Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures

This paper is the first part of an investigation of carbothermal nitridation of alumina at elevated nitrogen pressure. The effect of some process parameters on nitridation rate was studied at different temperatures (1300–1700 °C) and pressures (0.1–5 MPa). The experiments were performed in a graphite furnace permitting a controlled nitrogen flow through the sample holder, and continuous measurement of CO in the outlet gas. The products were characterized by X-ray diffraction, BET and elemental analyses. The nitridation rate was found to be a function of the process parameters. For a suitable temperature and pressure, e.g. 1600 °C and 1 MPa, pure AIN was obtained after just 1 h, with very low residual oxygen content (0.4 wt%). At a high gas-flow rate (⩾5 I min⁻¹), the optimum range of pressure was 0.5–1 MPa at 1550–1700 °C. The nitridation was hampered by even small amounts of CO added to the system. No intermediate phases formed under the experimental conditions studied. Thermodynamic calculations on the Al-O-C-N system were performed in a search for explanations of the results.     

Adsorption and diffusion of nitrogen,oxygen, argon,and methane in molecular sieve carbon at elevated pressures

Diffusion and equilibrium adsorption of nitrogen, oxygen, argon, and methane on molecular sieve carbons 3A and 5A were studied at 273, 303, and 323°K. The adsorbate pressure was varied over a pressure range of 0–1.3 MPa in a gravimetric setup. On a molar basis, methane had the highest adsorption capacity among the four gases studied. Oxygen and nitrogen had approximately similar equilibrium adsorption capacity. Equilibrium adsorption data were represented by the Langmuir model and the vacancy solution model (VSM). Langmuir equation showed deviations at high pressures. The isosteric heat of adsorption was found to increase slightly with an increase in surface coverage. Kinetic data showed that oxygen had the highest diffusion rate and methane the lowest. Diffusivities were found to be generally independent of pressure.     

The thermal conductivity of pure nitrogen and of mixtures of nitrogen and carbon dioxide at elevated temperatures and pressures

The thermal conductivities of nitrogen at 470 K and six mixtures of nitrogen and carbon dioxide at various temperatures have been measured as a function of pressure up to 25 MPa. The mixtures were measured at the following temperatures: one at 302 K, three at 380 K, one at 430 K, and one at 470 K. The data were used to test three prediction methods for the thermal conductivity of gas mixtures under pressure. Surprisingly good agreement was found with predictions using the corresponding-states method of Ely and Hanley. The predictions of the more theoretically based method of Mason et al. were low throughout, due partly to its use of the Hirschfelder-Eucken equation as the low-density limit, but also because the predicted density dependence rises too slowly. The simplified version of this method proposed by Svojskij gave slightly worse predictions, particularly at higher densities. The zero- density results for nitrogen are examined by comparing the zero-and first-density coefficients with the trends shown at lower temperatures.     

Nanoparticle effects on the crystallization of polyethylene at elevated pressures

We describe here phase and morphological changes that are induced when polyethylene-containing dispersion of nanoclay is isothermally crystallized in the pressure range of 0.1–150 MPa. To delineate and separate the effects of applied crystallization pressure from nanoparticle effects, a relative comparison is made between neat polyethylene and polyethylene containing dispersion of nanoclay under similar experimental conditions. X-ray diffraction studies support the evidence of monoclinic phase at high crystallization pressure in addition to commonly observed orthorhombic phase of polyethylene. The nucleation of monoclinic phase is encouraged by nanoclay even at low crystallization pressures. Furthermore, orientation effects were observed in the nanocomposite system. The equilibrium melting point is insignificantly influenced on the addition of nanoclay and weakly refines the structure, such that the crystallization pressure has no obvious effect. The weak thermodynamic interaction between nanoclay and polyethylene compared to polypropylene–nanoclay system is supported by the small shift in glass transition temperature and no changes in the modification of absorption bands of polyethylene in Fourier transform infrared spectrum.     

