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Pure SiO2 aerogel has a strong light transmittance in the infrared wavebands from 3.0 to 8.0 μm, and an opacifier could efficiently reduce aerogel's radiative thermal conductivity (λr), especially at high temperatures (>400 K). Consequently, the λr of different core/shell structured opacifiers is proposed, including micron hollow sphere opacifier (MHSOP), i.e., hollow carbon black/SiC/TiO2, and phase change material (PCM)/opacifier, i.e., VO2/SiO2, and Ge2Sb2Te5/SiC; further, their conductive λ model has also been established. The results showed that MHSOP could reduce MHSOPs-SiO2 aerogel composite's λ compared to traditional solid structure opacifiers; the effect of MHSOPs with a certain core–shell ratio on suppressing thermal radiation is equivalent to their solid structure opacifier at high-temperature. Adding SiC MHSOPs reduces aerogel composites' weight and thermal conductivity by 42.19 and 26.29%, while the shading effect of a core–shell ratio of over 0.75 is equivalent to the solid structure. Specifically, rutile-phased VO2/SiO2's λr is smaller than TiO2 MHOSP, and crystalline Ge2Sb2Te5/SiC doped aerogel exhibits good thermal insulation. The proposed micron hollow sphere opacifier and PCM/opacifier provide a novelty, lightweight, and high-efficiency method to restrain aerogel's infrared radiation and improve insulation performance at high temperatures.

Silica aerogel composite is an excellent thermal insulator for spacecraft under high-temperature and complex air environments. This study intends to evaluate SiC-doped silica aerogel’s thermal insulation performance under large temperature and air pressure differences. In this paper, the hot surface’s temperature response of SiC-doped silica aerogel with different content was studied at significant temperature differences (ΔT) when pressure changes instantaneously. Their thermal insulation performance was evaluated by analyzing the influence of pressure gradients on the unsteady-state heat transfer. When the cold surface’s temperature of the specimen keeps constant at 15 °C and ΔT = 171~912 K, the results demonstrate that the correlative thermal conductivities of silica aerogel with 1% and 5.84% SiC are 0.02223~0.04077 W·m−1·K−1 at P ≈ 10 Pa and 0.03165~0.04665 W·m−1·K−1 at P = 1 atm, respectively. The aerogel composite with 0% SiC showed the best thermal insulation performance at ΔT < 200 K and P ≈ 10 Pa, while the aerogel with 5.84% SiC became the best at ΔT > 700 K and P = 1 atm. In addition, the transient pressure decreases will significantly impair the heat transfer of the gas inside the aerogel, thereby weakening the gaseous thermal conductivity and improving the thermal insulation performance.