Guest size effects on a robust structure of semiclathrate hydrates and their thermophysical properties

The ability to tune the pore size, shape, and functionality of semiclathrate hydrates, host-guest materials formed from aqueous solutions of ionic guest materials and water, makes them attractive materials for thermal storage and gas storage applications. The flexibility of semi-clathrate hydrates and their guest-molecule-dependent reactions produce these unexpected and desirable properties. As an ionic guest, tetra-n-butylammonium cation is known for best-fit in hydrogen-bonded water structures. Few investigations have been conducted for other cations, while there are numerous candidates. Relationships between the molecular structures of ionic guest substances and their hydrate structure and relevant thermodynamic properties are yet to be understood. In this study, the semiclathrate hydrates formed with two variations of tetra-n-butylammonium chloride (N4444Cl) that are n-propyl, tri-n-butylammonium chloride (N3444Cl) and tri-n-butyl, n-pentylammonium chloride (N4445Cl) were investigated. Structure analyses found that both salts formed Jeffrey's type III tetragonal hydrate structure which is the same as that of tetra-n-butylammonium chloride hydrate, although their lattice parameters are significantly different. The present data found that this hydrate structure can cover a wide range of melting temperature compared to the other two main semiclathrate structures. The present N4445Cl hydrate is an example in which its melting temperature was adjusted to be suitable for air conditioning, i.e., ∼282 K, compared to that of the N4444Cl hydrate, the melting temperature of which is slightly too high for this purpose. The results provide insight that the thermal properties of the tetragonal P42/m hydrate structure can be widely tuned by ionic guests for various practical requirements.

Multi-phase retrieval of methane hydrate in natural sediments by cryogenic x-ray computed tomography

In this study, we observed natural methane (CH4) hydrate sediments, which are a type of unconventional natural gas resources, using x-ray computed tomography (CT). Because CH4 hydrates are formed by hydrogen bonding of water molecules with CH4, material decomposition becomes challenging when CH4 hydrates coexist with liquid or solid water in natural sediments. Tri-contrast (absorption, refraction, and scattering) imaging was performed via diffraction enhanced x-ray CT optics using monochromatic synchrotron x rays. The quantitative characterization of the contrast changes successfully enabled the decomposition of CH4 hydrates coexisting with frozen seawater (ice) in natural sediments obtained from the Okhotsk Sea. This study reveals complementary structural information about the microtexture and spatial relation among CH4 hydrates, ice, and pores by utilizing the distinct physical properties of x rays when passing through the materials. These results highlight the exceptional capabilities of high-resolution multicontrast x-ray tomography in materials science and geoscience applications.

Reversible Transition between Discrete and 1D Infinite Architectures: a Temperature-Responsive Cu(I) Complex with a Flexible Disilane-bridged Bis(pyridine) Ligand

Thermoresponsive molecular structure based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at -196 °C. The formation of these polymorphs is due to the flexibility of ligand. Cu-I bond formation is observed upon cooling the sample from -10 °C to -170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran-CO2 Binary Hydrate

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO₂ below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO₂ guests in small pentagonal dodecahedral 5¹² host water cages, not previously observed in the simple CO₂ hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I4₁/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

A Clathrate Hydrate Structure Hidden in Plain Sight

Clathrate hydrates have diverse crystal structures, and among them, the three (sI, sII, and sH) most prevalent ones cover nearly all known structures, while the norm is to consider other structures only when specific guest molecules are present. Here we report the observation of a hidden clathrate structure: the tetragonal structure (TS-I) in commonly formed gas hydrates, as evidenced from molecular dynamics simulations. We show that when two (or more) sI crystal grains with different growth directions come into contact or when the growth of a sI crystal encounters geometrical frustration, the TS-I results as a cocrystal. We give evidence that TS-I may also play an important role in the combination and/or transition between sI and sII. These results imply that this previously neglected structure may be commonly present whenever sI or sII is formed. This hidden structure must be identified, experimentally and in simulations; confining the possible structures may hinder an in-depth understanding of clathrate hydrates.

