Exploring the Effect of Porous Structure on Thermal Conductivity in Templated Mesoporous Silica Films

This work elucidates the effect of porous structure on thermal conductivity of mesoporous amorphous silica. Sol–gel and nanoparticle-based mesoporous amorphous silica thin films were synthesized by evaporation-induced self-assembly using either tetraethyl orthosilicate or premade silica nanoparticles as the framework precursors with block copolymers Pluronic P123 or Pluronic F127 as template. The films were characterized with scanning- and transmission-electron microscopy, two-dimensional grazing-incidence small-angle X-ray scattering, ellipsometric porosimetry, and UV–vis reflectance spectroscopy. The thermal conductivity of the mesoporous films, at room temperature and in vacuum, was measured by time-domain thermoreflectance. The films were 150 to 800 nm thick with porosities ranging from 9% to 69%. Their pore diameters were between 3 and 19 nm, and their thermal conductivities varied between 0.07 and 0.66 W/m.K. The thermal conductivity decreased strongly with increasing porosity and was also affected by the structure of the silica framework (continuous or nanoparticulate) and the pore size. A simple porosity weighted effective medium approximation was used to explain the observed trend in thermal conductivity. These results give new insight into thermal transport in nanostructured materials, and suggest design rules of the nanoscale architecture to control the thermal conductivity of mesoporous materials for a wide range of applications.