Simulation of the Action of a Permeable Barrier on an Ideal Incompressible Channel Flow

Plane ideal incompressible flow in a rectangular channel partitioned by a thin permeable barrier (lattice) is considered. In flowing through the lattice the stream suddenly (jumpwise) changes direction and loses energy. The flow is assumed to be vortical; the vorticity is discontinuous on the lattice. A mathematical formulation of the problem for the stream function is proposed in the form of a nonlinear elliptic equation with coefficients discontinuous on the lattice line. A numerical solution is constructed using the finite-element iteration method. The results of the numerical simulation show how the flow velocity profile in the channel can be controlled by means of permeable barriers.     

Molecular dynamics simulation of two-sided chemical modification of carbon nanoribbons on a solid substrate

The position of a graphene nanoribbon on a solid substrate allows the chemical modification of only one of the nanoribbon sides. A method was proposed that enables the chemical modification of the other side, too. It was numerically modeled how a nanoribbon separated from a substrate rolls up into a roll and how the roll unrolls on a flat substrate. The dependences of the number of coils and the radii of rolls forming by hydrogenation on the nanoribbon length and width were determined.     

Novel Copolymers of 4-Fluorostyrene. 11. Some Ring-substituted 1,1-dicyano-2-(1-naphthyl)ethylenes

Novel copolymers of trisubstituted ethylene monomers, ring-substituted 1,1-dicyano-2-(1-naphthyl)ethylenes, RC₁₀H₆CH?C(CN)₂ (where R is H, 2-OCH₃, 4-OCH₃) and 4-fluorostyrene were prepared by solution copolymerization in the presence of a radical initiator (ABCN) at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, ¹H and ¹³C-NMR. The order of relative reactivity (1/r ₁) for the monomers is (5.86) > 2-CH₃O (4.28) > 4-CH₃O (2.87). Relatively high T_g of the copolymers in comparison with that of poly(4-fluorostyrene) indicates a decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200–500°C range with residue (7.3–7.7% wt.), which then decomposed in the 500–800°C range.     

Dielectric photonic crystal with a near-zero effective refractive index in the microwave range

The microwave propagation in a 3D dielectric photonic crystal at the frequency range from 8.5 to 12GHz has been investigated. A frequency range where effective refractive index n _ef of the crystal approaches zero is found. In the frequency range where n _ef <0.2, themicrowave beam becomes collimated, sharpened, and free of diffraction noise. An excitation of a surface wave, propagating along the external surface of the structure, is revealed in same frequency range. This property of the photonic crystal structure can be used to screen and mask objects scanned by external radiation.     

Evaluation of dew-water density by the Monte Carlo method

To construct a physical model of water vapor condensation with the formation of dew water, a geometrically based Monte Carlo method was developed. The hit and geometrical density functions of random space filling with identical spheres were determined. The parameters of these functions is the minimum allowable approach OO _min of spheres and their excess in comparison with the space capacity. The specificity of dew-water molecules and hydrogen bonds (HBs) between them appears at 2/3L _b < OO _min < 2L _b, where L _b is the HB length. As the approach OO _min = L _b, the random filling density does not exceed the packing density in the sphere models of I _h and I _c ices. The densities characteristic of the sphere model of water molecule packing in these ices are achievable at OO _min ≈ 0.8L _b and/or a significant HB kink, as well as at vapor supersaturation (excess of molecules over the space capacity).     

Chemical potentials in a three-component, 1-dimensional liquid

The chemical potential of a dilute solute in a mixed solvent in a one-dimensional, three-component fluid model is calculated exactly. The results depend on three parameters derived from the interparticle potentials. An explicit formula for the leading deviations from ideality is obtained. For arbitrary cosolvent concentration, but still dilute solute, the chemical potential of the solute is obtained numerically. A lattice model of the same system is also treated exactly. The results depend on three parameters in precisely the same way as do those of the continuum model. However, the parameters have a different physical meaning. In the special case of a hard-core plus-square-well potential, the results of the two models become the same in the high-density limit.     

