A computational model to analyse the synthesis process of carbonado type synthetic polycrystalline diamond was performed through numerical simulation. In this model it is possible to analyse the voltage, the electrical power and the temperature distribution for different construction designs in both steady and nonsteady states. The simulation results allowed an understanding of the influence of the design on the temperature profile as well as the effect of increasing voltage in the temperature distribution during the process.

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Fine diamond particles (from 0. 5 to 2μm) were successfully coated with silicon of 6. 6, 13. 8 and 20. 0vol% by PVD method. These coated particles were sintered under 5. 5GPa at 1723K for 0. 5h with a cubic type ultrahigh pressure apparatus. Silicon carbide in each sintered diamond compact was formed between the diamond particles during sintering and acted as a bonding material for the diamonds. The obtained sinter having silicon carbide of 21. 8vol% was fully densified and showed very high hardness (about 9700 HV 1. 0) as high as the diamond sinter produced for industrial usage.

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Laser heating with nanosecond pulses was performed in DAC for the first time. The heated graphite transforms to a transparentdiamond-like state at pressures up to 35 GPa as follows from the micro-Raman studies. The scanning transmission electron microscopy (STEM) analysis shows the heated material is intermediate between the sp² and sp³ materials. Other observed phenomena are connected with ablation. Spherical droplets and nanocrystals were found at the surface as a product of condensation of the evaporated carbon. Possibly nitrogen (high-pressure medium) reacts with the species of ablated carbon. STEM analysis showed the incorporation of nitrogen in the heated sample.

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Three kinds of scandium-zinc intermetallic compounds, ScZn₂, Sc₃Zn₁₇ and Sc₁₃Zn₅₈ have been prepared from the elements under high pressure and high temperature. ScZn₂ crystallizes in the hexagonal system with lattice constants a=5. 2509Å, c=8. 4774Å, c/a=1. 6147, and unit cell valume 202. 42ų. Sc₃Zn₁₇ crystallizes in the cubic system with lattice constant a=13. 8406Å and unit cell volume 2661. 35ų. Sc₁₃Zn₅₈ crystallizes in the hexagonal system with lattice constant a=13. 744Å, c=13. 632Å, c/a=0. 9919, and unit cell volume 2230. 06ų. The usefulness of high pressure methods for the synthesis of intermetallic phases with highly volatile components could again be demonstrated here.

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InSb was synthesized from elements at various pressures up to 10 GPa using a laser heated diamond-anvil cell. The cubic as well as the high pressure phases were synthesized. Experiments above 8 GPa, wherein antimony exists in the tetragonal phase, have revealed no new phases of InSb.

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Ceramic-ceramic composites in the Ti-B-C system were fabricated by hot pressing of Ti and B₄C powders. The study describes a method to process fully dense ceramic-ceramic composites (without any sintering aids) through the formation of an intermediate soft phase at moderate pressures and temperatures. The results on the processing and properties of TiB₂-TiC and TiB₂-TiC-SiC composites are presented.

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High oxygen pressures have been developed in different media: gas (500 MPa), in liquid phase using oxidizing solutions (500 MPa) and in the solid phase through the "in situ" thermal decomposition of KClO₃. The main objective of the development of high oxygen pressures was the stabilization of the highest oxidation states of transition elements. For the same Mⁿ⁺ cation, the increase of the n value induces a decrease of the M-O distance. Such a variation being larger than the compression effect on oxides, the stabilization of the highest oxidation states of transition metals appears a fruitful route for investigating the correlations between the covalency of the chemical M-O bonds and the resulting physico-chemical properties of the corresponding oxides.

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A series of perovskite oxides of Ba_1-xEu_xTiO₃ (x=0. 0, 0. 1, 0. 2, 0. 3, 0. 4) were synthesized under 4. 0GPa and at 1090°C. The samples with x=0. 0 and 0. 1 are mixed phase of cubic and tetragonal, whereas those with x=0. 2, 0. 3 and 0. 4 are single phases. The structural transformation from tetragonal to cubic was observed at x=0. 3. Raman spectra of BaTiO₃ exhibited four bands which is consistent with tetragonal structure. For x=0. 2 and 0. 4 samples, only one band at low frequency was observed. XPS and EPR measurements indicated that Eu and Ti ions were in mixed valence.

