFPs, large model structures are necessary to perform the thermal analysis. We studied large models of a-SiO2:

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and a-Si:

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including the largest model of a-Si that we are aware of (105 atoms, right Figure). Thanks to Normand Mousseau for providing these a-Si structures.

Using these model structures of a-SiO2 and a-Si, we perform the thermal conductivity analysis using lattice dynamics (LD) calculations and molecular dynamics (MD) simulations.

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Using MD simulations and the Green-Kubo (GK) method for predicting thermal conductivity (kGK), we compare with LD calculations that predict the total vibrational conductivity (kvib, above Figure) for varying supercell sizes L. The results show clearly that the propagating contribution kpr is significant for a-Si but not a-SiO2. These results help to explain the recent experimental measurements by Regner at al.