Phase transformation of methane hydrate under high pressure

In situ Raman spectroscopy is employed to study the phase behavior of methane hydrate at high pressure. The structure 1 of methane hydrate can be maintained up to 950 MPa at 299 K. The transformation of structure I<-->structure H+water+CH4 occurs at 880 MPa and 323 K. The structure H of methane hydrate, however, decomposes to methane and water at 960 MPa and 348 K. The initiation mechanism of methane hydrate sI is also discussed.     

Structural, electronic, and dynamical properties of methane under high pressure

The electronic structure and lattice dynamical properties of solid methane under high pressure have been studied based on density functional theory. We identify a cubic structure with space group of I43m below 14 GPa, the Pmn2(1) structure in the range of 14-21 GPa, and the P2(1)/c structure from 21 to 65 GPa. Our obtained Raman spectra of the P2(1)/c structure agree well with the typical Raman active modes in the available experimental data. At 65 GPa, methane undergoes a phase transition from P2(1)/c to Pnma. The structures with P2(1)/c and Pnma symmetries are insulating, and under any pressure studied methane always remains in molecular form. For Pnma phase, the orientational ordering of CH(4) molecules varies significantly at 79, 88, and 92 GPa, and by further increasing pressure the rotation of the molecules freezes and orientational ordering remains unchanged.     

Activation of methane in microwave plasmas at high pressure

Methane is converted to C2 products in a microwave plasma under pressure up to 400 torr at maximum plasma power of 100 W. Steam is introduced with methane into the plasma zone in order to suppress coke formation. Major products are C2 hydrocarbons. Small amounts of benzene are also formed. Very small amounts of some unusual highly unsaturated hydrocarbons are also formed. Oxygenated products are CO and CO₂. The conversion and yields are related to experimental variables by an empirical second order linear model. The conversion of methane ranges from 10 to 60%. The yield of C2 products ranges from 5 to 68%. The major C2 product is acetylene.     

Protein aggregate structure under high pressure

We show that casein protein micelles--a complex protein/inorganic phosphate structure--can be subjected to high pressures (up to 350 MPa) while making in situ structural measurements using (ultra-)small angle neutron scattering, to give insight to the protein structure, aggregation and stability under pressure.     

Structural investigations under high pressure conditions

Because of its extremely high yield strength, hardness and relatively high transparency to high-energy x-ray radiation, the diamond anvil cell has become a favored device in high pressure research. This includes structural investigations where pressure induced phase transitions can be studied using x-ray diffraction techniques up to several Mbars when combined with a high-brilliance synchrotron source. In this paper several structural studies under high pressure are presented to show general trends. High pressure instrumentation has reached the stage at which hydrogen, according to theoretical calculations, can be metallized and thermodynamic conditions, (i.e. pressure and temperature) exceed those at the center of the earth.     

Crystallization kinetics of polyethylene under high pressure

The kinetics of isothermal crystallization of polyethylene under high pressures ranging from 840 to 5300 kg/cm² has been studied dilatometrically. The crystallization rate estimated from the half-time of the overall transformation increases markedly with pressure. The Avrami exponent n becomes smaller with increasing pressure. Values of n ≈ 2 for the crystallization at 840 and 1950 kg/cm², and n ≈ 1 at 5100 and 5300 kg/cm² were obtained. Differential scanning calorimetry and electron microscopy data are presented. It is concluded that extended-chain crystals grow rapidly, predominantly in one dimension, at high pressure. Relations between log k and T_m/T(ΔT) and T_m²/T(ΔT)² are nearly linear. Here, k is the crystallization rate constant from an Avrami equation, ΔT = T_m – T, T_m is the melting point, and T is the temperature of crystallization. From the dependence of the slope of the straight line on the crystallization pressure it is concluded that the surface energy of crystal nuclei decreases with increasing pressure.     

Emulsion polymerization of styrene under high pressure

Summary The emulsion polymerization of styrene was studied at 40 °C over a pressure range of 1 to 1000 bar. By measuring the ratio of the overall reaction rate under high pressure and at atmospheric pressure, the activation volume for the prouagation was obtained as –23.5 cm³/mol which was rather smaller than the literature values. The underestimation by the method of emulsion polymerization would be due to the character of polymerization processes and the emulsion polymerization method is not appropriate to determine the accurate value of the activation volume of propagation reactions.

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Zusammenfassung Es wird die Emulsionspolymerisation von Styrol bei 40 °C über den Druckbereich von 1 bis 1000 bar untersucht. Durch Messung des Verhältnisses der Reaktionsgeschwindigkeit unter hohem Druck und bei Atmosphärendruck wurde das Aktivierungsvolumen für die Ausbreitungsgeschwindigkeit zu –23,5 cm³/mol gefunden, ein Werr, der ziemlich kleiner als die Literaturwerte ist, Die Unterschätzung bei der Methode der Emulsionspolymerisation dürfte dem Charakter des Polymerisationsprozesses zuzuschreiben sein, und die Emulsionspolymerisation ist nicht geeignet, den exakten Wer für das Aktivierungsvolumen zu erhalten.

     

Simulations of polymer crystallization under high pressure

High pressure crystallization of polypropylene was studied by means of PVT measurements and computer simulations. The isothermal crystallization behaviour were described by using a model which takes into account the effect of pressure on the temperature dependence of nucleation rate and linear growth rate. The agreement between the simulation and the experiments was seen in the tendency that the crystallization was accelerated by the high pressure. The non-isothermal crystallization behavior was also simulated by applying a generalized Avrami equation. The simulation curves well reproduced the experimental values below relative crystallinity 0.5 and below 100 MPa.     

