1. Preparation of alumina aerogels (R refers to the alkoxyl group and X refers to the anion such as Cl^-)

2. Alumina-silica aerogels after heat-treatment^[39]

3. Si-modified alumina aerogels^[47]

4. (a) Porous structure^[5] and (b) elastic moduli of alumina aerogels

5. Synthesis of alumina aerogel composites

6. The aluminum borate whisker reinforced alumina-silica aerogel composite^[27]

7. Zirconia fiber-based porous skeleton and the alumina-silica aerogel composite reinforced by the skeleton^[37]

8. The macro (a) and micro morphologies (b,c) of the quartz fiber-based porous skeleton^[33]

9. Fiber reinforced alumina (alumina-silica) aerogel composites^[55,56,57]

10. Test site (a) and temperature curves (b) of the ceramic fiber reinforced alumina aerogel composite (15 mm in thickness) heated by a quartz lamp apparatus^[56]

11. High-temperature thermal conductivities of mullite fiber reinforced alumina-silica aerogel composites^[57]

12. Opacifier embedded mullite fiber felt reinforced alumina-silica aerogel composite^[68]

13. The mullite fiber felt reinforced alumina-silica aerogel composite doped with TiO₂ (a) High-temperature thermal conductivity^[47]; (b) High-temperature thermal conductivity of composites with different content of TiO₂^[54]; (c) Temperature (cold-face) of the composite (doped with 10wt% TiO₂) fired by the flame of butane spray gun at 1300 ℃ for 15 min^[54]; (d) Heat transfer mechanism of the composite (λ_s and λ_g refering to solid and gaseous thermal conductivity, respectively)^[54]

14. (a) Micro morphology of SiC modified mullite fibers; (b) Thermal conductivity of the alumina-silica aerogel composite reinforced by SiC modified mullite fibers^[69]

15. Thermal conductivity of typical pristine alumina aerogels and alumina aerogel insulation composites