The electrochemical behavior of the LaSrCuO<Subscript>4 − δ</Subscript>/Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface

The electrochemical behavior of the LaSrCuO_{4 − δ}/Ce_0.9Gd_0.1O_{2 − δ} interface is studied by impedance spectroscopy and cyclic voltammetry methods. By analyzing the dependence of the impedance frequency spectra on the oxygen partial pressure, the rate-determining stages of oxygen exchange are determined in the temperature interval of 500–900°C. For temperatures above 700°C, the adsorption of oxygen molecules and their dissociation to oxygen atoms are shown to make a substantial contribution to the polarization resistance of the overall electrode process, besides the charge-transfer resistance.     

Plastic deformation of glassy polymethylene: Computer-aided molecular-dynamic simulation

Molecular-dynamic simulation of low-temperature plastic deformation (T _def = 50 K, T _def/T _g ≤ 0.3) is studied for glassy polymethylene under the regime of active uniaxial compression and tension for a cell composed of 64 chains containing 100 -CH₂ groups in each (as united atoms) and with periodic boundary conditions. Thirty-two such cells are created, and, in each cell, polymethylene chains in the statistical coil conformation are independently constructed. The cells are subjected to isothermal uniaxial compression at T _def = 50 K by ɛ = 30% and by ɛ = 70% under uniaxial tension. In the course of loading, a σ-ɛ diagram is recorded, while the mechanical work spent on deformation, the changes in the overall potential energy of the system, and the contributions from various potential interactions (noncovalent van der Waals bonds, chemical links, valence and torsional angles) are estimated. The results are averaged over all 32 cells. The relaxation of stored potential energy and residual strain after complete unloading of the deformed sample is studied. The relaxation of stored energy and residual strain is shown to be incomplete. Most of this energy and strain is stored in the sample at the deformation temperature for long period. The conformational composition of chains and the average density of polymer glass during loading are analyzed. Simulation results show that inelastic deformations commence not with the conformational unfolding of coils but with the nucleation of strain-bearing defects of a nonconformational nature. The main contribution to the energy of these defects is provided by van der Waals interactions. Strain-bearing defects are nucleated in a polymer glass during tension and compression primarily as short-scale positive volume fluctuations in the sample. During tension, the average density of the glass decreases; during compression, this parameter slightly increases to ɛ ≈ 8% and then decreases. An initial increase in the density indicates that, during compression and at ɛ < 8%, coils undergo compactization via an increase in chain packing. During compression, the concentration of trans conformers remains unchanged below ɛ ≈ 8% and then decreases. During compression, it means that in a glass, coils do not increase their sizes at strains below ɛ ≈ 8%. During tensile drawing, coils remain unfolded below ɛ ≈ 35%; at higher strains, coils become enriched with trans conformers or unfold. At this stage, the concentration of trans conformers linearly increases. The development of a strain-induced excess volume (strain-bearing defects) entails an increase in the potential energy of the sample. Under the given conditions of deformation, nucleation of strain-bearing defects and an increase in their concentration are found to be the only processes occurring at the initial stage of loading of glassy polymethylene. The results of computer-aided simulation are compared with the experimental data reported in the literature.     

Simulation of ion transport in layered cuprate La2SrCu2O6

Oxygen diffusion in layered cuprate La₂SrCu₂O₆ has been simulated by the molecular dynamics method in the temperature range of 300–2500 K. The lattice is found to transform at temperatures above 1550 K; this transformation is accompanied by a change in the pair correlation functions. The abrupt change in the oxygen diffusion coefficient in the range of 1500–1550 K may indicate the presence of a phase transition to the superionic state. The motion of oxygen anions could be traced at the microscopic level. It has been proven for the first time that the La₂SrCu₂O₆ crystal lattice allows, along with displacements of O1 ions within the CuO₂ layer, their migration from the crystallographic positions to the intermediate unoccupied O3 positions. The motion of O2 anions is also fairly complicated: they move not only in their layer over the O2 positions but they also jump to the neighboring layer to occupy the O1 positions. The oxygen diffusion coefficient in layered cuprate La₂SrCu₂O₆ exceeds that in cuprates with perovskite structure and structure of the K₂NiF₄ type (at the same temperatures), which indicates that this material has good prospects for electrodes with mixed ionic-electronic conductivity.