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A procedure for the synthesis of calcium oxide has been developed, which consists in heat treatment of an aqueous solution of calcium acetate and d -glucose at 350 and then at 700 °C. The process parameters have been determined. It has been shown that when d -glucose is used and the reaction mixture is heat treated at 700 °C, highly dispersed calcium oxide with an average particle size of 77 nm is formed. CaO formed has been used as a precursor for the synthesis of priceite (Ca 2 (B 5 O 7)(OH) 5 · H 2 O) during hydrothermal treatment of CaO in an aqueous solution of boric acid. It has been found that the optimal conditions for the synthesis of monophase priceite under hydrothermal conditions include the temperature range of 170–200 °C and the heating time of 12 h. When heating priceite at 800 °C in air for 1 h, highly dispersed powder calcium bis(borate) Ca(BO 2) 2 has been isolated as the final product. It is shown that the use of synthesized powder CaO leads to the formation of fine-crystalline powders of calcium borates. Samples obtained at each stage of the proposed synthesis of highly dispersed calcium oxide and calcium borates, using the example of priceite and calcium bis(borate), have been studied by powder X-ray diffraction, SEM, TEM, BET, IR spectroscopy, and DTA. [Display omitted] [ABSTRACT FROM AUTHOR]

In the present work, a new method for the synthesis of a highly pure ceramic precursor for the synthesis of sapphire has been developed. The method consists in hydrothermal treatment of a mixture of gibbsite and nanosized powder γ-Al 2 O 3 in a solution of 1.5 wt % HCl at 200 °C followed by heat treatment of the resulting compact at 1700 °C. The conversion of a mixture of gibbsite and nanopowder γ-Al 2 O 3 into compact boehmite has been studied using atomic emission analysis, X-ray powder diffraction, infrared spectroscopy, scanning electron microscopy, differential thermal analysis, and the Brunauer–Emmett–Teller method. It has been found that the formation of boehmite compact is affected by the addition of nanosized powder γ-Al 2 O 3 and the duration of the autoclave treatment of the reaction mixture in a solution of 1.5 wt % HСl at 200 °C. The optimal results in the synthesis of a highly pure ceramic precursor were obtained when the mixture of gibbsite and 1 wt % of nanosized powder γ-Al 2 O 3 was treated for 7 h. The effect of the heat treatment conditions of the synthesized compact boehmite on the sintering density of a ceramic precursor has been studied. Image 1 • A high-purity ceramic precursor with a density of 3.45 g/cm3 was synthesized. • Hydrothermal treatment of gibbsite in an acidic medium followed by heating was used. • Addition of nano γ-Al 2 O 3 and the duration of the hydrothermal treatment affect the density of the ceramic precursor. • The synthesized ceramic precursor satisfies requirements for raw materials for deposition of sapphire. [ABSTRACT FROM AUTHOR]

The state and forms of water in dispersed aluminum oxides and hydroxides and products obtained during their treatment are investigated in this paper. It has been found that particles of dispersed aluminum oxides and hydroxides of various water forms are present on surface and in volume. Their classification based on the state of water and the values of Δ H of evaporation has been introduced: "surface-bound" water, "structural" water, and "single OH groups". For micron particles, the presence of predominantly "structural" water is exemplified for gibbsite (Al(OH) 3) and boehmite (AlOOH). For the defective structure of products of heat treatment of gibbsite in air at 250 and 800°С, a low dehydroxylation temperature and high values of Δ H of water evaporation have been found: 71.13 kJ/mol and 67.27 kJ/mol, respectively. For the γ-Al 2 O 3 nanopowder and the products of its treatment, the presence of "surface-bound", "structural" water, and water represented by "single OH groups" has been detected. It has been shown that the treatment of gibbsite or aluminum oxide in a supercritical aqueous fluid at a water/Al 2 O 3 molar ratio of less than 1.5 leads to the formation of tohdite 5Al 2 O 3 •H 2 O, the composition of which is characterized by a low content of structural water in the stable state over a wide temperature range. [Display omitted] • Temperature of reaction between steam and alumina determines the form of bound water. • Forms of water are classified as "surface-bound", "structural" and "single OH groups". • Density of the aqueous medium and water form determines the phase formed. • Products are bayerite, boehmite and todite, as well as γ-Al 2 O 3 and α-Al 2 O 3. • Exchange of oxide matrix with medium determines size and morphology of particles. [ABSTRACT FROM AUTHOR]