Hydrated γ-, γ’-, θ-, and κ-Al2O3 powders were precipitated with nitric acid, HNO3, from Na[Al(OH)4] alkaline solutions at pH = 5.0–5.5. Waste materials from α-B12Al and B12(C–Al–C) laboratory production were used. The precipitates were rinsed and dried at 420 → 570 K (Sspecific BET = 213 m2 ⋅ g–1, dCSD crystallites ≈ 10 nm, and dSEM agglomerates ≈ 200 nm). The samples were characterized by X-ray diffraction, X-ray diffractometry in the coherent scattering domain (CSD), fluorescence analysis, scanning electron microscopy (SEM), chemical elemental and phase analyses, and thermal desorption of nitrogen calculated by the BET method (Sspecific BET) for interpreting surface measurements of nonporous bodies. The concentrations of gas-forming elements (hydrogen, nitrogen, oxygen, and carbon) were determined by reductive and oxidative extraction in helium and oxygen flow, gas chromatography, and coulometry. The multiphase γ-, γ’-, θ-, and κ-Al2O3 samples treated at 1370–1470 K in air exhibited an α-Al2O3 structure. According to X-ray diffraction in hkl012 CSD, the α-Al2O3 crystallites had dCSD ≈ 48 nm. Based on SEM analysis, the sizes of the α-Al2O3 powder agglomerates did not exceed dSEM = 200–300 nm. The specific surface area of the powder, Sspecific BET, determined by thermal desorption of nitrogen calculated with the BET method, was equal to 8.6 m2 ⋅ g–1. The weight content of α-Al2O3 was 99.69%, while SiO2 impurities accounted to 0.31%, according to X-ray fluorescence analysis. The crystallites, as components of the alkaline γ-, γ'-, θ-, κ-Al2O3, and α-Al2O3 powder agglomerates, showed a lamellar shape. The thickness of the lamellas was close to the calculated dCSD values for crystallites. The submicron γ-, γ’-, θ-, and κ-Al2O3 particles had a ‘sandglass’ shape, determined by the dynamics of precipitating flat crystallites of the Al(OH)3 solid phase (nanosized thickness) as pH decreased. The α-Al2O3 agglomerates consisted of fused local particles connected by bridges.
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