Vol 61, No 6 (2019)

Plastic deformation of metals and polymethylmethacrylate under the action of dynamic load is analyzed based on the relaxation model of plastic deformation. The parameters of the relaxation model of plasticity are invariant with respect to the deformation history which allows, within a unified approach, obtaining any set of deformation curves, both monotonic, with varying yield limit, and nonmonotonic, with appearing and varying yield drop, as it is observed in experiment. Increase in the yield limit and hardening effect at high-rate and static deformation of high-strength 2.3Ni–1.3Cr steel is also simulated based on the relaxation model. Exemplified by DP600 steel and nanocrystal nickel, it is shown that the relaxation model of plasticity allows predicting the smooth transition to the plastic deformation stage at slow quasi-static loading ~10^–3 s^–1 and appearance of the yield drop at strain rate of 500–6000 s^–1. In addition, it is demonstrated that the developed approach allows modeling the similar effects also for the high-rate deformation of the polymethylmethacrylate. Thus, it is demonstrated on the example of specific materials that we may use the deformation history-invariant parameters of the relaxation model of plasticity for efficient prediction of the deformation dependences of the studied materials in a wide range of strain rates 10^–4–10⁴ s^–1.

The thermodynamics and kinetics of decomposition of a Fe–Cu alloy are investigated in the context of a simple ab initio parameterization model taking into account the concentration dependence of the magnetic contribution to the free energy. It is shown that taking into account a difference between the short-range and long-range magnetic orders near the Curie temperature is very important for calculating the solubility of copper in iron. The solubility of copper in bcc iron and the stability limit of a homogeneous state (physical spinodal) are evaluated via the kinetic Monte Carlo method. The impact of structural defects (dislocations and grain boundaries) on these curves, as well as on the kinetic time–temperature—transformation (TTT) diagram, is also discussed.

The electronic structure and optical properties of a binary intermetallic FeSb₂ compound are studied. The calculations of the band structure, that demonstrate the existence of a narrow ∼0.3 eV gap in the energy spectrum of this material, are performed in the approximation of a local electronic density. Spectral characteristics are studied by the ellipsometric method within a 0.22–18 μm range of wavelengths. It is shown that, in the region of interband transitions, the experimental optical conductivity of the compound is satisfactorily interpreted within the performed calculations of the density of electronic states.

Parametric amplification of a high-frequency radiation by point-like Josephson junctions in the cavity has been studied within a resistive model. It is shown numerically and analytically that near the subharmonic steps of the current–voltage characteristic there are areas of amplification of electromagnetic radiation. An one-dimensional array of the Josephson point junctions connected in series is treated. It is shown that the junctions in the array near the subharmonic steps of current–voltage characteristic are nonsynchronized and, accordingly, the amplification regions near the subharmonic steps of the array are missing.

One-phase powders of Ag_xPb_1 – xS cubic solid solutions with a maximum relative silver content to x = 0.12 were synthesized via the chemical codeposition from lead acetate and silver nitrate aqueous solutions in the presence of sulphidizer, as well as complexing and stabilizing agents. The thermal expansion of produced semiconductor solid solutions was first measured via the dilatometry in a temperature range of 295–580 K. As shown, substituting silver atoms for lead in Ag_xPb_1 – xS results in a slight decrease in thermal expansion value, because of alterations in unharmonism of atomic vibrations.

Symmetry aspects of the periodic model of a crystal with a point defect (a supercell model or a extended unit cell (EUC) model) are considered; splitting of the Wyckoff positions in a primitive crystal cell with the introduction of supercell and the change of the one-electron states classification over k-vector (Brillouin zone folding) are discussed. When considering a point defect in a crystal in the supercell model, it is necessary to take into account the symmetry of the one-electron states of the original crystal at the top of the valence band and at the bottom of the conduction band. The selected supercell should reproduce these states. Each specific selection of a supercell corresponds to a specific splitting of the Wyckoff positions of the original crystal and, as a result, the possibility to place a defect in positions with different point symmetry and to perform a calculation without taking into account the point symmetry of the crystal with a defect at all (site symmetry method). By the results of the calculation of a crystal with a defect in the supercell model, taking into account site symmetry, it is possible to determine the real symmetry of the crystal with a defect, which is essential for the interpretation of experimental data. A copper impurity in a lithium node in a LiCl crystal retains the cubic symmetry of the atom being replaced; the impurity of an iron atom in a titanium site with cubic local symmetry in a SrTiO₃ crystal lowers the symmetry to tetragonal; and the H site in the CsPbI₃ crystal is characterized by the complete removal of point symmetry with the formation of an \({\text{I}}_{2}^{ - }\) ion.

The influence of inhomogeneous strains on the occurrence of electric polarization of rare-earth compounds with a garnet structure has been analyzed. Polarization of the subsystem of rare-earth ions in the regions of inhomogeneous stress generated by edge and screw dislocations has been investigated. The electric-polarization distribution in inhomogeneously strained films of rare-earth garnets has been studied.