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Perovskite oxides of La_1-xNa_xTiO₃ (x=0. 05, 0. 1-0. 5) were synthesized under high-pressure and -temperature. XRD analysis showed that sample is cubic for x=0. 05 and pseudo-cubic for 0. 1≤x≤0. 5. Cell volumes of samples with 0. 1≤x≤0. 5 decrease as x increase, and the cell volume for x=0. 05 is less than that for x=0. 1. This implies that the deficiency of cation may exist in the samples. XPS and EPR measurements indicated that Ti ions were mixed valence of +3 and +4 and vacancy of A-site ions existed in samples. The valence state of Ti ions can be altered by high-pressure and -temperature.

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High pressures appear an important tool for the synthesis of new materials. Different routes are possible. -Through volume contraction, new phases can be prepared (ΔV<0). -The stabilization of thermally unstable materials can open the investigation of new diagrams. -The preparation of the highest oxidation states of transition metals through the development of high oxygen pressures is an important challenge for understanding the correlations between the covalency of the M-O bonds and the resulting physico-chemical properties of the corresponding oxides. -Recently, the development of solvothermal reactions has allowed the synthesis of new materials (as phyllosiloxides) and the development of original processes. Several examples will be selected for illustrating the different abilities of high pressure synthesis through the different routes described. A discussion will propose, for the future, some routes for developing pressures in Materials Science.

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The melting behavior of β-SiC with diamond structure was investigated under high pressures up to about 10 GPa using a flash-heating device. The peritectic temperature, at which the SiC decomposes into two phases of carbon saturated liquid Si and solid carbon (graphite) by a peritectic reaction, increases with pressure and the formation temperature of one liquid phase (l-SiC) also tends to increase with pressure. The solubility of carbon in liquid Si reach 50% at about 10 GPa and β-SiC melts directly into l-SiC.

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Solid-state metathesis (exchange) reactions provide a rapid method to produce high quality refractory materials under ambient conditions. An investigation into solid-state metathesis reactions under pressure using Bridgman anvils and a girdle-type apparatus has led to the bulk synthesis of GaN, TaN and CrN, which are inaccessible under ambient pressure/temperature conditions. For example, high quality, crystalline gallium nitride can be rapidly (< 1 sec) synthesized from the reaction of gallium (III) iodide and lithium nitride confined at a pressure of 4. 5 GPa and initiated. Characterization of these compounds includes powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, energy dispersive spectroscopy and scanning electron microscopy. The single-phase gallium nitride made by metathesis reactions under pressure displays significant photoluminescence intensity in the blue/ultraviolet region.

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Anewhigh-pressure phase of CrSb₂ has been synthesized from the marcasite-type ambient pressure phase by high-pressure/temperature treatment above 5. 5 GPa. The crystal structure of the high-pressure phase is body-centered tetragonal, which is assigned as CuAl₂-type structure. The bond nature in the high-pressure phase is metallic, in contrast to the mixed ionic and covalent character in the marcasite-type phase. The phase transition, marcasite-type to CuAl₂-type, accompanies 11% volume decrease and a change of d-electron character from localized to itinerant.

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Thallium based cupurate superconductors, TlSr₂Ca₂Cu₃O_y and TlSr₂Ca₃Cu₄O_y, were synthesized using a high-pressure technique. Synthesis condition was 2. 5 GPa, 1000°C for 4 hours for TlSr₂Ca₂Cu₃O_y, and 1. 5 GPa, 950°C for 4 hours for TlSr₂Ca₃Cu₄O_y. Lattice parameters were α=3. 823 Å and c=15. 33 Å for TlSr₂Ca₂Cu₃O_y, and α=3. 830 Å and c=18. 49 Å for TlSr₂Ca₃Cu₄O_y. The synthesis pressure and heating time were fairly critical to obtain single phases. T_c was 97 K and 90 K for TISr₂Ca₂Cu₃O_y, and TlSr₂Ca₃Cu₄O_y, respectively.

