Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Dynamic lateral transport of lipids, proteins, and self-assembled structures in biomembranes plays a crucial role in diverse cellular processes. In this study, we perform coarse-grained molecular dynamics simulations on a vesicle composed of a binary mixture of neutral and anionic lipids to investigate the lateral transport of individual lipid molecules and the self-assembled lipid domains upon an applied direct current (DC) electric field. Under the potential force of the electric field, a phase-separated domain rich in anionic lipids is trapped in the opposite direction of the electric field. The subsequent reversal of the electric field induces unidirectional domain motion. During the domain motion, the domain size remains constant, but a considerable amount of the anionic lipids is exchanged between the anionic-lipid-rich domain and the surrounding bulk. While the speed of the domain motion (collective lipid motion) shows a significant positive correlation with the electric field strength, the exchange of anionic lipids between the domain and bulk (individual lipid motion) exhibits no clear correlation with the field strength. The mean velocity field of the lipids surrounding the domain displays a two-dimensional (2D) source dipole. We revealed that the balance between the potential force of the applied electric field and the quasi-2D hydrodynamic frictional force well explains the dependence of the domain motions on the electric field strengths. The present results provide insight into the hierarchical dynamic responses of self-assembled lipid domains to the applied electric field and contribute to controlling the lateral transportation of lipids and membrane inclusions.

DOI： 10.1021/acs.jpcb.3c04351

Web of Science

Scopus

PubMed

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

γ-Carbonyl-substituted ε-caprolactones were found to predominantly isomerize to five-membered lactones rather than to form desired linear polyesters in ring-opening polymerization.

DOI： 10.1039/d3ra01025b

Web of Science

Scopus

PubMed

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The dynamics of hydration water (HW) in 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) was investigated by means of quasi-elastic neutron scattering (QENS) and compared with those observed in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The headgroup dynamics of DMPE was investigated using a mixture of tail-deuterated DMPE and D2O, and the QENS profiles were interpreted as consisting of three modes. The fast mode comprised the rotation of hydrogen atoms in –NH3+ and –CH2– groups in the headgroup of DMPE, the medium-speed mode comprised fluctuations in the entire DMPE molecule, and the slow mode comprised fluctuations in the membrane. These interpretations were confirmed using molecular dynamics (MD) simulations. The HW dynamics analysis was performed on a tail-deuterated DMPE and H2O mixture. The QENS profiles were analyzed in terms of three modes: (1) a slow mode, identified as loosely bound HW in the DMPC membrane; (2) a medium-speed mode similar to free HW in the DMPC membrane; and (3) a fast mode, identified as rotational motion. The relaxation time for the fast mode was approximately six times shorter than that of rotational water in DMPC, consistent with the results of terahertz time-domain spectroscopy. The activation energy of medium-speed HW in DMPE differed from that of free HW in DMPC, suggesting the presence of different hydration states or hydrogen-bonded networks around the phosphocholine and phosphoethanolamine headgroups.

DOI： 10.1063/4.0000184

Web of Science

Scopus

PubMed

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Correlation between the phase separation of membranes consisting of negatively charged and neutral phospholipids and cation distribution in aqueous solutions is presented.

DOI： 10.1039/d3sm00222e

Web of Science

Scopus

PubMed

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Biological membranes are functionalized by membrane-associated protein machinery. Membrane-associated transport processes, such as endocytosis, represent a fundamental and universal function mediated by membrane-deforming protein machines, by which small biomolecules and even micrometer-size substances can be transported via encapsulation into membrane vesicles. Although synthetic molecules that induce dynamic membrane deformation have been reported, a molecular approach enabling membrane transport in which membrane deformation is coupled with substance binding and transport remains critically lacking. Here, we developed an amphiphilic molecular machine containing a photoresponsive diazocine core (AzoMEx) that localizes in a phospholipid membrane. Upon photoirradiation, AzoMEx expands the liposomal membrane to bias vesicles toward outside-in fission in the membrane deformation process. Cargo components, including micrometer-size M13 bacteriophages that interact with AzoMEx, are efficiently incorporated into the vesicles through the outside-in fission. Encapsulated M13 bacteriophages are transiently protected from the external environment and therefore retain biological activity during distribution throughout the body via the blood following administration. This research developed a molecular approach using synthetic molecular machinery for membrane functionalization to transport micrometer-size substances and objects via vesicle encapsulation. The molecular design demonstrated in this study to expand the membrane for deformation and binding to a cargo component can lead to the development of drug delivery materials and chemical tools for controlling cellular activities.

DOI： 10.1021/jacs.2c12348

Scopus

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

Poly(ethylene glycol)-conjugated amphiphilic block copolymers, which contain degradable hydrophobic blocks connected by a rigid hydrogen-bonding motif (RHM), are developed to yield anisotropic nanoassemblies in a manner independent of the crystalline nature of the hydrophobic block. The all-atom and coarse-grained molecular dynamics simulations suggest that the anisotropic alignment of the block copolymers can be attributed to the π-πinteractions of the RHM and the crystallizable hydrophobic blocks help maintain the aligned structure. Light scattering analysis of the polymer assemblies demonstrates the formation of nonspherical assemblies by the RHM-containing block copolymers with an amorphous hydrophobic block; this indicates the strong contribution of the RHM to the directed assembly of the block copolymers, unlike crystallization-driven self-assembly. Enhanced cell proliferation is observed in cell cultures containing normal human fibroblasts in the presence of the anisotropic polymer assemblies with aspect ratios greater than 12 or lengths greater than 310 nm.

DOI： 10.1021/acs.macromol.1c01894

Web of Science

Scopus

researchmap

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Self-sorting and co-assembly of aqueous supramolecular fibres were formed using peptide amphiphiles having immiscible hydrophobic tails.

DOI： 10.1039/d1cc05560g

Web of Science

Scopus

PubMed

researchmap

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.1c09468

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.langmuir.1c00967

Language：English   Publishing type：Research paper (scientific journal)  

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

DOI： 10.1021/acs.langmuir.1c00727

Language：English   Publishing type：Research paper (scientific journal)  

Hydration states are a crucial factor that affect the self-assembly and properties of soft materials and biomolecules. Although previous experiments have revealed that the hydration state strongly depends on the chemical structure of lipid molecules, the mechanisms at the molecular level remain unknown. Classical and density-functional tight-binding (DFTB) molecular dynamics (MD) simulations are employed to determine the mechanisms underlying dissimilar water dynamics between lipid membranes with phosphatidylcholine (PC) and phosphatidylethanolamine (PE) head groups. The classical MD simulation shows that rotational relaxations of water are faster on the PE lipid than on the PC lipid, which is consistent with a previous experimental study using terahertz spectroscopy. Furthermore, DFTB-MD simulation of N(CH3)(4)(+) and NH4+ ions, which correspond to the different head groups in PC and PE, respectively, shows qualitative agreement with the classical MD simulation. Remarkably, the PE lipids and the NH4(+) ions break hydrogen bonds between water molecules in the secondary hydration shell. In contrast, the PC lipids and the N(CH3)(4)(+) ions bind water molecules weakly in both the primary and secondary hydration shells and increase hydrogen bonds between water. Our atomistic simulations show that these changes in the hydrogen-bond network of water molecules cause the different rotational relaxation of water molecules between the two lipids.

DOI： 10.1021/acs.langmuir.1c00417

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/physreve.103.042502

Language：Others   Publishing type：Research paper (scientific journal)  

<p>Chemical mechanical polishing (CMP) of Ga-face GaN is accelerated by the chemical reactions with OH radicals.</p>

DOI： 10.1039/d0cp05826b

Language：Others   Publishing type：Research paper (scientific journal)  

We reveal the generation of the "Graphene Arch-Bridge"on a diamond (111) surface by Si doping via first-principles calculations. The "Graphene Arch-Bridge"is different from a simple graphene structure because both its ends are pinned to the diamond surface, and it has an interesting arched-type curved structure. The large stress around the doped Si atom leads to the transition of the six-membered C ring to a five-membered C ring. The C atom excluded from the ring by this transition changes from an sp3 carbon to an sp2 carbon and generates the "Graphene Arch-Bridge"on the diamond (111) surface. These results suggest that the generation of the five-membered C ring by stress due to the Si doping is the reason why the "Graphene Arch-Bridge"is generated. Finally, we propose that the "Graphene Arch-Bridge"is the origin of the experimentally observed super-low friction of Si-doped diamond-like carbon (DLC). Furthermore, we suggest that the "Graphene Arch-Bridge"leads to the lower wear properties of Si-doped DLC compared with nondoped DLC because its ends of the bridge are pinned to the DLC surface.