Designing the structure and relevant properties of semiclathrate hydrates by partly asymmetric alkylammonium salts

Semiclathrate hydrates are host-guest materials that form from ionic guests and water. There are numerous options for ionic guests, such as quaternary ammonium salts, to tune the functional properties of these materials such as melting temperature, fusion heat, and gas capacity and selectivity. To design these materials, the stabilization mechanism of the side chains of quaternary ammonium salts must be understood based on both thermodynamic and crystallographic properties and relevant host-guest dynamics. In this paper, we studied semiclathrate hydrates formed from n-propyl, tri-n-butylammonium bromide (N₃₄₄₄Br) and tri-n-butyl, n-pentylammonium bromide (N₄₄₄₅Br). Their cation side chains are decremented or incremented from tetra-n-butylammonium (N₄₄₄₄ or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra-n-butylammonium bromide (N₄₄₄₄Br or TBAB) as an ionic guest, an increment of the carbon chain, i.e., N₄₄₄₅Br, caused disorders in its hydrate structure due to the oversizing of the cation. This suitably oversized cation selectively stabilized the orthorhombic structure, whose hydration number is relatively high. As a result, the fusion heat at the congruent composition of the hydrate phase was higher than that of the widely used N₄₄₄₄Br (TBAB) hydrates. The N₃₄₄₄Br hydrate showed both significantly decreased melting temperature and fusion heat compared to the N₄₄₄₄Br (TBAB) hydrates. The phase behaviour of the N₃₄₄₄Br hydrate was found to be analogous to that of the N₄₄₄₄Br (TBAB) hydrates. It was demonstrated that the semiclathrate hydrate structures and relevant properties can be modified by adjusting the alkyl side chain length of quaternary ammonium salts.

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Superheating of Structure I Gas Hydrates within the Structure II Cyclopentane Hydrate Shell

The superheated state of methane (CH₄) hydrate that exists under the surface ice layer can persist for considerable lengths of time, which showed promise as a method for storing and transporting natural gas. This study extends this further by coating sI CH₄ hydrate with one of several sII hydrates, thus eliminating the need for a defect-free continuous interface between the sI and sII hydrates. Gas hydrate crystals were kept intact above their dissociation temperature by immersing them in liquid cyclopentane (CP), as observed with powder X-ray diffraction and X-ray CT methods. It was observed that placing the CH₄ hydrate in CP converted the outer layer of CH₄ hydrate to a thin layer of CP hydrate at around 270 K under atmospheric pressure, which is ∼80 K higher than the usual dissociation temperature. It was also observed that sI CO₂ hydrate and C₂H₆ hydrate could be preserved by CP hydrate.

Transformation process of ice crystallized from a glassy dilute trehalose aqueous solution

Crystal growth of ice Isd occurring after crystallization of a glassy dilute trehalose aqueous solution is slower than that of ice Isd in a dilute glycerol solution and pure ice Isd, and ice Isd in trehalose aqueous solution survives to ∼230 K.

Structural CO2 capture preference of semiclathrate hydrate formed with tetra-n-butylammonium chloride

CO2 capture preference of semiclathrate hydrate analyzed by single crystal XRD. Asymmetrically distorted cages preferentially capture CO2.

Investigation of the thermodynamic properties of hydrates as cooling phase change materials for their implementation in electric vehicles

Electric vehicles (EVs) play key roles in realizing a sustainable society.

Advanced X-ray imaging at beamline 07 of the SAGA Light Source

The SAGA Light Source provides X-ray imaging resources based on high-intensity synchrotron radiation (SR) emitted from the superconducting wiggler at beamline 07 (BL07). By combining quasi-monochromatic SR obtained by the newly installed water-cooled metal filter and monochromatic SR selected by a Ge double-crystal monochromator (DCM) with high-resolution lens-coupled X-ray imagers, fast and low-dose micro-computed tomography (CT), fast phase-contrast CT using grating-based X-ray interferometry, and 2D micro-X-ray absorption fine structure analysis can be performed. In addition, by combining monochromatic SR obtained by a Si DCM with large-area fiber-coupled X-ray imagers, high-sensitivity phase-contrast CT using crystal-based X-ray interferometry can be performed. Low-temperature CT can be performed using the newly installed cryogenic system, and time-resolved analysis of the crystallinity of semiconductor devices in operation can be performed using a time-resolved topography system. The details of each instrument and imaging method, together with exemplary measurements, are presented.