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New compounds in the K-Ag, K-Ni, and K-Pd systems have been found to form at high pressure. Such compounds are of interest because it is possible that potassium may be incorporated into Earth's core, which is composed primarily of the transition metal iron. Furthermore, unusual crystal and electronic structures may result because of the novel chemical composition of the alkali metal-transition metal compounds.

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The direct elementary reactions among the first and second row elements often yield novel superhard, high energy density, and wide band-gap optical materials. The reactions of oxygen and nitrogen with boron and carbon have been investigated at high pressures and temperatures by using an integrated technique of diamond-anvil cell, laser-heating, x-ray diffraction, Raman spectroscopy. A wide range of products has been synthesized and characterized in-situ at high pressures, including α-CO₂, B₂O₃-I, B₂O₃-II, c-BN, h-BN, h'-BN, amorphous carbon nitrides. The elementary reactions occur exothermically and result in highly polycrystallized products with an exception in carbon-nitrogen reactions. The implication of the elementary reactions to energetic materials applications is discussed.

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BN nanotubes were found in laser heated cBN and hBN at nitrogen pressures up to 10. 1 GPa in a diamond anvil cell. The BN nanotubes were recrystallized from the BN-melts in a super critical nitrogen fluid. Their shape, bonding nature and stoichiometry were examined by high resolution electron microscopy and electron energy loss spectroscopy. The BN nanotubes did not contain any additional inclusion. The nanotubes typically exhibit 3-8 shells and stoichiometry of B/N ∼1.

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Based on the concept of functionally goaded materials, the thermoelectric conversion unit of SiGe with gaded electrodes was designed and fabricated by HIP sintering process in order to obtain a good electrical and mechanical contact. The composite of Si and MoSi₂ was used as the electrode, and the effect of the MoSi₂ volume fraction on the electrical resistivity was studied. The results show that the electrical resistivity decreases exponentially with increasing MoSi₂ content and reaches the order of 10^-4 Ω·cm when the volume fraction of MoSi₂ was beyond 25%. The graded (Si-MoSi₂) /SiGe obtained has dense microstructures and well bonded interfaces. The electrical resistivity continuously decreased from the SiGe to the surface electrode in the zone of gaded structure.

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In the ZrO₂-La₂O₃ system, metastable c-ZrO₂ solid solutions (ss) containing up to ≈27 mo1% La₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method The formation of La₂Zr₂O₇ proceeds via two reaction processes; (i) decomposition of c-ZrO₂ (ss) (c-ZrO₂ (ss) → La₂Zr₂O₇ + t-ZrO₂) above 1100°C and (ii) asolid-state reaction of t-ZrO₂ and La₂O₃ at 1200° to 1300°C. Submicron La₂Zr₂O₇ powder is producedat 1300°C. Dense La₂Zr₂O₇ ceramics (99. 5% of theoretical) with an average grain size of 10 μm have been fabricated by hot isostatic pressing (1500°C/2h/196MPa). Their fracture toughness and bending strength are 1. 9 MPa-m^1/2 and 172 MPa, respectively. They exhibit an electrical conductivity of 1. 5x10^-1 S-m^-1 at 1000°C.

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TiC-based composites were fabricated by dynamic pseudo isostatic compaction (DPIC) just after self-propagating high temperature synthesis (SHS). The pressing of 20-255MPa (velocity of piston: 60mm/sec) was performed through a sand medium. The process enables to simultaneously synthesize and density the TiC/Ti, TiC/Ti (Mo) composites from Ti, C, Mo powders. The sample could be taken out from a reaction vessel within a couple of minutes after ignition. The adiabatic combustion temperatures were also calculated as a function of the mixing ratio of the elemental powders in the reaction system. X-ray diffraction patterns of TiC/Ti and TiC/Ti (Mo) produced revealed that the portion of Ti consisted of α-Ti and β-Ti (Mo) respectively. The time delay between SHS and DPIC was affected greatly on the product density. Micro Vickers hardness, Young's modulus and bending strength of the composites were measured. In case of TiC/Ti, the values were 550[Hv], 230MPa, 1130MPa in the ratio of C/Ti=0. 3, respectively, and the density was 4. 54 g/cm³. In TiC/Ti (Mo) composite with 4. 92 g/cm³ of the density, the values were 610[Hv], 200MPa, 1050MPa in the ratio of C/Ti (Mo) =0. 3 and Mo/ (Ti, Mo) =0. 1, respectively. The wear resistance of TiC/Ti (Mo) composite was 2 times higher than TiC/Ti material.

