Structural Analysis of Aluminum Oxyhydroxide Aerogel by Small Angle X-Ray Scattering

The work presents studies on the microstructure and mesostructure of nanostructured aluminum oxyhydroxide formed as a high porous monolithic material through the surface oxidation of aluminum liquidmetal solution in mercury in a temperature- and humidity-controlled air atmosphere. The methods of X-ray diffraction analysis, thermal analysis, the low temperature adsorption of nitrogen vapors, transmission electron microscopy, small-angle and very small-angle neutron scattering, and small-angle X-ray scattering are used for comprehensive investigation of the samples synthesized at 25°С as well as that annealed at temperatures up to 1150°C. It is found that the structure of the monolithic samples can be described within the framework of a three-level model involving primary heterogeneities (typical length scale of rc ≈ 9–19 Å), forming fibrils (cross-sectional radius R ≈ 36–43 Å and length L ≈ 3200–3300 Å) or lamellae (thickness T ≈ 110 Å and width W ≈ 3050 Å) which, in turn, are integrated into large-scale aggregates (typical size R _c ≈ 1.25–1.4 μm) with an insignificant surface roughness. It is shown that a high specific surface (~200 m²/g) typical for the initial sample is maintained upon its thermal annealing up to 900°С, and it decreases to 100 m²/g after heat treatment at 1150°С due to fibrillary agglomeration.