Characterization of the Clathrate Hydrate Formed with Fluoromethane and Pinacolone: The Thermodynamic Stability and Volumetric Behavior of the Structure H Binary Hydrate

To reveal the relation of guest dynamics within the structure H clathrate hydrate and its macroscopic physical properties, experimental and computational works have been conducted on the system of fluoromethane (HFC-41) and pinacolone coexisting with water. The phase boundaries of the hydrate formed from HFC-41 and pinacolone within the pressure range of (0.25-2.48) MPa and the temperature range of (277-293) K were measured. The equilibrium hydrate formation pressure incorporating HFC-41 was lowered by adding the pinacolone as a large guest molecule compound to form a sH phase compared to the HFC-41 single hydrate. Powder X-ray diffraction measurements confirmed the formation of the structure H hydrate with the HFC-41 and pinacolone binary hydrate. The lattice constants of the sH hydrate were also measured to see the effect of the help guest molecular size, which showed a different trend from that of the previous studies of sH pinacolone hydrates. Molecular dynamics simulations of the binary sH phase indicate weak hydrogen bonding of the pinacolone molecules with the water in the cages in the phase with HFC-41. The oblate HFC-41 molecules showed strong orientational preference to the equatorial planes of the D' cages, which may explain some of the trends in the behavior of this phase.

Dissociation kinetics of propane-methane and butane-methane hydrates below the melting point of ice

Understanding the dissociation mechanism of gas hydrates below the melting point of ice is crucial for expanding the practical applications of solid hydrates in gas storage. The kinetic processes for gas hydrates have not been clarified, except for those of pure CH4 hydrate and CO2 hydrates. In this study, using in situ X-ray diffraction analysis, the low-temperature onset of the dissociation of C3H8 and C4H10 hydrate fine particles encapsulating CH4 as a secondary guest was investigated during temperature ramping. At ∼200 K, the C3H8 + CH4 hydrate, n-C4H10 + CH4 hydrate, and iso-C4H10 + CH4 hydrate all dissociated in a single step, similar to pure C3H8 and pure iso-C4H10 hydrate. The dissociation of C3H8 hydrate was also found to accelerate the dissociation of CH4 hydrate. Based on the experimental results, it was confirmed that the C3H8 and C4H10 molecules released from the dissociating hydrates accelerated hydrate dissociation.

Slow Crystal Growth of Cubic Ice with Stacking Faults in a Glassy Dilute Glycerol Aqueous Solution

Control of ice formation is an important issue as catastrophic ice growth influences our life activities and many industrial systems. We prepared a homogeneous glass of a dilute glycerol aqueous solution by a pressure liquid cooling vitrification method and examined the effect of solute on the ice formation of solvent water using a powder X-ray diffraction method. The solvent water immediately after the crystallization is composed of nanosized pure cubic ice (ice Ic). The crystal growth of ice Ic with stacking faults is much slower than that of pure water. The presence of glycerol molecules dispersing homogeneously may hinder crystal growth. The macroscopic segregation occurs rapidly during the transformation from stacking disordered ice to hexagonal ice. The results suggest that ice formation can be controlled by changing the solute type and concentration. This study has implications for thawing technology in cryobiology and frozen food engineering.

Temperature effects on the C-H symmetric stretching vibrational frequencies of guest hydrocarbon molecules in 512, 51262 and 51264 cages of sI and sII clathrate hydrates

C–H symmetric stretching vibrational frequencies of CH₄, C₂H₄ and C₂H₆ molecules encapsulated in 5¹², 5¹²6² and 5¹²6⁴ cages of structures I (sI) and II (sII) clathrate hydrates measured by Raman spectroscopy in the temperature range of 93–183 K was analysed. The slopes of the symmetric stretch vibrational frequencies under changing temperatures (Δv/ΔT) for CH₄, C₂H₄ and C₂H₆ molecules encapsulated in sII 5¹²6⁴ cages were smaller than those for molecules in sI 5¹²6² cages, although sI 5¹²6² cages are smaller than sII 5¹²6⁴ cages. We compared the results of Δv/ΔT in this study with the geometrical properties of each host water cage, and these comparisons suggest that the geometry of host water cages affects Δv/ΔT.

