Modulation of interfacial thermal transport between fumed silica nanoparticles by surface chemical functionalization for advanced thermal insulation

TL;DR

This study investigates how surface chemical functionalization of fumed silica nanoparticles affects interfacial thermal transport, demonstrating that tailored surface modifications can significantly suppress thermal conductivity for improved insulation.

Contribution

It introduces a comprehensive analysis of how different chemical surface modifications influence thermal boundary conductance and conductivity in silica nanocomposites, guiding optimal design for thermal insulators.

Findings

Surface functionalization can modulate thermal conductivity by up to 50%.

Optimal chain length and chemical structure significantly influence thermal suppression.

Analytical models and simulations support the experimental results.

Abstract

Since solid-state heat transport in a highly porous nanocomposite strongly depends on the thermal boundary conductance (TBC) between constituent nanomaterials, further suppression of the TBC is important for improving performance of thermal insulators. Here, targeting a nanocomposite fabricated by stamping fumed silica nanoparticles, we perform a wide variety of surface functionalization on fumed silica nanoparticles by silane coupling method and investigate the impact on the thermal conductivity (Km). The Km of the silica nanocomposite is approximately 20 and 9 mW/m/K under atmospheric and vacuum condition at the material density of 0.2 g/cm3 without surface functionalization, respectively, and the experimental results indicate that the Km can be modulated depending on the chemical structure of molecules. The surface modification with a linear alkyl chain of optimal length significantly suppresses Km by approximately 30%, and the suppression can be further enhanced to approximately 50% with the infrared opacifier. The magnitude of suppression was found to sensitively depend on the length of terminal chain. The magnitude is also related to the number of reactive silanol groups in the chemical structure, where the surface modification with fluorocarbon gives the largest suppression. The surface hydrophobization merits thermal insulation through significant suppression of the TBC, presumably by reducing the water molecules that otherwise would serve as heat conduction channels at the interface. On the other hand, when the chain length is long, the suppression is counteracted by the enhanced phonon transmission through the silane coupling molecules that grows with the chain length. This is supported by the analytical model and present simulation results, leading to predict the optimal chemical structure for better thermal insulation.