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Hot pressing effect on critical current density (J_c) is studied on Bi₂Sr₂CaCu₂O_x (Bi-2212) superconducting materials. Hot pressing is effective to hasten the grain growth, align the grains, and improve the bulk density of the Bi-2212 materials. In consequence of hot pressing, J_c increases by more than 10 times at 77K under zero magnetic field. Hot pressing effect is enhanced by incorporation of large single crystalline Bi-2212 whiskers. J_c is 3. 1×10³ A/cm², which is about 8 times higher than that for the hot pressed Bi-2212 material without the whiskers, at 77K under zero magnetic field for the Bi-2212 whisker composite. This is due to the haste of grain growth and improvement of grain alignment around the whiskers.

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For the development of new advanced carbon materials, carbon/metal composites were prepared by using HIP for impregnation. Different metals, Sb, Al, Ag and Cu were impregnated into open pores of carbon materials at a temperature, 100∼150°C higher than the melting points of these impregnants, undera pressure of 10∼12 MPa. Sb-impregnated graphite exhibited gas imperviousness and good performance for sliding component or low friction coefficient and high wear resistance. Al-impregnated carbon material has a high mechanical strength and elastic modulus with moderate thermal expansion coefficient. Cu-impregnated carbon materials which has excellent properties for sliderfor pantograph of electric car has been developed.

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Two types of TiAl intermetallic compound matrix composites were fabricated by the combustion synthesis. A powder mixture of titanium, aluminum, and carbon was used to synthesize in-situ a dispersed Ti₂AlC particle composite. Theother material was a fiber reinforced composite, prepared through a pseudo-HIP process. X-ray analysis and microstructure observation were performed to evaluate the reaction products. In the Ti-Al-C system, a dual phase TiAl matrix dispersed with Ti₂AlC particles was obtained by HIP'ing at 1473K. In the TiAl/Si-Ti-C-O fiber composite case, no interface product was detected, but when the synthesis temperature reached 1273K, the Ti₂AlC was detected as the reactant.

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An innovated volumetric dilatometer was developed using buoyancy under high pressure gas. The advantage of this volumetric dilatometer is independence of the number and shape of samples, which is the characteristic highly demanded among industrial fields. An aluminum block as sample with the volumetric dilatometer was placed in the HIP chamber using argon gas as pressure medium to examine the voltage output change with respect to the pressure and temperature change of the HIP chamber. The voltage output change corresponding to sample weight change caused by argon gas density change was monitored by this volumetric dilatometer.

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Features involved in the simulations both on continuum basis and on discrete particle basis are reviewed. It is of importance to make an attempt to carry out these two approaches and compare with experimental observations, so that we would be able to bridge the gap between the microscopic and macroscopic behaviours of particle assemblies. In the simulation by particulate modeling, an assembly of particles is compacted with various ratios of strain rate in three directions, and the constitutive behaviour is investigated. It is thus shown that the yield surfaces agrees qualitatively well with experimental observations which have already been reported and with the ones predicted by the theories so far developed.

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Al₂O₃-ZrO₂ (95/5 wt%a) composite powders have been prepared by the coprecipitation technique. Both normal sintering and capsule-free HIP sintering have been performed under various sintering conditions. The combination of normal sintering at 1798 K and HIP sintering at 1723 K enhances the bending strength of the composites to 958 MPa. Bending strength above 1 GPa is succeeded by capsule HIPing at 1623 K. The enhancement of bending strength is discussed in connection with the sintering conditions.

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We discuss the hypothesis that, in addition to the known critical point in water (below which two fluid phases-a lowerdensity gas and a higher-density liquid-coexist), there exists a "second" critical point at low temperatures (below which two liquid phases-a higher-density liquid and a lower-density liquid-can coexist). We also discuss briefly some of the evidence relating to this hypothesis. This evidence is rather tentative at the present time, and is largely based on a growing number of computer simulations using the ST2 and TIP4P intermolecular potentials. We also discuss selected experimental results that are consistent with this hypothesis.

