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

Fig. 2 Alumina-silica aerogels after heat-treatment[39] (a) Specific surface areas of aerogels with different molar ratios of Si; (b) TEM photos of the pristine alumina aerogel (AA-1300) and alumina-silica aerogel calcined at 1300 ℃(ASA81-1300); (c) Distribution and atomic ratio of Si and Al in the aerogel with a theoretical Si/Al molar ratio of 0.125:1 (Bars representing 20 nm; the red and green color representing Si and Al, respectively)

Fig. 3 Si-modified alumina aerogels[47] (a) Deposition modification of the gels; (b) Macro and (c) Micro changing of pristine and Si-modified alumina aerogels after heat-treatment at 1300 ℃

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

Fig. 5 Synthesis of alumina aerogel composites

Fig. 6 The aluminum borate whisker reinforced alumina-silica aerogel composite[27] (a) Macro and (b) micro morphology; (c) Mechanical properties

Fig. 7 Zirconia fiber-based porous skeleton and the alumina-silica aerogel composite reinforced by the skeleton[37] Micro morphology of (a) skeleton and (b) composite, and (c) their mechanical properties

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

Fig. 9 Fiber reinforced alumina (alumina-silica) aerogel composites[55,56,57] (a) Influence of mass fraction of fibers on mechanical properties[55]; (b) Behavior of composites under compressive and bending loadings[56]; (c) Morphology and (d) bending/tensile strengths of composites with different fiber densities[57]

Fig. 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]

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

Fig. 12 Opacifier embedded mullite fiber felt reinforced alumina-silica aerogel composite[68] (a) Macro morphology; (b) Temperature curves of the composite (20 mm in thickness) during one-face heating test

Fig. 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 (λs and λg refering to solid and gaseous thermal conductivity, respectively)[54]

Fig. 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]

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