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Since solid-state heat transport in a highly porous nanocomposite stronglydepends on the thermal boundary conductance (TBC) between constituentnanomaterials, further suppression of the TBC is important for improvingperformance of thermal insulators. Here, targeting a nanocomposite fabricatedby stamping fumed silica nanoparticles, we perform a wide variety of surfacefunctionalization on fumed silica nanoparticles by silane coupling method andinvestigate the impact on the thermal conductivity (Km). The Km of the silicananocomposite is approximately 20 and 9 mW/m/K under atmospheric and vacuumcondition at the material density of 0.2 g/cm3 without surfacefunctionalization, respectively, and the experimental results indicate that theKm can be modulated depending on the chemical structure of molecules. Thesurface modification with a linear alkyl chain of optimal length significantlysuppresses Km by approximately 30%, and the suppression can be further enhancedto approximately 50% with the infrared opacifier. The magnitude of suppressionwas found to sensitively depend on the length of terminal chain. The magnitudeis also related to the number of reactive silanol groups in the chemicalstructure, where the surface modification with fluorocarbon gives the largestsuppression. The surface hydrophobization merits thermal insulation throughsignificant suppression of the TBC, presumably by reducing the water moleculesthat otherwise would serve as heat conduction channels at the interface. On theother hand, when the chain length is long, the suppression is counteracted bythe enhanced phonon transmission through the silane coupling molecules thatgrows with the chain length. This is supported by the analytical model andpresent simulation results, leading to predict the optimal chemical structurefor better thermal insulation.

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