Kazushige Nagashima

High-pressure structures of methane hydrate observed up to 8 GPa at room temperature

Three high-pressure structures of methane hydrate, a hexagonal structure (str.A) and two orthorhombic structures (str.B and str.C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str.I) decomposed into the str.A and fluid at 0.8 GPa. The str.A transformed into the str.B at 1.6 GPa, and the str.B further transformed into the str.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structural changes occurring with increasing pressure were observed reversibly with decreasing pressure. The symmetric stretching vibration, ν1, of the methane molecule observed in the Raman spectra changed along with the structural changes. The bulk moduli, K0, for the str.I, str.A, and str.C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively. The difference in the bulk moduli implies the difference in fundamental structure of the high-pressure structures.Three high-pressure structures of methane hydrate, a hexagonal structure (str.A) and two orthorhombic structures (str.B and str.C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str.I) decomposed into the str.A and fluid at 0.8 GPa. The str.A transformed into the str.B at 1.6 GPa, and the str.B further transformed into the str.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structural changes occurring with increasing pressure were observed reversibly with decreasing pressure. The symmetric stretching vibration, ν1, of the methane molecule observed in the Raman spectra changed along with the structural changes. The bulk moduli, K0, for the str.I, str.A, and str.C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively. The difference in the bulk moduli implies the difference in fundamental structure of the high-pressure structures.

Nonequilibrium effect of anisotropic interface kinetics on the directional growth of ice crystals

Directional growth experiments were carried out on ice crystals in a water-KC1 solution system. It is well known that the effect of anisotropic interface kinetics causes tilting of the cell axes. The nonequilibrium effect of anisotropic interface kinetics, namely the kinetic supercooling ΔTk, was estimated by observing the tilt angle of the cell axes, based on the theoretical prediction of linear stability analysis for small perturbations on a planar interface during directional growth. It was found that ΔTk of an ice crystal during directional growth is minimum in the 〈1120〉 direction, i.e., the dendrite tip growth direction, and maximum in the 〈1010〉 direction. Therefore, it is shown that the preferred growth direction determined by anisotropic interface kinetics coincides with the direction by anisotropic interface free energy.

In situ observation of gas hydrate behaviour under high pressure by Raman spectroscopy

The dynamic process and kinetics of gas hydrate formation and dissociation have been studied by means of Raman spectroscopy. The formation and dissociation of methane and carbon dioxide hydrates as well as the phenomenon of interchange of the components were observed as functions of time, temperature and pressure conditions. From the direct observation, it was found that the reformation of carbon dioxide hydrate occurred at the surface and inside the sample of methane hydrate as well as producing ice crystals.

Time development of a solute diffusion field and morphological instability on a planar interface in the directional growth of ice crystals

Abstract The solute distribution in front of directionally growing ice crystals in a water–KCl solution was observed in situ as well as the three-dimensional interfacial patterns, using a Mach–Zehnder interferometer. We give a direct evidence for the constitutional supercooling and discuss the wavelength selection of perturbation. The morphological instability occurred much earlier than the establishment of the steady-state condition of diffusion field. The steady-state analysis using Mullins–Sekerka theory could not predict the wavelength of perturbation. However, when the measured values of solute distribution and growth rate at the time of instability are substituted for the parameters assuming the quasi-steady state, it is found that the analysis precisely predicts the experiments.

Laser trapping of ice crystals

Toward application to crystal physics, we demonstrate optical trapping of ice crystals as well as supercooled water droplets using counterpropagating laser beams. Confinement of an ice crystal is evidenced by the angular distribution of laser light deflected from the crystal faces. The average trap time, limited by air currents, is 5s for ice crystals and much longer for water drops.

Methane hydrate, amoeba or a sponge made of water molecules

Abstract A new technique fabricating single crystals and polycrystalline aggregates of methane hydrate at room temperature under high pressure was established using a diamond anvil cell. In-situ observations by optical microscopy and X-ray diffractometry revealed high-pressure behavior up to 5.5 GPa, including growth, a compression process associated with changes in cage occupancy, and decomposition into high-pressure ice and solid methane. Interesting features such as an amoeba-like motion at crystallization and sponge-like behavior with pressure changes were observed. Cage occupancy, the so-called hydration number, was estimated from the relative intensity of the X-ray diffraction pattern, and changes in cage occupancies dependent on pressure were clearly observed.