Temperature effects on the C–H symmetric stretch of hydrocarbons in various cages of sI and sII clathrate hydrates were observed.

Correction: X-ray CT observation and characterization of water transformation in heavy objects

Correction for 'X-ray CT observation and characterization of water transformation in heavy objects' by Satoshi Takeya et al., Phys. Chem. Chem. Phys., 2020, 22, 3446-3454, DOI: 10.1039/c9cp05983k.

Effect of temperature and large guest molecules on the C-H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates

Large molecules such as 2-methylbutane (C₅H₁₂) or 2,2-dimethylbutane (C₆H₁₄) form structure H (sH) hydrates with methane (CH₄) as a help gas. In this study, the Raman spectra of the C–H symmetric stretch region of CH₄ enclathrated within various sH hydrates and structure I CH₄ hydrates were analyzed in the temperature range 137.7–205.4 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH₄ in host–water cages increased because of varying temperature, and the sizes of the host–water cages also increased; variation of CH₄ in small cages was less than in larger cages. Comparing the variations of the C–H symmetric stretching frequencies of CH₄ in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH₄ molecule in water cages.

Temperature effect on C–H symmetric stretching frequencies of CH₄ in water cages of sI and sH clathrate hydrates were clarified.

X-ray CT observation and characterization of water transformation in heavy objects

Nondestructive observations and characterization of low-density materials composed of low-Z elements, such as water or its related substances, are essential for materials and life sciences. However, visualizing these compounds and their phase changes is still challenging. In this study, an approach to X-ray imaging of water-related substances in heavy objects without the use of contrast agents is proposed. The implementation of the approach is based upon X-ray phase shift, in which the optimal photon energy is simulated for high-contrast X-ray imaging. Proof of concept is provided by observations of resins, water, and clathrate hydrates such as CO₂ hydrate and tetrahydrofuran (THF) hydrate in an aluminum container by diffraction-enhanced X-ray imaging with synchrotron X-rays of 35 keV. These results suggest that the proposed approach is a unique method for visualizing the transformation of these clathrate hydrates and is also applicable to in situ observations of other objects composed of multiphase materials with small density differences.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Retraction: Effect of temperature and large guest molecules on the C–H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates

Retraction of ‘Effect of temperature and large guest molecules on the C–H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates’ by Akihiro Hachikubo et al., RSC Adv., 2018, 8, 3237–3242, DOI: 10.1039/c7ra12334e.

X-Ray attenuation and image contrast in the X-ray computed tomography of clathrate hydrates depending on guest species

In this study, X-ray imaging of inclusion compounds encapsulating various guest species was investigated based on the calculation of X-ray attenuation coefficients.

Structure and Density Comparison of Noble Gas Hydrates Encapsulating Xenon, Krypton and Argon

Understanding the effect of guest species on the host framework is important for the development of structure-based properties of inclusion compounds. Herein, the crystal structures of the noble gas hydrates encapsulating Xe, Kr, and Ar were studied by powder X-ray diffraction measurements. The crystal structures and hydration numbers of these noble gas hydrates were solved by Rietveld refinements using optimized models with the direct-space technique. It was revealed that host cage size of these hydrates changed depending on the type of guest species even though their unit-cell parameters were the same. Based on the structure models obtained, the densities of Xe, Kr, and Ar gas hydrates were also determined to be 1.837, 1.445 and 1.097 g/cm³ at 93 K, respectively. Our findings, from a crystallographic point of view, may give insight into further understanding the thermodynamic stability and physical properties of gas hydrates encapsulating small guests.