DOI： 10.1021/acs.jpcc.0c09716

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1246/cl.200323

Language：English   Publishing type：Research paper (scientific journal)  

Hydrogels are biocompatible polymer networks; however, they have the disadvantage of having poor mechanical properties. Herein, the mechanical properties of host-guest hydrogels were increased by adding a filler and incorporating other noncovalent interactions. Cellulose was added as a filler to the hydrogels to afford a composite. Citric acid-modified cellulose (CAC) with many carboxyl groups was used instead of conventional cellulose. The preparation began with mixing an acrylamide-based αCD host polymer (p-αCD) and a dodecanoic acid guest polymer (p-AADA) to form supramolecular hydrogels (p-αCD/p-AADA). However, when CAC was directly added to p-αCD/p-AADA to form biocomposite hydrogels (p-αCD/p-AADA/CAC), it showed weaker mechanical properties than p-αCD/p-AADA itself. This was caused by the strong intramolecular hydrogen bonding (H-bonding) within the CAC, which prevented the CAC reinforcing p-αCD/p-AADA in p-αCD/p-AADA/CAC. Then, calcium chloride solution (CaCl2) was used to form calcium ion (Ca2+) complexes between the CAC and p-αCD/p-AADA. This approach successfully created supramolecular biocomposite hydrogels assisted by Ca2+ complexes (p-αCD/p-AADA/CAC/Ca2+) with improved mechanical properties relative to p-αCD/p-AADA hydrogels; the toughness was increased 6-fold, from 1 to 6 MJ/m3. The mechanical properties were improved because of the disruption of the intramolecular H-bonding within the CAC by Ca2+ and subsequent complex formation between the carboxyl groups of CAC and p-AADA. This mechanism is a new approach for improving the mechanical properties of hydrogels that can be broadly applied as biomaterials.

DOI： 10.1021/acs.biomac.0c01095

Language：English   Publishing type：Research paper (scientific journal)  

Understanding atomic-scale wear is crucial to avoid device failure. Atomic-scale wear differs from macroscale wear because chemical reactions and interactions at the friction interface are dominant in atomic-scale tribological behaviors, instead of macroscale properties, such as material strength and hardness. It is particularly challenging to reveal interfacial reactions and atomic-scale wear mechanisms. Here, our operando friction experiments with hydrogenated diamond-like carbon ( DLC) in vacuum demonstrate the triboemission of various hydrocarbon molecules from the DLC friction interface, indicating its atomic-scale chemical wear. Furthermore, our reactive molecular dynamics simulations reveal that this triboemission of hydrocarbon molecules induces the atomic-scale mechanical wear of DLC. As the hydrogen concentration in hydrogenated DLC increases, the chemical wear increases while mechanical wear decreases, indicating an opposite effect of hydrogen concentration on chemical and mechanical wear. Consequently, the total wear shows a concave hydrogen concentration dependence, with an optimal hydrogen concentration for wear reduction of around 20%.

DOI： 10.1126/sciadv.aax9301

Language：Others   Publishing type：Research paper (scientific journal)  

Stress Transmitters at the Molecular Level in the Deformation and Fracture Processes of the Lamellar Structure of Polyethylene via Coarse-Grained Molecular Dynamics Simulations

DOI： 10.1021/acs.macromol.9b00636

Language：Others   Publishing type：Research paper (scientific journal)  

Coarse-grained molecular dynamics simulation for uptake of nanoparticles into a charged lipid vesicle dominated by electrostatic interaction

DOI： 10.1103/PhysRevE.100.012407

Language：Others  

Ionic Conductivity in Ionic Liquid Nano Thin Films
PMID: 30199622

DOI： 10.1021/acsnano.8b06386

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1246/cl.180450

Language：English   Publishing type：Research paper (scientific journal)  

Diamond-like carbon (DLC) is a promising solid lubricant used as a protective coating to reduce friction against alumina. Friction properties of DLC/alumina are strongly affected by temperature. To improve the friction performance of DLC, we investigate the friction behaviors of DLC/alumina at various temperatures and reveal the mechanisms by using our tight-binding quantum chemical molecular dynamics method. We observe an interesting volcano-type temperature dependence of friction coefficients in our friction simulations. Friction coefficients of DLC/alumina are low and show little change at 300–600 K because no tribochemical reactions occur at the interface. However, as the temperature increases, friction coefficients increase at 600–800 K and subsequently decrease at 800–1000 K. At 600–800 K, interfacial C-O and C-Al bonds between two substrates are formed during friction, leading to a high friction coefficient. Interestingly, further increment of temperature to 800–1000 K induces the graphitization of DLC. The graphite-like surface suppresses the interfacial bond formation, reducing the friction coefficient. We reveal that the volcano-type temperature dependence of friction coefficients is due to the tribochemical reactions generating interfacial bonds at 600–800 K and the graphitization of DLC reducing the number of interfacial bonds at 800–1000 K.

DOI： 10.1016/j.carbon.2018.03.034

Language：English   Publishing type：Research paper (scientific journal)  

First-principles calculation of activity and selectivity of the partial oxidation of ethylene glycol on Fe(0 0 1), Co(0 0 0 1), and Ni(1 1 1)
To recycle ethylene glycol (HOCH2CH2OH) fuel in alkaline fuel cells, active and selective catalysts for partially oxidizing HOCH2CH2OH to glycolic acid (HOCH2COOH) and oxalic acid ((COOH)2) are required at the anode
in other words, complete oxidation of HOCH2CH2OH to CO2 prevents ethylene glycol recycling. We investigate catalyst activity and selectivity for oxidizing HOCH2CH2OH to HOCH2COOH on Fe(0 0 1), Co(0 001), and Ni(1 1 1) via first-principles calculations. We calculate the oxidation reaction path from HOCH2CH2OH to HOCH2COOH without C–C bond dissociation to avoid CO2 generation. Partial oxidation of HOCH2CH2OH to HOCH2COOH without C–C bond dissociation proceeds as follows: O–H bond dissociation of HOCH2CH2OH to generate HOCH2CH2O
C–H bond dissociation of HOCH2CH2O to generate HOCH2CHO
C–H bond dissociation of HOCH2CHO to generate HOCH2CO
and OH addition to HOCH2CO to generate HOCH2COOH. The activation energies for O–H bond dissociation of HOCH2CH2OH and C–H bond dissociation of HOCH2CH2O and HOCH2CHO on Fe(0 0 1) are 20.2, 22.8, and 35.2 kcal/mol, respectively, which are the lowest of the three surfaces. Thus, Fe(0 0 1) is most active. To determine the selectivity, we compare the bond dissociation activation energies. The activation energies for C–C bond dissociation of HOCH2CH2OH and HOCH2CH2O on Fe(0 0 1) (66.7 and 39.5 kcal/mol, respectively) are higher than those for O–H bond dissociation of HOCH2CH2OH (20.2 kcal/mol) and C–H bond dissociation of HOCH2CH2O (22.8 kcal/mol), implying that the O–H bond of HOCH2CH2OH and C–H bond of HOCH2CH2O dissociate before the C–C bond dissociation during oxidation on Fe(0 0 1). In contrast, the activation energies for C–H and C–C bond dissociation of HOCH2CHO (35.2 and 32.8 kcal/mol, respectively) are similar. The C–H and C–C bonds therefore dissociate during HOCH2CHO oxidation. On Co(0 0 0 1) and Ni(1 1 1), the activation energies for C–C bond dissociation of HOCH2CH2O and HOCH2CHO are lower than those for their C–H bond dissociation. Therefore, Fe(0 0 1) is more active and selective than Co(0 0 0 1) and Ni(1 1 1) for the partial oxidation of HOCH2CH2OH to HOCH2COOH.

DOI： 10.1016/j.jcat.2018.03.017

Language：English   Publishing type：Research paper (scientific journal)  

Molecular simulations are powerful tools for revealing the properties of polymers at the molecular level. In particular, coarse-grained molecular dynamics simulations are useful for elucidating the deformation and fracture processes of polymers. However, in the case of crystalline polymers, it is difficult to reproduce experimentally observed structures and mechanical properties using these models. This review describes our recent investigations into the deformation and fracture processes of crystalline polymers using coarse-grained molecular dynamics simulations. We were able to successfully reproduce the lamellar structure of polyethylene, which is a fundamental structural feature of this polymer, and obtain a stress–strain curve that exhibited good consistency with that observed experimentally. The molecular dynamics simulations revealed that void generation in the amorphous layers was caused by the movement of the chain ends, which is difficult to observe through experiments. The conditions required to reproduce the experimentally observed structure and mechanical properties using molecular simulations are also discussed.

DOI： 10.1038/s41428-018-0067-1

Language：English   Publishing type：Research paper (scientific journal)  

Double-network (DN) gels consisting of highly and slightly cross linked networks exhibit good mechanical properties, complicated deformation behavior, and fracture processes owing to the existence of a large number of influential parameters. To determine the effects of these factors at the molecular level to improve the mechanical properties further, we studied the fracture processes of DN gels using a coarse-grained molecular dynamics simulation. First, we propose a modeling method for DN gels consisting of highly (first) and slightly (second) cross-linked networks. Then, we stretch the DN gels and investigate the effects of the network ratio, chain length, and first- and second-network structures on the mechanical properties. During the stretching, the stress increases with the bond breaking in the first network. Then, the stress further increases with the simultaneous bond breaking in the first and second networks when they are entangled with one another. Finally, the bond breaking in the first network stops, and only the bond breaking in the second network occurs. The second network remains at a high strain, which prevents rupture of the gel. We find that (i) a low concentration of the first network is necessary for the gel to exhibit the properties of both the first and second networks, (ii) a tense first network increases the Young's modulus, and (iii) the second network with a long chain length and separated cross linking points increases the peak stress and ductility. We have therefore successfully elucidated the effects of the network structures on the mechanical properties of DN gels.

