Effective Thermal Conductivity Model of Micron Hollow Sphere or Phase Change Material/Opacifier-SiO₂ Aerogel Composites

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 (λ_r), especially at high temperatures (>400 K). Consequently, the λ_r 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 λ_r 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.