Single-file transport of water molecules through a carbon nanotube

Recent molecular dynamics simulations of water transport through the interior channel of a carbon nanotube in contact with an aqueous reservoir showed that conduction occurred in bursts with collective water motion. A continuous-time random-walk model is used to describe concerted transport through channels densely filled with molecules in a single-file arrangement, as also found in zeolites, as well as ion channels and aquaporins in biological membranes. Theoretical predictions for different collective properties of the single-file transport agree with the simulation results.     

Atomic-core dynamics and the electronic structure of some endo-and exohedral complexes of fullerenes with light elements

The atomic and electronic structure of some endo-, exo-, and endo-exohedral complexes of the fullerene C₆₀ with various guest atoms and molecules (He_n, H₂, and Li₂) are investigated using semiempirical and nonempirical quantum-chemical methods. The atomic core dynamics is studied by the method of molecular dynamics. It is shown that guest atoms and molecules in fullerene polyhedra acquire an orbital angular momentum due to the correlated motion of nuclei above the low-energy barriers of the potential surface within the carbon polyhedron even at low temperatures (from 4 to 78 K). The emergence of orbital angular momenta of nuclei of guest atoms and molecules is attributed to a change in the contribution of the orbital angular momentum of electrons to the potential surface of the complexes. The motion of Li ions in a polyhedron leads to blurring of the top of the valence band and to the emergence of a charge polarization wave in the carbon polyhedron.     

Variations of chemical bonds in silver halides under high pressures

ABSTRACT

Rocksalt structured AgI (rs-AgI), which appears under pressures between 0.4 and 11.3?GPa, shows high ionic conductivity as high as that in α-AgI, especially at high temperatures. Microscopic origins of ion conduction mechanisms have not been clarified until now and are therefore investigated using the discrete variational Xα (DV-Xα) cluster method. Comparable studies for AgCl and AgBr, which are known as high ionic conductors just below melting temperatures and form the rocksalt structure, are also done. Ionic interactions between a mobile Ag ion and remaining ions are almost the same between those under different pressures, while covalent interactions between the mobile Ag ion and the remaining Ag ions change drastically when the mobile Ag ion is migrating. Similar results are also obtained for AgCl and AgBr. The covalent interactions between the mobile Ag ion and the remaining Ag ions, which should affect the Ag ion migration, play important roles in not only rs-AgI but AgBr and AgCl.     

Defect silicene and graphene as applied to the anode of lithium-ion batteries: Numerical experiment

Mechanical properties and stability of two layers of defect silicene supported by graphene sheets, between which the lithium ion passes under an electrostatic field, are studied by the molecular dynamics method. Defects are mono-, di-, tri-, and hexavacansies. Graphene and silicene edges are rigidly fixed. Graphene sheets contacting with silicene take a convex shape, deflecting outward. Mono- and divacancies in silicene tend to a size decrease; larger vacancies exhibit better stability. The ion motion control using an electric field becomes possible only using perfect silicene or silicene with mono- and divacancies. The ion penetrated through larger defects, and its motion in the silicene channel becomes uncontrolled. When the ion moves in the channel, the most strong stress spikes appear in silicene containing monovacancies. In the case of fixed edges, perfect silicene intercalated with a lithium ion is inclined to accumulate larger stresses than silicene containing defects.     

Modeling and simulation of Li-ion conduction in poly(ethylene oxide)