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In tetravalent network materials like quartz or ice the main interactions are the strong bonds that connect each vertex to four others and the repulsive forces that keep non-bonded vertices further apart. Those interactions, modelled by square wells, are shown to be sufficient to account for the stability of ice-like crystals, with tetrahedral bond angles, at low temperature; endothermic melting of the crystal to a denser water-like amorph at intermediate temperatures; exothermic collapse of the supercooled crystals at high pressure, to a glass, without breaking any bonds; expansion on cooling of both the crystal and the amorph; and a line of spinodal instabilities in the water-like supercooled amorph where the compressibility, heat capacity and negative expansivity diverge.

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Molecular dynamics simulations have been carried out at constant pressure and temperature to examine phase behaviors of supercooled water. The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density liquids and a fragile-strong transition are accounted for by reconciling an idea introducing a second critical point separating two liquid states with a conjecture that low temperature state is a different phase from normal water.

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When ice Ih in an emulsion is compressed below 250 K, it melts to supercooled liquid water, avoiding the formation of other crystal phases [1]. Here, we create emulsified high-pressure ices tinder high pressure and low temperature, and measure their temperature while these ices are decompressed at a constant rate at different temperatures. We detect metastable melting points of high-pressure ices, and identify their melting lines. We find what could be possibly two new ice phases, and discuss the relationship between decompression-induced melting and decompressioninduced amorphization.

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The results of an extensive set of molecular dynamics simulations of the SPC/E water model are described. The states examined cover a range of densities from greater than ambient at 1200 kg/m³ to states near the critical density at 300 kg/m³ and over a range of temperatures from 297 K to 823 K. The emphasis is on hydrogen bonding measures and how these measures vary with density and temperature. The mean number of bonds per molecule, the distribution of the lifetimes of the bonds and the size of hydrogen bonded clusters are reported.

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The proton chemical shift of water is measured at temperatures up to 400°C and densities of 0. 19, 0. 29, 0. 41, 0. 49, and 0. 60 g/cm³. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas . The chemical shift is related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule in the supercritical densities . The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is estimated from the number of hydrogen bonds. we perform the proton chemical shift measurements for a water molecule as a function of the density. than that in the gas phase, and this enhancement of supercritical water by fitting the experimentally determined number of hydrogen bonds.

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Molecular dynamics calculations of water have been done successfully over a wide PρT-range using a polarizable potential model. Gas-liquid coexistence curve evaluated by Maxwell equal area rule and, estimated. the critical point showed little improvement compared with even filed-charge models. However, when the calculated and experimental structures were compared at the same thermodynamic state reduced by the critical point, the polarizable model presented a better prediction than the fixed-charge models.

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The dielectric relaxation in water and heavy water have been measured in the temperature and pressure ranges up to 750°C and 120 MPa by using a newly developed microwave spectroscopy in the frequency range up to 40 GHz. The dielectric relaxation time τ is deduced from observed transmission spectrum on the assumption that the dielectric constant obeys the Debye relaxation. Below about 350°C τ rapidly decreases with increasing temperature irrespective of pressure, while at higher temperatures r changes little with temperature and increases rapidly with decreasing density.

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Recent developments in high-pressure diffraction techniques using both neutron and synchrotron x-ray sources now allow accurate studies of ices over a wide range of pressures. Results are revealing complex and unexpected behaviour. The disorder in ice VII is found to be different from that previously thought and to give rise to a complex Hbond geometry. The ammonia molecules in ammonia phase IV are found to orientationally ordered with a bifurcated H-bond geometry which becomes more distorted with increasing pressure. And hydrogen sulphide phase-I' is found to have a partially ordered structure where H-bonding plays a more significant role than at ambient pressure.

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The pressure dependence of acoustic velocities and refractive index of liquid H₂O and D₂O have been determined up to 1. 05 GPa at room temperature. For the tetragonal ice VI from P = 1. 05 to 2. 10 GPa, we present the detailed method of the determination of six elastic constants, which are compared with the recent result at 0. 72 GPa and 271 K by Tulk et al.

