A new route for the synthesis of nitrides: The solvothermal preparation of GaN

Abstract

A new solvothermal route for the synthesis of nitrides is proposed using liquid NH3 as solvent in supercritical conditions, Such a preparation method was applied to the synthesis of GaN using gallium metal as starting material.

GaN is a wide band-gap semi-conductor (3.4eV). It is a very attractive nitride due to its various applications in micro- and opto-electronics [1,2]. Consequently, many research groups are interested in synthesising GaN. Two methods have been principally developed:

(i) synthesis of thin films by epitaxy [3,4]

(ii) synthesis of bulk GaN by high pressure method [5,6].

The new proposed process leads to fine microcrystallites of GaN with the wurtzite-type structure. The chemical purity can be optimised versus the synthesis mechanism. The size and shape of the crystallites would be influenced by the nature of the nitriding additive and the thermodynamical conditions (pressure and temperature) used for the synthesis.     

Numerical study on combustion of H2—O2 in a supersonic reactive expansion in a nozzle

A study on expansion flow inside a nozzle considering full mechanism chemistry of hydrogen and oxygen was carried out. In this study, a full implicit scheme for turbulent reactive flow was obtained by combining the second order TVD scheme of Yee and Harten (1987, Implicit TVD schemes for hyperbolic conservation laws in curvilinear coordinates. American Institute of Aeronautics and Astronautics Journal, 25(2), 266–274) with the efficient implicit lower-upper scheme of Shuen and Yoon (1989, Numerical study of chemically reacting flows using a lower-upper symmetric successive overrelaxation scheme. American Institute of Aeronautics and Astronautics Journal, 27(12), 1752–1756). The species equations, Navier–Stokes equations and turbulence model were implemented in the numerical scheme and solved in conjunction with full detailed finite rate chemistry. The numerical scheme is verified by comparison with experimental results of a converging–diverging nozzle. Effects of inlet pressure, temperature and fuel-oxidant mass ratio on nozzle flow field were studied. Variation of chemical species under different conditions was investigated by considering a chemical mechanism. Results show that increasing inlet pressure increases the rate of reactions due to increasing the concentration of reactants. For lower inlet pressure the radical H increases slightly in the diverging part of the nozzle, while for higher pressures it decreases along the nozzle. Inlet fuel–oxidant mass ratio affects the variation of all species with a greater effect for a near stoichiometric ratio. It was also shown that a higher inlet temperature provides a more enhanced reaction zone in the diverging part of the nozzle.     

Effects of Hydrostatic and Chemical Pressures on Impurities Determined through Optical Parameters

A survey on the influence of hydrostatic and chemical pressures upon optical properties due to Transition-Metal impurities in insulators is offered in this article. Aside from recalling the theoretical framework where the properties of impurities in insulating materials are reasonably explained particular attention is paid to the dependence of crystal field and charge transfer transitions upon the impurity-ligand distance, R . By means of 10 Dq dependent d-.d transitions or the energy of charge transfer transitions it is pointed out that j R variations around 0.1 pm can be detected. The R dependence of 10 Dq is shown to determine also the corresponding dependence of Huang-Rhys factors and Stokes shift. The Huang-Rhys factor, S A , of the A 1 local mode is found to increase when R does a conclusion supported by recent experimental data on Cs 2 NaScCl 6 : Cr 3+ under hydrostatic pressure. The increase of the Stokes shift when R increases observed in the series of cubic fluoroperovskites doped with Mn 2+ is also discussed. It is pointed out that for the same j R variation, j y A changes produced by chemical pressures can be bigger than those produced through a hydrostatic pressure.     

Pressure dependence of the weak itinerant ferromagnetism of Y(Co1-xAlx)2

Abstract

Y(Co_1-xAl_x)₂ is a weak itinerant ferromagnet for 0.12 < x < 0.20 with a maximum Curie point of about 25 K near x = 0.15. The pressure dependence of the Curie point for samples with 0.14 < x < 0.18 was measured in the temperature and pressure range 5–25 K and 0–8 kbar using a liquid filled lock cell. It was found that T_c was linear in pressure and extrapolated to OK at P_c = 9 ± 1 kbar for all samples. The volume expansion from YCo₂ to x = 0.15 is equivalent to a chemical pressure of -40 kbar so ferromagnetism cannot occur at the lattice constant of YCo₂.     

