Probing optical and acoustic phonons in heated nano-Si/epoxy composites

TL;DR

This study investigates how optical and acoustic phonons in heated silicon nanoparticle-epoxy composites respond to different heating methods, revealing insights into thermal transport, phonon behavior, and composite thermal conductivity.

Contribution

It combines Raman and Brillouin spectroscopy with modeling to analyze phonon dynamics and thermal conductivity in Si NP-epoxy composites under local and global heating.

Findings

Local heating causes greater phonon softening and damping than global heating.

Thermal conductivity increases with Si NP loading, from 0.09 to 0.46 W/(mK).

Interfacial thermal resistance dominates heat transport, limiting composite conductivity.

Abstract

Understanding the thermal response of optical and acoustic phonons is crucial for designing functional polymer nanocomposites. We study silicon nanoparticle (Si NP)-epoxy composites using combined Raman and Brillouin spectroscopy under local(laser-induced) and global(stage-controlled) heating. Raman spectra reveal THz longitudinal optical(LO) phonon softening and spectral broadening under local heating, indicating nanoscale hot-spots and interfacial scattering. Brillouin data track GHz longitudinal acoustic(LA) phonons, showing temperature- and concentration-dependent evolution of elasticity and damping. Contrasting heating methods unravels Si loading thresholds for isolated thermal absorbers, thermal percolation, acoustic attenuation and elastic homogenization. Local heating induces greater phonon softening and damping than global heating, with this disparity amplified at higher loadings by thermal gradients and interfacial dissipation. Global heating correlates with viscoelastic relaxation, showing intensified acoustic attenuation near the glass transition. Raman thermometry coupled with finite-element opto-thermal modeling allows evaluation of thermal conductivity of the composites characterized by increase from 0.09 to 0.46 W/(mK) for 0.07 up to 2 wt% of Si NPs, respectively, outperforming SiC nanowires at 2 wt% [D. Shen et al, Sci. Rep. 7, 2606 (2017)] despite bulk conductivity of Si being more than 3 times smaller than that of SiC. However, effective heat conductivity of our nanocomposites remain far below bulk Si, confirming that interfacial thermal resistance, not filler conductivity, governs heat transport.