Experimental and analytical analyses of the thermal conductivities and high-temperature characteristics of silica aerogels based on microstructures

An analytical heat transfer model based on scanning electron microscopy, Brunauer-Emmett-Teller and pycnometry measurements and a 3D random diffusion-limited cluster-cluster aggregation structure is proposed to calculate the temperature-dependent microstructural parameters and thermal conductivities of silica aerogels. This model is a pure prediction model, which does not need experimentally fitted empirical parameters and only needs four measured structural parameters as input parameters. This model can provide high-temperature microstructural and thermophysical properties as well as theoretical guidelines for material designs with optimum parameters. The results show that three stages occur during the thermal evolution processes of the aerogel structure with increasing temperature from 300 to 1500 K. The current analytical model is fully validated by experimental data. The constant structure assumptions used in previous heat transfer models are found to cause significant errors at higher temperatures as the temperature-dependent structure deformation significantly increases the aerogel thermal conductivity. The conductive and total thermal conductivities of silica aerogels after high-temperature heat treatments are much larger than those with no heat treatment.