The ultraviolet (UV) absorption spectra were studied in UV–Vis–IR quartz glasses exposed to the rhenium ion irradiation at the energy of 30 keV. An increase in ion fluence (5 × 10¹⁵–5 × 10¹⁷ cm²) is found to result in a characteristic fan-shaped broadening of the optical absorption edge of glasses. The results were interpreted in the context of the generalized Urbach rule (the approximation of ion radiation induced quasi-dynamic disorder). The effective radiation disorder cross section of a glass matrix was evaluated from the dose dependence of characteristic Urbach energy. This parameter was found to vary between 1.07 × 10^–17–1.2 × 10^–18 cm² with increasing fluence. The optical gap width of implanted samples was estimated via the Tautz method for allowed direct interband transitions. The relationship between the second-order deformation potential constants, revealing the correlation between the Urbach energy and the optical gap width in a SiO₂ quartz glass matrix was established using the equivalence principle of static and dynamic structural disorder.

This article is devoted to the effect of a magnetic field of 0.7 T on the state of an aluminum alloy whose creep increases after exposing to a magnetic field, exhibiting a “magnetic memory.” The alloy contains FeAl microinclusions identified by electron microscopy, magnetometry, and Mössbauer spectroscopy. Possible mechanisms of changing the states of ferromagnetic microinclusions and aluminum matrix around them under a magnetic field are discussed.

We study the dc magnetization of a Bi_1.94Fe_0.06Se₃ single crystal in the temperature range of 1.9 to 400 K in applied magnetic fields up to 70 kOe for two sample orientations in the applied field. We register three magnetic phase transitions and observe considerable anisotropy of the magnetic properties. The coercive force varies with temperature in a nonmonotonic manner. For the sample in a paramagnetic state, we estimate its magnetic moment per iron atom; its Curie–Weiss temperature is found to be negative. Arrot–Belov plots are constructed.

The specific heat, thermal expansion, permittivity, and electrocaloric effect in bulk of BaTiO₃ (BT) samples in the form of nano- (nBT-500 nm) and micro- (mBT-1200 nm) ceramics fabricated using spark plasma sintering and solid-state plasma techniques have been investigated. The size effect has been reflected, to a great extent, in the suppression of the specific heat and thermal expansion anomalies and in the changes in the temperatures and entropies of phase transitions and permittivity, and a decrease in the maximum intensive electrocaloric effect: \(\Delta T_{{{\text{AD}}}}^{{\max }}\) = 29 mK (E = 2.0 kV/cm) for nBT and \(\Delta T_{{{\text{AD}}}}^{{\max }}\) = 70 mK (E = 2.5 kV/cm) for mBT. The conductivity growth at temperatures above 360 K leads to the significant irreversible heating of the samples due to the Joule heat release in the applied electric field, which dominates over the electrocaloric effect.

An increase in the velocities of automobile and aerospace transport and, simultaneously, the necessary of decreasing their masses increase the requirements to the operating reliability of used materials and constructions. In this case, the operation of a material under dynamic impact loads remains in nonnormative ranges. This work presents a comprehensive technique of studying and predicting the behavior of materials under conditions of dynamic tension. A possibility of increasing the strength properties of materials by severe plastic deformation (SPD) in a wide range of varying parameters of external loads and the fi-xation of these changes in the framework of the proposed technique is demonstrated using Al–Mg and Al‒Cu–Mg aluminum alloys as examples.

In crystals of yttrium orthoaluminate doped with the ¹⁵¹Eu isotope, Eu²⁺, Cr³⁺, Gd³⁺, and Мо³⁺ paramagnetic centers are detected. The fine structure parameters of the spin Hamiltonian are determined or refined for these centers. The orientation behavior of previously unobserved forbidden transitions is considered for the Cr³⁺ centers. It is shown that two Mo³⁺ signals that are detected in magnetic fields directed parallel to the crystallographic axes do not belong to different molybdenum centers, but are two intradoublet transitions of the same center.

Transition metal doped cesium lead halide (CsPbI₃) perovskite compounds were studied for application in photovoltaic solar cells. Electronic structures, chemical shifts of ²⁰⁷Pb and ¹²⁷I-NMR, vibration modes in infrared and Raman spectra of transition metals (Mn²⁺, Fe²⁺ or Cu²⁺)-doped CsPbI₃ perovskite compounds were studied by the first-principles calculation using density functional theory. The CsPb(Fe)I₃ perovskite crystals had a slight perturbation of crystal field in the coordination structure. The electron density distribution was delocalized on the 5p orbital of I atom, the 3d orbital of Fe atom and the 6p orbital of Pb atom. The first excited process was based on ligand metal charge transfer from the 5p orbital on I atom to the 3d orbital of Fe atom. The chemical shifts of ¹²⁷I-NMR were associated with the electron correlation of electron-nuclear spin interaction and nuclear quadrupole interactions based on electron field graduate. The asymmetric vibrations of Pb–I bonds stretching mode related to electron conductivity with scattering of the carrier diffusion as phonon effectiveness. The slight perturbation of the coordination structure in the CsPb(Fe)I₃ perovskite crystal will improve the photovoltaic and optical properties.