DOI： 10.1021/acs.macromol.8b00124

Language：English   Publishing type：Research paper (scientific journal)  

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

DOI： 10.1021/acsami.7b07551

Language：English   Publishing type：Research paper (scientific journal)  

Ni sintering in the Ni/YSZ porous anode of a solid oxide fuel cell changes the porous structure, leading to degradation. Preventing sintering and degradation during operation is a great challenge. Usually, a sintering molecular dynamics (MD) simulation model consisting of two particles on a substrate is used; however, the model cannot reflect the porous structure effect on sintering. In our previous study, a multi-nanoparticle sintering modeling method with tens of thousands of atoms revealed the effect of the particle framework and porosity on sintering. However, the method cannot reveal the effect of the particle size on sintering and the effect of sintering on the change in the porous structure. In the present study, we report a strategy to reveal them in the porous structure by using our multi-nanoparticle modeling method and a parallel large-scale multimillion-atom MD simulator. We used this method to investigate the effect of YSZ particle size and tortuosity on sintering and degradation in the Ni/YSZ anodes. Our parallel large-scale MD simulation showed that the sintering degree decreased as the YSZ particle size decreased. The gas fuel diffusion path, which reflects the overpotential, was blocked by pore coalescence during sintering. The degradation of gas diffusion performance increased as the YSZ particle size increased. Furthermore, the gas diffusion performance was quantified by a tortuosity parameter and an optimal YSZ particle size, which is equal to that of Ni, was found for good diffusion after sintering. These findings cannot be obtained by previous MD sintering studies with tens of thousands of atoms. The present parallel large-scale multimillion-atom MD simulation makes it possible to clarify the effects of the particle size and tortuosity on sintering and degradation.

DOI： 10.1021/acsami.7b07737

Language：English   Publishing type：Research paper (scientific journal)  

Coarse-grained molecular dynamics simulations can model the deformation and fracture processes of the lamellar structure in polyethylene on a molecular scale; however, the simulations have not been performed due to the difficulty in building the structure and the limitations of small-scale simulations on the order of 104 beads. Thus, we propose a crystallization method for a large-scale lamellar structure on the order of 106 beads. The highly oriented lamellar structure is stretched in a coarse-grained molecular dynamics simulation. The stress and variation of crystallinity during stretching parallel and perpendicular to the crystal direction agree with experimental results, confirming the validity of our simulation results. During stretching parallel to the crystal direction, the amorphous layers crystallize and the crystalline layers fragment. We also find that the movement of the polymer chain ends from amorphous to crystalline layers, which is difficult to observe experimentally, increases the compression and generation of voids in the amorphous layers.

DOI： 10.1021/acs.macromol.6b02613

Language：English   Publishing type：Research paper (scientific journal)  

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp(3) content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp(2) planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.

DOI： 10.1038/srep46394

Language：Others  

Effect of Polarity of a Substrate on ZnO Crystal Growth Process by Molecular Dynamics Simulation

DOI： 10.2477/jccj.2016-0056

Language：English   Publishing type：Research paper (other academic)  

Water lubrication has been attracting attention for environment-friendly society due to low CO2 emission. Furthermore, carbon-based materials such as diamond-like carbon (DLC) show the low friction properties in water lubrication due to the oxidation reaction on the surface in pre-sliding. However, the influence of oxidation reactions on low friction mechanism is still unclear. In this study, we clarify the structure change of DLC with the oxidation reaction in the pre-sliding using first-principles calculation, which suggests the low friction mechanism of DLC in water lubrication. The results show the structure change from sp(3) carbon (Csp(3)) to sp(2) carbon (Csp(2)) by the oxidation reaction on the surface. Furthermore, the Csp(2) rich surface in water lubrication indicates the smooth sliding. We suggest that the structure change from Csp(3) to Csp(2) would affect low friction properties of DLC in water lubrication.

Language：English   Publishing type：Research paper (other academic)  

Sintering of Ni particles in the Ni-based anode is a major obstacle to the widespread use of solid oxide fuel cell because the sintering induces the degradation in the anode. The large amount of water vapor in the fuel is known to accelerate the degradation during the operation. However, the detailed accelerated sintering mechanism is unclear. In this study, to clear the accelerated sintering mechanism during the initial stage of the sintering by water vapor, we investigated the adsorption and dissociation of water molecules on the Ni surface as well as the effect of the terminations of O, H, and OH on the interaction between the Ni clusters by density functional theory because the sintering of Ni particles is started by the Ni particles approaching each other due to the attractive interaction between Ni particles at the initial stage. In our adsorption and dissociation calculations, increasing the amount of water vapor facilitates the adsorption of H2O molecule on the Ni surface due to the H-bond interaction. Meanwhile, the activation energy for the dissociation of H2O molecules on the Ni surface is also lowered with increasing the amount of H2O molecules. Then, we calculated the interaction between Ni cluster, and O terminated, OH terminated, and H terminated Ni clusters. We observed that the attractive interaction between Ni cluster and O terminated Ni cluster is larger than that between two Ni clusters in vacuum. Enhancing the attractive interaction is not observed in the H terminated and OH terminated Ni clusters. It indicates that two Ni clusters approaching each other is faster under the water vapor environment than that in vacuum due to the strong attractive interaction caused by the termination of O. Thus, we suggest that the generation of O termination plays an important role in the sintering at the initial stage under the water vapor environment.

Language：English   Publishing type：Research paper (scientific journal)  

Biomembranes, which are mainly composed of neutral and charged lipids, exhibit a large variety of functional structures and dynamics. Here, we report a coarse-grained molecular dynamics (MD) simulation of the phase separation and morphological dynamics in charged lipid bilayer vesicles. The screened long-range electrostatic repulsion among charged head groups delays or inhibits the lateral phase separation in charged vesicles compared with neutral vesicles, suggesting the transition of the phase-separationmechanism from spinodal decomposition to nucleation or homogeneous dispersion. Moreover, the electrostatic repulsion causes morphological changes, such as pore formation, and further transformations into disk, string, and bicelle structures, which are spatiotemporally coupled to the lateral segregation of charged lipids. Based on our coarse-grained MD simulation, we propose a plausible mechanism of pore formation at the molecular level. The pore formation in a charged-lipid-rich domain is initiated by the prior disturbance of the local molecular orientation in the domain.

DOI： 10.1103/PhysRevE.94.042611

Language：English   Publishing type：Research paper (scientific journal)  

We study fracture processes of amorphous and semicrystalline polymers with a coarse-grained molecular dynamics simulation. In the amorphous state, the stress caused by strain mainly arises from the loss of the attractive interaction in the voids. However, in semicrystalline polymers, the elongation of bonding is the dominant factor and it causes much more stress than that in an amorphous state. This is because growth of the voids is prevented by the amorphous regions and it is difficult to relax the folded polymers.

DOI： 10.1088/0965-0393/24/5/055006

Language：Others  

Large-scale coarse-grained molecular dynamics simulation based on MPI parallel computing: Mechanical property of polymers in molecular scale

Language：English   Publishing type：Research paper (scientific journal)  

We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO2 abrasive grain in aqueous H2O2. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H2O2, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H2O2 molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu Cu bonds. The dissociation of the Cu Cu back bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)(2) molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H2O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H2O2 and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.

DOI： 10.1021/acsami.5b11910

Language：English   Publishing type：Research paper (scientific journal)  

We used our etching simulator [H. Ito et al., J. Phys. Chem. C, 2014, 118, 21580-21588] based on tight-binding quantum chemical molecular dynamics (TB-QCMD) to elucidate SiC etching mechanisms. First, the SiC surface is irradiated with SF5 radicals, which are the dominant etchant species in experiments, with the irradiation energy of 300 eV. After SF5 radicals bombard the SiC surface, Si-C bonds dissociate, generating Si-F, C-F, Si-S, and C-S bonds. Then, etching products, such as SiS, CS, SiFx, and CFx (x = 1-4) molecules, are generated and evaporated. In particular, SiFx is the main generated species, and Si atoms are more likely to vaporize than C atoms. The remaining C atoms on SiC generate C-C bonds that may decrease the etching rate. Interestingly, far fewer Si-Si bonds than C-C bonds are generated. We also simulated SiC etching with SF3 radicals. Although the chemical reaction dynamics are similar to etching with SF5 radicals, the etching rate is lower. Next, to clarify the effect of O atom addition on the etching mechanism, we also simulated SiC etching with SF5 and O radicals/atoms. After bombardment with SF5 radicals, Si-C bonds dissociate in a similar way to the etching without O atoms. In addition, O atoms generate many C-O bonds and COy (y = 1-2) molecules, inhibiting the generation of C-C bonds. This indicates that O atom addition improves the removal of C atoms from SiC. However, for a high O concentration, many C-C and Si-Si bonds are generated. When the O atoms dissociate the Si-C bonds and generate dangling bonds, the O atoms terminate only one or two dangling bonds. Moreover, at high O concentrations there are fewer S and F atoms to terminate the dangling bonds than at low O concentration. Therefore, few dangling bonds of dissociated Si and C atoms are terminated, and they form many Si-Si and C-C bonds. Furthermore, we propose that the optimal O concentration is 50-60% because both Si and C atoms generate many etching products producing fewer C-C and Si-Si bonds are generated. Finally, we conclude that our TB-QCMD etching simulator is effective for designing the optimal conditions for etching processes in which chemical reactions play a significant role.