Polyethylene oxide (PEO) containing a lithium salt (e.g., LiI) serves as a solid polymer electrolyte (SPE) in thin-film batteries and its ionic conductivity is a key parameter of their performance. We model and simulate Li⁺ ion conduction in a single PEO molecule. Our simplified stochastic model of ionic motion is based on an analogy between protein channels of biological membranes that conduct Na⁺, K⁺, and other ions, and the PEO helical chain that conducts Li⁺ ions. In contrast with protein channels and salt solutions, the PEO is both the channel and the solvent for the lithium salt (e.g., LiI). The mobile ions are treated as charged spherical Brownian particles. We simulate Smoluchowski dynamics in channels with a radius of ca. 0.1 nm and study the effect of stretching and temperature on ion conductivity. We assume that each helix (molecule) forms a random angle with the axis between these electrodes and the polymeric film is composed of many uniformly distributed oriented boxes that include molecules with the same direction. We further assume that mechanical stretching aligns the molecular structures in each box along the axis of stretching (intra-box alignment). Our model thus predicts the PEO conductivity as a function of the stretching, the salt concentration and the temperature. The computed enhancement of the ionic conductivity in the stretch direction is in good agreement with experimental results. The simulation results are also in qualitative agreement with recent theoretical and experimental results.     

Modeling and simulation of Li-ion conduction in poly(ethylene oxide)

Polyethylene oxide (PEO) containing a lithium salt (e.g., LiI) serves as a solid polymer electrolyte (SPE) in thin-film batteries and its ionic conductivity is a key parameter of their performance. We model and simulate Li⁺ ion conduction in a single PEO molecule. Our simplified stochastic model of ionic motion is based on an analogy between protein channels of biological membranes that conduct Na⁺, K⁺, and other ions, and the PEO helical chain that conducts Li⁺ ions. In contrast with protein channels and salt solutions, the PEO is both the channel and the solvent for the lithium salt (e.g., LiI). The mobile ions are treated as charged spherical Brownian particles. We simulate Smoluchowski dynamics in channels with a radius of ca. 0.1 nm and study the effect of stretching and temperature on ion conductivity. We assume that each helix (molecule) forms a random angle with the axis between these electrodes and the polymeric film is composed of many uniformly distributed oriented boxes that include molecules with the same direction. We further assume that mechanical stretching aligns the molecular structures in each box along the axis of stretching (intra-box alignment). Our model thus predicts the PEO conductivity as a function of the stretching, the salt concentration and the temperature. The computed enhancement of the ionic conductivity in the stretch direction is in good agreement with experimental results. The simulation results are also in qualitative agreement with recent theoretical and experimental results.     

Recent studies in heavy ion induced fission reactions

Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion and fission process. From the various investigations carried out in the past years, it is now well recognized that there is large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission, fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated. We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM) calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the K-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources: (i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing collective modes in the fissioning nucleus. One of the collective modes — the tilting mode depends on the K quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively that the collective statistical spin modes get strongly suppressed for high K values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies on the mass-energy correlations of fission fragments at near-barrier bombarding energies.     

Solid state lithium ion conductors: Design considerations by thermodynamic approach

The most common previously employed methods of designing useful solid state lithium ion conductors (SSLICs) are reviewed and a new approach for the rational design of advanced SSLICs is described, which makes use of thermodynamic considerations. The described method is based on the Gibbs energy of formation of binary compounds of substitutional or additional cations (including dopants) and is demonstrated by the improvement of the lithium ion conductivity of SSLICs having perovskite-, NASICON- and Li₄SiO₄-type structures. Dopant metal oxides with higher negative Gibbs energies of formation than that of the parent metal oxide increase commonly the lithium ion conduction. The stronger binding forces of the oxide ion with the dopant cation result in an electrostatic shielding of the attractive forces between the lithium ions and the anions which facilitates the ionic motion. Irrespective of the crystal structure, it is expected that this thermodynamic rule holds also for other mobile ionic species. Paper presented at the 8th Euroconference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Development of oxygen ion conductors: One relevant tendency

In the past ten years a series of developments in the field of materials with dominant oxygen ion conduction introduced a variety of new systems. The most relevant aspect is a shift from ionic transport mechanisms based on the motion of oxygen vacancies, to mechanisms based on mobile oxygen interstitials. In parallel, structures identified with dominant oxygen ion conduction moved from the relatively simple fluorite related ones to more complex materials, namely with the apatite-type and related structures, including coexistence of ionic and covalent bonding. This shift is understood as a major development, opening a wide range of alternative solutions for further exploitation. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

