Hydrothermal formation of diamond from chlorinated organic compounds

The unique properties of diamond are well known a long time ago and therefore are extensively used. However, the most general aspects of diamond origin are not yet clearly determined. Hydrothermal synthesis as any heterogeneous reactions occurring under the conditions of high-temperature–high-pressure (> 100 °C, > 1 atm) in aqueous solutions in closed system has a growing interest among the scientists in particular due to a possibility of synthesis of new phases or crystals growth. During this study the diamond particles or films were produced through hydrothermal interaction of 1,1,1-Trichloroethane and 10 M NaOH in the presence of diamond or cubic BN seeds at temperature 300 °C and pressure 1 GPa. Synthesized diamond was thoroughly characterized by TG–DTA, SEM (EDX) and had (220) preferable orientation according to XRD pattern. These results demonstrate that the hydrothermal growth of diamond in like manner with quarts and emerald is possible.     

Extraction of edingtonite from a natural zeolite under hydrothermal conditions

In the present study, edingtonite has been extracted from natural zeolite clinoptilolite by simulating the natural hydrothermal conditions in the laboratory, under the influence of solutions with different concentrations of Ba⁺² and Na⁺, varying from 0.5 to 2.8 mol/L, at 150 °C. In this work, the essential hydrothermal conditions have been provided by hydrothermal autoclaves. The natural and laboratory prepared samples were characterized by XRD, XRF and SEM methods.     

Intercalation of kaolinite under hydrothermal conditions

Intercalation of kaolinite with dimethylsulphoxide (DMSO) was carried out under hydrothermal conditions using a Parr bomb. The intercalated complex has been characterized by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and thermogravimetric analysis techniques. It was found that the intercalation reaction is completed in 20 min and a high yield, 90%, of kaolinite/DMSO intercalate was obtained. An expansion of the original c-axis spacing of 0.714 nm in kaolinite to 1·124 nm, as a result of incorporation of DMSO molecules into the kaolinite interlayers, has been observed by XRD studies. FT-IR analyses of the intercalated complex have established the nature of interactions between the host structures and the incoming DMSO molecules. The hydrothermal method of intercalation developed in this study can also be used for other systems.     

Synthesis and thermal behaviour of nanostructured ZrO2 powders obtained under hydrothermal conditions

Nanocrystalline zircone (ZrO2) powders were synthesized from zirconium oxyacetate aqueous solution treated up to 240 degrees C under autogeneous pressure. Depending on experimental conditions, colloidal particles of 26-150 nm are formed by aggregation of zirconia crystallites. The effect of some experimental conditions over the colloidal particle size or zirconia phase composition was studied. The powder sintering behaviour is also presented.     

Topotactic crystallisation of calcite under hydrothermal conditions

It is shown that control of the phase of calcium carbonate produced during hydrothermal crystallisation can be achieved by the use of titania as a catalyst. In this work it was demonstrated that if either amorphous or the rutile phase of titania is added the aragonite polymorph of calcium carbonate is produced to the exclusion of all other phases. When anatase is the major titania form present then the calcite phase of calcium carbonate is favoured though some aragonite is still produced. The selectivity towards calcite is enhanced by increasing the hydrothermal processing temperature from 120°C to 135°C. It is suggested that the titania phase present catalyses the crystallisation by providing nucleation sites for the calcium carbonate to grow from.     

Metal elution from Ni- and Fe-based alloy reactors under hydrothermal conditions

Elution of metals from Ni- and Fe-based alloy (i.e. Inconel 625 and SUS 316) under hydrothermal conditions was investigated. Results showed that metals could be eluted even in a short contact time. At subcritical conditions, a significant amount of Cr was extracted from SUS 316, while only traces of Ni, Fe, Mo, and Mn were eluted. In contrast, Ni was removed in significant amounts compared to Cr when Inconel 625 was tested. Several factors including temperature and contact time were found to affect elution behavior. The presence of air in the fluid even promoted elution under subcritical conditions.     

