Keiji Yamahara

Molecular dynamics study of the thermal behaviour of silica glass/melt and cristobalite

Abstract The thermal behaviour of amorphous silica is compared with that of a high temperature form of cristobalite using molecular dynamics (MD) simulations, in order to understand the thermal behaviour of silica glass from an atomistic point of view. The MD simulations reproduce the α–β transition of cristobalite, the negative thermal expansion of β-cristobalite, and the density maximum of the silica melt. The thermal expansion of cristobalite is closely related to the distances between the first neighbour silicon atoms, which depend on the average Si-O–Si angle. On the other hand, the thermal expansion of amorphous silica is not directly correlated with the variation in the average Si–O–Si angle. The positive thermal expansion of silica glass is ascribed to the deformation of network-forming rings, not to the variations in the lengths of the nearest neighbour Si–O or Si–Si bonds. The density anomaly of the silica melt is caused by two opposite factors in the density variation with decreasing temperature: densification due to the increase in number of bridging bonds, and opening of the tetrahedral network in which 5- to 7-membered rings become dominant with a reduction in smaller rings.

Molecular dynamics simulation of the structural development in sol–gel process for silica systems

Abstract The mechanism of the gel formation process has been studied using molecular dynamics (MD) simulation. Two characteristic systems considered are silicic acid, Si(OH)4, and monomethoxy silicic acid, Si(OMe)(OH)3, which are regarded as differing from each other in the extent of hydrolysis. The interatomic potential used is that of Feuston and Garofalini [B.P. Feuston, S.H. Garofalini, J. Phys. Chem., 94 (1990) 5351–5356.], except for a slight modification about methoxy groups. Microscopic features of the structural development are discussed. There is a slow cluster growth during the early stage, a rapid cluster growth follows this due to cluster–cluster aggregation, and the network structure becomes denser with time through the repeated bond-formation and bond-breaking. Finally, the trend of phase separation is observed. The presence of the methoxy groups interferes with the aggregation and the formation of the dense network structure. Having some problems to be examined about set temperature, system size and dilution effect, MD simulation has proved effective for understanding the structural development in sol–gel process for silica systems.

Suppressed Carbon Deposition Behavior in Nickel/Yittria-Stablized Zirconia Anode with SrZr0.95Y0.05O3 − α in Dry Methane Fuel

The distribution of carbon deposition on the cross section of a nickel/yittria-stablized zirconia (Ni/YSZ) anode after an accelerated degradation test in dry methane was analyzed using a scanning electron microscope equipped with an energy dispersive X-ray. The area near the anode top surface side was favored for carbon deposition compared with that near the electrolyte side, suggesting that the electrochemically produced H 2 O and oxygen near the triple phase boundary near the anode top surface side were insufficient for carbon removal reaction. To enhance the reaction of carbon with these two species, a proton-conductor SrZr 0.95 Y 0.05 O 3―α (SZY) was used to modify the Ni/YSZ anode by the infiltration method. The Ni/YSZ anode infiltrated with SZY (Ni/YSZ―SZY) showed a unique structure in which the distribution of SZY was mainly localized near the anode top surface side. In conclusion, the modification of the Ni/YSZ anode by the addition of the proton-conductor SZY improved the operational stability of the anode in dry methane.

Molecular packing analysis of benzene crystals. Part 2. Prediction of experimental crystal structure polymorphs at low and high pressure

Abstract Benzene crystal structures were obtained by pseudoannealing initial structures sampled from a trajectory of constant moles, volume, and temperature molecular dynamics (NVT MD). The crystal structure of the observed low pressure phase I was reproduced as the most stable state and other crystal structures, which included high pressure phase III, were obtained as metastable states at zero pressure. The pressure dependence of enthalpy was analyzed for these crystal structures, and the relative stability among these crystal structures is discussed, at pressures up to 4 GPa. The observed low pressure phase I was the most stable state up to 1.4 GPa and the high pressure phase III was the most stable state above 1.4 GPa. The reported high pressure phase II was not obtained in this work at any pressure up to 4 GPa.

Effects of aluminum impurity on the structural relaxation in silica glass

Abstract To elucidate effects of Al impurity on the glass-forming process in silica glass, the structural relaxation in Al-containing silica glass, with alkali ions of only trace levels, was investigated by observing the fictive temperature. The fictive temperature was determined by infrared (IR) absorption analysis. Al, even at concentrations lower than 10 wtppm, increases the relaxation time and the activation energy of the α -relaxation. It also suppresses the sub-relaxational process due to OH ions. These results indicated that Al should have other effects on structural relaxation than alkali–aluminate complex formation, as has been thought to be the cause for an increase in the α -relaxation time and thus the viscosity of silica glass. Furthermore, the structural relaxation does not merely depend on the number of non-bridging oxygen atoms in the glass network.