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Elastic properties of different ice phases were studied in the low-temperature-high-pressure region (77<T<200 K, 0<P<2 GPa). The main attention was paid to the elastic characteristics of low-density (lda) and high-density (hda) amorphous H₂O phases. Considerable softening of the transverse acoustic velocity and the corresponding shear modules G was found in the vicinity of lda-hda transition during both heating of the Ma phase and compression of the lda phase.

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Morphologies of ice IV crystals were observed and their melting pressure (P) -temperature (T) relations were determined in a hydrothermal diamond-anvil cell at pressures up to 1 GPa. Water sample and a chip of BaTiO₃ were sealed in the sample chamber of the cell, and were observed under a microscope during isochoric cooling-heating cycles. Images of the sample, together with time and T, information, were recorded continuously on VCR tapes. The tetragonal to cubic phase transition in BaTiO₃ observed in heating was used to provide pressure and water density information for the sample. Our ice IV melting P-T data are in excellent agreement with those predicted by the equation formulated previously based on the data collected at pressures between 0. 4 and 0. 6 GPa.

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Infrared absorption spectra of D₂O ice were measured up to 140 GPa. The O-D stretching frequency rapidly decreases with pressure toward zero. The transition pressure, which could be defined as a pressure where the stretching frequency becomes zero, in D₂O ice was determined to be 94 GPa. This transition pressure is higher by 26 GPa than that in H₂O ice. A large difference in the transition pressure may be attributed to anharmonic contribution to the potential and consequently to the stretching vibrations.

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The structural pressure dependence of the high-pressure ices VII and VIII has been measured up to 20 GPa by neutron powder diffraction. Measurements of the atomic thermal motion reveal that, while it is strongly reduced by pressure in both phases, the stepwise change in the oxygen thermal motion at the VIII/VII transition is pressure independent within the precision of the measurements. Also, the O-D bondlength in ice VIII changes by less 0. 002 Å to 20 GPa. These results suggest that the site-separation of the disordered oxygen atoms in ice VII and the direction of the disorder displacement remain closely similar to those found at 5 GPa.

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In-situ neutron powder diffraction experiments on D₂O ices III and V are reported. They were performed in gas pressure cells using argon as a pressure medium. The pressure and temperature dependency of lattice constants and the deuteron ordering were established. It turns out that both ice III and V are increasingly ordered on lowering the temperature or increasing the pressure. Even close to the melting point both phases are far from being fully disordered.

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Visual observations of ice I_h growing in pure water and measurements of the melting curve on which the ice does not grows nor melts are performed using the pressure jump method. The resulting solidliquid equilibrium points exhibit a discontinuity at -16°C and 165 MPa, where the growth shapes also change to a hexagonal plate from a circular disc as a result of roughening transition of the (1010) interfaces. The agreement between discontinuity and roughening points is discussed.

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Clathrate hydrates of N₂ and CO₂ were investigated in situ as a function of gas pressure by neutron powder diffraction. Gas pressure cells developed specifically for this work were employed. The pressure dependency of the filling of the different cages in these clathrate hydrates was established. In contrast to some earlier indirect evidence the small cages in CO₂ clathrate hydrate are partly filled, however the generally accepted Langmuir-behaviour of the filling is violated in this case. Likewise, instead of the usually assumed filling of the large cages with only one N₂ molecule a double occupancy of these cages is observed for N₂ clathrate hydrate.

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Stability of gas hydrate in pure H₂O, THF/H₂O and CH₃OH/H₂O solutions were measured by high pressure optical vessel. From the measurement of stability conditions for the gas hydrate against temperature and pressure, the order of hydrate crystal stability is as follows ; THF solution>pure H₂O>CH₃OH solution. The resulting solvent effect on the stability of the gas hydrate is closely related with the microscopic molecular clustering structure in solutions observed through the liquid cluster beam mass spectrometer. The stable clusters with dodecahedral structure (H₂O) ₂₁H⁺ (THF) _n were observed in high probability in the THF solution. On the other hand, water structure is strongly influenced by methanol and hydrogen-bonding networks of water molecules were broken in the high concentration region of methanol solution. The difference of the stability of hydrate crystal in these solutions seems to be caused by clustering structure difference of these aqueous solutions.

