Transparent, mechanically strong, thermally insulating cross-linked silica aerogels for energy-efficient windows

Silica aerogels are transparent and excellent thermal insulators; however, application in energy-efficient windows has been elusive. The main reason is fragility that renders handling and processing extremely difficult. One way to overcome the fragility issue of silica aerogels is via cross-linking technology, in which a polymer is applied conformally over their entire skeletal framework. Extremely strong materials may be obtained by that approach; however, the penalty is loss of transparency. This study was designed to reach a compromise among those competing requirements. Aerogels based on co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (APTES) were prepared and were cross-linked with a triisocyanate (Desmodur N3200A). The two independent variables, the total silane concentration in the sol and the concentration of the monomer in the cross-linking bath, were varied according to a full-factorial model. The nanostructure was characterized with SEM and SAXS. The porous structure with N2 sorption. Properties of interest with respect to the intended application were the thermal conductivity, the elastic modulus, and the haze. The latter has two components, from the bulk and the surface. Cross-linking places the polymer inside secondary aggregates that behave as core–shell particles with increased scattering, due to refractive index mismatch. Surface haze can be minimized by molding with clean glass plates and careful handling. Guided by our statistical model, we demonstrate optimal samples with bulk densities in the range of 0.25–0.33 g cm−3, porosities in the range of 76–83% v/v, bulk haze around 10%, Young’s modulus in the range of 30–70 MPa, and thermal conductivities in the range of 18–22 mW m−1 K−1. Those samples were strong enough to be machined with conventional tools (band saws, diamond cutters), they displayed sufficient clarity, and their thermal conductivity was lower than that of still air.