Technical Physics Letters最新文献

Abstract

Using a developed computational algorithm based on multilevel recomposition of autonomous blocks with Floquet channels, mathematical modeling of diffraction of the fundamental mode has been carried out on a magnetic nanostructure based on a 3D array of magnetically functionalized carbon nanotubes (MFCNTs) in a waveguide depending on the bias field under the ferromagnetic resonance conditions in the millimeter wavelength range. Using the developed special computational algorithm for determining bifurcation points of the nonlinear Maxwell operator (Maxwell’s equations together with the Landau‒Lifshitz equation), the thresholds of parametric excitation of the non-exchange magnetostatic waves and dipole-exchange spin waves in a periodic 2D array of MFCNTs have been calculated depending on values of the bifurcation parameters (pump wave amplitude and frequency).

Abstract

The article deals with the modeling of high-energy cavitation processes, such as shock waves, cavitation erosion, bubble glow (sonoluminescence), etc., in a high-intensity acoustic field. It is shown that the well-known model based on the Keller–Miksis and Bjerknes equations does not correspond to a number of experimental data obtained in the study of a “single” cavitation bubble pulsating motionlessly in the antinode of a standing wave and an “ordinary” bubble moving in a cavitation cloud. To eliminate these inconsistencies, a new system of equations is proposed, which additionally takes into account the nonequilibrium processes of vapor evaporation and condensation and the imperfection of the vapor–gas mixture in the bubble, as well as the translational motion of the bubble. It is shown that with rapid compression of the bubble, the vapor inside it does not have time to condense and strongly damps this compression. The resulting equation explains the strong dependence of the intensity of “single” bubble glow on the temperature of the liquid. Contradictions in the description of the translational motion of bubbles associated with the application of the Bjerknes equation are eliminated. It is shown that a translationally moving bubble is compressed much weaker than a stationary one, since in the compression phase the energy of the radial motion of the bubble flows into the energy of translational motion. This allows us to explain the reason for the difference in the mechanisms of light emission from bubbles of different types. A “single” bubble emits light at maximal compression due to heating of the vapor–gas mixture up to 5000–10 000 K. Bubbles in a cavitation cloud move progressively, and their glow, in the absence of strong compression, is caused by micro-discharges in the vapor–gas phase during deformation of the bubble surfaces.

Abstract

Co–Cr–Fe–Mn–Ni alloys with a change in the manganese and iron concentrations from 5 to 35 at % providing the optimum ratio between the strength and ductility have been investigated. Using a comprehensive study of the structure and mechanical properties of the samples, data on the effect of the elemental composition on the micro- and nanohardness of the Co–Cr–Fe–Mn–Ni alloys have been obtained and an optimum Mn-to-Fe ratio ensuring high strength has been determined. The structure and the phase and chemical compositions of the materials have been examined by X-ray diffraction analysis and scanning electron microscopy.

Abstract

The development of the double selective doping technique has stimulated interest in the optical properties of semiconductor nanostructures containing the H^‒-similar impurity centers and their molecular complexes. Interest in the optical properties of quantum dots with the (D_{2}^{ - }) centers in an electric field is due, first of all, to the possibility of effective control of both the binding energy of impurity states and the photoexcitation spectra of molecular impurities. Depending on the quantum dot radius and the spatial configuration of impurity molecules, the (D_{2}^{ - }) photoexcitation band can be in the visible, IR, or terahertz frequency range, which significantly expands the range of instrumental applications of quantum dots with impurity states. Therefore, great interest is presented by quasi-zero-dimensional structures with the (D_{2}^{ - }) impurity states, which can be used to create IR and terahertz receivers. The aim of this study is to theoretically investigate the features of the spectra of intracenter optical transitions in quasi-zero-dimensional structures with the (D_{2}^{ - }) centers in an electric field. The binding energy of the (D_{2}^{ - }) states has been calculated by the zero-radius potential method in the effective mass approximation. The expression for the coefficient of impurity absorption of light has been obtained in the dipole approximation within the perturbation theory. It has been shown that the violation of symmetry in the arrangement of the ({{D}^{0}}) centers leads to the removal of degeneracy between the g and u terms. It is shown that an external electric field leads to a decrease in the splitting between the g- and u-terms. It has been established that the photoexcitation spectrum is a band the position of which depends on the external electric-field strength. The quasi-zero-dimensional structures with the (D_{2}^{ - }) centers in an external electric field can be used to create IR and terahertz detectors with controllable characteristics.