Molecular simulation of chemical reaction equilibria by Kinetic Monte Carlo

ABSTRACT

We describe a new algorithm for the molecular simulation of chemical reaction equilibria, which we call the Reactive Kinetic Monte Carlo (ReKMC) algorithm. It is based on the use of the equilibrium Kinetic Monte Carlo (eKMC) method (Ustinov et al., J. Colloid Interface Sci., 2012, 366, 216–223) to generate configurations in the underlying nonreacting system and to calculate the species chemical potentials at essentially zero marginal computational cost. We consider in detail the typical case of specified temperature, T and pressure, P, but extensions to other thermodynamic constraints are straightforward in principle. In the course of this work, we also demonstrate an alternative method for calculating simulation box volume changes in NPT ensemble simulations to achieve the specified P. We consider two sets of example reacting systems previously considered in the literature, and compare the ReKMC results and computational efficiencies with those of different implementations of the REMC algorithm (Turner et al., Molec. Simulation, 2008, 34, 119–146).     

Chemical transformations of benzene under high pressure and at high temperature

Abstract

In order to understand the chemical transformations of organic compounds composed of aromatic rings under high pressure and at high temperature, we have studied the benzene behavior up to 25 GPa and 550°C by Raman scattering. The chemical transformations and the phase diagram have been established. This point was worth investigating because the phases of benzene before the chemical transformations influence the characteristics of the recovered compound at atmospheric pressure.     

A New High Pressure - Low Temperature Process for the Synthesis and the Crystal Growth of Gallium Nitride

Gallium nitride can be synthesized by a solvothermal route using ammonia as solvent and a nitriding additive (such as hydrazine hydrochloride NH 2 NH 3 Cl or sodium azide NaN 3 ) encapsulated in melt gallium as chemical reagents. The involved temperature and pressure are respectively 600 °C and 150 MPa. The synthesized products are then characterized by X-Ray Diffraction and Scanning Electron Microscopy. The synthesis parameters influence the powder morphology and purity.     

Evaluation of Ph and granite/hot water interaction under geothermal conditions

Abstract

The in-situ laboratory measurement of pH in a granite-hot water reaction system has been undertaken under flowing conditions. The calculation of fluid pH and multi-component chemical equilibria in the granite-hot water reaction system was made, using the calculation program SOLVEQ92. In this study, the chemical equilibrium relationship of granite and hot water interaction was also deduced using the some program. The measurement of in-situ pH in the granite-hot water reaction system was undertaken in the temperature and pressure range of 100–250°C and 20MPa respectively. The in-situ pH value shows good agreement with the pH calculation value with regard to multi-component chemical equilibria. As a result of this work, it is reasonable to suggest that the pH of reservoir fluids in geothermal system can be readily estimated by use of the SOLVEQ92 calculation program, which provides a ‘calculated’ pH value.     

Non-Conventional Behaviour of Castor Oil Under High Pressure

Castor oil shows non-conventional physical and chemical behaviour when submitted to pressure. This paper gives a brief presentation of the specific physical properties of castor oil under pressure up to 1.0 GPa. The changes of permittivity, dielectric loss and the structure of the high-pressure induced phases of castor oil are also described.     

High pressure since P. W. Bridgman - A personal view

Abstract

In this paper an attempt is made to review high pressure research over the past three or four decades from a personal viewpoint. Focus is first on two technical developments; the introduction of shock wave techniques in solids, and the extension of the static pressure range by a factor of 20–30.

The major emphasis is on the introduction of high pressure into the mainstream of modern science through the concept that pressure is a powerful, indeed an essential tool in understanding molecular and electronic properties of matter. These properties are classified under the headings (q) structure (x-rays) (b) molecular dynamics (c) pressure tuning spectroscopy (d) chemical and electronic rate phenomena (e) chemical synthesis.     

High pressure treatment of pharmaceutical drug delivery and related systems

Abstract

Solids like layer silicates (kaolinite and montmorillonite) as well as ZnO are in use in pharmacy. They are pressure treated and subsequently analyzed by means of ESR spectroscopy. The structural changes indicated by Fe³⁺ ions and paramagnetic defects are reponsible for the enhanced chemical activity of the pressurized systems.

Especially active Fe-O-species are formed which can react with intercacalated organic molecules. The most probable first step of such reactions will be a solid state single electron transfer. Spin probes and spin traps were used to prove this statements. The activation of the drug delivery systems by mechanical treatments is of importance for the stability of the drugs incorporated.     

