Radiative characteristics of opacifier-loaded silica aerogel composites

• A model is proposed to evaluate radiative characteristics of aerogel composites.
• The model agrees well with the published experimental data.
• Radiative characteristics of six opacifiers with various sizes are investigated.
• Optimal opacifier category and size are evaluated.
• A novel gradient design is proposed to reduce radiative heat transfer of composite.

Radiative characteristics of opacifier-loaded silica aerogel composites such as specific spectral extinction coefficient and Rossland mean extinction coefficient were usually calculated by the Fourier infrared spectral experiment and the Beer law. For the composites, it needs lots of experiments to find the proper opacifier categories, contents, and sizes, hence, the optimal design becomes difficult. Based on this reason, this work proposes a theoretical method with four sub-models to evaluate the radiative characteristics of opacifier-loaded silica aerogel composites. First, the Fourier infrared spectral experiment and the modified Kramers–Krönig (K–K) relation are used to calculate the basic optical constants of the opacifier (complex refractive index). Second, the extinction efficiency of a single opacifier particle is calculated based on its complex refractive index. Third, the spectral and Rossland extinction coefficients of opacifier particle assemble are calculated by using extinction efficiency and mass fraction of opacifier. Finally, the spectral and Rossland extinction coefficients and radiative heat conductivity of the composite are obtained. The radiative characteristics of six kinds of opacifiers with various particle diameters are investigated by using the present models. The results show that optimal opacifier and its diameter are strongly temperature-dependent. The optimal diameter of opacifier reduces with increased temperature, and SiC is the best choice due to its high-temperature stability. A gradient design of composite is proposed based on the temperature-dependent optimal opacifier and its diameter, which significantly reduces radiative heat transfer compared to the traditional design.