Том 61, № 10 (2019)

The charge state of a disordered system of metallic nanoparticles on a conducting substrate is analyzed. The theoretical model that describes thermostimulated tunnel transitions of electrons between the particles and also the electron transitions between the particles and the substrate due to the difference in the work functions has been built. The model takes into account the interaction of the charges of the nearest particles and also the interaction of the particle charges with the image charges in the substrate. The Monte Carlo computer simulation for this model makes it possible to obtain the distribution pattern of the charge in structures with different nanoparticle density in a one-layer coating on the substrate at various values of the dielectric permittivity of the environment and various values of the difference of the work functions of particles and the substrate. The numerical results for structures with Ni and Pt nanoparticles on a carbon substrate, which are often used as catalysts of chemical processes, are presented as an example.

The influence of various doses of electron irradiation on the dielectric permeability and specific conductance of ternary nonlinear optical crystals of AgGaSe₂ at various frequencies of the measuring field in a range of temperatures of 100–300 K is studied. It is found that the irradiation of single crystals leads to a decrease in the values of dielectric permeability and a significant increase in conductance. It is shown that dielectric permeability and conductance increase with the growth in temperature. It is found that the presence of several types of conductance is characteristic for the crystals of AgGaSe₂. Substantial frequency dispersion of the dielectric properties of the studied crystals is found.

The structural features of the formation of radiation defects in proton-implanted layers of silicon plates during their heat treatment have been studied. New data on the nature, the characteristics, and the microdefect concentration in Si crystals irradiated with protons with energies 100 + 200 + 300 keV with the total dose 2 × 10¹⁶ cm^–2 and the evolution of the defect structure during heat treatment have been obtained by high-resolution three-crystal X-ray diffraction and transmission electron microscopy over wide temperature range from 200 to 1100°C.

In this paper, we employ SiC super hard ceramics that are widely used for the manufacture of individual means of protecting people from point impact effects and MgAl₂O₄ ceramics transparent in a wide spectral range that are used for protective screens of optical devices of aircraft, which are exposed to solid dust particles and atmospheric precipitation. Here, we study generation and relaxation of microcracks under impact action by using acoustic emission and electromagnetic emission, respectively. Since the mechanism of generation of these types of emissions has a different origin, then a comparison of the emission activity of one and another type allowed identifying general and individual patterns of impact damage to SiC and MgAl₂O₄ solid ceramics.

The solution to the problem of calculating the magnetostatic interaction energy of domain walls in uniaxial magnetics with a uniform magnetization distribution inside the domains is given. In carrying out the calculations, the principle of equivalent currents is used, assuming a uniform distribution of magnetization and its representation by equivalent currents flowing along the domain walls and along the surface. Analytical expressions for the mutual induction of two rectangular conductors with an arbitrary aspect ratio have been obtained. Results may be helpful in determining equilibrium configurations of domain structures in magnetic elements of spintronic devices, magnetic sensors and magnetic memory.

The response of the magnetic moment of a magnetically uniaxial nanoparticle and a planar array of such nanoparticles to a short Gaussian magnetic field pulse is studied in the presence and absence of its modulation. The periodic dependence of the response duration and the final orientation of magnetic moments on the pulse duration and its peak value are revealed and analyzed. The effect of the weak bias magnetic field and the pulse field deviation from the transverse orientation on remagnetization processes is studied. It was shown that the effect of the dipole–dipole interaction leads to modulation of the response to the pulse exposure.

The magnetic properties (field range H = 0–50 kOe, temperature range T = 3–300 K), and structural features of a sodium-doped carbon composite material based on fullerene C₆₀ and thermally exfoliated graphite (TEG) are studied. The material is obtained with different ratios of the components by sintering at a pressure of 7 GPa and T = 600°C, at which it is found that significant amorphization of the crystal lattice of the initial C₆₀ occurs. The dia-, para-, and ferromagnetic components (M_D, M_PM, and M_FM) were separated from the total magnetic moment of the samples under study. It is found that a sodium dopant has no effect on the magnetic properties of the composite. Analysis of the M_PM(H) field dependences by using the Brillouin function for the fullerene-containing sample (i.e., without TEG) makes it possible to determine the quantum number of the total angular momentum of paramagnetic (PM) centers. Its value is found to be J = 1, which corresponds to elementary magnetic moment μ_PM = 2μ_B of a PM center. The concentration of PM centers is estimated at the level of N_PM ≈ (2–5) × 10¹⁸ g^–1 for most samples, including the material without TEG. The introduction of TEG into the initial composition and an increase in its proportion in the composite leads to a strong increase in the magnetic moment, which is explained by an increase in both the J value and the concentration of PM centers.

