Stress concentration in a three-layer plate with two cylindrical cavities

The stressed state of a thick three-layer plate with two circular holes is studied. Resolving functions are expressed in terms of Kolosov-Muskhelishvili integral potentials and modified Bessel functions. The problem is reduced to solving an infinite set of linear algebraic equations.Translated from Teoreticheskaya i Prikladnaya Mekhanika, No. 18, pp. 25–28, 1987.     

Temperature Effect on the Lithium Electrode Kinetics

The effect of temperature on the kinetics of processes occurring on metallic lithium covered by an ion-conducting passive film, in contact with a LiClO₄ solution in propylene carbonate, is studied by the pulsed voltammetry method. Symmetrical anodic and cathodic polarization curves for a lithium electrode in the temperature range –35 to 80°C include a portion of ohmic current j _ohm caused by intrinsic ionic conductivity of the passive layer. Following an increase in overvoltage E, the j _ohm portion is replaced by a portion of injection current j _inj with a characteristic exponential dependence j _inj E ⁿ, where n 2 and varies with temperature. The reason for this is presumed to be some structural imperfectness (structural disordering) of the passive-layer material, which leads to the emergence of dispersions in the hop distances and heights of energy barriers for charge carriers. When calculating a current–voltage curve, stochastic charge carrier transport in a disordered solid, which is characterized by a wide dispersion of times of interpoint hops, leads to the exponential function j _inj E ⁿ with n depending on temperature as follows: T: n = 1 + (a – b/T)^–2. Experimental data nicely fit this model. Comparing experimental j vs.E curves with theoretical equations permits the determination of a set of microscopic transport parameters, which include the average hop distance and effective localization radius of the charge, the frequency of attempts at hopping, and the average energy-barrier height.     

Determination of diffusion coefficient of lithium in substituted LiMn1.95Cr0.05O4 spinel using impedance technique

Electrodiffusion properties of chromium-substituted lithium-manganese spinel Li _x Mn_1.95Cr_0.05O₄ intended for application as a cathodic material for lithium-ion batteries is studied. The studies are carried out at 25°C using the electrochemical impedance spectroscopy technique in alkyl-carbonate electrolyte. In the analysis of impedance spectra, the apparatus of electric equivalent circuits was employed to determine surface layer resistances, double electric layer capacitance, differential intercalation capacity, chemical diffusion coefficient D of lithium, and other electrode characteristics. The issues of substantiating the choice of electric equivalent circuits and correct interpretation of their elements are discussed; dependences of the calculated model parameters on the electrode potential (lithium concentration in the electrode) are analyzed. The chemical diffusion coefficient of Li⁺ in Li _x Mn_1.95Cr_0.05O₄ found on the basis of the impedance spectra is in the range of 10^?9 to 10^?12 cm²/s under electrode potential variation in the range of 3.5–4.5 V (vs. Li/Li⁺) with a pronounced minimum of D in the middle of this range. Repeated cycling of the electrode is accompanied by a gradual increase in resistance of the solid-electrolyte interphase (SEI).     

Diffusion aspects of lithium intercalation as applied to the development of electrode materials for lithium-ion batteries

The creation of new electrode materials and the modification of existing ones are important trends in the development of lithium-ion batteries. Of special significance is to evaluate their diffusivity, i.e., the ability of providing transfer of the electroactive component. Such electrochemical techniques as cyclic voltammetry, electrochemical impedance spectroscopy, potentiostatic intermittent titration technique, and galvanostatic intermittent titration technique are used for this purpose. The values of chemical diffusion coefficient D estimated in similar electrode materials are shown to scatter by several orders of magnitude. Principal causes of this rather considerable scattering are discussed, including the uncertainty of diffusion area estimations and the use of various approaches to deriving equations to calculate D. Our conclusions are illustrated by examples of D estimations in the electrode materials Li _x C₆, Li _x Sn, Li _x TiO₂, Li _x WO₃, LiM _y Mn_2?y O₄, and LiFePO₄.     

Electrochemical properties of LiMn<Subscript>2−<Emphasis Type="Italic">y</Emphasis></Subscript>Me<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>4</Subscript> (Me = Cr,Co, Ni) spinels as cathodic materials for lithium-ion batteries

A number of spinel phases with a general formula LiMn_2?y Me _y O (Me = Cr, Co, Ni) was synthesized using the melt-impregnation and sol-gel methods. All synthesized materials were subjected to electrochemical testing of their suitability as cathodes in lithium-ion batteries. Cyclic voltammograms were used in the testing. The cathode materials prepared using the melt-impregnation method showed the highest initial discharge capacity (up to 120 mA h/g) and stable operation during the cycling. The partial substitution of chromium and cobalt atoms for manganese gives positive effect: the spinel structure is stabilized during the cycling. The double doping of the Li-Mn-O system with small amounts of Co and Ni results in the stabilizing of the discharge capacity. An overstoichiometry excess of lithium in Co- or Cr-doped spinels also favors the increasing of the discharge capacity and slows down its decaying during the cycling.     

Real-time monitoring of all-optical poling of azo-dye polymer thin film

Simultaneous writing and probing of photoinduced second-order susceptibility chi((2)) in an azo-dye polymer thin film is demonstrated. The method is based on the fact that tensor properties of chi((2)) provide different planes of polarization for seeding and signal second harmonics. Our technique allows the chi((2)) value to be monitored in real time, completely eliminating the distortion of optical poling that can occur in traditional probing configurations. The possibility of attaining high values of chi((2)) in a DR1-PMMA guest?host system under low seeding intensities is shown.     

Solution of a multipoint boundary-value problem for a system of linear ordinary differential equations with holomorphic coefficients

Necessary and sufficient conditions are established for the existence of a unique holomorphic solution of a boundary-value problem for a system of linear ordinary differential equations with holomorphic coefficients and general linear boundary conditions. A procedure for finding this solution is investigated.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 42, No. 1, pp. 125–128, January, 1990.     

Effect of boron additive on the formation of the carbon fiber structure

Effect of boron on the process of directed recrystallyzation of a nanostructured fibrous carbon material in a high-temperature thermomechanical treatment was studied by means of X-ray diffraction analysis.     

An electrochemical study on the substituted spinel LiMn1.95Cr0.05O4

Cyclic voltammetry, galvanostatic charge?Cdischarge technique, potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS) were used to study the behavior of a LiMn_1.95Cr_0.05O₄ (substituted lithium?Cmanganese spinel) electrode in nonaqueous electrolytes at 25 °C. Quantitative and qualitative changes of the electrode transport parameters as functions of lithium concentration were analyzed. Several equivalent circuits are discussed; the results obtained by different methods are compared. The PITT and EIS results are in good agreement; the chemical diffusion coefficient D varies within 10^?14?C10^?9 cm² s^?1 depending on the lithium content in the Li_xMn_1.95Cr_0.05O₄ electrode.