DOI： 10.1039/c5cp06515a

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the growth mechanisms and structures of hydrogenated amorphous silicon carbide (a-SixCyHz during chemical vapor deposition (CVD) by using density-functional tight-binding molecular dynamics (DFTB MD) and statistical thermodynamics (ST) calculations. Our multiscale modeling from an atomic to an experimental scale allows us to bridge the gap between micro- and macroscopic knowledge. As in any compound, the degree of chemical order in a-SixCyHz is of practical importance. However, the origin of chemical order and effects of composition on the degree of chemical order remain unknown. First, CVD simulations are performed by the impingement of CH3 and SiH3 radicals on a Si(001)-(2 X 1):H surface with DFTB MD. The initial growth process consists of an abstraction-adsorption mechanism, where a CH3 or SiH3 radical abstracts a H atom and forms a dangling bond (DB) on the surface, and a subsequent CH3 or SiH3 radical is adsorbed on the DB. A surface-adsorbed CH2 species with a DB is inserted into a neighboring Si-Si bond converting it to a Si-C bond. The bond rearrangement simultaneously transfers the DB from the C to Si atoms. A CH3 radical is then adsorbed on the Si atom, forming a Si-C bond. The absence of DBs on C atoms reduces the opportunity for forming C-C bonds. Therefore, the bond-rearrangement and DB transfer mechanism explain why Si C bonds are preferentially formed instead of Si-Si and C-C bonds. Second, to model CVD growth of a-SixCyHz on a macroscopic scale, we develop a ST model for a-SixCyHz, calculating the bonding fractions. Importantly, we show that the degree of chemical order depends on the H atom fraction. The bonding fractions obtained by ST results show excellent agreement with those obtained by DFTB MD calculations. Our DFTB MD and ST models predict the relationship between the composition and the degree of chemical order, and bridge a gap between micro- and macroscopic observations. The models can be used for designing materials for other covalent and ionic compounds at micro- to macroscales.

DOI： 10.1021/acs.jpcc.5b08561

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the effect of charge on the membrane morphology of giant unilamellar vesicles (GUVs) composed of various mixtures containing charged lipids. We observed the membrane morphologies by fluorescent and confocal laser microscopy in lipid mixtures consisting of a neutral unsaturated lipid [dioleoylphosphatidylcholine (DOPC)], a neutral saturated lipid [dipalmitoylphosphatidylcholine (DPPC)], a charged unsaturated lipid [dioleoylphosphatidylglycerol (DOPG((-)))], a charged saturated lipid [dipalmitoylphosphatidylglycerol (DPPG((-)))], and cholesterol (Chol). In binary mixtures of neutral DOPC-DPPC and charged DOPC-DPPG((-)), spherical vesicles were formed. On the other hand, pore formation was often observed with GUVs consisting of DOPG((-)) and DPPC. In a DPPC-DPPG((-)-)Chol ternary mixture, pore-formed vesicles were also frequently observed. The percentage of pore-formed vesicles increased with the DPPG((-)) concentration. Moreover, when the head group charges of charged lipids were screened by the addition of salt, pore-formed vesicles were suppressed in both the binary and ternary charged lipid mixtures. We discuss the mechanisms of pore formation in charged lipid mixtures and the relationship between phase separation and the membrane morphology. Finally, we reproduce the results seen in experimental systems by using coarse-grained molecular dynamics simulations.

DOI： 10.1103/PhysRevE.92.062713

Language：Japanese  

Development and application of a double-network gel modeling method for fracture processes using a coarse-grained molecular dynamics simulation
We examine the mechanism of fracture processes in a double-network (DN) gel model by using a coarse-grained molecular dynamics simulation. Initially, we develop a modeling method for DN gel containing both slightly and highly cross-linked networks, and then stretch the DN gel model. During stretching, the highly cross-linked network begins to dissociate at a strain of 1.0, increasing the stress. At strains from 4.0 to 5.0, the slightly and highly cross-linked networks simultaneously dissociate and the stress decreases. Then, the dissociation of the highly cross-linked network stops and only the slightly cross-linked network dissociates at a strain of 12.0, while the stress remains almost the same. We reveal that characteristics of each type of network gradually appear in the DN gel. Next, we change the polymer chain length to reveal its influence on the mechanical properties of the gel. An increase in the length of the slightly cross-linked network chains improves the strength of the DN gel, whereas that of the highly cross-linked network chains does not affect its strength. An increase in the slightly cross-linked network chain length increases the number of entanglements, leading to the increase in strength.

DOI： 10.2477/jccj.2015-0053

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the different dynamics of the stress-induced dissociation and recombination reactions in a model of polyethylene by a first-principles molecular dynamics simulation at the B3LYP/6-31g(d) level. The dissociation under external forces acting on the chemical reaction site at 300 K follows the same pathway as the one calculated by the static first-principles method because it has a similar activation barrier to that of the static first-principles calculation. On the other hand, in the recombination process, thermal fluctuations causes collisions between hydrogen atoms at the chain ends. Furthermore, when external forces do not directly act on the chemical reaction site, two different dissociation processes are observed. On the other hand, recombination process is not observed due to rarely contact of the radical carbon. These results indicate that dissociation and recombination dynamics are very different, showing the importance of the dynamic calculation. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemphys.2015.08.007

Language：Others   Publishing type：Research paper (scientific journal)  

<p>The effects of the ceramic type and porosity on the sintering and degradation in Ni/YSZ and Ni/ScSZ anodes are unveiled by a recently developed multi-nanoparticle sintering simulation method based on molecular dynamics simulation.</p>

DOI： 10.1039/c5ta05575j

Language：English   Publishing type：Research paper (scientific journal)  

To improve the tribological performance of polytetrafluoroethylene (PTFE) resin sliding against a metallic surface, it is important to understand the chemical behavior of PTFE in this sliding system. The tribochemical reaction of PTFE on an aluminum surface has been strenuously studied by a series of computational chemistry methods [Onodera, T., et al. J. Phys. Chem. C 2014, 118, 5390-5396, and Onodera, T., et al. J. Phys. Chem. C 2014, 118, 11820-11826]. One of the most important insights was that PTFE reacted tribochemically with the oxidized surface of aluminum as a Lewis acid catalyst, forming a fluoride on the aluminum surface. The aluminum fluoride formed was a cause of decreasing tribological performance of PTFE because of less formation of a transfer film. In regard to this tribochemical reaction, it was suggested that preventing the fluoride formation is a key to improving the tribological performance of PTFE sliding against an aluminum surface. In this study, to investigate fluoride formation by a tribochemical reaction, the catalytic effect Of an oxidized aluminum surface was investigated experimentally and theoretically. Two phases of an oxidized aluminum surface, namely, the alpha and gamma phases of alumina, were chosen for investigating the catalytic tribochemistry of PTFE. A thermogravimetric analysis showed that the gamma-alumina surface potentially exhibited a stronger catalytic effect in regard to PTFE since the reaction took place at lower temperature. The effect of the catalytic reaction on the tribological performance of PTFE was then investigated by a pin-on-disk tribometer. The results of this investigation show that the amount of wear of PTFE on the gamma-alumina surface was higher than that on the a-alumina surface. By observing the wear scar on alumina surfaces by a scanning electron microscope (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), it was clarified that the transfer film formed on the gamma-alumina surface was less abundant, while it was regularly formed and more abundant on the a-alumina surface. In other words, the antiwear performance of PTFE was decreased because a lower amount of transfer film was formed by the catalytic effect during friction. In addition, a density-functional-theory (DFT) calculation also showed a stronger catalytic effect on the gamma-alumina surface because the energy barrier for the chemical reaction producing fluoride was lower than that on the a-alumina surface. On the basis of these results, it was suggested that controlling the catalytic reaction of PTFE on the sliding surface is one of the ways to improve the antiwear performance of PTFE.

DOI： 10.1021/acs.jpcc.5b01370

Language：English   Publishing type：Research paper (other academic)  

Sintering of Ni particles in the Ni/Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
anode is a major obstacle to the widespread use of solid oxide fuel cell. In this study, we investigated the dopant effect on the diffusion of a Ni atom on the ZrO&lt
inf&gt
2&lt
/inf&gt
surface with dopants (Y and Al) by density functional theory calculations in order to inhibit the sintering. The most stable adsorption sites of the Ni atom on the Al-doped and Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surfaces are the vicinity of the twofold-coordination oxygen atom and the vicinity of an oxygen vacancy, respectively. It is found that the most stable adsorption energy on the Al-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface is larger than that on the Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface. The analysis of diffusion path based on the potential energy surfaces of the Ni atom on the two surfaces shows that the energy barrier for the diffusion of the Ni atom on the Al-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface is larger than that on the Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface. The diffusion of the Ni atom on the Al-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface is more difficult than that on the Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface. This is because the Ni atom strongly bound to the twofold-coordination oxygen atom and the Ni atom is constrained in the Al-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface. Thus, the Ni sintering on the Al-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface is inhibited compared to that on the Y-doped ZrO&lt
inf&gt
2&lt
/inf&gt
surface.

