Examining the Role of Atomic Scale Heterogeneity on the Thermal Conductivity of Transparent, Thermally Insulating, Mesoporous Silica-Titania Thin Films

Abstract

This paper examines the effect of compositional heterogeneity on the thermal conductivity of transparent, mesoporous silica-titania composites that contain either 10 or 20 mol % titania. The relative hydrolysis rates of the silica and titania precursors were modified to control their compositional heterogeneity, while the ratio of polymer to inorganic precursors (silica + titania) was varied to control the porosity of the films. All films were optically transparent at thicknesses up to 1 mu m with transmittance above 90% and haze below 5% at visible wavelengths. It was found that the heterogeneity of the titania species in the 10 mol % titania samples could be easily tailored from highly dispersed titania to a composition with heterogeneous silica-rich and titania-rich domains. By contrast, samples with 20 mol % titania always showed a heterogeneous titania distribution. The results show that mesoporous films with more homogeneously distributed titania had a lower thermal conductivity at all porosities, likely due to increases in propagon and diffuson scattering as a result of the increased number density of titania heteroatom scattering centers. These results increase our understanding of heat carrier propagation in amorphous materials and add to the design rules for creating amorphous, optically clear, low thermal conductivity materials.