Growth Mode Transition of Tetrahydrofuran Clathrate Hydrates in the Guest/Host Concentration Boundary Layer

Clathrate hydrates are known to form a thin film along a guest/host boundary. We present here the first report of tetrahydrofuran (THF) clathrate hydrate formation in a THF/water concentration boundary layer. We found that the THF-water system also forms a hydrate film separating the guest/host phases. The lateral growth rate of the film increases as supercooling increases. The thickness of the film at the growth tip decreases as supercooling and the lateral growth rate increase. These tendencies are consistent with reports of experiments for other hydrates and predictions of heat-transfer models. After film formation and slight melting, two types of growth modes are observed, depending on temperature T. At T = 3.0 degrees C, the film slowly thickens. The thickening rate is much lower than the lateral growth rate, as reported for other hydrates. At T < or = 2.0 degrees C, however, the growth mode transitions spontaneously from film growth to continuous nucleation, and an agglomerate of small polycrystalline hydrates forms in each phase. Grain boundaries in the film and pore spaces in the agglomerate act as paths for permeation of each liquid. Timing when continuous nucleation starts is dominantly controlled by the time of initiation of liquid permeation through the film. Digital particle image velocimetry analysis of the agglomerate shows that it expands not by growth at the advancing front but rather by continuous nucleation in the interior. Expansion rates of the agglomerate tend to be higher for the cases of multipermeation paths in the film and the thinner film. We suppose that the growth mode transition to continuous nucleation is caused by the memory effect due to slight melting of the hydrate film.

Effect of Inhibitor Methanol on the Microscopic Structure of Aqueous Solution

Several experimental investigations have been carried out to study the stability of methane hydrates under various conditions. 1–7 Methanol has been used as an inhibitor of gas hydrate formation for a long time. It is well known that the equilibrium temperature of gas hydrates is reduced by the addition of methanol. 1 The addition of some cyclic ethers, such as THF, also changes the equilibrium condition of gas hydrates 1 and they are added mainly to promote gas hydrate formation. Recently, studies were carried out to clarify the effect of methanol on hydrate formation on a molecular level. 6,7 From a microscopic viewpoint, these compounds are expected to change the clustering structure of liquid phase from that of pure water, leading to the change of the crystallization properties of gas hydrates. Therefore, it is important to determine the correlation between the crystallization properties and the microscopic liquid structure of an aqueous solution in order to gain further understanding of the gas hydrate formation mechanism. We previously reported that the stable clusters that were formed from methanol–water and THF–water mixtures have rather different structures. 8 However, this measurement was carried out only for comparatively low concentration solutions.

Structure and dynamics of empty cages in xenon clathrate hydrate

We performed molecular dynamics calculations of xenon clathrate hydrate to investigate the effects of empty cages on the structure and dynamics of the surrounding lattice. The distinct structure and dynamics of the empty cages, and cages including Xe, which coexist in the lattice, were analyzed. The results show that the ellipsoidal tetrakaidecahedral cage shrinks along the minor (100) axis and expands along the major (100) axis due to the absence of Xe from the cage, whereas the dodecahedral cage shrinks isotropically. These distortions of the empty cages cause a reduction in the lattice constant and an enhancement of the thermal vibrations of the surrounding lattice. The vibrational density of states shows that the hydrogen bonds consisting of the tetrakaidecahedral cage are strengthened by the absence of Xe, whereas those of the dodecahedral cage are weakened. These results show differing mechanisms of guest-host interaction for the two types of cages including Xe. Repulsion is the dominant guest-host interaction for the dodecahedral cage, as proposed by previous studies. For the tetrakaidecahedral cage, however, attractive interaction is dominant along the major (100) axis, whereas repulsive interaction is dominant along the minor (100) axis. The present results suggest that a small number of empty cages can affect not only the local structures but also the macroscopic properties of the crystal. It is concluded that the distortions of the empty cages are one of the important factors governing the density and phase equilibrium of clathrate hydrates.

Formation of Periodic Layered Pattern of Tetrahydrofuran Clathrate Hydrates in Porous Media

Directional growth of tetrahydrofuran (THF) clathrate hydrates was studied in a mixture of glass beads and a stoichiometric THF-water solution. Results showed that disseminated pore space type hydrates formed in a mixture containing 50-microm beads. However, a pure hydrate layer formed pushing the beads in a mixture containing 2-microm beads (frost heaving of hydrates). As the growth proceeded, new layers were formed repeatedly, leading to the eventual formation of a periodic layered pattern. It was found that as the growth rate increased, both the thickness of a hydrate layer and the interval between the neighboring layers decreased according to power laws. The effects of the applied temperature gradient and the weight ratio of the solution and glass beads were also systematically studied. Further, the possibility of applying our model experiments to the formation of natural methane hydrates was discussed.