中文摘要：

二氧化硅气凝胶广泛用于各种航天器、飞行器的热防护中,添加遮光剂或纤维能大大改善其在高温下的隔热性能.本文通过计算辐射特性得到遮光剂和纤维不同温度下的最佳掺杂粒径,在复合材料等效热导率最小时得到最佳掺杂量,并应用优化结果设计了具有温度梯度的多层掺杂方案.研究发现：遮光颗粒和纤维的最佳掺杂直径随温度增加而减小,掺杂量随温度增加而增加,研究的4种遮光剂（碳黑、SiC、ZrO2和TiO2）中,碳黑具有最好遮光效果,但高温下会被氧化,SiC在高温下具有较好的遮光效果;石英纤维多层掺杂具有最小有效热导率,碳黑与SiC遮光剂和石英纤维共同掺杂对辐射传热的抑制最强.气凝胶复合材料背温实验结果有效证实了掺杂优化方法的正确性.

英文摘要：

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, Zr O2 and Ti O2）, 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 opt