Investigation of the reactions of SiCl4 and O2 at elevated temperatures by infra-red spectroscopy

The modified chemical vapour deposition (MCVD) process for making fibre optic preforms uses direct oxidation of SiCl₄, together with dopant chlorides to produce core glass. This reaction has been studied by direct infra-red absorption spectrophotometry of the flowing gas system. The spectrum of the SiCl₄ reactant is recognizable, its decrease with increasing temperature was measured, and the absorption spectra of SiO₂ and of silicon oxychloride reaction products were recognized in the flowing gas stream. The oxychlorides began to appear in the spectra at 900° C and increased with temperature to 1110° C. Their concentration decreased drastically between 1110° C and 1160° C above which only the spectrum of SiO₂ was observed. The identities of the oxychloride molecular species produced are uncertain, but small amounts of Si₂OCl₆ and the cyclic Si₄O₄Cl₈ were present together with several unidentified higher molecular weight compounds. The particulate matter formed in the MCVD process shows infra-red spectra varying with conditions of formation. At temperatures below 1160° C the particulate matter shows absorption bands attributable to silicon oxychlorides. At higher temperatures, the particulate matter is free of oxychloride bands and shows only the proper SiO₂ spectrum. If moisture is present, these spectra are modified.     

Investigation of NO detection in flames by the use of polarization spectroscopy

Various aspects of detection of NO molecules in a premixed H(2)/N(2)O flame were investigated by the use of polarization spectroscopy. The signals were recorded in the A(2)Σ(+)-X(2)II (0, 0) band of NO at 226 nm. Excitation spectra for circularly and linearly polarized pump beams are presented to illustrate the influence of the polarization state. The signal dependencies on the pump- and the probe-beam intensities were also investigated, as well as the influence of the setup geometry with respect to the pump-beam focusing and the crossing angle between the pump and the probe beams. A preliminary evaluation of temperature based on spectra recorded by the use of the Boltzmann plotting approach was performed. Finally the possibilities of two-dimensional imaging of the NO molecules were demonstrated.     

Calculation of the thermodynamic properties of nonpolar wet gases at elevated pressures

A calculation procedure and results forsteam solubility in compressed gases and the thermodynamic properties of wet gases at elevated pressures using the Lennard-Jones potential and semi-empirical rules for combining potential parameters are presented.All-Russian Scientific-Research Center for Materials Standardization, Information, and Certification, Moscow, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 6, pp. 887–896, November–December, 1995.     

Thermoelectric properties of silicon at high pressures in the region of the semiconductor-metal transition

The thermo emf in Czochralski grown silicon single crystals annealed at 450°C was experimentally studied in a range of pressures up to 16 GPa in a chamber with synthetic diamond anvils. There is a correlation between the curves of thermo emf versus pressure, the semiconductor-metal transition pressure, and the mechanical properties (microhardness, compressibility) of samples with various oxygen content. The values of thermo emf in the high-pressure metallic phases have been determined.     

Disorder in the molecular conformation of crystalline isotactic polypropylene at high pressures

Crystalline IPP was compressed non-hydrostatically and the Raman spectra of CC-chain coupled bands monitored. The spectra indicated an increase in chain kinking with increasing pressure.     

Flame acceleration and the development of detonation in fuel-oxygen mixtures at elevated temperatures and pressures

Experimental measurements of the conditions required for the development of detonation in a 7 mm tube following ignition by a low energy spark are reported. There are then compared to previous experimental propagation limit criterion using theoretical predictions of detonation cell sizes based on a one-dimensional detonation length scale computed using a detailed chemical kinetic scheme. Technical difficulties precluded direct cell size measurements. Ethylene-oxygen and hydrogen-methane-oxygen mixtures were investigated as well as methane-ammonia-oxygen, at initial pressures and temperatures in the ranges 1-7 bar and 293-540 K, respectively. The likelihood of detonation in ethylene-air mixtures in 150 mm and 50mm pipes at ambient initial conditions is also discussed in relation to published cell width data.The results indicate that whilst detonation cell width predictions do not provide a quantitative measure of the conditions for which detonation may develop in a pipe of given diameter, for prescribed initial conditions, predicted detonation cell size data does provide useful qualitative guidance as to possible hazardous compositions, particularly if preliminary experimental safety testing is thought to be necessary.     

The limits of flammability of mixtures of ammonia,hydrogen and methane in mixtures of nitrogen and oxygen at elevated temperatures and pressures

As part of a program investigating the flammability characteristics of ammonia in mixtures with air, nitrogen and ammonia side-products, the flammabili The flammable regions are not as large as those found in a previous study for mixtures of ammonia and hydrogen, but are still remarkably wider than the     

Investigation of the thermodynamic properties of nitrogen at pressures up to 8 kbar and temperatures up to 1800°K

A procedure is described and the results are given of the experimental determination of the density of nitrogen at high pressures and temperatures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 2, pp. 280–286, August, 1977.     