The effect of hydrostatic pressure on phase transitions in lead hafnate (PbHfO₃) has been studied by the methods of X-ray diffraction and X-ray diffuse scattering. The measurements were performed in the temperature range of 182 < T < 316°C and the pressure range of 1.6 < P < 2.6 GPa. Four regions characterized by different X-ray diffraction patterns and corresponding to an antiferroelectric phase, phases with a long-wavelength modulation of the lead sublattice, and phases with different distortions of the oxygen sublattice are identified. In the temperature region immediately above the region of phases with a long-wavelength modulation, a temperature-dependent maximum in the distribution of diffuse scattering was detected, which means that such a modulation is formed due to condensation of an incommensurate soft mode.

This paper presents the results of X-ray diffraction studies on a single crystal of lead zirconate titanate PbZr_0.993Ti_0.007O₃ in the region of existence of an intermediate ferroelectric phase. In addition to the known superstructure reflections of the M type q_M = \(\left\{ {\frac{1}{2},\frac{1}{2},0} \right\}\) and the first-order satellite reflections q_M + {δ, δ, δ}, unknown second-order satellites have been observed near q_M and near the Bragg reflections. Structural model of regular system of antiphase domains is used for diffraction calculation. The model is shown to describe the first- and second-orders satellite reflections in the vicinity of q_M, but it cannot explain the appearance of satellites around the main Bragg peaks. A possible origin of the system of the superstructures observed in the intermediate phase is discussed.

Static cylindrical bending of nanofilms is considered in linear and nonlinear formulations. The frequency spectrum of bending vibrations and the parametric resonance are determined. In this case, two surface effects are taken into account. The first one is associated with different elastic properties in the surface layer and in the bulk of the material. It is manifested in stretching and bending of nanometer-thick films. The second effect is due to the difference, caused by bending, between the areas of the convex and concave surfaces subjected to gas pressure. The greater the ratio of the mean pressure to the elastic modulus of the material and the ratio of the length of the film to its thickness, the stronger this effect. The loading conditions of the end surfaces of the film are also important, as well as the strain over the film thickness under the action of the mean pressure. A positive mean pressure leads to an increase in effective stiffness, a decrease in deflection, and an increase in natural frequencies. A negative mean pressure reduces stiffness and natural frequencies. It is shown that, in this case, film bending may occur as a result of the longitudinal in stability. Oscillations of the mean pressure lead to a parametric amplification of bending vibrations. These results cannot be obtained on the basis of the classical equations of bending of plates and films.

A theoretical model is proposed that describes the hardening mechanism of ultrafine-grained aluminum obtained by high pressure torsion after low-temperature annealing. Within this model, the hardening is due to the successive transformation of the grain-boundary dislocation structure. In particular, plastic deformation is occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the grain body, and the formation of walls of climbing grain-boundary dislocations along opposite grain boundaries. The energy characteristics and critical stresses for emission of lattice dislocations are calculated. Theoretical dependences of the flow stress on the plastic strain, which demonstrate good qualitative and quantitative agreement with experimental data, are constructed.

The effect of X-ray radiation on the valence state of europium ions in the Y₂O₃ matrix was studied by EPR. The alternation of the charge of europium ions (Eu³⁺ → Eu²⁺ → Eu³⁺) in the Y₂O₃ structure was detected. The mechanism of europium reduction to the divalent state under the action of X-ray irradiation is described.

Using experimental second-rank parameters of the spin Hamiltonian of the rhombic centers Gd³⁺ and Eu²⁺ in three garnets and the superposition approximation formalism, relaxation of the nearest environment of impurity ions is estimated.