Oxygen self-diffusion in rutile under hydrothermal conditions

Oxygen self-diffusion coefficients have been determined for synthetic and natural rutile single crystals under hydrothermal conditions at 100 MPa total pressure and in the temperature range 873–1373 K. The diffusion coefficients are lower than the results from dry gas exchange studies would predict. Between 973 and 1373 K the results can be characterized by two linear Arrhenius relationships. D=1.14×10^–11 exp(–168.8 kJ mol^–1/RT) m^–2s^–1 for the natural rutile, and D=2.41×10^–12 exp(–172.5 kJ mol^–1/RT) m²s^–1 for the synthetic crystal. The results have been interpreted in terms of a defect model involving the dissolution of water in rutile as substitutional hydroxyl defects on oxygen lattice sites, with a solution enthalpy in the range 81–106 kJmol^–1.     

Stability of lead magnesium niobate under hydrothermal conditions

Low-crystalline pyrochlore powder of lead magnesium niobate was treated under various hydrothermal conditions. The perovskite phase crystallized in pure water above 580 °C, but hydrothermal treatments at high temperatures for a long time decreased the amount of the perovskite phase and increased that of the cubic pyrochlore phase. The addition of lead oxide enhanced the formation of the perovskite, but not magnesium oxide. The perovskite phase was not stable in many salt solutions even with the addition of lead oxide, but its stability was increased in solutions consisting of large anions and cations.     

Corrosion of silicon nitride ceramics under hydrothermal conditions

Corrosion behaviour of Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as sintering aids was investigated under hydrothermal conditions at 200–300 C and saturated vapour pressures of water for 1–10 days. Hydrothermal corrosion resulted in the dissolution of the Si₃N₄ matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si₃N₄ ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5–108 kJ mol^–1, and the bending strength of the corroded samples decreased from 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm^–2, and then was almost constant up to a weight loss of 0.012 g cm^–2.     

Synthesis of Sb2O3 nanorods under hydrothermal conditions

Sb₂O₃ nanorods were successfully prepared via a mild hydrothermal route based on the reactions between SbCl₃ and NH₃·H₂O in aqueous solution at 120-180 °C for 12 h. The as-prepared Sb₂O₃ nanorods were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and X-ray photoelctron spectroscopy (XPS). Results showed that NH₃·H₂O played a significant role in the formation of Sb₂O₃ nanorods. The presence of NH₃·H₂O could greatly favor the reaction progress toward the right-hand side and led to the orientation growth of Sb₂O₃ nanorods. A possible mechanism for the formation of Sb₂O₃ nanocrystallites was discussed.     

Preparation of well-crystallized BaSnO3 powders under hydrothermal conditions

Cubic perovskite-type barium stannate (BaSnO₃) powders were successfully prepared under hydrothermal conditions. Hydrothermal treatment of a highly reactive co-precipitated stannic hydroxide gel and barium hydroxide resulted to form a well-crystallized single phase of barium stannate powder via an intermediate phase of BaSn(OH)₆. The optimum hydrothermal conditions of 573 K for 120 s were found for the formation of pure phase of BaSnO₃. The synthesized product had a fine microstructure, with a grain size of ∼7 μm. The formation of BaSnO₃ strongly depends on the reaction temperature.     

Cluster mechanism of diamond synthesis under severe cavitation conditions

The process of diamond synthesis is considered as the formation and growth of nanoclusters in the presence of phonon excitations in their crystal lattices. Theoretical estimations obtained within this approach qualitatively agree with experimental data on the cavitation synthesis of a diamond.     

Synthesis and crystallographic study of Pb-Sr hydroxyapatite solid solutions by high temperature mixing method under hydrothermal conditions

The solid solutions in the system of Pb and Sr hydroxyapatite, Sr_10−xPb_xHAp (x = 0-10), were successfully synthesized by high-temperature mixing method (HTMM) at 160 °C for 12 h under hydrothermal conditions. The samples were characterized by X-ray diffraction, chemical analysis and electron microscopic observation, and the site of the metal ions in the solid solutions was analyzed with the Rietveld method. The lattice constants, both a and c, of the solid solutions varied linearly with Pb content. It was found that Pb ions in the solid solutions preferentially occupied the M(2) site in the apatite structure. HTMM gives Sr-Pb HAp solid solutions much better crystallization. However, due to the formation of intermediate compound of Pb₃O₂(OH)₂ in the Pb(NO₃)₂·4H₂O solution before mixing with (NH₄)₂HPO₄ solution at 160 °C, HTMM causes the decrease of crystallization of the samples with high Pb content.