Tsutomu Uchida

Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance: Variations in crystal morphology

This paper reports on our interpretation of our visual observations of the variations in macroscopic morphology of hydrate crystals growing in liquid water saturated with a guest substance prior to the hydrate formation. The observations were made in a high-pressure cell charged with liquid water and gaseous CO2. They revealed distinct variations in the morphology of hydrate crystals depending on the system subcooling ΔT sub, the temperature deficiency inside the cell from the triple CO2–hydrate–water equilibrium temperature under a given pressure. When ΔT sub ≳ 3 K, a hydrate film first grew along the CO2–water interface; then hydrate crystals with dendritic morphology grew in large numbers into the liquid-water phase from that hydrate film. When ΔT sub ≲ 2 K, the dendritic crystals were replaced by skeletal or polyhedral crystals. We present a non-dimensional index for such variations in hydrate crystal morphology. This is based on the idea that this morphology depends on the growth rate of hydrate crystals, and their growth rate is controlled by the mass transfer of the hydrate–guest substance (CO2 in the present experiments), dissolved in the bulk of liquid water, to the hydrate crystal surfaces. The morphology variations observed in the present and previous studies are related to this index.

Measurement of the Density of CO2 Solution by Mach-Zehnder Interferometry

Abstract: The density of CO2 solution was measured by using Mach‐Zehnder interferometry in the pressure range from 5.0 to 12.5 MPa, at temperatures from 273.25 to 284.15 K, and CO2 mass fraction in solution up to 0.061. It was found that the density difference between the CO2 solution and pure water at the same pressure and temperature is monotonically linear with the CO2 mass fraction. The slope of this linear function, calculated by experimental data fitting, is 0.275.

Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.

Freezing Properties of Disaccharide Solutions: Inhibition of Hexagonal Ice Crystal Growth and Formation of Cubic Ice

Numerous studies have been undertaken to preserve living bodies and cells from freezing by addition of cryoprotective agents including natural substances (e.g., sugars and proteins) and synthetic chemicals (e.g., glycerol and dimethyl sulfoxide). Trehalose and sucrose, which consist of fructose and glucose rings connected by a glycosidic bond, are naturally occurring disaccharide compounds found in cryoprotectants. Trehalose is primarily found in animals capable of enduring cold temperatures, whereas sucrose is typically found in plants (Crowe et al., 1988). Since these molecules are too large to permeate the biomembrane, the cryoprotective mechanism is considered to be different from those of cellpermeable substances, such as glycerol and dimethyl sulfoxide. One of the considerable mechanisms of disaccharide molecules operating as a cryoprotective agent of living cells is that they protect the lipid bilayer by making the hydrogen bonding during extracellular ice formation. Based on the interactions between disaccharide molecules and the lipid bilayer, it has been suggested that the molecular mechanism underlying this cryoprotective effect is the hydrogen bonding of trehalose molecules to the bilayer head group (Sum et al., 2003). A simulation of the interaction of the lipid bilayer with trehalose has revealed that only marginal changes occur in the lipid bilayer. Another considerable mechanism of disaccharide molecules of the cryoprotective effect for living cells is that they inhibit the growth of ice crystals in extracellular space. To reveal the inhibition mechanism of disaccharides on ice crystal growth, the interaction between the disaccharide and water molecules has been investigated both by macroscopic observations (for example, Sei et al., 2001; 2002; Sei & Gonda, 2004), where the melting point of disaccharide solutions has been determined, and by microscopic observations (for example, Wang & Tominaga, 1994; Kanno & Yamazaki, 2001; Akao et al., 2001; Branca et al., 1999a) where the interaction has been investigated via Raman or infra-red spectroscopy. Sei et al. (2001; 2002) found that the melting points of trehalose and sucrose solutions were lower than those expected by the molar freezing point depression. They explained this phenomenon by considering the ratio of the hydrated water around disaccharide molecules

Estimations of Interfacial Tensions Between Liquid CO2 and Water from the Sessile-Drop Observations

Publisher Summary Greenhouse gases, such as CO 2 , have become a serious problem for mankind. To offset the CO 2 emissions into the atmosphere, sequestration of CO 2 in the deep ocean has been proposed. The behavior of CO 2 injected into seawater can be predicted from the CO 2 -H 2 O phase diagram and a depth profile of the seawater temperature. In addition, the interfacial tension between liquid CO 2 and water (or sea water) is an important factor for understanding the behavior of the injected CO 2 droplets into deep water. However, CO 2 hydrate formation on the droplets at depths deeper than 400 m makes the prediction of CO 2 dissolution difficult due to insufficient knowledge of the relevant physical parameters. CO 2 hydrate is an ice-like clathrate compound formed from CO 2 and water under suitable conditions of low temperature T and high pressure P. This crystalline compound will form at the interface between the injected liquid CO 2 and seawater and can reduce the dissolution rate of CO 2 into seawater. The interfacial tension between liquid CO 2 and water or NaCl solution was measured by a simple sessile-drop method at pressures up to 25 MPa and at temperatures of 278 and 288 K. The interfacial tension between liquid CO 2 and pure water was approximately 38 mN m -1 at 288 K and 5 MPa with a small pressure dependence, whereas the values between liquid CO 2 and 3 wt% NaCl solution were more than 10 % larger than those between liquid CO 2 and pure water. At 278 K, CO 2 hydrate is stable and the interfacial tension has larger pressure dependence. This might be related to the supersaturation prior to hydrate formation.

The Effect of Magnetic Atom Substitution on the Tunneling Conductance in Skutterudite CoSb3 Semiconductors

Electron tunneling experiments were performed on n-Co 0.9 Ni 0.1 Sb 3 -Al oxide-Al junctions measured at 4.2 K. A V-shape tunneling conductance curve with small zero bias offset is observed, which can be associated with disorder states. Comparing with the results on p-Co 0.9 Fe 0.1 Sb 3 samples, it has been clarified that the Fe substitution leads to a stronger three-dimensional disorder state as compared to the case of Ni substitution. Magnetic susceptibility measurements suggest that the observed three-dimensional disorder state is mainly magnetic in nature.