Thermodynamic Properties and Crystallographic Characterization of Semiclathrate Hydrates Formed with Tetra-n-butylammonium Glycolate

Semiclathrate hydrates are a crystalline host-guest material, which forms with water and ionic substances such as tetra-n-butylammonium (TBA) salts. Various anions can be used as a counter anion to the TBA cation, and they can modify thermodynamic properties of the semiclathrate hydrates, which are critical for applications, for example, cold energy storage and gas separation. In this study, the semiclathrate hydrates of the TBA glycolate were newly synthesized. Measurements for melting temperatures and a heat of fusion and a crystal structure analysis were performed. In comparison with the other similar materials, such as acetates, propionates, lactates, and hydroxybutyrates, the glycolate greatly changed the melting temperature and the heat of fusion. The preliminarily determined crystal structure showed that the glycolate anion builds a relatively porous structure compared to the previously reported hydrates formed with hydroxycarboxylates. The present study showed that substitution of a hydrophobic group by a hydrophilic group is an effective method to control the thermodynamic properties as well as to improve environmental, biological, and chemical properties.

Correction: Carbon nanotube-copper exhibiting metal-like thermal conductivity and silicon-like thermal expansion for efficient cooling of electronics

Correction for 'Carbon nanotube-copper exhibiting metal-like thermal conductivity and silicon-like thermal expansion for efficient cooling of electronics' by Chandramouli Subramaniam and Kenji Hata et al., Nanoscale, 2014, 6, 2669-2674.

Stability and characterization of the structure II binary clathrate hydrate of the refrigerant trans-1,3,3,3-tetrafluoropropene + methane

A new clathrate hydrate formed with trans-1,3,3,3-tetrafluoropropene and methane was characterized by phase equilibrium and PXRD measurements and MD simulations.

Anisotropy of dodecahedral water cages for guest gas occupancy in semiclathrate hydrates

Anisotropic dodecahedral cages in semiclathrate hydrates.

Feasibility study of phase-contrast X-ray laminography using X-ray interferometry

For fine observation of laminar samples, phase-contrast X-ray laminography using X-ray interferometry was developed. An imaging system fitted with a two-crystal X-ray interferometer was used to perform the observations, and the sectional images were calculated by a three-dimensional iterative reconstruction method. Obtained images of an old flat slab of limestone from the Carnic Alps depicted fusulinids in the Carboniferous period with 3 mg cm^-3 density resolution, and those of carbon paper used for a fuel-cell battery displayed the inner fibrous structures clearly.

Retracted Article: Effect of temperature and large guest molecules on the C–H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates

Large molecules such as 2-methylbutane (C₅H₁₂) or 2,2-dimethylbutane (C₆H₁₄) form structure H (sH) hydrates with methane (CH₄) as a help gas. In this study, the Raman spectra of the C–H symmetric stretch region of CH₄ enclathrated within various sH hydrates and structure I CH₄ hydrates were analyzed in the temperature range 83–183 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH₄ in host water cages increased because of varying temperature, and the sizes of the host water cages also increased; variation of CH₄ in small cages was less than in larger cages. Comparing the variations of the C–H symmetric stretching frequencies of CH₄ in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH₄ molecule in water cages.

Temperature effects on C–H symmetric stretching frequencies of CH₄ in water cages of sI and sH clathrate hydrates were clarified.

Thermodynamic stabilization of semiclathrate hydrates by hydrophilic group

Introducing hydrophilic groups into carboxylates is a way to modify semiclathrate hydrate frameworks and change the properties of the hydrates.

CO₂ processing and hydration of fruit and vegetable tissues by clathrate hydrate formation

CO2 hydrate can be used to preserve fresh fruits and vegetables, and its application could contribute to the processing of carbonated frozen food. We investigated water transformation in the frozen tissue of fresh grape samples upon CO2 treatment at 2-3 MPa and 3°C for up to 46 h. Frozen fresh bean, radish, eggplant and cucumber samples were also investigated for comparison. X-ray diffraction indicated that after undergoing CO2 treatment for several hours, structure I CO2 hydrate formed within the grape tissue. Phase-contrast X-ray imaging using the diffraction-enhanced imaging technique revealed the presence of CO2 hydrate within the intercellular spaces of these tissues. The carbonated produce became effervescent because of the dissociation of CO2 hydrate through the intercellular space, especially above the melting point of ice. In addition, suppressed metabolic activity resulting from CO2 hydrate formation, which inhibits water and nutrient transport through intercellular space, can be expected.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Preservation of carbon dioxide clathrate hydrate in the presence of trehalose under freezer conditions