Molecular packing analysis: prediction of experimental crystal structures of benzene starting from unreasonable initial structures

Abstract The crystal structures of benzene were analyzed to study whether it is possible to predict actual crystal structures. The minimization of energy and enthalpy were carried out from four types of initial structures. These initial structures included two experimental structures of actual phases and two unreasonable structures created artificially. From an artificial initial structure, we were able to obtain a similar local minimum structure to an observed low pressure phase by energy minimization at zero pressure. The enthalpy minimum structures at 2.5 GPa, which were obtained from two artificial initial structures, were in good agreement with the observed structure of a high pressure phase. We made sure that the corresponding structure of each observed phase was the most stable structure at each pressure but was the metastable structure at the other pressure. The isostress-isoenthalpic molecular dynamics (MD) calculations were carried out starting from each energy minimum structure at two temperatures (147 K, 242 K) lower than the melting point. The crystal structure corresponding to the observed low pressure phase (the energy global minimum) was stable in the trajectory of MD calculations. In the MD calculation starting from local minimum structures, the unit cell deformed greatly and the positions of benzene molecules in the unit cell shifted far from their initial positions.

Molecular dynamics studies on thermal behavior of an MFI-type zeolite

Abstract Molecular dynamics (MD) simulations of an MFI-type zeolite have been performed at various temperatures. Crystal structures and vibrational spectra obtained from the simulations were in good agreement with the experimental data. The temperature induced phase transition between orthorhombic and monoclinic phases, which was already known from experimental measurements, was also reproduced. The potential model used in this study was thus sufficiently applicable to molecular simulations of this type of zeolites. Our MD simulations suggest Pn symmetry for the monoclinic phase instead of experimental P21/n. A small change in the thermal expansion coefficient was found on the phase transition. In the higher temperature range, another remarkable change was found: the thermal expansion coefficient became negative. This indicates the existence of a higher temperature orthorhombic phase.

Effect of Proton Conductor SrCe0.95Yb0.05O3 − α on Electrochemical Characteristics of Nickel/Gadolinium-Doped Ceria Anode in Dry Methane

Ni/Gd 0.2 Ce 0.8 O 2―δ (Ni/GDC) anodes were infiltrated with various amounts of the proton conductor SrCe 0.95 Yb 0.05 O 3―α (SCYb) with the goal of improving the performance of solid oxide fuel cells. First, the electrochemical characteristics of the Ni/GDC and Ni/GDC-SCYb anodes were evaluated in humidified hydrogen (1% H 2 O) fuel and in dry methane fuel at 1173 K, and then the durability of the anodes against carbon deposition was evaluated using an accelerated degradation test. Results revealed that the maximum power density of the Ni/GDC-SCYb anode was 50% higher than that of the Ni/GDC anode, namely 0.648 W/cm 2 in humidified H 2 and 0.438 W/cm 2 in dry methane. In conclusion, the performance of Ni/GDC anodes can be improved in either humidified hydrogen fuel or dry methane fuel by infiltration with SCYb.

Surface of Silica Glass Reacting with Silicon Melt: Effect of Raw Materials for Silica Crucibles.

The density of brownish spots formed on the surface of silica glass through the reaction with silicon melt is studied using three types of silica glass, that are made from natural quartz powder, synthetic silica powder, and Al-doped synthetic silica powder. The density of the brownish spots was higher in natural quartz glass (NQG) than in synthetic silica glass (SSG) and in Al-doped synthetic silica glass (ASG). The result is consistent with that of the devitrification of silica glass observed in the interior of the bulk glass after a heat treatment. Aluminum and calcium are detected as aggregates in the devitrification spots of NQG. It is noted that ASG, containing a comparable concentration of well-dispersed aluminum, shows a significantly lower density of brownish spots and internal devitrification spots than NQG. Thus, it is considered that the formation of the brownish spots originates from the devitrification of silica glass caused by aggregates of impurities.

In situ observation of the Si melt-silica glass interface concerning CZ-Si crystal growth

Abstract An in situ observation was carried out for investigating the interfacial phase of so-called ‘brownish rings’ formed at the Si melt–silica glass interface concerning Czochralski (CZ) silicon crystal growth. It is found that some small dots appeared at the interface immediately after the Si melting, and grew gradually. The growth rate of the interfacial phase increases with increasing temperature, and it is almost the same when the silica dissolution rate is lower than a critical value. It can be concluded, therefore, that the growth rate of the interfacial phase does not depend on the silica dissolution rate in a practical CZ-Si crystal growth by considering that the silica dissolution rate in CZ-Si crystal growth is also lower than the critical value. Crystalline phase forms from the central region of the interfacial phase, and the amount of the crystalline phase increases with increasing temperature and reaction time.