Transparent MgAl₂O₄:Dy³⁺ ceramic samples were synthesized by spark plasma sintering. Their optical properties, photo- and cathodoluminescence, the luminescence decay kinetics, and the effect of temperature on spectral and kinetic characteristics of luminescence were studied. The luminescence of Dy³⁺ ions (458 nm (⁴I_15/2 → ⁵H_15/2), 480 nm (⁴F_9/2 → ⁶H_15/2), 573 nm (⁴F_9/2 → ⁶H_13/2), 665 nm (⁴F_9/2 → ⁶H_11/2), and 750 nm (⁴F_9/2 → ⁶H_9/2)), impurity Cr³⁺ ions (²E_g → ⁴A_2g), and intrinsic defects (F⁺-centers, 460 nm) was detected. Thermal quenching of luminescence in the visible spectral range was observed as the temperature varied from 100 to 300 K. It is demonstrated that the patterns of quenching for impurity and intrinsic centers differ in MgAl₂O₄:Dy³⁺ ceramics. The mechanisms of thermal quenching are discussed.

A simple method of pyrolysis of an aerosol solution of yttrium, aluminum, and cerium nitrates with the addition of urea or citric acid, followed by brief annealing, is developed to obtain a highly dispersed aluminum–yttrium garnet powder that demonstrates intense photoluminescence in the visible light region. Characterization of the synthesized samples (photoluminescence spectra, X-ray diffraction analysis, Raman spectra) showed that intense photoluminescence is achieved only in a narrow window of process conditions, namely, the concentration of nitrate solution and the concentration of citric acid or urea. The photoluminescence intensity increases upon annealing synchronously with an increase in the crystallite size, which, along with the optimal cerium concentration (~0.5 at %), is a determining factor for obtaining high-quality samples. The synthesized powders had intense photoluminescence and high optical perfection, as evidenced by the observation of whispering gallery modes.

A dynamic matrix of rare-gas crystals is constructed within the model of deformable and polarizable atoms based on the nonempirical short-range repulsion potential taking into account the three-body interaction and deformation of electron shells of dipole-type atoms within the pair and three-body approximations. Ab initio calculations of the phonon energy for compressed rare-gas crystals are performed at two and ten mean-value points of the Chadi–Kohen method in a wide pressure range. It is shown that the contribution of three-body forces to the phonon frequencies due to overlap of electron shells of neighboring atoms is small as compared to the pair interaction even at a high pressure and most pronounced for Xe. The contributions of the deformation of electron shells within the pair and three-body approximations differ for different mean-value points and increase with an increase in pressure. The zero-point energies calculated by the Chadi–Kohen method for crystals of the Ne–Xe series are compared with the known experimental data at p = 0 and calculation results obtained by other researchers.

This work is aimed at general analytical representation of the conditions necessary for generation of layered periodic doping superstructures resulting from the solid-state phase transition accompanied by variation in admixture concentration. Using the capillary-wave model, the explicit expression for the region of temperature gradient and averaged interface velocity, inside which the interface velocity transits to the regime of periodic oscillations, is obtained. The formula for the spatial period of the doping superstructure whose appearance is caused by these oscillations is derived.

Based on the hydrodynamic model, the kinetics of wetting phase transitions in nanoscale liquid films on the substrate surface is analyzed. In the range of metastable states, the features of the formation of equilibrium clusters are studied and corresponding film thickness distributions are calculated. In the range of unstable states, the kinetics of the phase transition resulting in cluster formation is analyzed. In the early stage, regions shaped as holes with a film thickness close to the equilibrium one are formed. Hole coalescence leads to a film material redistribution followed by clustering. For this process, the kinetics of the average hole size and concentration is calculated. For formed clusters, the kinetics of the average radius, average height, concentration, and their radius and height distribution function are studied.

The electrical conductivity of thin polycrystalline VNi_xO₂ films has been studied in a wide range of temperatures, which covers the regions of both metallic and insulator phase. It is shown that the metal–insulator phase transition temperature decreases as the nickel concentration increases, and the temperature range of coexistence of the phases increases monotonically. The temperature dependence of the conductivity of the insulating VNi_xO₂ phase is explained using the hopping conduction model that takes into account the influence of thermal atomic vibrations on the resonance integral. Parameter ε has been calculated as a function of the doping level of VO₂.

Al-doped single wall carbon nanotube with Stone Wales defect was theoretically analyzed, two different orientations of chiral (8, 4) carbon nanotubes was doped among the joints of defective carbon rings. Density functional theory was implemented to study structural and electronic properties of Al-doped chiral carbon nanotubes. Doping bond lengths as well as their geometrical structure were determined at the different orientations. The electronic properties were also illustrated by evaluation band of the gap energies at each possible doping site. Our results indicated that not only Al-doping tune the band structure, but also dopant site played a crucial rule on manipulating physical properties of chiral carbon nanotubes.

