Improvement of thermal insulation performance of silica aerogels by Al2O3 powders doping

Silica aerogel is deemed as a kind of high-performance thermal insulation materials. However, the existence of macropores in the structure is always ignored in the research and application of aerogels. Thus the thermal insulation performance of silica aerogels could be further improved if the macropores are reduced. In this work, nano-sized Al2O3 powders are explored as nano fillers to reduce the macropore volume fraction in silica aerogels by filling the big voids among the silica aggregates, and further lower the thermal conductivity. The experimental results showed that the macropore volume fraction (VMAC) was dramatically reduced from 63.05% to 23.12% with the addition of Al2O3 powders ranging from 0.0 g to 1.0 g. This trend was also verified by the variation of (VT*-VBET) and (VBET/VT*). Accordingly, the thermal insulation performance was improved due to the reduction of macropores in aerogels. The lowest thermal conductivity of Al2O3-doped aerogels reached 7.41 mW/(m K) in contrast with that of pure silica aerogels (9.00 mW/(m K)), which was a significant decline for aerogel-based materials due to the gaseous heat transfer being further weakened. Moreover, the increment of thermal conductivity from 7.41 to 9.71 mW/(m K) with the Al2O3 powders increasing could be attributed to the enhancement of solid heat transfer in the system. The variation of experimental thermal conductivity was in good agreement with the result of theoretical calculation. This study proposed an innovative idea to improve the thermal insulation of aerogel under ambient conditions.