Yu. D. Ivakin

Phenomenology of corundum crystal formation in supercritical water fluid

This paper describes a solid phase transformation of aluminium hydroxide into oxyhydroxide (boehmite), and then into well formed fine crystals of corundum (α-Al 2 O 3 ) in the medium of a supercritical water fluid at a temperature of 450 °C and a pressure of 30 MPa. Using electron microscopy the development and processing of corundum crystal formation from incipient crystals to crystal growth and their perfecting is traced.

Role of precursor compaction and water vapour pressure during synthesis of corundum in supercritical water

The present paper is devoted to studying the influence of water vapour pressure on the α-Al 2 O 3 phase nucleation process and on the size distribution and doping level of the synthesized fine crystalline corundum. The effect of preliminary compaction of precursors and pressure variation on the corundum synthesis process in water vapour was also investigated. The products of synthesis were studied by various physical and chemical methods. It was found that the rise of water vapour pressure during synthesis leads to diminution of the concentration of nucleation centres and promotes increase of the corundum crystal size. The higher the pressure the more perfect the corundum crystals that are formed, having lower concentration of defects.

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.

Barioferrite BaFe12O19: A new mineral species of the magnetoplumbite group from the Haturim Formation in Israel

A new mineral barioferrite—a natural analogue of synthetic barium ferrite Ba Fe123+O19—has been identified in the central part of a metamorphosed barite nodule in the rock of the Haturim Formation (Mottled Zone) on the southern slope of Mount Ye’elim in Israel. The mineral is associated with barite, calcite, magnetite, and maghemite and occurs as tiny platy crystals up to 3 × 15 × 15 μm and their irregular aggregates. Barioferrite is black with streaks of brown, and its luster is submetallic. Its Calculated density is 5.31 g/cm3. The mineral is brittle; cleavage is absent. IR absorption bands (cm−1) are observed at 635 (shoulder), 582, 544, 433, and 405 (shoulder). Barioferrite is characterized by ferrimagnetic behavior. Under a microscope in reflected light, barioferrite is grayish white with brownish red internal reflections, the pleochroism is weak (from gray-white on Ro to gray-white with a brown tint on Re), and the bireflectance is weak with distinct anisotropy. The reflectance values of Ro/Re, % (λ, nm) are 24.51/22.80 (470), 24.17/22.25 (546), 23.65/21.68 (589), and 22.67/20.85 (650). The chemical composition (electron microprobe, wt %; the ranges are given in parentheses) is BaO 13.13 (12.5–13.8), Fe2O3 86.47 (85.5–87.5), and 99.60 in total. The empirical formula is Ba0.95Fe12.033+O19. Barioferrite is hexagonal with space group P63/mmc, a = 5.875 (3) Å, c = 23.137 (19) Å, V = 691.6 (5) Å3, and Z = 2. The strongest lines of the X-ray powder diffraction pattern [d, Å, (I, 5) (hkl)] are 2.938(46) (110), 2.770(100) (107), 2.624 (84) (114, 200), 2.420(44) (203), 2.225(40) (205), and 1.627(56) (304, 2.0.11). The holotype specimen of barioferrite is deposited at the Mineralogical Museum of St. Petersburg State University; its catalogue number is 1/19436.

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.

Spectroscopic features of intrinsic defects in silica glasses fused from different raw materials

Abstract Spectral characteristics of silica glasses produced from three different quartz materials by electrical fusion under vacuum, in crucible furnaces, and by flame fusion were studied by optical spectroscopy, photoluminescence and ESR spectroscopy. The effect of the annealing on the optical properties of silica glasses and quartz raw materials was investigated. Silica glass samples were annealed at 1100°C, nd samples of raw quartz were annealed at temperatures of 1600, 1650, 1700 and 1750°C. It was found that the main types of intrinsic defects in these glasses were the oxygen vacancies, and that their spectral characteristics are dependent on the local environment determined by impurities in the initial material and by the technique of glass fusion.

Synthesis and investigation of crystalline modifications of silicon dioxide

The synthesis of α-quartz and other crystalline modifications of silicon dioxide (crystobalite and SiO2-X phase) from amorphous silicon dioxide was carried out via hot steam treatment in the presence of the activating agent, tetra-N-methylammonium hydroxide (TMAH), in an amount of 0.2 wt.-% SiO2. The mass spectrometric study and the electron paramagnetic resonance (EPR) study lead us to the conclusion, that under conditions of hot steam treatment TMAH interacts with surface hydroxides of silicon dioxide and modifies the surface forming amino and methoxy groups. The latter interact with SiO2 matrix-based hydroxyls contributing to the formation of interphase contacts. The electron microscopic study and X-ray analysis show, that the structuring of amorphous silicon dioxide proceeds by the mechanism of solid phase crystallization via the consolidation and ordering of SiO2 structural elements.

Stepwise synthesis of fine crystalline Ce-doped yttrium aluminum garnet in water medium in subcritical and supercritical conditions

In this paper the continuous stepwise method of a production of fine crystalline yttrium aluminum garnet doped with cerium (YAG: Ce3+) in supercritical water fluid (SCWF) are represented. The synthesis was carried out in water medium in two stages: first in subcritical conditions and then in an atmosphere of supercritical water fluid. The stoichiometric mixture of yttrium oxide and aluminum hydroxide in a water solution of cerium nitrate was maintained the certain time at 280°C and under vapor water pressure 6.3 MPa. Then temperature and pressure were risen up to a supercritical condition (T = 392–400°C,

Hydrothermal synthesis of barium hexaferrite

P_{H_2 O}

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

= 22 MPa). The concentration of cerium ions in reaction medium was changed in the interval 0.012–0.706 wt %. The products, obtained on various stages of synthesis, were investigated by physical-chemical methods. During the first stage, the crystals of boehmite and yttrium hydroxide under hydrothermal conditions were arising, and eventually poorly formed YAG: Ce3+ were appearing. At this stage, the diffusion of cerium ions into intermediate products takes place. Because of this, at the second step of synthesis, in supercritical conditions, YAG: Ce3+, phosphor with high luminescence intensity at 530 nm, was obtained. In supercritical conditions well-faceted crystals of 0.5–3.0 μm with rhombododecahedral habitus were produced.