Reentrant excitation in an analog-digital hybrid circuit model of cardiac tissue

We propose an analog-digital hybrid circuit model of one-dimensional cardiac tissue with hardware implementation that allows us to perform real-time simulations of spatially conducting cardiac action potentials. Each active nodal compartment of the tissue model is designed using analog circuits and a dsPIC microcontroller, by which the time-dependent and time-independent nonlinear current-voltage relationships of six types of ion channel currents employed in the Luo-Rudy phase I (LR-I) model for a single mammalian cardiac ventricular cell can be reproduced quantitatively. Here, we perform real-time simulations of reentrant excitation conduction in a ring-shaped tissue model that includes eighty nodal compartments. In particular, we show that the hybrid tissue model can exhibit real-time dynamics for initiation of reentries induced by uni-directional block, as well as those for phase resetting that leads to annihilation of the reentry in response to impulsive current stimulations at appropriate nodes and timings. The dynamics of the hybrid model are comparable to those of a spatially distributed tissue model with LR-I compartments. Thus, it is conceivable that the hybrid model might be a useful tool for large scale simulations of cardiac tissue dynamics, as an alternative to numerical simulations, leading toward further understanding of the reentrant mechanisms.     

四氟化碳分子电离解离过程中的异构化动力学

采用离子动量成像谱仪研究了能量为1.0 ke V的电子束碰撞条件下CF₄分子的解离动力学.实验上,对解离离子的三维动量进行了成像测量,通过离子飞行时间关联谱识别了CF₄²⁺异构化生成F₂⁺分子的两个通道:F₂⁺+CF₂⁺与CF⁺+F₂⁺+F,得到了两个通道的离子动能及动能释放分布.对于其中的三体解离通道,我们进一步采用Dalitz图与Newton图等三体动力学分析方法对解离碎片的动量关联进行了分析.该通道以两个带电离子的背对背出射为主,中性的F原子作为旁观者只得到极小的反冲动量.     

Dynamical significance in alkali metasilicate glasses

In this work, we elucidate the effect of the less mobile ions on the dynamics of the more mobile ions by molecular dynamics simulations of lithium ions motion in lithium metasilicate glass by freezing some randomly chosen lithium ions (5%, 10% and 25%) at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density–density correlation function, F_s(k,t), and in the diffusion coefficients. On the other hand, there is no significant change in the structure. These results show many similarities to the mixed alkali effect (MAE) with mixing of the small content of foreign alkali (10% and 25% of K₂O), where large reduction of the dynamics was also observed in both experiments and MD simulations. This immobilization of faster ions causes the large MAE as already discussed in relation to the mechanism of the cooperative ion jump motions. Although of lesser importance, the ion dynamics are influenced by the matrix of oxygen atoms, because the jump motions of Li ions are assisted by the localized motions of oxygen atoms.     

On the Effect of the Surface State and Geometry of the Discharge- Chamber Edges and Sizes of Morozov’s Stationary Plasma Thruster upon its Long-Term Operation

The results of long-term tests of Morozov’s stationary plasma thrusters are presented. It is revealed how the surface state and geometry of the discharge chamber’s edges influence the thruster’s parameters. It is shown that, during the ground tests of thrusters with cylindrical geometry of the acceleration channel under initial stage of operation, material sputtered from the discharge chambers’ walls is deposited onto the nearanode segment of the walls. Films of deposited material fail during thruster operation causing fragment formation, which jut out towards the discharge volume and disturb the motion of drifting electrons in the area of their acceleration. As a result the thruster reactive force and specific impulse decrease. The way in which the forming fragments influence thruster performance and operation is examined. It is shown that it decreases under long-term operation and significant channel widening since the ion flux to the wall and the quantity of the sputtered material decrease, and since the profile of the walls changes due to their wear and cleaning effect of the discharge. As a result the thruster’s parameters are restored to a level close to the initial one. It is shown that the dynamics of thruster parameters variation in space and during ground tests is different. This means that it is necessary to simulate more properly the conditions of thruster operation in space when conducting ground development tests. Thrusters with a long lifetime should be designed with widening of the acceleration channel beyond the loop which surrounds the magnetic system so that areas of acceleration and the erosion of walls are located in the widened part of the channel.     

