Asymptotics of the front motion in the reaction-diffusion-advection problem

A singularly perturbed initial boundary value problem is considered for a parabolic equation that is known in application as the reaction-diffusion-advection equation. An asymptotic expansion of solutions with a moving front is constructed. This asymptotics is proved by the method of differential inequalities, which is based on well-known comparison theorems and develops the ideas of formal asymptotics for constructing upper and lower solutions in singularly perturbed problems with internal and boundary layers.     

Dusty plasma structures in He-Kr DC glow discharge

Ion drift in gas mixtures has certain properties that can be used to generate ion flows with desired characteristics. For example, when the field is strong, ion heating is significant, and there is a large difference in atomic weight between ions and atoms, the ion velocity distribution can be highly anisotropic. Ion distribution anisotropy, in turn, can cause a substantial change in properties of dust structures in plasmas. Experiments on dusty plasma structures in glow discharge in mixtures of light and heavy gases (helium and krypton) are performed, and results of numerical simulations of ion and electron drift in the mixture are presented.     

Influence of the temperature modulation of an incident flux on the properties of films of cadmium selenotellurides synthesized under sharply nonequilibrium conditions

Results of structural and electrical investigations of the films of cadmium selenotellurides synthesized by evaporation and condensation in vacuum under sharply nonequilibrium conditions are presented. The modulating effect of a nonuniform thermal field on the vapor phase flux has been revealed. The possibility of controlling the stoichiometry of the film composition with the use of a thermal field applied on the path of the thermal phase from the evaporator to the substrate has been demonstrated. Curves of condensation and temperature dependences of the conductivity, Hall coefficient, and Hall mobility are presented.     

New scandium rhodium boride Sc<Subscript>4</Subscript>Rh<Subscript>17</Subscript>B<Subscript>12</Subscript> with a framework structure: synthesis,crystal structure,and properties

The new scandium rhodium boride Sc₄Rh₁₇B₁₂ was synthesized by arc-melting of the elements followed by annealing in inert atmosphere. The crystal structure of Sc₄Rh₁₇B₁₂ was solved using single crystal X-ray diffraction data. The structure can be described as a three-dimensional frame-work formed by trigonal prisms [BRh₆] and [BRh₅Sc] with isolated boron atoms inside the prisms and trigonal prisms [BRh₅B] representing the coordination polyhedra of paired boron atoms. The temperature dependences of the magnetic susceptibility and specific resistance of Sc₄Rh₁₇B₁₂ revealed that the compound is a Pauli paramagnet and shows metal-like specific resistance.     

Target-aimed synthesis of anion-deficient perovskites

The brownmillerite-type A 2B 2O 5 structure is considered as a parent one, giving rise to different derivatives: layered double perovskites, A-site and anion-vacancy-ordered perovskites, and the perovskite-like compounds with crystallographic shear planes. The structural relationships and synthesis pathways for these classes of materials are discussed with particular attention to the ordering at the A or B sublattices, anion vacancy ordering, and their mutual interaction.     

Electrotransport properties of SOFC cathode materials based on lanthanum cuprate doped with praseodymium and strontium oxides

A complex study of thermal, conducting, and electrocatalytic properties of cuprates La_1.8?xPr_xSr_0.2CuO_4–δ (х = 0.2; 0.4) with the K₂NiF₄ structure is carried out in order to assess their prospects as the cathode materials for solid-oxide fuel cells. The thermal analysis reveals stability of samples heated up to 950°С in air. The conductivity of cuprates measured in the temperature range of 100–900°С and the partial oxygen pressure from 10^–3 to 1 atm is of the metallic nature and varies from 70 to 40 S/cm in the temperature interval of 500–900°С in air. The studies of chemical stability of cuprates with respect to solid electrolytes demonstrate the absence of their chemical interaction with Ce_0.9Gd_0.1O_1.95 (GDC) at 900°С and with La_0.8Sr_0.2Ga_0.85Mg_0.15O_3–δ (LSGM) at 1000°C after 25 h annealing. For La_1.6Pr_0.2Sr_0.2CuO_4–δ electrodes deposited on the surface of GDC or LSGM solid electrolytes, the studies of electrocatalytic activity in the oxygen reduction reaction demonstrate that the smallest polarization resistance is typical of electrodes deposited on the GDC surface.     

Generalized Nernst layer model for convective-diffusion transport. Numerical solution for bromide ion electroreduction on inactive rotating disk electrode under steady state conditions

The process of electroreduction of bromate anion BrO₃ ^- from aqueous solutions on catalytically inactive (e.g., carbon) electrodes is theoretically described in the framework of the generalized Nernst layer model in which the Nernst-layer thickness is chosen independently for each system’s component according to the Levich formula. For this system, the numerical algorithm is developed for solving the system diffusion- kinetic equations for the case of excessive content of protons in solution and one-dimensional transport (corresponding to RDE) under stationary conditions. The results are compared with conclusions of the approximate analytical theory proposed for the same system in our recent study (J. Electroanal. Chem., 2016, vol. 779, p. 146). The closeness of the numerical and analytical data makes it possible to conclude that both approaches can be used for solving this problem. Deviations are observed only when the approximations lying in the basis of the corresponding analytical relationships are violated.     

Exploring Route for Pyrophosphate-based Electrode Materials: Interplay between Synthesis and Structure

New sodium vanadium(III) hydrogenphosphate hydrate NaV(HPO₄)₂(H₂O)_0.5 and sodium vanadium(III) hydrogenphosphate β-NaV(HPO₄)₂ were prepared in mild hydrothermal conditions. The crystal structures of NaV(HPO₄)₂(H₂O)_0.5 [space group Cc, Z = 4, a = 8.46174(19) Å, b = 9.52583(19) Å, c = 8.69376(15) Å, β = 110.9553(11) °, V = 654.41(2) ų] and β-NaV(HPO₄)₂ [space group C2/c, Z = 4, a = 7.8681(3) Å, b = 9.8451(3) Å, c = 8.5180(2) Å, β = 107.626(2) °, V = 628.85(3) ų] were solved and refined from X-ray powder diffraction data. Both compounds were used as precursors in a new route for the preparation of attractive cathode material β-NaVP₂O₇. The formation of the structure motive providing fast sodium ion diffusion at the first synthesis stage and its further conservation upon stepwise dehydration was revealed. The oversized Na⁺-embedding channels are stabilized by site-coordinated water in NaV(HPO₄)₂(H₂O)_0.5 structure. The topology resemblance and difference in known sodium vanadium(III) complex phosphates are discussed.     

Autonomous magnetocumulative power supply

Helical magnetocumulative generators (MCGs) fed from explosive piezo generators (EPGs) are the most compact power supplies with an output energy of 1–10 kJ. EPGs are successfully coupled with MCGs in the operation mode and in structural parameters; these generators are easy to operate and do not require transformers or commutators. We report on the results of designing an autonomous small-size power supply based on EPGs and MCGs. Owing to bidirectional impact loading of the piezoceramic block of an EPG, the energy at the piezo generator output attains values of ～25 J and is transferred to the helical MCG with an initial inductance of ～1000 μH. The power supply is constructed in the form of a monoblock with a volume of ～2 dm³ or in the form of two separate devices connected by a high-voltage cable.