Vol 59, No 1 (2017)

A model of the grain-boundary self-diffusion process in metals undergoing phase transitions in the solid state is proposed. The model is based on the ideas and approaches of the theory of nonequilibrium grain boundaries. It is shown that the range of application of basic relations of this theory can be extended, and they can be used to calculate the parameters of grain-boundary self-diffusion in high-temperature and low-temperature phases of metals with phase transition. Based on the constructed model, activation energies of grainboundary self-diffusion in titanium and zirconium are calculated, and their anomalously low values in the low-temperature phase are explained. The calculated activation energies of grain-boundary self-diffusion are in good agreement with experimental data.

The nonlinear characteristics of high-temperature superconductors of the Bi–Sr–Ca–Cu–O system have been experimentally investigated in the temperature range of the superconducting transition under the influence of a harmonic alternating magnetic field. The effect of the generation of odd harmonics in the signal of response to a harmonic alternating magnetic field for multiphase high-temperature superconductors containing regions with different values of the critical temperature in their bulk has been observed for the first time. The mechanism of harmonic generation in a superconductor in the resistive state, which is associated with the switch effect, i.e., with the redistribution of eddy current density between the local regions of the superconductor, has been considered.

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.

X-ray diffraction, dielectric, piezoelectric, and pyroelectric studies have been performed on ceramic samples of (1–2x)BiScO₃ · xPbTiO₃ · xPbMg_1/3Nb_2/3O₃ (0.30 ≤ x ≤ 0.46) perovskite-like solid solutions. The solid solution symmetry was found to vary from the rhombohedral (x ≤ 0.38) to tetragonal (x ≥ 0.42) as x increases. The samples with 0.30 < x ≤ 0.42 have properties characteristic of relaxor ferroelectrics, namely the existence of a wide peak in the temperature dependence of the dielectric permittivity at T_mε = 390–440 K that shifts to higher temperatures as the frequency increases, narrow unsaturated dielectric hysteresis loops, and an electric field-induced transition to the ferroelectric state at 318 K. The observed features of the dielectric, piezo-, and pyroelectric properties of these solid solutions are explained by the fact that they are relaxor ferroelectrics.

The relaxation of the electric current in a metal–nitride–oxide–semiconductor structure has been measured experimentally. The experiment has been compared with the calculation based on the two-band conduction model and the multiphonon mechanism of the ionization of traps. The upper estimate obtained for the recombination cross section from the comparison of the experiment with the calculation is found to be 5 × 10^–13 cm².

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t_x, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ t_Co ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t_Co = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t_Co = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.

Series of spontaneous magnetization jumps have been observed on the background of a continuous relaxation of the magnetic moment of a molecular ferrimagnet in a constant magnetic field. A statistical analysis of the set of data has demonstrated the presence of at least two modes in the distribution of magnetization reversal jumps over their amplitude. It has been found that the number of modes in the distribution depends on the magnetic field and temperature and characterizes a discrete energy spectrum of nonlinear spin ensembles formed at low temperatures. The continuous component of the magnetic relaxation corresponds to the motion of domain-wall arrays in the temperature range from 20 to 50 K and to relaxation processes in the spin-soliton lattice at temperatures ranging from 2 to 10 K.

The longitudinal dielectric, piezoelectric, and elastic characteristics of ferroelectric RbH₂PO₄ and antiferroelectric NH₄H₂PO₄ crystals are calculated in terms of the modified model of proton ordering with allowance for the piezoelectric coupling in an approximation of a four-particle cluster. The results obtained agree well with known experimental data. Along with this, the electromechanical properties of a Rb_0.2(NH₄)_0.8H₂PO₄ single crystal undergoing an antiferroelectric phase transition have been studied experimentally over a wide temperature range. The model predictions and the experimental data agree qualitatively.