The electrostatic stabilization of bulk nanobubbles is investigated taking into account the structure of a double layer consisting of diffusion and Stern regions, with the permittivity of the latter being significantly lower than that of the former. It is shown that a narrow hydrated Stern layer significantly increases the negative electrostatic pressure and, at certain values of the surface charge, can compensate for the huge compressive Laplace pressure by preventing the diffusion dissolution of bulk nanobubbles.

High-cycle fatigue tests of VT1-0 titanium samples were carried out under conditions of exposure to a constant magnetic field of various magnitudes and without it. It is shown that the use of a constant magnetic field with induction B = 0.3, 0.4, and 0.5 T leads to a multiple increase in the average number of cycles before the destruction of titanium samples VT1-0 by 64, 123, and 163%, respectively. Using scanning electron microscopy, it was found that the structure of a sample destroyed under fatigue testing conditions, regardless of the test mode, has three characteristic zones: a fatigue crack growth zone, an accelerated crack growth zone, and a fracture zone. It was found that the width of the fatigue crack growth zone depends on the magnetic field induction and reaches its maximum values (h = 264 μm) at B = 0.4 T, and during fatigue tests without a magnetic field, h = 182 μm. This indicates an increase in the critical crack length (the width of the fatigue crack growth zone) by a factor of 1.45. It is shown that the average distance between fatigue striations in titanium samples depends on the value of the magnetic induction of the magnetic field and decreases from 0.78 μm in the absence of a field to 0.49 μm at B = 0.5 T. The formation of a subgrain (fragmented) structure in the zone of fatigue crack growth in a titanium sample was established. The subgrain sizes correspond to the distance between the fatigue striations, which has a retarding effect on the movement of a microcrack. Taken together, the revealed facts indicate a higher resistance of the material to the propagation of a fatigue crack and an increase in its service life during fatigue tests in a magnetic field.

The aim of this study is to develop, construct, and implement methods for solving a nonlinear diffraction problem. The effect of a nonlinear medium specified by the Kerr law ({{k}^{2}}(x) = k_{1}^{2} + alpha {{left| {u(x)} right|}^{2}}) on the propagation of a wave through an object is examined. The differential and integral forms of the problem and the nonlinear integral equation are presented. The problem is solved on different bodies using different computational grids, and plots of convergence of iterative processes and graphical results are presented. Explicit and implicit methods for solving the integral equation are compared.

The article describes the peculiarities of the formation of the geometry of the deformation zones and the structure of samples made of pure-iron in surface plastic deformation (SPD) by a multiradius roller (MR-roller). The SPD MR-roller leads to a complex kinematic introduction of a deforming tool into the surface layer of a blank and the appearance of a deformation hearth of a curvilinear shape. The results of X-ray structural analysis of pure-iron are presented. It is shown that processing of SPD MR-roller with maximum force results in a significant increase in the microdistortion of the crystal grid, which is characteristic of the nanoscale structure.

A new wire-arc additive manufacturing (WAAM) based on Cold Metal Transfer (CMT) welding process is used to fabricate iron-chromium-aluminum (Fe–Cr–Al) alloy by simultaneously feeding two separate wires. A wall of Fe–Cr–Al containing approximately 4.2% Al and 6.5% Cr was deposited over a steel substrate. Chemical composition test performed on the deposited alloy indicated a uniform material distribution of element throughout the deposited wall. It was also found that an increase in Al content or decrease in Cr content improved the hardness of the Fe–Cr–Al alloy. A macro and microstructural characterization revealed that the top region of the wall contained equiaxed grains, whereas columnar grain in the middle region and some acicular precipitates of Fe₃AlC_0.5 phase at the bottom were observed. The phases at different locations within the built wall were very different, especially, in the top section and bottom section. In the top and middle sections Fe₃C carbide were found. The Cr₃C₂ carbides were found in the top section. No (Cr,Fe)_xC_y carbides were found at the bottom of the buildup wall. A comparison of bottom and top sections indicated that the (Fe,Cr)_xC_y carbides have the ability to prevent cracks from occurring.

The authors have considered the gas-dynamic influence upon the mechanical properties, chemical composition, microhardness, and geometry of single-rim welds made of 30ХГСА steel when welding with a consumable electrode under a double-jet gas shield. Their regressional relationships on the selected controlled welding parameters have been developed. It has been found that the gas-dynamic effect of a dynamic shield gas jet has a controlling influence on the formation of welds made of alloy-treated 30ХГСА steel.