Investigating Phonon Transport in Silicon-Silica Aerogel Nanocomposites via Molecular Dynamics Simulations

This study investigates the thermal transport characteristics of silicon-silica aerogel nanocomposites via Reverse Non-Equilibrium Molecular Dynamics (RNEMD) simulations, addressing critical demands in thermoelectric efficiency. The findings reveal a substantial reduction in lattice thermal conductivity for Si nanowires (NWs) embedded in highly porous silica aerogels. Specifically, simulations indicate a decrease in thermal conductivity, κ_nw, from 2.16 to 1.8 W/m.K, a 16.7% reduction, when smooth NWs are embedded in a silica aerogel with 69.09% porosity. This observed reduction diverges from predictions by the Diffuse

Mismatch Model (DMM), suggesting a need for refined theoretical frameworks. To further elucidate the impact of system geometry on phonon dynamics, thermal transport properties in Lennard-Jones (LJ) solid analogs, focusing on argon crystal/aerogel nanocomposites, were examined. Utilizing the LJ interatomic potential alongside the Green-Kubo formalism, this analysis underscores the efficiency of aerogel-embedded NWs in reducing thermal conductivity

through enhanced phonon scatterings. Phonon lifetimes (τ_ph) were examined across a range of temperatures and porosities to unveil the influence of structural modifications on phonon dynamics. Focusing on the low-frequency regime (0.2 to 0.78 THz) at 10K as a representative example, the analysis reveals pronounced variations in τ across different argon-based structures. In bulk argon, phonon lifetimes decrease from 170 to 21 ps within this frequency band. A similar trend appears across configurations, with τ_ph decreasing from 92.5 to 9.47 ps in nanowires, from 58.8 to 9.72 ps in aerogels with 64.5% porosity, and from 35.2 to 8.18 ps in composites. Narrowing to the upper frequency limit (0.78 THz), τ_ph decreases by ∼14% from nanowire to composite, dropping from 9.47 to 8.18 ps. This trend is consistently observed across temperatures < 10K. The integration of Normal Mode Analysis (NMA) further illuminates phonon mode interactions, providing insights into frequency-dependent scattering in nanocomposites. This research offers foundational insights into the structural influences on thermal transport, with significant implications for advancing thermoelectric applications.