Hideki Minagawa

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.

Formation of a linear LiOH compound on Cu(001): reaction of H2O with Li adatoms at low coverages

The reaction of H2O with Li adatoms at low coverages on Cu(001) has been studied at room temperature by using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and work-function measurement. The coverage (θ) of Li was 0.125 except for the experiment of the work function (θ = 0.25). The O 1s XPS spectra from the H2O exposed Li/Cu(001) surface showed a single peak at 531.5 eV. It was found by comparing the area of the O 1s peaks of H2O/Li/Cu(001) with that of (√2 × 2√2) R45°OCu(001) that the stoichiometry of oxygen to Li is 1:1. HREELS spectra showed a strong peak at 600 cm−1 and small peaks at 3600, 1300 and 1100 cm−1. The work function increased with increasing H2O exposure. These observations and the results of previous studies lead to the conclusion that the reaction product as a result of interaction of H2O with Li adatoms on Cu(001) is a linear triatomic molecule of LiOH which sits on the surface upright with Li down. The reaction scheme is expressed as Li(a) + H2O(a) → LiOH(a) + H(a), where (a) denotes adspecies. The strong 600 cm−1 peak in HREELS spectra is assigned to the Li-OH stretching mode. The intense loss-peak indicates the LiOH molecule formed on Cu(001) has an ionic-bond character. In fact the effective dynamic charge of the Li-OH stretching vibration is estimated to be ∼ 0.5e, and this is larger than that of the Li stretching vibration in the Li/Cu(001) system, ∼ 0.3e. Force constants of the LiOH admolecule are estimated. A transition state for the reaction is proposed.

New Method of Assessing Absolute Permeability of Natural Methane Hydrate Sediments by Microfocus X-ray Computed Tomography

The structure of natural-gas hydrate sediments was studied using a microfocus X-ray computed-tomography (CT) system. The free-gas spaces, sand particles, and hydrates or ices were identified from the obtained three-dimensional (3-D) images. We used CT data to analyze a continuous pore, which allows gas and water flow. The absolute permeability of sediment samples correlated well with horizontal-channel density in terms of direction. The grain-size distribution in sediment samples depended on the spread of flow channels. The average area and length of a channel evidently have little effect on absolute permeability. We determined that absolute permeability increased with the ratio of horizontal- to vertical-channel numbers. It was clear that the number ratio of the horizontal to vertical channels is a predominant factor that determines absolute permeability in similar porosity ranges. These results indicate that the pore network in sediments can be useful for assessing permeability.

Distribution of Hydrate Saturation Ratios in Artificial Methane Hydrate Sediments Measured by High-Speed X-Ray Computerized Tomography

A high-speed X-ray computerized tomography (CT) system was developed for measuring the texture of methane hydrate sediment. The system enabled time dependent nondestructive observations of the hydrate sediment in a volume of 100×100 mm3 within 40 s. The spatial variation of mass density and the volume fraction of an artificial methane hydrate sediment that is either ice or hydrate, hereafter referred to as the hydrate saturation ratio within methane hydrate sediments, were measured using the X-ray CT system. The three-dimensional (3D) distribution change of density and hydrate saturation caused by the hydrate dissociation were also visualized on the basis of the measured gray scale values of the CT images taken before and after their dissociation. The technique allows us observations of the dissociation process of methane hydrate sediment under experimental conditions which are the same as conditions in the natural environment, and will allow the simulation of methane gas production from wells in both oceanic and permafrost areas.

Study on in-situ carbon coating in JIPP T-IIU

The effectiveness of the in-situ carbon coating (carbonization) has been demonstrated to reduce the radiation loss by iron impurities during ICRF heating in the JIPP T-IIU tokamak. As a result of carbonization, the total radiation loss decreased down to one fifth of the RF power, which resulted in an increase in electrons and total stored energy compared with these conditions before carbonization. The thickness of the carbon layer was 300–900 A, and its toroidal uniformity was within a factor of 3, although only one anode and one gas-inlet were used. A thin carbide layer is formed between the C-film and the stainless steel substrate with carbonization at room temperature. The hydrogen concentration is 40–50 at.% in the carbon layer. Deposition of carbon was observed on window materials. The deposition rate was relatively less on electrical insulators compared to the deposition rate on metals.