DOI： 10.1149/06801.3187ecst

Language：English   Publishing type：Research paper (scientific journal)  

Thin-film Si grows layer by layer on Si(001)-(2x1):H in plasma-enhanced chemical vapor deposition. Here we investigate the reason why this occurs by using quantum chemical molecular dynamics and density functional theory calculations. We propose a dangling bond (DB) diffusion model as an alternative to the SiH3 diffusion model, which is in conflict with first-principles calculation results and does not match the experimental evidence. In our model, DBs diffuse rapidly along an upper layer consisting of Si-H-3 sites, and then migrate from the upper layer to a lower layer consisting of Si-H sites. The subsequently incident SiH3 radical is then adsorbed onto the DB in the lower layer, producing two-dimensional growth. We find that DB diffusion appears analogous to H diffusion and can explain the reason why the layer-by-layer growth occurs.

DOI： 10.1038/srep09052

Language：English   Publishing type：Research paper (other academic)  

We developed a chemical mechanical polishing (CMP) simulator based on tight-binding quantum chemical molecular dynamics method and applied it to gallium nitride (GaN) CMP process. We successfully clarified the chemical reaction dynamics at the friction interface between the GaN substrate and an abrasive grain in pure water; surface-adsorbed H2O molecules, OH groups, and H atoms are generated by the adsorption and dissociation of water on the GaN surface. Our developed CMP simulator is capable of predicting the CMP processes controlled by chemical reactions.

Language：English   Publishing type：Research paper (other academic)  

To elucidate the chemical mechanical polishing (CMP) performance of perovskite oxide abrasive grain for glass, we investigated electronic states of CaZrO3 and SrFeO3 by the first-principles calculation. The calculation results show that the Zr and Fe atoms in CaZrO3 and SrFeO3 take low valence states. We suggest that the metal atoms in perovskite oxide are effective for the glass polishing since low-valent metal atoms can weaken the Si-O bond of the glass surface by electron donation based on our reported CMP mechanism of glass by CeO2 abrasive grains.

Language：Others  

Tribo-Chemical Reaction of Molybdenum Dithiocarbamate on Diamond-Like Carbon Films: Quantum Chemical Molecular Dynamics Simulation

DOI： 10.2477/jccj.2014-0034

Language：English   Publishing type：Research paper (scientific journal)  

The plasma etching of SiO2 by CF2 and CF3 radicals is investigated by using our etching simulator based on tight-binding quantum chemical molecular dynamics method. During etching simulations, C-F and Si-O bonds dissociate and C-O and Si F bonds are generated. Moreover, CO, CO2, COF, and COF2 molecules form, which is consistent with previous experimental studies. We also examine the dependence of the etching mechanism of CF2 and CF3 radicals on the kinetic energy of irradiation. At a low kinetic energy of 10 eV, a CF2 radical dissociates more Si-O bonds than a CF3 radical does. This is because the high chemical reactivity of the CF2 diradical accelerates the etching process. At a high kinetic energy of 150 eV, a CF3 radical dissociates more Si-O bonds than a CF2 radical does. This is because a CF3 radical generates a greater number of reactive F atoms than a CF2 radical does and thus forms more Si-F bonds. Thus, we conclude that our etching simulator modeled the different SiO2 etching mechanisms of CF2 and CF3 radicals, which arose from the different chemical reactivities of radicals and F atoms at different kinetic energies. This is the first quantum chemistry study to model complicated chemical reactions, which are induced by the attack of many radical species, and clarify the different SiO2 etching mechanisms for CF2 and CF3 radicals.

DOI： 10.1021/jp5015252

Language：English   Publishing type：Research paper (scientific journal)  

The super-low friction mechanism of fluorine-terminated diamond-like carbon (F-terminated DLC) is investigated by using our tight-binding quantum molecular dynamics code and compared with that of hydrogen-terminated DLC (H-terminated DLC). Under a contact pressure of 1 GPa, F- and H-terminated DLC show smooth sliding and low friction coefficients of 0.07 and 0.04, respectively. The ion radius of fluorine is larger than that of hydrogen, which leads to the larger asperity of the F-terminated DLC surface. Thus, the friction coefficient of F-terminated DLC is slightly larger than that of H-terminated DLC. We also perform friction simulations under contact pressures of 3 and 7 GPa. Under a contact pressure of 3 GPa, the friction coefficients are 0.09 and 0.13 for F- and H-terminated DLC, respectively. F-terminated DLC shows the same friction behavior as seen under a contact pressure of 1 GPa, whereas the C-C bond formation reaction is observed at the interface of H-terminated DLC under a contact pressure of 3 GPa, leading to a slightly higher friction coefficient than when under a contact pressure of 1 GPa. Thus, under a contact pressure of 3 GPa, F- and H-terminated DLC show different friction behaviors. Furthermore, under a high contact pressure of 7 GPa, bond formation and dissociation are observed at the friction interface in F- and H-terminated DLC. C-C bond formation is observed more frequently in H-terminated DLC than in F-terminated DLC, and the lifetime of C-C bonds in H-terminated DLC is much longer. At this higher pressure, H-terminated DLC shows a high friction coefficient of 0.42 due to strong C-C bonds at the friction surface, whereas F-terminated DLC shows a low friction coefficient of 0.08. The strong repulsive interaction at the interface of F-terminated DLC that arises from the large negative charge and ion size of fluorine maintains the distance between DLC films under a high contact pressure. This prevents strong C-C bond formation at the friction surface, which results in the low friction properties of F-terminated DLC. We suggest that the friction properties of DLC films under a high contact pressure are improved by F termination.

DOI： 10.1039/c4ra04065a

Language：English   Publishing type：Research paper (scientific journal)  

Understanding tribochemical reaction mechanisms is necessary to develop a novel resin material that can easily slide on metallic parts. For this purpose, the chemical reaction dynamics between poly(tetrafluoroethylene) (PTFE) resin and an aluminum surface were studied by using a quantum chemical molecular dynamics simulation [Onodera, T., et al. J. Phys. Chem. C 2014, 118, 5390-5396]. The study showed that the PTFE tribochemically reacted with the oxidized surface of aluminum, forming two chemical products, namely, aluminum fluoride and depolymerized PTFE with a carbon double bond at the terminus of the PTFE polymer chain. The carbon backbone was exposed by changing to a double bond configuration, although that in genuine PTFE is fully covered by fluorine atoms. The subsequent chemical reaction of the polymer that reacts with gaseous molecules in the atmosphere (i.e., nitrogen, oxygen, and water vapor) was first studied by density functional theory (DFT). The DFT calculation results show that the chemical reaction of PTFE with water vapor was the most likely to occur and that a carboxyl group was finally formed on the terminus of the PTFE chain. The effect of the chemical reaction with water vapor on formation of a PTFE transfer film on which directly affects tribological performance of the focusing system, was then investigated by a classical molecular dynamics method. By forming a carboxyl group as a reaction product with water vapor, the amount of PTFE transfer film on an aluminum fluoride surface (one of the tribochemical reaction products) was increased. On the other hand, less genuine PTFE (without a carboxyl group) was transferred to the aluminum fluoride. This study clarified that the transfer film is formed easily by the reaction of PTFE with atmospheric water vapor, which thereby improves the tribological performance of the PTFE/aluminum lubrication system.

DOI： 10.1021/jp503331e

Language：English   Publishing type：Research paper (scientific journal)  

To develop a novel shearing resin material, it is necessary to understand the mechanism of friction-induced chemistry during the friction process. For this purpose, the chemical reaction of the polytetrafluoroethylene (PTFE) resin on an aluminum surface during friction was first focused on and investigated by a quantum chemical molecular dynamics method. From our simulation, an aluminum atom on a native oxide of aluminum surface led to a tribochemical reaction, which included defluorination of PTFE and aluminum fluoride formation. It was inferred that the aluminum surface acted as a catalytic Lewis acid in which the fluorine atom was removed from the PTFE polymer chain. The tribological performance of the investigated system was reduced by the forming of aluminum fluoride since a self-lubrication by PTFE was inhibited by an interfacial electrostatic repulsion. On the basis of our study, it was suggested that the key to increase tribological performance was a chemical reaction between reactive defluorinated PTFE and environmental water vapor to form a novel functional group on the PTFE chain.

