Doping method for opacifier and fiber in aerogel composites

Silica aerogel as a super-insulating material is widely used in thermal protection system of spacecraft and aircraft. However, the significant radiative heat transfer causes obvious deterioration in the insulation capability of silica aerogel at high temperature because the pure silica aerogel is almost transparent to infrared radiation in such a spectrum range. Doping fibers or opacifiers could greatly improve the thermal insulation properties at high temperature. In this paper, we determine the optimal temperature-dependent size for typical opacifiers and silica fibers by calculating radiative properties. Suppose that the opacifier particles are spherical and the fiber is an infinite cylinder. According to the Mie scattering theory, the extinction efficiency factor of single opacifier particle and single fiber could be obtained. Then, the extinction coefficient could be calculated which characterizes thermal insulation performance of aerogel composites. The results show that the radiative thermal conductivity increases with increasing temperature, thus smaller particles and thinner fibers are more suitable for doping at high temperature. In addition, the carbon black has the best extinction characteristic in the four types of studied opacifiers (carbon black, SiC, ZrO₂ and TiO₂), but it will be oxidized at high temperature. Compared with TiO₂ and ZrO₂, the insulating performance of the SiC opacifier is more effective at high temperature. Although doping opacifier or fiber in silica aerogel provides a better extinction function, it might increase the heat conduction. Therefore, we could achieve an optimal doping amount at the minimum effective thermal conductivity, in other words, the highest insulating capability. The effective thermal conductivity in doped silica aerogel is equal to the sum of the conductive thermal conductivity and the radiative thermal conductivity. The optimal temperature-dependent doping amount is obtained by minimizing the effective thermal conductivity. The results show that the optimal doping of both opacifier and fiber amount in silica aerogel increases with the temperature. In addition, the carbon black behaves the strongest ability to inhibit heat transfer when the temperature is below 600K, while the SiC doped silica aerogel has the most effective performance in suppressing heat transfer when the temperature is higher than 600K. Optimized results were applied to design multi-layer doping with a temperature gradient. Each layer is doped with the corresponding optimal size and amount of opacifiers and fibers according to the temperature. Assuming that the temperature changes linearly in the direction of heat transfer, we studied four multi-layer solutions as follows. Solution 1 is the only SiC opacifier multi-layer doping. Solution 2 is SiC and carbon black opacifier multi-layer doping. Carbon black opacifier only exists in the temperature below 600K, otherwise the SiC is doped, which could ensure that only a single type of opacifier was doped for each layer. Solution 3 is the only silica fiber multi-layer doping. Solution 4 is the optimal co-doping of opacifier and silica fiber, which is similar to Solution 2 but mingled with silica fiber in each layer. The results show that the silica fiber only doping has the smallest effective thermal conductivity, and the co-doped silica aerogel with carbon black, SiC opacifier and silica fiber has the smallest radiative thermal conductivity. To verify the above results, the back temperature curve of doped silica aerogel was measured. Silica aerogel doped with various types of opacifiers and SiC in different particle sizes and doping amounts were considered. In addition, the back temperature of SiC-multi-layer doped silica aerogel was investigated. The predictions agree with experimental results qualitatively.