The temporal characteristics of the dynamic fracture of the titanium alloy VT-6 have been investigated under high-speed loading conditions. A relationship has been established between the process of dynamic tension of the specimen according to the Kolsky method and the surface erosion damage. A numerical analysis of the experimental data has been carried out. The method of erosion loading has been further developed as a tool for the dynamic testing of strength properties of materials.

The kinetics of decomposition of a polycrystalline Fe–Cu alloy and the formation of precipitates at the grain boundaries of the material have been investigated theoretically using the atomistic simulation on different time scales by (i) the Monte Carlo method implementing the diffusion redistribution of Cu atoms and (ii) the molecular dynamics method providing the atomic relaxation of the crystal lattice. It has been shown that, for a small grain size (D ~ 10 nm), the decomposition in the bulk of the grain is suppressed, whereas the copper-enriched precipitates coherently bound to the matrix are predominantly formed at the grain boundaries of the material. The size and composition of the precipitates depend significantly on the type of grain boundaries: small precipitates (1.2–1.4 nm) have the average composition of Fe–40 at % Cu and arise in the vicinity of low-angle grain boundaries, while larger precipitates that have sizes of up to 4 nm and the average composition of Fe–60 at % Cu are formed near grain boundaries of the general type and triple junctions.

Crystals of the composition Ca_1–xEr_xF_{2 + x} (0 < x ≤ 0.02) have been grown from the melt by the Bridgman method. The absorption, luminescence excitation, and photoluminescence spectra of this material in the case of the Stokes and anti-Stokes mechanisms of the optical excitation from two radiation sources simultaneously have been analyzed. It has been found that the visible luminescence manifests itself upon sequential absorption of two resonance infrared photons with different energies. This effect is associated with the fact that, in this material, there are metastable states of the excited levels ⁴I_11/2 of the Er³⁺ ions, from which transitions to higher energy levels occur due to the absorption of infrared photons.

The ab initio calculations have been carried out for the crystal structure and Raman spectrum of a single crystal of lutetium pyrosilicate Lu₂Si₂O₇. The types of fundamental vibrations and their frequencies and intensities in the Raman spectrum for two polarizations of the crystal have been determined. The calculations have been performed within the framework of the density functional theory (DFT) using the hybrid functionals. The ions involved in the vibrations have been identified using the method of isotopic substitution. The results of the calculations are in good agreement with the experiment.

The thermodynamic properties and phase transitions in a two-dimensional strongly diluted threevertex antiferromagnetic Potts model on a triangular lattice have been investigated using the Monte Carlo method. The systems with linear dimensions of L × L = N, where L = 18–48, have been considered. It has been shown using the method of fourth-order Binder cumulants that, upon the introduction of nonmagnetic impurities into the spin system described by the two-dimensional antiferromagnetic Potts model, the firstorder phase transition changes to a second-order phase transition.

Сointerсalated materials are studied, obtained by introducing copper into a TiSe₂ lattice preintercalated with transition metals M = Mn, Fe, Co, or Ni. The analysis of the state of cointercalated systems at 950°C shows that copper reduces manganese and iron, but it is incapable of reducing cobalt or nickel. To explain the results, the values of the binding energy of hybrid states M3d/Ti3d are compared.

The structural characteristics, magnetic properties, and processes of magnetic heating in an alternating magnetic field of magnetic nanoparticles (MNPs) Co_0.5Zn_0.5Fe₂O₄ (cobalt–zinc ferrite, CZF) are studied to explore the possibilities of their application in medicine, namely, for magnetic hyperthermia treatment (the heating of particles with external alternating magnetic field). CZF magnetic nanoparticles were obtained by coprecipitation using sodium hydroxide (NaOH) as a precipitating agent. Based on the data obtained by transmission electron microscopy in the transmission geometry, it is found that CZF magnetic nanoparticles have an almost spherical shape with an average particle size of 13 nm. X-ray diffraction and Mössbauer studies showed that CZF magnetic nanoparticles are single-phase, and their structure corresponds to a cubic spinel structure. The saturation magnetization M_s of CZF nanoparticles is measured at room temperature using a vibrating sample magnetometer. The possibility of heating CZF magnetic nanoparticles with an external alternating magnetic field was studied using an induction heating system. The specific absorption rate is determined by applying an external alternating magnetic field in the range of 167.5 to 335.2 Oe at a fixed frequency of 265 kHz. It is found that the maximum amount of heat (114.98 W/g) is produced at a concentration of 5 mg/L under a field of 335.2 Oe.