DOI： 10.1021/jp412461q

Language：English   Publishing type：Research paper (scientific journal)  

In this Communication, we use density functional theory (DFT) to examine the fracture properties of ceria (CeO2), which is a promising electrolyte material for lowering the working temperature of solid oxide fuel cells. We estimate the stress-strain curve by fitting the energy density calculated by DFT. The calculated Young's modulus of 221.8 GPa is of the same order as the experimental value, whereas the fracture strength of 22.7 GPa is two orders of magnitude larger than the experimental value. Next, we combine DFT and Griffith theory to estimate the fracture strength as a function of a crack length. This method produces an estimated fracture strength of 0.467 GPa, which is of the same order as the experimental value. Therefore, the fracture strength is very sensitive to the crack length, whereas the Young's modulus is not. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4869515

Language：English   Publishing type：Research paper (other academic)  

Understanding of the sintering mechanism of nickel nanoparticles in the Ni/YSZ anode is requested for long-term durability of solid oxide fuel cell. To elucidate the effects of a porous structure of the Ni/YSZ anode on the sintering, we investigated the sintering process in the Ni/YSZ anodes with different porosities by using our developed multi-nanoparticle molecular dynamics simulation method. We found that the nickel nanoparticle size significantly increased although the YSZ nanoparticles showed little change during the sintering in all the Ni/YSZ models. Since the decrease in the porosity leads to the decrease in the pore size of the YSZ framework, the sintering of the nickel nanoparticles in the Ni/YSZ model with the less porosity is more prevented by suppressing the neck growth in the small pore. On the other hand, the excessively large porosity increases the distance between the nickel nanoparticles and prevents the nickel nanoparticles from contacting.

DOI： 10.1149/05701.2459ecst

Language：Others  

Quantum chemical molecular dynamics simulation on atomistic mechanisms of SiO2 etching process by fluorocarbon radicals

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

We use tight-binding quantum chemical molecular dynamics to investigate the crystal growth mechanisms of H-terminated Si(001)-(2 x 1) during plasma-enhanced chemical vapor deposition of SiH3 and Sill, radicals. We find that crystal growth by SiH3 radical deposition consists of two stages: (1) the first SiH3 radical abstracts a surface-terminating H atom and produces a dangling bond, and (2) a second SiH3 radical is adsorbed on the dangling bond. Thus, at least two SiH3 radicals are required for generating a new Si Si bond. Interestingly, during SiH2 deposition, a SiH2 radical can be directly adsorbed onto a H-terminated site without H abstraction by another SiH2 radical. Thus, one SiH2 radical is sufficient for generating a new Si Si bond. This SiH2 radical crystal growth mechanism is different from the SiH3 radical mechanism. The direct adsorption process consists of a two-step chemical reaction: (1) the SiH2 radical abstracts a surface-terminating H atom and produces a dangling bond and a SiH3 radical, and (2) the SiH3 radical is adsorbed on the dangling bond. In addition, the crystal growth rate for SiH2 radicals is higher than that for SiH3 radicals, because generating one new Si Si bond requires either a single SiH2 radical or two SiH3 radicals. However, our simulations reveal that SiH2 deposition produces defective Si thin films because many dangling bonds are formed during crystal growth. Compared with SiH2 deposition, SiH3 deposition should therefore produce Si thin films of higher quality.

DOI： 10.1021/jp4021504

Language：English   Publishing type：Research paper (scientific journal)  

We have developed a molecular dynamics (MD) simulation method to investigate the sintering of nickel nanoparticles in the nickel and yttria-stabilized zirconia (Ni/YSZ) anode of a solid oxide fuel cell (SOFC). The conventional sintering model consists of only two or three nickel nanoparticles. Therefore, it does not reflect the properties of the porous structure of the Ni/YSZ anode or reproduce realistic sintering. Our Ni/YSZ multi-nanoparticle MD simulation method uses a multi-nanoparticle model based on the porosity and are packed randomly in the simulation cell, and compressed to achieve the correct porosity. Furthermore, because the reliable potential parameters for MD simulation between nickel and YSZ have not been reported, we determine reliable interatomic potential parameters between nickel and YSZ by using the nonlinear least-squares method to fit the Morse potential function to interaction energies obtained by density functional theory. The sintering simulation using our Ni/YSZ multi-nanoparticle model and out potential parameters reveal that the YSZ nanoparticle framework suppresses the sintering of nickel nanoparticles by disrupting the growth of the neck between two nickel nanoparticles. The previously reported model of two nickel nanoparticles did not produce these results. Our multi-nanoparticle MD simulation method is effective for investigating the realistic sintering process in the porous structure of the Ni/YSZ anode and for designing durable anode structures for SOFCs.

DOI： 10.1021/jp310920d

Language：English   Publishing type：Research paper (scientific journal)  

The plasma etching of SiO2 by CF2 radicals was investigated using a newly developed etching process simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD). CF2 radicals were continuously irradiated on the SiO2(001) surface and then the dissociations of the C-F and Si-O bonds were observed. We also observed the generation of CO and CO2 molecules and Si-F bonds, which is in good agreement with previous experiments. The formation of etching holes was realized after the continuous irradiation of CF2 radicals. Furthermore, the effect of radical velocity on etching efficiency was also examined. The ratio of penetration depth to the width of irradiated atoms was examined for the evaluation of etching efficiency. The ratio increases as the irradiation velocity of CF2 radicals increases. Our TB-QCMD etching process simulator is capable of predicting etching rate and aspect ratio depending on the velocity of irradiated radicals. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.026502

Language：English   Publishing type：Research paper (scientific journal)  

We have developed a crystal growth simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD) method and applied it to our crystal growth simulator to plasma-enhanced chemical vapor deposition (PECVD) for silicon thin film growth via SiH3 radicals on hydrogen-terminated Si(001). We successfully simulated the abstraction of a surface hydrogen atom by irradiated SiH3 radical and the formation of a dangling bond on the hydrogen-terminated Si(001) surface. SiH3 radical was subsequently adsorbed on this dangling bond. When these processes were repeated, the thin film grew. Thus, a detailed mechanism was successfully found for the chemical reaction and electron transfer dynamics of silicon thin film growth by PECVD.

DOI： 10.1021/jp3002542

Language：English   Publishing type：Research paper (scientific journal)  

Recently, much attention has been given to diamond-like carbon (DLC) as a solid-state lubricant, because it exhibits high resistance to wear, low friction and low abrasion. Experimentally it is reported that gas environments are very important for improving the tribological characteristics of DLC films. Recently one of the authors in the present paper, J.-M. Martin, experimentally observed that the low friction of DLC films is realized under alcohol environments. In the present paper, we aim to clarify the low-friction mechanism of the DLC films under methanol environments by using our tight-binding quantum chemical molecular dynamics method. We constructed the simulation model in which one methanol molecule is sandwiched between two hydrogen-terminated DLC films. Then, we performed sliding simulations of the DLC films. We observed the chemical reaction of the methanol molecule under sliding conditions. The methanol molecule decomposed and then OH-termination of the DLC was realized and the CH3 species was incorporated into the DLC film. We already reported that the OH-terminated DLC film is very effective to achieve good low-friction properties under high pressure conditions, compared to H-terminated DLC films. Here, we suggest that methanol environments are very effective to realize the OH-termination of DLC films which leads to the good low-friction properties.

DOI： 10.1039/c2fd00125j

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the folding transition between an elongated coil state and a compact state on a single polymer chain confined in a small space with different stiffness with the aid of Monte Carlo simulation. In a flexible polymer, the folding transition is retarded in a confined space. In contrast, the transition is promoted for a semiflexible chain, in which the discontinuity of the volume change occupied by a single chain is diminished by confinement. This unique confinement effect is interpreted in terms of conformational entropy and self-avoiding repulsive interaction.

DOI： 10.1103/PhysRevE.84.021924

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the effect of the torsional rigidity of a semiflexible chain on the wrapping transition around a spherical core, as a model of a nucleosome, the fundamental unit of chromatin. Through molecular dynamics simulation, we show that the torsional effect has a crucial effect on the chain wrapping around the core under the topological constraints. In particular, the torsional stress (i) induces the wrapping/unwrapping transition, and (ii) leads to a unique complex structure with an antagonistic wrapping direction which never appears without the topological constraints. We further examine the effect of the stretching stress for the nucleosome model in relation to the unique characteristic effect of the torsional stress on the manner of wrapping.