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The process of gas hydrate formation and dissociation in a pressurized solution has been studied, using a specially designed experimental apparatus. It was found that the phase behavior showed a typical hysteresis due to the kinetic process of hydrate formation and dissociation, and that the environmental conditions for hydrate nucleation were mainly affected by the states of liquid phase. The mechanism of nucleation and crystal growth, of gas hydrate is also discussed based on the change in the clustering structure of the liquid phase.

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The structure of supercritical water and aqueous solutions has been examined up to 510°C and 40 MPa with in situ laser Raman spectroscopy. Just near the critical point, the maximal concentration of the symmetric OH stretching is obtained although the breakdown of the hydrogen-bonding network occurs. Consequently, short-lived tetrahedral configurations accompanied by large fluctuations of the structure are significantly formed and become possible along with dimers monomers. The spectra of an aqueous zinc nitrate solution reveal that the replacement of H₂O molecules in the solvation shell of Zn²⁺ by a nitrate ion is significantly promoted at higher temperatures above 300°C at 30 MPa. The average number of water molecules bound to the zinc ion is decreased to about 3 at 380°C from 6.

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The ₂H NMR spin-lattice relaxation times (T₁) of D₂O molecules and the ¹H chemical shifts (δ) of water in tert-butyl alcohol (TBA) -and urea-D₂O solutions up to 8 mol% were measured at 7. 7, 29. 9, and 48. 4°C up to 294 MPa in order to elucidate the pressure and temperature effects on the hydrophobic hydration. The pressure coefficient of T₁ in the TBA solution was smaller than that in pure water, and decreased monotonously with an increase in the TBA content. The δ (H₂O) in the TBA solution exhibits a downfield shift relative to that in neat D₂O and the difference in the δ (H₂O) between TBA solution and neat D₂O becomes larger with compression. These results suggest that the structure of the hydration shell is not so bulky as the tetrahedral open structure of water. On the other hand, the results in urea solutions were in marked contrast to those in TBA solutions.

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In order to elucidate the pressure effect on the hydrophobic hydration shell, the limiting molar conductivities of tetraalkylammonium ions (Me₄N⁺ to Bu₄N⁺) in H₂O and D₂O were determined at 5°C as a function of pressure up to 196. 1 MPa. The rotational correlation times of D₂O molecules in pure D₂O (τ_c⁰) and coordinated to Me₄N⁺ and Bu₄N⁺ ions (τ_c) were also estimated at 6. 9°C up to 294. 2 MPa from ²H NMR spin-lattice relaxation times (T₁). The residual friction coefficient (Δζ_obs) obtained from the limiting molar conductivity of the ion (λ⁰) became larger with increasing ionic radius and pressure except for Pr₄N⁺ and Bu₄N⁺ ions in D₂O at lower pressure. These results suggest that the structure of the hydrophobic hydration shell is not weakened by pressure as much as that of bulk water, which is in qualitative agreement with the pressure dependence of τ_c (Bu₄N⁺) /τ_c⁰. However, a larger value of τ_c (Me₄N⁺) than that of τ_c⁰ does not always correspond to a negative value of Δζ_obs (Me₄N⁺) indicating a structure breaking property of this ion.

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A new vibrating-tube densitometer for measuring densities of fluids in the temperature range from ambient to 580 K and the pressure range up to 35 MPa is described. A novel design of driving and pick-up systems of a metal vibrating tube was employed. The driving system is based on passing the driving electrical signal directly to the electrically insulated vibrating tube, the photo-electric pick-up system employs glass-fibre optics with light source and sensor. New values of partial molar volumes at infinite dilution in water for several derivatives of benzene obtained by means of the densitometer are presented.

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The effect of pressure on the crystal-nematic and nematic-isotropic transition temperatures, T_CN and T_NI, was investigated for MBBA (n- (4-methoxybenzylidene) -4-n- butylaniline), EBBA (n- (4-ethoxybenzylidene) -4-n-butylaniline), and binary mixtures containing these compounds such as (MBBA+EBBA), and (MBBA, EBBA + C₆H₆, C₆H₁₂) at temperatures from 263 to 363 K and pressures up to 30 MPa. The measurements were performed using a high-pressure optical vessel designed for direct visual observation with the aid of a video microscope, and a three-terminal cylindrical capacitor for dielectric study.

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