Ab initio study of the crystalline structure and the phonon spectrum of R₂TiO₅ (R = Nd, Sm) crystals is conducted using the density functional theory. The calculations are carried out using the hybrid functional, which takes into account the contribution of nonlocal exchange to the Hartree–Fock formalism. Coordinates of ions in the unit cell and lattice constants, as well as frequencies and types of fundamental vibrations, intensities of lines of Raman and IR reflectance spectra are determined. The elastic constants of R₂TiO₅ are calculated for the first time.

The Monte Carlo replica technique is used to study phase transitions and the thermodynamic and critical properties of the three-dimensional Heisenberg antiferromagnetic model on the body-centered cubic lattice with the inclusion of the interaction of the nearest and the next-nearest neighbors. The studies are performed for the proportions of the values of the exchange interactions of the nearest and the next-nearest neighbors in the range of values of k [0.0, 0.6]. The phase transition character is analyzed based on the histogram method. The overall set of static critical indices is calculated in terms of the theory of the finite-dimensional scaling. It is shown that the class of the critical behavior universality of this model is conserved in the above range of values of k.

We present the data of studies on the structure, phase states, and magnetic properties of magnetic nanoparticles (MNPs) of magnesium ferrite spinel (MgFe₂O₄), synthesized by ultrasonic aerosols pyrolysis. Primary single-phase MNPs with an average size of 9.6, 11.5, and 14.0 nm, synthesized from precursors at concentrations of 0.06, 0.12, and 0.24 M, respectively, agglomerate into tightly aggregated spherical particles (secondary particles) with sizes of 206, 300, and 340 nm, respectively. Primary particles inside the spheres do not interact with each other and are in a superparamagnetic state. There is a layer on the surface of the particles, the magnetic structure of which differs from the structure of the inner part of the MNP; this is explained by the formation of a canted spin structure or a spin glass state in the surface layer of the MNPs. MgFe₂O₄ nanospheres obtained from a precursor at a concentration of 0.06 M are most promising as valid sources of heat in magnetic hyperthermia therapy.

This report presents the results of studying an anomalous increase in the conductivity in the Cr/polymer/Cu structure near the antiferromagnet/paramagnet phase transition in chromium. As a polymer, poly(diphenylene phtalide) in which the charge instability effect was observed before is used. It is found that, when approaching the phase transition temperature, current fluctuations in the system increase and switch the conductivity of the structure to a high-conducting state with anomalously low resistance near the phase transition. The results are interpreted with the allowance for a change in the charge carrier injection near the phase transition temperature in chromium.

Using the small-angle synchrotron X-ray diffraction method at the research facility “BELOK” of the Kurchatov synchrotron radiation source at the NRC Kurchatov Institute, it was shown that the first-order solid phase transition in tetracosane C₂₄H₅₀ passes according to a heterogeneous mechanism in a narrow temperature range ΔT ~ 1 K in accordance with the theory of diffuse phase transitions.

In this paper, we described numerically several scenarios of formation of vortex flows (VF) in microsized hybrid-oriented liquid crystal (HOLC) channels with orientation defects using a nonlinear generalization of the classical Ericksen–Leslie theory that allows taking into account termomechanical contribution, both in the expression for the shear stress and in the entropy balance equation. An analysis of the numerical results showed that there are two or one vortices in the HOLC channel although two vortices directed towards each other are generated at the initial stage of the VT formation Thermomechanically Excited Vortical Flow.

The electronic and optical properties of gold fullerenes are studied in the framework of the Hubbard model. The expressions of the Fourier transforms of anticommutator Green functions have been obtained for gold fullerenes Au₁₆ and Au₂₀, the poles of which determine the energy spectrum of the system under consideration. The calculations are performed for the thermodynamic means that characterize jumps of electrons from a nanosystem site to a neighboring site, the correlation functions demonstrating the possibility of existing two d electrons with oppositely oriented spin projections on the same site of the fullerenes consisting of gold atoms. The optical absorption spectra are presented. The optical absorption peaks of ions \({\text{Au}}_{{20}}^{ - }\) and \({\text{Au}}_{{16}}^{ - }\) correspond to a near-infrared spectral region, where the light absorption by blood or a soft tissue is vanishingly small; thus, these ions can be used as a new class of contrast improvements and phototherapeutic means for diagnostics and treatment of cancer.

The analytical expressions for the density of states and the occupation numbers are obtained for simple models of carbon nanostructures (graphene–boron nitride lateral heterostructure, decorated zigzag edges of semi-infinity graphene and graphene nanoribbons, and decorated carbyne). The main attention is placed to the strong-coupling regime of the nanostructures with a semiconducting substrate. The numerical estimations are given for the SiC substrate.