DOI： 10.1103/PhysRevE.82.031909

Language：English   Publishing type：Research paper (scientific journal)  

Nucleosome is a fundamental structural unit of chromatin, and the exposure from or occlusion into chromatin of genomic DNA is closely related to the regulation of gene expression. In this study, we analyzed the molecular dynamics of poly-nucleosomal arrays in solution by fast-scanning atomic force microscopy (AFM) to obtain a visual glimpse of nucleosome dynamics on chromatin fiber at single molecule level. The influence of the high-speed scanning probe on nucleosome dynamics can be neglected since bending elastic energy of DNA molecule showed similar probability distributions at different scan rates. In the sequential images of poly-nucleosomal arrays, the sliding of the nucleosome core particle and the dissociation of histone particle were visualized. The sliding showed limited fluctuation within similar to 50 nm along the DNA strand. The histone dissociation occurs by at least two distinct ways: a dissociation of histone octamer or sequential dissociations of tetramers. These observations help us to develop the molecular mechanisms of nucleosome dynamics and also demonstrate the ability of fast-scanning AFM for the analysis of dynamic protein-DNA interaction in sub-seconds time scale. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.ultramic.2010.02.032

Language：English   Publishing type：Research paper (scientific journal)  

The folding transition of single, long semiflexible polymers was studied with special emphasis on the chain length effect using Monte Carlo simulations. While a relatively short chain (10-25 Kuhn segments) undergoes a large discrete transition between swollen coil and compact toroid conformations, a long chain (50 Kuhn segments) exhibits an intrachain segregated state between the disordered coil and ordered toroid. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2008.06.068

Language：English   Publishing type：Research paper (scientific journal)  

The conformational behavior of a giant DNA mediated by condensing agents in the bulk solution has been investigated through experimental and theoretical approaches. Experimentally, a pronounced conformational hysteresis is observed for folding and unfolding processes, by increasing and decreasing the concentration of condensing agent (polyethylene glycol) (PEG), respectively. To elucidate the observed hysteresis, a semiflexible chain model is studied by using Monte Carlo simulations for the coil-globule transition. In the simulations, the hysteresis loop emerges for stiff enough chains, indicating distinct pathways for folding and unfolding processes. Also, our results show that globular state is thermodynamically more stable than coiled state in the hysteresis loop. Our findings suggest that increasing chain stiffness may reduce the chain conformations relevant to the folding pathway, which impedes the folding process.

DOI： 10.1063/1.2759925

Language：Others  

Conformational Hysteresis on A Giant DNA Molecule

Event date： 2022.5

Language：Others  

Country：Other  

Fracture Process of Polymer Materials by Molecular Simulation

Event date： 2022.5

researchmap

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Japanese  

Country：Other  

Language：English  

Country：Other  

Mechanical Properties of Semicrystalline Polymers at the Molecular Level by Coarse-Grained Molecular Dynamics Simulation

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

Language：Others  

Country：Other  

researchmap

Language：Japanese  

5PM3-PMN-019 Quantum Chemical Molecular Dynamics and First-Principles Study on Chemical Mechanical Polishing of Gallium Nitride
We performed the chemical mechanical polishing (CMP) process simulations of GaN(0001) by a SiO_2 cluster in aqueous NaOH and H_2O_2 solution. In aqueous NaOH solution, OH" ions adsorbed on the Ga-top site of the GaN surface. In aqueous H_2O_2 solution, OH radicals adsorbed on the hollow site of the GaN surface and weakened the Ga-N back-bond. Therefore, we suggest that the OH radical is effective for GaN-CMP as compared with the OH ion. However, we did not observe the subsequent chemical reactions and the polishing process of the GaN surface in both environments. We performed the CMP process simulation of GaN(0001) by a SiO_2 cluster with many OH radicals produced by a catalyst. After the OH radicals occupied all the hollow sites of the GaN surface, and another OH radical attacked a Ga atom of GaN surface, we observed the diffusion of the OH species adsorbed on the GaN surface into the GaN bulk and generation of gallium oxide and N_2 molecules.

Language：Japanese  

6PM3-PMN-018 Computational Simulation on Change of Surface Property in Water Lubrication of Silicon Carbide
Friction of silicon carbide (SiC) films under water environment shows a low friction coefficient experimentally. Understanding of the mechanism is essential to improve friction characteristics of the SiC films. However, it is difficult to directly obtain atomic-scale dynamics with chemical reactions by experiments. In this study, our purpose is to reveal the chemical reactions of the SiC surface under water lubrication by our first-principles molecular dynamics (FPMD) and tight-binding quantum chemical molecular dynamics (TB-QCMD) methods. First, we performed the simulation of SiC sliding in water by our FPMD method and then Si-OH and C-H bonds were generated on the surface. Next, we also simulated the larger model of SiC in water environment by our TB-QCMD method. Then, the generation of Si-O-Si bonds and the growth of Si-O-Si bond chains were observed. We suggest that this growth relates to the low friction property of the SiC surface.

Language：English  

6AM2-C-7 The Structure Transition of Si Doped Diamond Like Carbon Using Computational Simulation
Si doped DLC films as solid lubricant exhibit excellent tribological properties of low friction and high wear resistance. One of the low friction reasons is the structure change from sp^3 carbon to sp^2 carbon on the surface of DLC films by Si doping. In this study, to reveal the mechanism of the structure change of the DLC surface by Si doping, we investigate the surface structure of the Si doped diamond using the first-principles calculation. The results show the structure change from six-membered ring to five-membered ring on the surface of Si doped diamond since a Si atom has larger atomic radius than a C atom. Furthermore, the structure change from sp^3 carbon to sp^2 carbon results in the generation of graphene, which would affect the low friction of Si doped DLC films.

Language：Japanese  

6AM2-C-6 Low Friction Mechanism of Carbon Nitride Based on Tight-Binding Quantum Chemical Molecular Dynamics and First-Principles Molecular Dynamics Simulations
We performed friction simulations of hydrogen terminated carbon nitride (H-CNX) using our first-principles molecular dynamics (FPMD) and tight-binding quantum chemical molecular dynamics (TB-QCMD) simulators. In FPMD simulation, H-CN_X showed low friction coefficient of 0.15 and 0.10 at 5 and 10 GPa, respectively. This is because terminated hydrogen atoms prevented the bond generation at the films interface. In TB-QCMD simulation, under 1 GPa, H-CN_X and H-DLC also showed low friction coefficients of 0.05 because terminated hydrogen atoms prevented the bond generation at the films interface. On the other hand, under 5 GPa, while H-CN_X showed a low friction coefficient of 0.07, a friction coefficient of H-DLC takes a high value of 0.42. Although C-C bonds were generated at the interface of H-DLC films under high pressure, nitrogen atoms of H-CN_X prevented C-C bond generation at the interface. We found that H-CN_X is more stable and shows lower friction than H-DLC under high pressure.

Language：Japanese  

6AM2-C-4 CMP Process Analysis of α-Al_2O_3 Substrate by Silica Abrasive Grain via Computational Method
In order to reveal a chemical mechanical polishing (CMP) mechanism of a sapphire substrate by a colloidal silica abrasive grain, we investigated a polishing process of an α-Al_2O_3(0001) surface by a SiO_2 cluster under water environment by a first-principles calculation. The results show that the mechanical pressing by the SiO_2 cluster and the chemical reaction with a H_2O molecule introduce the break of the Al-O bond of the α-Al_2O_3(0001) surface. In addition, after the chemical reaction with H_2O, an Al(OH)_3 molecule is generated and desorbs from the α-Al_2O_3 surface in the CMP process.

Language：Japanese  

I131 Clarification of Mechanism for Decomposition of Perfluorinated Polymer Electrolyte by First-Principles Molecular Dynamics Method
To clarify the degradation mechanism of the perfluorinated polymer electrolytes in polymer electrolyte fuel cells, we studied the decomposition process of the end group of these polymers using the first-principles molecular dynamics method. We successfully clarified that the polymer end model of CF_3CF_2CF_2OH reacts with a hydroxyl radical to generate a water molecule and CF_2O. After CF_2O was desorbed, polymer end model became CF_3CF_2・. CF_3CF_2・reacts with another hydroxyl radical to reproduce the hydroxyl end group. Therefore, it was revealed that cyclic degradation reactions like the unzipping mechanism occur when CF_3CF_2CF_2OH reacts with hydroxyl radicals. Furthermore, it was revealed that the CF_2O desorption reaction is inhibited by surrounding water molecules, while the HF desorption reaction from CF_3CF_2CF_2OH is promoted.

Language：Japanese  

I132 A First-Principles Investigation on Catalyst Activity of Pt-Ru Alloy Nano Particle in Polymer Electrolyte Fuel Cell
A PtRu alloy nano-particle attracts attention as an anode catalyst in a polymer electrolyte fuel cell because of its higher tolerability for CO poisoning than a Pt nano-particle. To reveal the mechanism of the high tolerability for CO in the PtRu alloy nano-particle, we investigated adsorption process of a CO molecule on the Pt and PtRu alloy nano-particles by first-principles calculation. The results show that the bond length of the CO molecule adsorbed on the PtRu alloy nano-particle is longer than that on the Pt nano-particle. This indicates that the PtRu alloy nano-particle oxidizes a CO molecule to a CO_2 molecule more easily than the Pt nano-particle. In addition, the CO molecule is more difficult to adsorb on the Pt atom in the PtRu alloy nano-particle than that in the Pt nano-particle.

Language：Japanese  

J044013 Chemical Aging Effect on Toughness of Polyethylene by First-Principles Calculation and Coarse-Grained Molecular Dynamics Simulation
We study the degradation process and toughness of polymer composites by first-principles calculation and coarse-grained molecular dynamics simulation. To clarify the effect of water at the filler surface on the degradation of polymers, we calculate H abstraction of polyethylene by OH and H radicals. Activation barriers are 3.2 and 5.0 kcal/mol and these reactions easily occur at room temperature. Therefore, water molecules generate radical and cause degradation. Then, we study the toughness of melt and semicrystalline polymers with filler against the stretching by coarse-grained molecular dynamics simulation. In melt polymer, fillers increase the toughness but the toughness of degraded one decrease more than that without fillers. On the other hand, in semicrystalline polymer, fillers decrease the toughness but the toughness of degraded one decrease less than that without fillers.