Auto-ignition and upper explosion limit of rich propane-air mixtures at elevated pressures

The auto-ignition limits of propane-air mixtures at elevated pressures up to 15 bar and for concentrations from 10 mol% up to 70 mol% are investigated. The experiments are performed in a closed spherical vessel with a volume of 8 dm3. The auto-ignition temperatures decrease from 300 degrees C to 250 degrees C when increasing the pressure from 1 bar to 14.5 bar. It is shown that the fuel concentration most sensitive to auto-ignition depends on initial pressure. A second series of experiments investigates the upper flammability limit of propane-air mixtures at initial temperatures up to 250 degrees C and pressures up to 30 bar near the auto-ignition area. Finally the propane auto-oxidation is modelled using several detailed kinetic reaction mechanisms and these numerical calculations are compared with the experimental results.     

Carbon nanotube-induced structure and phase evolution in polymer-based nanocomposites crystallized at elevated pressures

The dispersion of small weight fraction of carbon nanotube (CNT) in Nylon 6,6 introduces a significant difference in the structure and phase evolution during crystallization at ambient and elevated pressures. In the nanocomposite, the γ-phase is promoted at low crystallization pressure of ～0.1–25 MPa and is in striking contrast to pure Nylon 6,6, where γ-phase is nucleated only at crystallization pressures exceeding ～50 MPa. The differences in the behavior of Nylon 6,6 and its nanocomposites is attributed to CNT–polymer interface driven nucleation, which is also responsible for significant reduction in spherulite size and increase in crystallinity. The nanoindentation behavior of the nanocomposite is assessed via nanoscale deformation experiments, which indicated that a significantly higher indentation-force is required for the Nylon 6,6–CNT nanocomposite as compared to Nylon 6,6 to produce a constant displacement. The observed significantly higher modulus and hardness is primarily CNT-induced effect. This observation is of particular relevance to functional devices because they are most likely to experience force in the nanonewton range, which can induce deformation at the micro- and/or nanometer scale.     

Soft X-ray generation and its role in breakdown of air gap at elevated pressures

Fast runaway electrons, which appear during the electric breakdown in an air-filled discharge gap under the action of a short-front voltage pulse, generate 525-eV photons from the K-shell of oxygen. This soft X-ray emission effectively generates new electrons in the region of enhanced field. The proposed mechanism may explain the fast streamer propagation and the formation of a diffuse discharge at atmospheric pressure.     

Effect of simple stress on the glass transition of polymers at high pressures

Experimental studies, which have been carried out in this laboratory, showed the yield strength in tension, compression, and shear in the rubbery and the glassy states increased with increasing hydrostatic pressure. Moreover, the Young's modulus also increased with pressure and the amount of the increase across the glass transition temperature (T _g) at a given pressure can be as large as three orders of magnitude in the case of elastomers. An extension of the Gibbs-Dimarzio theory is proposed to account for the effect of applied stress on the glass transition temeprature of glass-forming polymers. When a simple stress, such as tensile, compressive or shear stress, is applied to a polymer, the T _g will decrease, compared to a polymer without applied stress. A glass-forming polymer in the vicinity of the transition would behave differently from that predicted by rubber elasticity. The partition function taking into account the effect of stress is suggested to be $$\Gamma = \Sigma W(f, n_0 ){\text{ }}\exp {\text{ }}[ - \beta (PV + U - \sigma V\varepsilon )]$$ where the strain ?=σ (f ? f ₀) in which f and f ₀ are the fraction of flexed bonds with and without stress, respectively. Furthermore, by this model, the Young's modulus across the transition, E _L and E _G, can be evaluated. The Young's modulus increases with increasing pressure at lower and moderate pressure range but the increase is rather small at very high pressure range.     

Theoretical study of the thermodynamic properties of nitrogen at high temperatures and pressures

A theoretical analytical equation of state is derived and used to compute new thermodynamic data for nitrogen at high tempratures and pressures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 6, pp. 975–983, December, 1987.