随机中毒对神经元网络时空动力学行为的影响

神经元细胞膜上的离子通道能够被一些有毒的化学物质阻断. 离子通道阻断会降低离子通道的电导率和激活通道数, 影响神经元的放电活动, 进而影响神经网络时空模式的动力学行为. 本文采用具有周期边界的近邻耦合Hodgkin-Huxley神经元网络, 数值研究了钠离子和钾离子通道随机中毒时神经网络时空模式的演化过程. 发现钠离子和钾离子通道随机中毒可以导致螺旋波破裂. 通过分析网络的放电概率, 发现钠离子通道随机中毒降低了神经网络的兴奋性, 且其对中毒的敏感程度与噪声强度有关; 钾离子通道随机中毒增强了神经网络的兴奋性. 与均匀的通道中毒相比, 随机通道中毒的神经网络具有更丰富的动力学行为. 最后, 采用无流边界条件对神经网络进行数值仿真, 得到了类似的结果. 该研究更真实地反映神经系统中毒时整体兴奋性的变化, 从另一个方面揭示离子通道中毒对网络时空行为的影响, 有利于更进一步理解离子通道在网络整体行为中的作用. 关键词： 神经网络 离子通道 随机中毒 时空动力学     

Dynamics of Adsorbate Islands with Nanoscale Resolution

Surface catalytic processes produce, under certain conditions, small clusters of adsorbed atoms or groups, called islands which, after they have been formed, move as individual entities. Here we consider the catalytic reduction of NO with hydrogen on platinum. (i) Using video field ion microscopy, we observe the dynamic motion of small hydroxyl islands on the Pt(001) plane; despite changes in their morphology, the islands dimensions are confined to values corresponding to 10 to 30 Pt atoms suggesting cooperative effects to be in operation. (ii) We construct an automaton (or lattice Monte-Carlo) model on the basis of a set of elementary processes governing the microscopic dynamics. The agreement between the simulation results and the experimental observations suggests a possible mechanism for the formation and dynamics of hydroxyl islands.     

Two separate Li ionic motions observed by Li and B NMR in xLi2S + (1 − x)B2S3 glassy fast ionic conductors

Studies of ion dynamics in the highly conductive glassy fast ionic conductor (FIC) xLi₂S + (1 − x)B₂S₃ (x = 0.65 and 0.70) were made with NMR nuclear spin lattice relaxation (NSLR) R₁(ω, T) of both mobile ⁷Li and immobile ¹¹B ions, and ⁷Li NMR line narrowing δν(T). The possible dependence of ion dynamics on the short range order structures (SRO) and the distribution of activation energies (DAE) in this highly conductive FIC was investigated. Two Gaussian DAE were employed to fit ⁷Li NSLR data, where each Gaussian DAE was correlated to a separate ¹¹B NSLR in a BS₃ and in a BS₄ group. The long range diffusion of Li ions among BS₃ groups and a seemingly localized ionic hopping motion around BS₄ group is suggested as a microscopic model for the ion dynamics in thioborate glasses, namely a ‘two channel relaxation’.     

Experimental investigation of the conduction phase of themicrosecond plasma opening switch

Results of experimental studies of the conduction phase of the microsecond plasma opening switch are presented. It is shown that during this phase, translation of the current channel in the plasma near the anode takes place with anomalously high velocity. The ion component of the current reaches 25-30% of the total value, and the current streamlines late in the conduction phase acquire a considerable slant in the axial direction. The ion current behind the current channel makes up more than 30% of the total current there. The ion current density reaches a maximum during the conduction phase and decreases slowly during switching