Thick Transparent Nanoparticle-Based Mesoporous Silica Monolithic Slabs for Thermally Insulating Window Materials

This paper presents a novel template-free water-based sol gel method to synthesize thick transparent and thermally insulating mesoporous silica monolithic slabs by gelation and drying of a colloidal suspension of silica nanoparticles under ambient conditions. For the first time, mesoporous silica slabs were synthesized on perfluorocarbon liquid substrates to reduce adhesion and enable the gels to shrink freely during aging and drying without incurring significant stress that could cause fracture. The free-standing nanoparticle-based mesoporous silica slabs were disks or squares, with thickness between 1 and 6 mm and porosity around 50%. The slabs had high transmittance and low haze in the visible spectrum due to small nanoparticles (6-12 nm) and pore size (<10 nm), narrow pore size distribution, and optically smooth surfaces (roughness <15 nm). The slabs' effective thermal conductivity of 104-160 mW m(-1) K-1 at room temperature was smaller than that of other mesoporous silicas with similar or even larger porosity reported in the literature. This was attributed to the slabs fractal structure and high mass fractal dimension. The mechanical properties were similar to those of common polymers. The simple synthesis is readily scalable and offers promising materials for window solutions and solar-thermal energy conversion, for example.