To investigate the preservation of CO₂ clathrate hydrate in the presence of sugar for the novel frozen dessert, mass fractions of CO₂ clathrate hydrate in CO₂ clathrate hydrate samples coexisting with trehalose were intermittently measured. The samples were prepared from trehalose aqueous solution with trehalose mass fractions of 0.05 and 0.10 at 3.0 MPa and 276.2 K. The samples having particle sizes of 1.0 mm and 5.6–8.0 mm were stored at 243.2 K and 253.2 K for three weeks under atmospheric pressure. The mass fractions of CO₂ clathrate hydrate in the samples were 0.87–0.97 before the preservation, and CO₂ clathrate hydrate still remained 0.56–0.76 in the mass fractions for 5.6–8.0 mm samples and 0.37–0.55 for 1.0 mm samples after the preservation. The preservation in the trehalose system was better than in the sucrose system and comparable to that in the pure CO₂ clathrate hydrate system. This comparison indicates that trehalose is a more suitable sugar for the novel frozen carbonated dessert using CO₂ clathrate hydrate than sucrose in terms of CO₂ concentration in the dessert. It is inferred that existence of aqueous solution in the samples is a significant factor of the preservation of CO₂ clathrate hydrate in the presence of sugar.

Observation of the growth process of icy materials in interparticle spaces: phase-contrast X-ray imaging of clathrate hydrate

The visualization of temperature-controlled crystal growth and dissociation of tetrahydrofuran clathrate hydrates and ice in the interparticle spaces between beads is presented. Phase-contrast X-ray imaging using synchrotron X-ray radiation is a unique technique to study clathrate hydrates coexisting with both ice and liquid water and is used here to observe tetrahydrofuran hydrate and ice formation in situ. The nondestructive images obtained reveal a morphology change of tetrahydrofuran clathrate hydrate grown under isothermal temperature conditions at 253 K, which may be caused by the thermal history of crystallization of the clathrate hydrate. In addition, the water freezing process in the interparticle spaces between is observed using phase-contrast X-ray imaging. This method is useful for understanding the kinetics of clathrate hydrates in interparticle spaces.

Characterization of the ionic clathrate hydrate of tetra-n-butylammonium acrylate

The ionic clathrate hydrate of tetra-n-butylammonium (TBA) acrylate was characterized using single-crystal X-ray diffraction, elemental analysis, and nuclear magnetic resonance (NMR) spectroscopy. The crystal structure of TBA acrylate was Jeffrey’s type III and tetragonal P42/n, with a 33.076(7) × 33.076(7) × 12.170(2) Å3 unit cell. The volume of the unit cell was 13315(5) Å3, which is almost twice that of the ideal structure. The TBA cation was disordered and located in two types of fused cages. Although the acrylate anion was located in a pentagonal dodecahedral cage neighboring the TBA cation, there is a residual acrylate anion that could be around the other TBA cation in the unit cell. Solid-state 13C NMR spectra showed that the TBA cation was clearly disordered at 173 K, but not at 239 K. NMR peaks from the acrylate anion were not observed at either temperature. This is probably because of the strong restriction on the acrylate anion by hydrogen bonding with the lattice water. Some of the characteristics of the anion and cation of the ionic guest incorporated in the hydrate structure have yet to be defined. Further research is needed to clarify complexation of the ionic clathrate hydrate and the ionic guest, and the resulting structure.

Effect of nitrogen atom substitution in cyclic guests on properties of structure H clathrate hydrates

The effect of substituting nitrogen heteroatoms in the cyclohexane ring of methylcyclohexane on the structure and guest dynamics of structure H (sH) clathrate hydrates with methane help gases are studied through experimental synthesis, powder X-ray diffraction (PXRD) measurements, and classical molecular dynamics simulation of methylcyclohexane and 1-methylpiperidine. The PXRD measurements were performed for temperatures in the range of 138 to 183 K, and the a axis and c axis lattice constants were determined in this temperature range. The PXRD results show the different thermal expansivity of lattice constants in both sH hydrate cages. Simulations on methylcyclohexane and 1-methylpiperidine are performed, and the effects of methane cage occupancy on the lattice constants were studied by simulations with 100% and 80% of the small and medium cages occupied by methane. The sH phases do not expand isotropically, and the a and c lattice constants can vary over a range of 0.015 Å and 0.06 Å, respectively, depending on the large cage guest and methane occupancy. Hydrogen bonding between the 1-methylpiperidine nitrogen atom and cage water in the sH hydrate are not observed in the simulations, and differences in behavior of the two sH hydrates are likely related to the differences in geometry of the large guests and occupancies of methane in the small and medium sH cages.