Aluminum-doped zinc oxide thin films have been grown by atomic layer deposition at a temperature of 200°C. Using X-ray diffraction, it has been established that the ZnO:Al thin films exhibits the reflections from the (100), (002), (110), and (201) ZnO hexagonal phase planes. The (101) and (102) planes have also been detected by electron diffraction. The ZnO:Al thin films grow smooth with a root-mean-square roughness of R_q = 0.33 nm and characteristic nanocrystallite sizes of ~70 and ~15 nm without additional aluminum or aluminum oxide phases. The transmission at a wavelength of 550 nm with regard to the substrate has been found to be 96%. The refractive indices and absorption coefficients of the ZnO:Al thin films in the wavelength range of 250–900 nm have been determined. The maximum refractive indices and absorption coefficients have been found to be 2.09 at a wavelength of 335 nm and 0.39 at a wavelength of 295 nm, respectively. The optical band gap is 3.56 eV. The resistivity, Seebeck coefficient, and power factor of the ZnO:Al thin films are ∼1.02 × 10^–3 Ω cm, –60 μV/K, and 340 μW m^–1 K^–2 at room temperature, respectively. The maximum power factor attains 620 μW m^–1 K^–2 at a temperature of 200°C.

An explanation is proposed for the difference of the electric-strength properties of polymers in dc and ac electric fields. The energy release upon the recombination of electrons and holes injected into a polymer dielectric is considered as a factor accelerating the processes of electric aging of these dielectrics in an ac electric field. It is shown that the nonradiative relaxation of electron excited states, which causes the bond scission in macromolecules and the formation of free radicals, leads to the formation of deep electron traps in the polymer dielectric; as a result, the macromolecule ionization in an electric field is accelerated due to the transition of electrons into these traps. In the solid-state plasma, screening effect, which decreases the molecule ionization potential, appears. As a result, the macromolecule ionization rate, i.e., the rate of charge carrier formation, increases, which leads to a decrease in the lifetime of a polymer dielectric in an ac electric field as compared to the polymer lifetime in a dc electric field.

We analyze heat effects associated with the solid-state phase transition in ultrahigh molecular weight polyethylene from the unstable monoclinic phase into thermodynamically stable orthorhombic phase. The model proposed here for the structure of supramolecular formations of monoclinic phase does not contradict to earlier X-ray diffraction data for this polymer.

The results of doping of a carbon composite material, in which fullerenes are located in a conductive matrix based on thermally exfoliated graphite, with a sodium dopant are presented. Charge transfer processes taking place in samples with different initial ratios of components are studied. It turns out that the electrical resistivity of the samples increases with the introduction of sodium and an increase in its content, since the mobility of the main charge carriers, which are holes as in the undoped material, decreases. The concentration of charge carriers in different types of samples varies in both directions and can increase by more than an order of magnitude. It is concluded that Na plays an ambiguous role. It can contribute not only to the generation of free electrons, but also to an additional increase in the concentration of various defects that can generate free holes and can affect, being effective traps and scattering centers, all types of charge carriers.

We have studied the structure of epitaxial graphene obtained as a result of thermal desorption of the silicon carbide surface under conditions of vacuum synthesis and in Ar medium by reflection electron diffraction. As a result of the study, a significantly more uniform buffer layer coating of the SiC surface by epitaxial graphene has been found when forming in inert medium on the surface of 4H- and 6H-SiC(0001) polytypes compared with the synthesis of graphene in high vacuum. The quality of the coating has been shown to depend on the degree of perfection of the original single crystal.

The heat capacity of synthesized iron tantalate FeTa₂O₆ was measured in the temperature range of 323–1103 K by the ratio method using a thermal analyzer combining thermogravimetry and scanning calorimetry. The temperatures of phase transformations were determined. Structural changes and thermal expansion of oxide were studied in the range of 300–1173 K by high-temperature X-ray diffraction. The temperature dependences of the unit cell parameters were approximated by third order polynomials. The values of thermal expansion coefficients and anisotropy factors were calculated based on the obtained data.