Language：Japanese  

J061055 First-Principles Investigation on Reaction Activity in Alkaline Fuel Cell with Ethylene Glycol
To develop the inexpensive non-noble highly-active and selective catalyst of the alkaline fuel cell (AFC), we investigated ethylene glycol oxidation on transition metal surfaces such as Fe(001), Co(0001), and Ni(111) based on first principles calculation. To clarity which surface has high activity for ethylene glycol oxidation, we calculate the activation energies for O-H and C-H bond dissociations on all surfaces. Then, we find the lowest activation energies for O-H and C-H bond dissociations on Fe(001). We also calculate the activation energies for C-C bond dissociations on Fe(001) and Ni(111) to reveal the selectivity of the catalyst. The comparison among the activation energies for O-H, C-H, and C-C bond dissociations shows the higher selectivity for the ethylene glycol oxidation on Fe(001) than that on Ni(111). Thus, Fe(001) is expected for the highly-active and selective catalytic material of the AFC.

Language：Japanese  

J061054 Degradation Process Simulation of Lithium-Ion Battery by Quantum Chemical Molecular Dynamics Method
To clarify the degradation mechanism of lithium-ion battery cathode, we studied the degradation process of the cathode caused by a chemical reaction with ethylene carbonate (EC) via our tight-binding quantum chemical molecular dynamics (TB-QCMD) method. We simulated the dynamics of the chemical reaction of LiCoO_2(010) surface and EC. In addition, our calculated atomic bond population revealed that a Co-O bond of the LiCoO_2(010) surface weakened when an O atom bound with a C atom of EC. This weakening of the Co-O bond indicates that the O atom of the surface is easily removed from the surface and the Co atom is reduced. It is known that the structural change of the cathode after the reduction of the Co atom causes the degradation of the lithium-ion battery cathode. Thus, we suggested that the adsorption of EC resulting in the reduction of Co atoms was the initial process of the degradation of the lithium-ion battery cathode.

Language：Japanese  

J061033 First-Principle Molecular Dynamics Simulation of Decomposition Process in Side Chain of Perfluorosulfonic Acid Polymer Electrolyte
To clarify the degradation mechanism of perfluorosulfonic acid (PFSA) membranes in polymer electrolyte fuel cells, we studied the decomposition process of PFSA side chain by radical species using the first-principles molecular dynamics method. This approach is possible to simulate real-time reactions from a viewpoint of atomic scale and reveal novel reaction pathways that traditional static methods may overlook. We investigated the reactions of OH and H radicals with the side chain model of CF_3CF_2CF_2SO_3H. In the reaction with a OH radical, a water molecule was generated when the OH radical abstracted the hydrogen atom from the side chain. An analysis of the interatomic distances and potential energy profiles revealed that the hydrogen bond between the OH radical and the oxygen atom in the sulfo group lowered the activation energy for the hydrogen abstraction. On the other hand, a H radical reacted with not the hydrogen atom but the oxygen atom in the sulfo group even though the activation energy for the reaction with the oxygen atom is higher than that with the hydrogen atom. This is because the hydrogen atom can escape from the approach of the H radical due to the small radius and mass, while the oxygen atom reacts with the radical due to the large radius and mass relative to the hydrogen atom. Moreover, this result indicates that it is important for clarification of the degradation mechanism to take account of not only statics but also dynamics.

Language：Japanese  

J061012 Investigation of the Degradation Process of the Ni/YSZ Electrode Using Computer Simulation
Sintering of nickel nanoparticles during the long-term operation leads to the degradation of the Ni/YSZ anode. To prevent the sintering, understanding of the sintering process in the Ni/YSZ anode with a porous structure is needed. In this work, to investigate the effect of porosity of the Ni/YSZ anode on the sintering, we modeled the Ni/YSZ multi-nanoparticle models with the porosity of 0.25 and 0.45 and simulated the sintering in the models by using our developed multi-nanoparticle molecular dynamics simulation method that can consider the effects of the properties of the porous structure such as porosity and YSZ nanoparticle framework, etc. It was revealed that the YSZ framework prevented the sintering of nickel nanoparticles in the small pore of the YSZ framework. Furthermore, it was found that the degree of sintering of nickel nanoparticles in the Ni/YSZ model with the porosity of 0.25 is smaller than that of 0.45, because the decrease in the porosity leads to the decrease in the pore size of the YSZ framework. Thus, we suggest that decreasing the porosity of Ni/YSZ anode can inhibit the sintering of nickel nanoparticles.

Language：Japanese  

Effect of fillers on the degradation and toughness of polymer composites

Language：Japanese  

J056032 The Degradation Simulation of the Polymer Electrolyte Membrane Based on First-Principles Molecular Dynamics
To clarify the degradation mechanism of proton exchange membrane (PEM) in a polymer electrolyte fuel cell (PEFC), we studied the degradation process of Nafion widely used as PEM by the first-principles molecular dynamics (FPMD) method. FPMD calculations were carried out using our "Violet" program, based on density functional theory (DFT) with B3LYP exchange-correlation functional and 6-31G (d) basis sets. We successfully clarified the dynamics of the reactions in which the main chain end was decomposed by OH radicals. Moreover, it was revealed that the reaction pass differs whether there are water molecules or not in the reaction system. In order to elucidate the difference, we calculated activation energies in these reactions by DFT method. The activation energy of the reaction with water molecules is lower than that of without a water molecule. Therefore, we conclude that water molecules work as a catalyst in the degradation reaction.

Language：Japanese  

J056046 Computational Simulation on Fracture Properties of Gadolinia Doped Ceria Electrolytes for Solid Oxide Fuel Cell
In this paper, we discuss the fracture properties of gadolinia doped ceria (GDC) electrolytes for solid oxide fuel cell (SOFC), based on molecular dynamics (MD) simulation and density functional theory (DFT). The ceria based materials like a GDC are expected as electrolytes to lower the operating temperature, however, the short lifetime of GDC prevents their wide applications. Therefore, we executed the tensile test using the MD simulation and DFT calculation and obtained the Young's modulus and the fracture stress of the GDC. Both indicate phase transition where the oxygen coordination number of Ce^<4+> ions changes from eight to six after the yield point. Furthermore, we calculated the Young's modulus and the fracture stress as functions of oxygen concentration. This result shows that the Young's modulus decreases linearly when the oxygen concentration increases, while the fracture stress is insensitive to the changes. This tendency is in good agreement with experimental data qualitatively. We also discuss the effect of steam condition on the fracture properties of GDC, and find the crack advance of GDC is accelerated by the chemical reaction with H_2O molecules.

Language：Japanese  

J056045 Molecular Dynamics Simulation of Dopant Effects on Sintering Process in Solid Oxide Fuel Cell Anode
The dopant effects on the sintering process of nickel nanoparticles in the nickel and doped zirconia cermet anode were investigated by using molecular dynamics (MD) simulation. In this study, we unraveled the differences of the sintering process, the surface area of the nickel nanoparticles, and the triple phase boundary (TPB) length between the Ni/YSZ and the Ni/ScSZ systems. According to the simulation results, we observed that the extent of sintering in the Ni/YSZ is larger than that in the Ni/ScSZ due to the dopant effect. These observations revealed that the sintering on the YSZ surface takes place more easily than that on the ScSZ surface. Then, the mechanism of the dopant effect on the sintering of nickel nanoparticles is investigated by using density functional theory (DFT). It was found that the low adsorption energy between the nickel and the doped zirconia decreases the movement of the nickel nanoparticles and inhibits the sintering of the nickel nanoparticles on the doped zirconia surface.

Language：Japanese  

J056034 High-Speed Screening of Ethylene Glycol Oxidation Catalyst for Alkaline Fuel Cells via Computational Science Method
In this study, to elucidate highly-selective catalyst materials for an oxidation reaction of an ethylene glycol (HOCH_2CH_2OH) to an oxalic acid ((COOH)_2), we performed high-speed screening of effective metal catalyst for partial oxidation of HOCH_2CH_2OH via first-principles calculations. Here, to screen effective element for the partial oxidation process, we investigated the oxidation reactivity of the ethylene glycol on the Fe(001), Ni(111), Co(0001), Cu(111), and Pt(111) surfaces. Fe, Ni, and Co are found to be effective for the dissociations of O-H and C-H bonds of HOCH_2CH_2OH, which are elementary reactions process of the above partial oxidation. In addition, the dissociation of the C-H bond is found to require the step structure on the metal catalyst surface. Furthermore, we revealed that Pt completely oxidizes an ethylene glycol to a carbon dioxide since the C-C bond is broken by Pt at the oxidation process. Therefore, we suggest that highly-selective catalyst materials for the partial oxidation of the ethylene glycol consist of Fe, Ni, and Co.

Language：Japanese  

24aAH-4 Chemical Aging Effect on the Fracture Process of Semicrystalline Polymers Studied by Molecular Dynamics Simulation

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Academic society, research group, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc.