Function of the Non-Crystalline X-Ray Amorphous Phase of Highly Dispersed Powder of CaO in Adsorption of CO2 and H2O

Russian Journal of General Chemistry - Highly dispersed CaO has been synthesized by heating an aqueous solution of calcium nitrate and D-glucose up to 800°C. Calcium oxide is formed with an...     

Kinetic model and mechanism of Y3Al5O12 formation in hydrothermal and thermovaporous synthesis

Abstract

Kinetics and mechanism of fine-crystalline yttrium-aluminum garnet (YAG) formation under hydrothermal and thermovaporous treatment were investigated. It was synthesized from the stoichiomctric mixture of oxides in the temperature range 200-400°C at pressures of water vapor 4.0-26MPa. It was established that in quasiequilibrium with water vapor conditions the synthesis of YAG proceeds with formation of intermediate substance with Y(OH)₃ structure and amorphous aluminous component. The diffusion of this aluminous component into the Y(OH)₃ matrix resulted in the reorganization of oxygen sublattice accompanied with dehydroxylation. The kinetics of YAG formation is described by the equation of solid-phase transformation with the limiting stage of nucleation. Synthesized YAG contains 5-7% of water, which corresponds to a hydrogarnet structure. The study of luminescence properties of YAG doped with Nd³⁺ or Cr³⁺ ions has allowed to determine the positions of hydroxyl groups and oxygen vacancies in the structure.     

State of manganese ions in the structure of corundum synthesized in supercritical water fluid

The kinetics and formation mechanism of doped corundum (α-Al₂O₃) from hydrargillite (γ-Al(OH)₃) in supercritical water fluid (SCWF) in the presence of manganese ions are studied. It was ascertained that due to the reversible dehydroxylation in an aqueous medium, solid-phase transformation of hydrargillite into boehmite (γ-AlOOH) and then into corundum occurs with the formation of well-faceted corundum micro-crystals that are uniformly doped with manganese. It was found that when Mn²⁺ or MnO₄⁻ ions are introduced into the reaction medium, Mn⁵⁺, Mn⁴⁺, Mn³⁺, and Mn²⁺ ions are observed in the synthesized corundum. Meanwhile, the manganese ions form a complex defect in the corundum structure, which comprises oxygen vacancies and hydroxyl groups. The defects in corundum that emerge upon doping with manganese in SCWF are different from those in corundum doped during high-temperature synthesis.     

Scientific basis of technology of fine-crystalline quartz and corundum

Abstract

Authors have investigated the mechanism and the kinetics of fine-crystalline quartz and corundum synthesis in supercritical water. The thermovaporous treatment of raw materials has been carried out in laboratory (v=20 ml) and technical (v = 2m³ and v=4m³) autoclaves at temperatures between 390 and 410°C and vapor pressures of water from 20 to 27 MPa in the presence of miacroadditives of activators. The samples of products after thermovaporous treatment have been studied by X-ray diffraction, optical and electron microscopy, mass-spectroscopy, ESR-, IR-spectroscopy, thermal analysis and pycnometry. It was shown that microadditives of activator into silica or alumina might not only accelerate the attainment of the best hydroxylation, but also initiate the transformation of precursors in an ordered way. The control of the transformation mechanism by introducing activators into the starting materials has allowed us to obtain fine-crystalline quartz and corundum with desired properties: various habitues and size of crystals. On the basis of these data, technology of fine-crystalline quartz and corundum has been developed.     

The kinetics and mechanism of doped corundum structure formation in an water fluid

The kinetics and mechanism of corundum formation from hydrargillite in an water medium under sub- and supercritical conditions in the presence of manganese ions was studied. The conclusion was drawn that corundum structure formation with the insertion and uniform distribution of manganese ions occurred thanks to solid-state mobility, which appeared under the conditions of reversible dehydroxylation in the interaction of a solid matrix with an water fluid. Complex defects containing Mn²⁺, Mn³⁺, and Mn⁴⁺ ions along with hydroxyl groups and oxygen vacancies were formed when corundum was doped with manganese ions in different charge states because of redox processes in a supercritical water fluid. Corundum doped with manganese exhibited ferromagnetic properties at room temperature.     