Carbon nanotube-copper exhibiting metal-like thermal conductivity and silicon-like thermal expansion for efficient cooling of electronics

Increasing functional complexity and dimensional compactness of electronic devices have led to progressively higher power dissipation, mainly in the form of heat. Overheating of semiconductor-based electronics has been the primary reason for their failure. Such failures originate at the interface of the heat sink (commonly Cu and Al) and the substrate (silicon) due to the large mismatch in thermal expansion coefficients (∼300%) of metals and silicon. Therefore, the effective cooling of such electronics demands a material with both high thermal conductivity and a similar coefficient of thermal expansion (CTE) to silicon. Addressing this demand, we have developed a carbon nanotube-copper (CNT-Cu) composite with high metallic thermal conductivity (395 W m(-1) K(-1)) and a low, silicon-like CTE (5.0 ppm K(-1)). The thermal conductivity was identical to that of Cu (400 W m(-1) K(-1)) and higher than those of most metals (Ti, Al, Au). Importantly, the CTE mismatch between CNT-Cu and silicon was only ∼10%, meaning an excellent compatibility. The seamless integration of CNTs and Cu was achieved through a unique two-stage electrodeposition approach to create an extensive and continuous interface between the Cu and CNTs. This allowed for thermal contributions from both Cu and CNTs, resulting in high thermal conductivity. Simultaneously, the high volume fraction of CNTs balanced the thermal expansion of Cu, accounting for the low CTE of the CNT-Cu composite. The experimental observations were in good quantitative concurrence with the theoretically described 'matrix-bubble' model. Further, we demonstrated identical in-situ thermal strain behaviour of the CNT-Cu composite to Si-based dielectrics, thereby generating the least interfacial thermal strain. This unique combination of properties places CNT-Cu as an isolated spot in an Ashby map of thermal conductivity and CTE. Finally, the CNT-Cu composite exhibited the greatest stability to temperature as indicated by its low thermal distortion parameter (TDP). Thus, this material presents a viable and efficient alternative to existing materials for thermal management in electronics.

Increasing molecular O3 storage capacity in a clathrate hydrate

Ozone storage capacity in the clathrate hydrate exceeds 2 mass%.

Hydration structures of lactic acid: characterization of the ionic clathrate hydrate formed with a biological organic acid anion

Lactic acid is incorporated in the ionic clathrate hydrate showing various water clustering patterns.

Characterization of tetra-n-butylphosphonium bromide semiclathrate hydrate by crystal structure analysis

The crystal structure of the semiclathrate hydrate of tetra-n-butylphosphonium bromide is reported, and the thermophysical property is characterized by comparison with an analogous hydrate.

Ca-VII: a chain ordered host-guest structure of calcium above 210 GPa

The recently discovered high pressure phase VII of calcium [M. Sakata et al., Phys. Rev. B 83, 220512(R) (2011)] has the highest superconducting transition temperature (T(c)) of 29 K among all the elements. Understanding the cause for such a high T(c) state is necessary to clarify its crystal structure. The structure of this phase was determined by an x-ray powder diffraction experiment and a density functional theory calculation and was not found to be the usual host-guest type but consisted of a 2×2 supercell in the tetragonal ab plane with a commensurate host-guest ratio of 4/3 along the c axis containing 128 atoms.

Synthesis and characterization of a structure H hydrate formed with carbon dioxide and 3,3-dimethyl-2-butanone

Experiments were carried out to synthesize and characterize a structure H clathrate hydrate containing CO(2) and 3,3-dimethyl-2-butanone (pinacolone) by means of phase equilibrium and powder X-ray diffraction measurements. Molecular dynamics simulations of this structure H hydrate were performed to understand the nature of guest-host molecular interactions.