  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.