A New Method for Producing a Nanosized γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powder

A new method for producing a nanosized γ-Al₂O₃ powder was proposed, by which a saturated solution of aluminum oxychloride and sucrose was subjected to sequential heat treatment to 350°C to form a transient species and then to 800°C to form a nanosized γ-Al₂O₃ powder. The optimal treatment parameters were determined. Stages of the process were identified. The transient species and the nanosized γ-Al₂O₃ powder were studied.     

Erratum to: Function of the Non-Crystalline X-Ray Amorphous Phase of Highly Dispersed Powder of CaO in Adsorption of CO2 and H2O

Russian Journal of General Chemistry - Erratum     

Induced formation of corundum crystals in supercritical water fluid

The formation of fine-crystalline corundum (α-Al₂O₃) from hydrargillite in supercritical water fluid (SCWF) at 400°C and 26.8 MPa in the presence of corundum particles was studied. It was found that the α-Al₂O₃ particles added to the reaction medium accelerated the formation of fine-crystalline corundum due to an increase in the rate of nucleation. In this case, both the buildup of added corundum grains, and the generation of new corundum crystals from boehmite were observed. The formation of new crystals depended on distance between the grains of the additive. Depending on this distance, the newly formed crystals consisted of two or three fractions with different average particle sizes. It was concluded that the buildup of the fused corundum particles in SCWF occurred due to the uptake of corundum nuclei formed in a surrounding layer of boehmite. For the added particles of fused corundum with an average size of 2.82 µm, the thickness of a layer from which the buildup occurred was 17.47 µm. The corundum nuclei formed at large distances from the growing particles became the centers of growth for new corundum crystals of the smallest sizes. The role of SCWF consisted in increasing the spatial and structural mobility of the reactants.     

Synthesis of corundum doped with cerium in supercritical water fluid

The properties of fine crystalline corundum doped with cerium (α-Al₂O₃: Ce³⁺) during synthesis in a supercritical fluid have been studied. The synthesis of corundum has been carried out by the thermal treatment of hydrargillite, Al(OH)₃, at T = 415°C and {ie290-1} MPa in reaction media that contained from 0.001 to 0.25 wt % of cerium. Cerium ions are incorporated into the boehmite lattice during the transformation of hydrargillite into boehmite, which forms fine monocrystals of the doped corundum with a size from 20 to 50 μm. The size of the corundum crystals increases with increasing pressure and increasing concentration of cerium. The synthesized α-Al₂O₃: Ce³⁺ reveals a luminescent band in the UV region at 352 nm and a blue band at 421 nm. The intensity of the cerium ion luminescence in corundum increases with an increase in the water fluid pressure during synthesis. The follow-up annealing of α-Al₂O₃: Ce³⁺ at 1400°C in a vacuum leads to a decrease in the luminescence. It has been concluded that spectrally active complex structures that include cerium ions, oxygen vacancies, and hydroxyl groups are produced in the media of a supercritical water fluid upon the synthesis of boehmite and corundum. Exposure to high temperatures causes their transformation.     

Increasing Crystallization Rate in Growing Single Crystals of Quartz

A new method was proposed to increase the crystallization rate in growing single crystals of quartz. In this method, the crystallization is accelerated by using surfactants (polyethyleneamine, tetramethylammonium base, and polyethyleneimine) added to a soda-alkaline solution of standard concentration. Within studied ranges of pressures and concentrations of components of the solution, polyethyleneimine at a concentration of 0.002–0.05 wt % is the most efficient crystallization accelerator, which can increase the quartz crystal growth rate on the average by a factor of 2.