Transparent, thermally insulating coatings can be used for energy saving architectural glass or in solar thermal applications. By increasing phonon scattering in a material, its thermal conductivity can be lowered. The most common way that this is done is by adding porosity to a material which increases interfacial scattering and reduces the total material density. These ideas are used in highly porous silica aerogels with ultra-low thermal conductivities. Unfortunately, these materials significantly scatter light making them unsuitable for applications that require high optical transparency. In this work we will build on these ideas in silica based films by using porosity to reduce thermal conductivity. Importantly, we keep all structural features less than 20 nm to maintain high optical transparency. Using a polymer templating and removal process, highly transparent, mesoporous thin films of SiO2 or SiO2:TiO2 with porosities up to 70% were studied to identify structural parameters that minimize thermal conductivity. Overall, porosity was the dominant parameter affecting the thermal conductivity, but the type of framework precursor, wall thickness between pores and heteroatom incorporation in the mesoporous films also significantly affected the thermal conductivity. Films made from sol-gel or nanoparticle-based SiO2 precursors had similar thermal conductivities when the wall thicknesses were comparable. For films made of sol-gel precursors, there was a strong dependence of the thermal conductivity on the overall wall thickness, while for the nanoparticle-based system, nanoparticle size dominated the thermal conductivity. The addition of TiO2 to the mesoporous SiO2 system decreased the thermal conductivity. Overall we now have a general understanding of how tune the thermal conductivity of mesoporous SiO2 films while retaining the high transparency of the system.

Examining the effect of nanoscale architecture on the thermal conductivity of mesoporous silica thin films

2019 Fall National Meeting

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