The multilayer amorphous nanostructure [(CoFeB)₆₀C₄₀/SiO₂]₂₀₀ of alternating composite and dielectric layers was obtained by ion-beam sputtering on a rotating pyroceramic substrate of two targets, one of which was a Co₄₀Fe₄₀B₂₀ metal alloy plate with graphite inserts. The dielectric interlayers of SiO₂ were sputtered from a quartz plate (second target). The thicknesses of bilayers of the multilayered nanostructure (MNS) (6 nm), consisting of metal–carbon composite layers (CoFeB)₆₀C₄₀ approximately 4 nm in thickness and a silicon oxide dielectric interlayers with a thickness of approximately 2 nm, were determined by small-angle diffraction. The results of experimental layer-by-layer study without destroying the MNS by ultrasoft X-ray spectroscopy (USXES) showed a significant deviation of the stoichiometric composition of the dielectric interlayers from stoichiometry sputtered quartz towards decreasing oxygen concentration with the formation of SiO_1.3 suboxide. As a result of simulation of the Si L_2,3 spectra of silicon using reference spectra of known phases, the concentration of the silicon suboxide phase in the amorphous dielectric interlayers reaches about half of the interlayer content, the second half of which is accounted for SiO2 dioxide. A “shielding” effect of carbon in the metal layers is manifested in the absence of silicide formation at the interfaces of the multilayer structure under study and should help to increase the anisotropy of their electromagnetic properties.

The structure and thickness of zinc and cadmium chalcogenide semiconductor films are studied by X-ray radiography. The film thickness is shown to be comparable with the half-value layer depth. The electrical conductivity of the films increases upon heating in the hydrogen atmosphere and decreases upon heating in carbon oxide. The opposite trend is observed in the ratio between the electrical conductivity and band gap of the initial and oxidized film surfaces.

A decrease in the electrical durability, which is defined as an amount of time required for dielectric breakdown at a constant electric field strength, of polyethylene and Lavsan (polyethylene terephthalate) films under tensile loading is registered in a temperature range from 100 to 300 K. It is established that the pulling apart of the axes of neighbor chain molecules in consequence of tensile loading gives rise to a decrease in the energy level of the intermolecular electron traps. In the amorphous region of a polymer, this accelerates the release of electrons from the traps through over-barrier transitions at higher temperatures ranging from about 230 to 350 K and quantum tunneling transitions at lower temperatures in the range from about 80 to 200 K. As a result, the time required for the formation of a critical space charge, i.e., the waiting period of dielectric breakdown, decreases, which means a reduction in the electrical durability of polymers.

Molecular-dynamics simulation of the evolution of bipartite bimetallic clusters consisting of 390 atoms during bombardment by Ar_n (n = 1, 2, 13) clusters with initial energies from 1 eV to 1.4 keV is performed. Binary Cu–Au and Ni–Al clusters consisting of equal atomic fractions of corresponding elements were used as a target. As a result of simulation, the temperatures, changes in the potential energy, sputtering yields, and intensities of collision-stimulated displacement of atoms through the interface of monometallic parts of binary clusters, depending on the size and energy of incident particle, are obtained.

The energy spectrum of C₈₂ fullerene (isomer no. 3 of C₂ symmetry) is calculated within the Hubbard model in the approximation of static fluctuations. Based on the energy spectrum, optical absorption spectra of this isomer in neutral and anionic states with one, two, three, and four additional electrons are simulated. The calculated optical spectra in neutral and monoanionic states are compared with known experimental spectra.