Transmission electron microscopy (TEM) images of clathrate hydrates were obtained with their electron diffraction patterns. A tetrahydrofran (THF) hydrate sample was used as the specimen, which was prepared by quenching or slow crystal growth methods from THF aqueous solution. The sample temperature was maintained at approximately K under a high vacuum condition (10^-5Pa) because clathrate hydrates are unstable at low pressure and are too brittle to undergo electron irradiation. Crystallographic structure characterization procedures from one diffraction pattern were proposed because we were able to obtain only one diffraction pattern of clathrate hydrate for each real image, owing to its instability under TEM observation conditions and to limitations of the apparatus. The validity of the proposed procedures was confirmed by ice crystal observations. The real image indicated that THF hydrate samples were either thin film-like specimens or small granular ones (less than nm in diameter). Even if THF hydrate had the morphology of a thin film, the diffraction pattern had spots similar to those of a single crystal. Type-II cubic structures of THF hydrates were identified by the electron diffraction patterns. Although most of the sample was found to be hexagonal ice Ih crystals, THF hydrates existed approximately of the specimen.

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In this study, the cage occupations of guest molecules in gas hydrates formed from methane-carbon dioxide, methane-ethane, and methane-propane mixtures are estimated using cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance (CP-MAS 13C NMR) technique. The 13C chemical shifts of the guest molecules depended on the type of hydrate cage. The cage occupations of two guest components were estimated from the integrated intensity ratio of the obtained NMR signals. In the methane and carbon dioxide system, the number of methane molecules in the large cages of structure I tended to decrease with an increase in the concentration of carbon dioxide as a second component. On the other hand, in the methane-ethane and methane-propane systems, the large cages were predominantly occupied by C2H6 or C3H8 rather than CH4 molecules. The cage occupancies of methane-ethane and methane-propane gas hydrates were almost full in the case of large cages, but a few small cages remained vacant.

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The dissociation heat of gas hydrates is one of the important parameters that control the formation/dissociation speed of the gas hydrate. We investigated the effect of the ethane concentration on the dissociation heat of the mixed-gas (methane and ethane) hydrate. The dissociation heat of the mixed-gas hydrate [kJ mol-1] was within the range between the dissociation heat of pure methane hydrate and that of ethane hydrate, and it increased with the ethane concentration. Only several % of ethane composition was quite effective to increase the dissociation heat. The hydration number, which changed according to the crystal structure (sI/sII hydrates), determined the dissociation heat during the transition from hydrate to gas and water. The dissociation heat of the mixed-gas hydrate [kJ kg-1] was 6-30 % greater than that of pure methane and ethane hydrates during the transition from hydrate to gas and ice, and 3-8% greater during the transition from hydrate to gas and water.

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We carried out “in situ” observation of the growth of the Xe clathrate hydrate crystal on an ice surface at various pressures and temperatures. Xe hydrate nucleated and grew along the ice surface. The growth morphology changed with an increase in excessive pressure as follows: polygonal, middle type and circular. The analysis of growth rate and morphology indicated layer by layer growth at excess pressure below 0.06MPa. At excess pressures below 0.02MPa, the growth of the Xe hydrate would be controlled by the formation of a screw dislocation. The growth mechanism changed to adhesive growth at excess pressure above 0.06MPa, because of roughening, and the corresponding growth morphology changed to circular.

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[in Japanese]

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[in Japanese]

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火星や氷衛星に見られる流動地形や断層地形の形成過程を明らかにするためには，氷・岩石混合物 のレオロジーを知ることが重要となる．本研究では氷・シリカ混合物の変形実験を行い，力学強度に 対するシリカ含有率と温度の影響を調べた．シリカは直径1μmのものを用いて，その含有率は0か ら63vo1％ まで変化させた．実験は－10℃から－25 ℃まで制御した低温室で等歪速度一軸圧縮変形 実験を行い，歪速度は2.9×10-3s-1から8.5×10-7s-1まで変化させた．今回求めた流動則は，応力-歪 み曲線上の最大応力σmaxと歪速度εの関係式ε＝A・σmax nで表される．実験の結果，温度が－10℃の 場合，シリカ含有率が0.4vol％ から4vol％では最大応力が純氷とほぼ同じとなりnは約3となっ た．一方，シリカ含有率が15vol％ 以上になると， 最大応力はシリカ含有率が増加するほど大きくな りnは約6と大きくなった．温度を変化させた実験では，シリカ含有率が15vo1％ の場合，nに温度 依存性はなく約6.2となった．また， シリカ含有率が29vol％ と63vol％の場合は，脆性一塑性境界が 純氷の境界よりも30 ～50 ℃高くなった．

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[in Japanese]

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