Coke formation during high pressure catalytic partial oxidation of methane to syngas

Summary Carbonization of the surface of alumina-based catalysts has been studied with respect to the composition of the catalysts and conditions of the propionitrile ammonolysis. It was shown that the surface concentration of carbon increases with the increase in temperature and with time of the reaction and depends on the catalysts nature in the order: Al-Zr(5)-O &lt; Al-Zr(40)-O &lt; Al-O &lt; Al-Mg-O. The surface concentration of the Br&ouml;nsted acidic sites follows the same sequence.     

Synthesis of 5-ketotetrazoles under high pressure

Conclusions The reaction of -ketonitriles with hydrazoic acid and organic azides at high pressure is an effective method for the synthesis of 5-acyltetrazoles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1168–1170, May, 1988.     

Synthesis of titanium sulfides under high pressure

Titanium sulfides were synthesized in the temperature range 600–750°C and in the pressure range 10–200 MPa with an apparatus used for hydrothermal synthesis. Stoichiometric TiS₂ cannot be prepared under atmospheric pressure, but was synthesized under high pressure. Also the phase relationship between TiS₂ and TiS₃ has been established under high pressure up to 200 MPa, and the pressure-temperature phase diagram for TiS system is presented.     

Lamellar thickening of polyethylene under high pressure

Single crystal mat (SCM) samples of polyethylene (PE) were prepared from dilute solution of p-xylen, then they were annealed at pressures of 200 and 500 MPa. Lamellar thickness of the original and annealed SCM samples was measured by small-angle X-ray scattering method. Orientation of the molecular chain in those SCM samples was investigated by wide-angle X-ray diffraction pattern. From these X-ray measurements, annealing temperature dependence of the lamellar thickness, i.e., lamellar thickening, under high pressure was obtained. Melting process of the SCM samples was also investigated at 200 and 500 MPa by high pressure differential thermal analysis. Then correspondence between the lamellar thickening and the melting process was studied. The lamellar thickness increases markedly with approaching to the melting temperature of the orthorhombic crystal even in the high pressure region where the high pressure phase (hexagonal phase) appears. The annealing temperature dependence curve of the lamellar thickness at 200 MPa can be superimposed on the curve at 500 MPa by shifting the curve along the temperature scale by 47 K. Large scale lamellar thickening occurs in the orthorhombic crystal phase in the high pressure region. The formation process of extended-chain crystal is discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 535–543, 1997     

Carbon dioxide and methane adsorption at high pressure on activated carbon materials

Granular and monolith carbon materials were prepared from African palm shell by chemical activation with H₃PO₄, ZnCl₂ and CaCl₂ aqueous solutions of different concentrations. Adsorption capacity of carbon dioxide and methane were measured at 298 K and 4,500 kPa, and also of CO₂ at 273 K and 100 kPa, in a volumetric adsorption equipment. Correlations between the textural properties of the materials and the adsorption capacity for both gases were obtained from the experimental data. The results obtained show that the adsorption capacity of CO₂ and CH₄ increases with surface area, total pore volume and micropore volume of the activated carbons. Maximum adsorption values were: 5.77 mmol CO₂ g^?1 at 273 K and 100 kPa, and 17.44 mmol CO₂ g^?1 and 7.61 mmol CH₄ g^?1 both at 298 K and 4,500 kPa.     

DTA measurements on polymers under high pressure

The construction of a high-pressure (up to 800 MPa), low-temperature (173–600 K) differential thermal analysis (DTA) apparatus is described. Some results on the phase transitions in polyethylene and poly(diethylsiloxane) under high pressure are presented.

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Zusammenfassung Mittels DSC wurde im Bereich 323 bis 1273 K die spezifische Wärme von Bi₄Ge₃O₁₂- und Bi₄Ti₃O₁₂-Einkristallen untersucht. Unter Zuhilfenahme des normalen Wärmekapazitätskurvenverlaufes für Bi₄Ge₃O₁₂ wurde derjenige Temperaturbereich bestimmt, in dem in Bi₄Ti₃O₁₂ die anomale Umwandlung von einer polarisierten in eine unpolarisierte Phase verläuft. Wärmeeffekt und Entropieänderung für diese Umwandlung wurden ermittelt.

     

Free volumes in solids under high pressure

A review of experimental work on ortho-positronium (o-Ps) lifetime in solids under the pressures up to 1 GPa is presented. Among the effects observed at high pressure one can mention: the disappearance of the energy level for Ps at the reduction of free volume size; pressure induced phase transitions; variation of Ps formation intensity with time; increase of o-Ps lifetime after intercalation of high pressure gas to the paraffin samples.     

High-resolution optical spectroscopy of benzophenone under high pressure

Phosphorescence spectra of benzophenone have been investigated with a diamond anvil cell. At 2 K the spectra remain sharp under high pressure. The pressure shift of the origin exhibits a plateau around 25 kbar. This and other spectral observations are interpreted.     

NQR of hydrogen bonded crystals under high pressure

The work presents the results of NQR under high hydrostatic pressure for the following ferroelectrics: KDA, KD^xA, LHA, LDA and AHCA. Most of the below presented discussions concerns the behaviour of these ferroelectrics and the explanation of this behaviour on the basis of pseudo-spin-lattice coupling model.