Temperature and pressure dependencies of the crystal structure of the organic superconductor (TMTSF) 2 ClO 4

The crystal structure of (TMTSF)₂ClO₄ has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of b ^* /2 superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV. Received 10 April 2000 and Received in final form 27 September 2000     

Influence of Synthesis and Composition conditions on Strength Characteristics of Synthetic Carbonado-Type Diamonds

Polycrystalline diamonds carbonado were synthesized under pressure 6.0-12.0 GPa. Ni-Cr-Mo alloy and pure Ni were used as the catalysts. The strength was tested by static compression technique according to GOST 9206-80. Diamond powders APK 4 with the grit size 500/400 and 400/315 were made by crushing of the bulk polycrystalline samples and this diamond powders were under investigation. Influences of the synthesis pressure and the chemical treatment on strength of diamond carbonado were studied.     

Growth of diamond thin film on tool materials from a gas phase

Abstract

In the present paper, diamond films have been synthesized on tungsten carbide, sintered diamond and high pressure diamond by hot filament chemical vapour deposition method from the mixture gas of methane and hydrogen, and growth features of diamond were studied.     

Efficiency of an electron-beam-pumped chemical laser with an SF6-H2 working mixture

Results are presented from a study of HF lasers pumped by non-chain chemical reactions initiated by a radially convergent and by a planar electron beam. The main channels of formation of vibrationally excited HF molecules are analyzed. The distribution of the energy density of the radiation in the output beam of a wide-aperture laser is measured. In 30 liters of a mixture of SF₆:H₂=8:1 at a pressure of 1.1 atm an output energy of ∼200 J is obtained at an ∼11% efficiency with respect to the energy deposition. It is shown that the admixture of a buffer gas of neon or argon improves the uniformity of the radiation energy distribution in the output beam of an HF laser pumped by a non-chain chemical reaction and initiated by an electron beam, and it also increases the output energy. Zh. Tekh. Fiz. 69, 76–81 (January 1999)     

Pressure dependence of the absorption edge at the E0 gap of mocvd - Grown ZnTe films

Abstract

We have investigated the direct gap absorption of 1μm thick ZnTe-epilayers grown on GaAs substrates by metalorganic chemical vapour deposition (MOCVD). Free ZnTe-layers were obtained by selective etching. The absorption coefficient was measured up to about 50000 cm^?1 in a diamand anvil cell in the temperature range from 115–300 K. The spectra near the direct gap E₀ are dominated by a sharp excitonic structure. Its change with pressure is evaluated by a model which allows to determine the pressure shift of the gap energy dE₀/dP and the change of the Rydberg energies of the excitons dR*/dP.^[1]     

Physical mechanisms for the development of lightning discharges between a thundercloud and the ionosphere

An investigation is made of the influence of changes in atmospheric pressure with altitude and the thundercloud geometry on the development of lightning propagating upward to the ionosphere. It is shown that the mechanism for the development of high-altitude lightning does not differ from that for the formation and propagation of ordinary lightning between a thundercloud and the ground. It is established that high-altitude lightning forms as a result of a reduction in pressure with altitude and can only take place from thunderclouds located at high altitudes. Zh. Tekh. Fiz. 69, 134–137 (April 1999)     

Modeling pressure-ionization of hydrogen in the context of astrophysics

Abstract

The recent development of techniques for laser-driven shock compression of hydrogen has opened the door to the experimental determination of its behavior under conditions characteristic of stellar and planetary interiors. The new data probe the equation of state (EOS) of dense hydrogen in the complex regime of pressure ionization. The structure and evolution of dense astrophysical bodies depend on whether the pressure ionization of hydrogen occurs continuously or through a “plasma phase transition” (PPT) between a molecular state and a plasma state. For the first time, the new experiments constrain predictions for the PPT. We show here that the EOS model developed by Saumon and Chabrier can successfully account for the data, and we propose an experiment that should provide a definitive test of the predicted PPT of hydrogen. The usefulness of the chemical picture for computing astrophysical EOS and in modeling pressure ionization is discussed.     

The influence of etching parameters on the sensitivity of plastics

Abstract

It seems that the critical rate of primary ionization for track registration must also depend on the etching conditions. Simple experimental methods are presented to show a possible change in registration sensitivity as a function of the chemical parameters. It was proved, from investigations of α-particle tracks, that the sensitivity of a cellulose acetate detector can considerably depend on the concentration and type of etching reagent. Quantitative data are given on the ratio of the rate of chemical etching along the track and on the detector surface as a function of concentration of NaOH solution for tracks of fission fragments registered in plastics of cellulose acetate, polycarbonate and polyethylene-terephthalate.