Thermal Insulation Performance of Silica Aerogel Composites Doped with Hollow Opacifiers: Theoretical Approach

Silica aerogels demonstrate great promise in thermal insulation applications, such as energy-efficient buildings, cold-chain transportation, and aerospace engineering. However, the application of pure silica aerogels is limited in high temperature applications (>500 K) due to their transparency in the wavelength of 2−8 µm. The conventional strategy is to dope silica aerogel with solid spherical opacifiers (e.g., SiC, TiO2, and ZrO2) to increase their extinction coefficient; however, incorporating solid opacifiers into silica aerogel matrix improves the structural density of silica aerogel composites. Herein, we propose to improve the extinction coefficient of the silica aerogel by using hollow opacifiers. A theoretical model was developed to investigate the parameters including the outer diameter, shell thickness, and mass fraction on both the radiative thermal conductivity and total thermal conductivity of the silica aerogel composite doped with hollow opacifiers. Our results indicate that doping hollow opacifiers can enable the silica aerogel matrix to achieve lower radiative thermal conductivity when compared to matrices doped with optimally sized solid opacifiers. The total thermal conductivity of silica aerogel doped with hollow opacifiers could be lower than that of the silica aerogel doped with optimally sized solid opacifiers. This work contributes to the understanding of heat transfer within porous materials and guides the structural design of high-temperature thermally insulating materials.