Synthesis of Si–Ge Alloy by Rapid Cooling in Short-Duration Microgravity

We solidified Si–Ge alloys by rapid cooling in short-duration microgravity. When the Si–Ge melt was solidified by contact with the copper chill block (cooling rate; 100 K/s), the samples solidified in microgravity and on the ground had large grains (about 0.2 mm diameter), and were segregated. When we splat-solidified Si–Ge melt on the copper chill block by using argon gas pressure (4×105 Pa), the solidified sample contained less Ge than the starting material because of the heterogeneity of the Ge component in the melt. When we splat-solidified the Si–Ge melt in microgravity, we obtained a layer with a fine structure (less than 1 µm diameter) on the side contacting the copper chill block because of the high cooling rate (>5000 K/s). A 100-µm-thick layer with fine structure was obtained by doping with P; this layer was much thicker than that in the nondoped sample. The thermal conductivity of the sample splat-solidified in microgravity was lower than that of the arc-melted sample.

Water Permeability of Porous Media Containing Methane Hydrate as Controlled by the Methane-hydrate Growth Process

This chapter seeks to clarify the relation between fluid permeability and methane-hydrate saturation (Sh). The ultimate purpose is to estimate the theoretical expression by the equation K = K0(1 Sh)N (where K0 is the apparent permeability at Sh = 0 and N is a constant) for input into methane-hydrate numerical simulators. However, the permeability of hydrate-bearing sediment strongly depends on the hydrate saturation, grain-size distribution, porosity, pore-size distribution, hydrate formation method, and so on. To clarify the relation between the permeability and methane-hydrate saturation, we measured the water permeability of methane-hydrate-bearing sediments with different hydrate saturations for three contrasting methane-hydrate formation methods: (1) the connate water reaction method, (2) the gas diffusion method, and the (3) cementing method. The results demonstrate that the rate of decrease in the apparent water permeability (AWP) with increasing methane-hydrate saturation differs for each method of gas-hydrate formation. In addition, the values of K and N in the theoretical expression K = K0(1 Sh)N were estimated for each production method, and a different N value was obtained for each hydrate formation method. It is apparent that the method of gas-hydrate formation leads to a contrasting geometry of methane-hydrate growth at the pore scale and in turn affects the macroscopic AWP saturation relations.

Study on carbon coating films produced in Heliotron-E

In the Heliotron-E device, carbonization experiments were successfully performed by using a DC glow discharge in a mixture gas of hydrogen and methane. The properties of carbon films produced on surface probes were analyzed by Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy (RS). Film thickness varied between 17 and 40 nm, and almost no metal impurities were found in the film layer. Both the radial and the toroidal distributions of the film thickness were analyzed. The toroidal uniformity of the carbon film was within a factor of two. The structure of carbon film produced in Heliotron-E was amorphous carbon, while that of TEXTOR film was a mixture of amorphous and graphite. According to the depth profile analysis, the carbon film could hardly be eroded by main discharges. But D2 or He discharge cleaning effectively removed the film. In addition, after the main discharge with Ti-flashing in the carbonized chamber, the formation of TiC in the film region was observed.

In-situ analysis of the carbonization process and surface modification of the carbon layer by tokamak discharges in TEXTOR

Abstract Surface analysis has been carried out on samples exposed to carbonization, discharge cleaning and tokamak discharges with Auger Electron Spectroscopy (AES). A carbon layer of ∼ 30 monolayers on an Inconel sample was neither fully removed nor contaminated during exposure to 134 tokamak discharges at the liner position. The carbon layer was also not removed with exposure to 25 discharges at 1 cm behind the main limiters. Deposition of metals and oxygen was observed on the top surface of the C-layer in this case. Deposition of carbon on silicon samples was found with a carbonized wall after exposure to tokamak discharges. From a semi-quantitative discussion of these experimental results, it has been concluded that the C-layer on the liner, on Faraday shields and on limiters have long life time, and redeposition processes contribute to maintaining the carbon layer. Removal of the carbon layer by glow-discharge cleaning has also been demonstrated.

Study of metal deposition on low Z samples during ECR and glow discharge cleaning in TEXTOR

Abstract A POCO graphite sample was exposed to an electron cyclotron resonance (ECR) plasma for 5 h. Surfaces of the sample were analyzed by AES. No metal deposition was found. Another graphite sample was exposed to an RF-assisted glow (RG) discharge for 6 h. Iron and nickel were found with AES, at surface concentrations of 3% and 1%, respectively. The space potential measured by a Langmuir probe was less than 20 V for ECR plasma and 265 V for RG discharge. In the case of the ECR discharge, the impact energy of ions on the wall is lower than the threshold energy of metal sputtering. Actually, no metal contamination of the carbon sample occurred.