Том 61, № 8 (2019)

The melting and crystallization processes in some binary alloys based on the Ag–Cu model system have been studied by high-resolution differential scanning calorimetry. The crystallization and melting is found to be accompanied by heat flow jumps as the liquidus line is achieved or the melting process is completed. During crystallization, these phenomena are assumed to be related to the formation in a liquid of a great number of local microvolumes (concentration fluctuations) enriched in the component leading the crystallization. Their appearance precedes the process of spontaneous formation and growth of many crystals of a new phase. It is established that our concepts on the crystallization and melting processes in binary metallic alloys should be complemented.

The Co/TiO₂ multilayer thin films have been deposited by e-beam evaporation method on glass substrates in vacuum and annealed in air at 773 K for 1 h. The crystal structure, surface morphology, optical properties, electrical properties and magnetic properties of Co/TiO₂ multilayer thin films have been systematically investigated. The results showed that the particle sizes were significantly reduced when the samples are annealed and the enlargement of particle size occurred when the samples thickness is increased. The spectroscopic analysis exhibited enhanced transmittance and higher optical band gap of annealed sample than the as-deposited one and it was decreased with sample thickness. The resistivity measurement confirmed resistivity decreament with temperature. Furthermore, by investigating the magnetic properties, room temperature ferromagnetism was observed.

The results of the comparative study of the modification of the temperature dependences of the Nernst–Ettingshausen coefficient Q in the normal phase of two series of the samples of the Y_1 – xPr_xBa₂Cu₃O_y and Y_0.85 – xPr_xCa_0.15Ba₂Cu₃O_y compositions due to praseodymium doping are presented. The specific features of dependences Q(T) and Q_300 K(T) appeared as additional calcium ions exist in the YBa₂Cu₃O_y lattice have been revealed and analyzed. It is shown that dependences Q(T) and the temperature dependences of the thermopower coefficient obtained for the same samples previously are completely described in terms of the narrow-band model for both systems under study. The charge carrier mobilities and the degrees of asymmetry of the dispersion law are determined using a quantitative analysis of the experimental data. It is shown that the existence of additional calcium ions in the lattice does not influence a change in the asymmetry of the dispersion law with an increase in the praseodymium content, but strongly influences the character of changing the mobility. The found dependence of the mobility on the level of doping in the two systems and the observed difference in the characters of the changes in the absolute values of the thermopower coefficient and the Nernst–Ettingshausen coefficient in the Y_0.85 – xPr_xCa_0.15Ba₂Cu₃O_y system are explained on the basis of the analysis of the mechanisms of influence of the energy spectrum parameters on the charge carrier mobility in the YBa₂Cu₃O_y system.

The electric polarization in a zero magnetic field and in a magnetic field of 12 kOe is measured in a Gd_xMn_1 – xSe solid solution in the percolation concentration region in the temperature range 80–380 K. For composition x = 0.15, the polarization hysteresis and the dependence of the remanent polarization on magnetic field and temperature are determined. The hysteresis is explained in the model of migration polarization and the magnetoelectric effect is explained in the Maxwell–Wagner model.

Two main polytype transformations in silicon carbide, namely, 2H → 6H and 3C → 6H, have been studied by ab initio methods. It has been shown that the intermediate phases with trigonal symmetry P3m1 and monoclinic symmetry Cm make it much easier to move the close-packed layers in such transitions by breaking them up into separate stages. It has been found that these two polytype transformations proceed in completely different ways. The links being relocated noticeably tilted compared to their initial position at the transition 2H → 6H, which allows the compression of the SiC links in the plane (\(11\bar {2}0\)). The transition 3C → 6H is carried out through the formation of Si–Si and C–C auxiliary links, living for a short time and helping densely packed layers to swap places. As a result, the activation barrier of the transformation 2H → 6H (1.7 eV/atom) is significantly less than the activation barrier of the transformation 3C → 6H (3.6 eV/atom), which means that the second transition should occur at the temperatures by 750–800°C higher than the first one. The energy profiles of this polytypic transformations, as well as the geometry of all intermediate and transition phases have been calculated. It has been shown that all the transition states have monoclinic symmetry.

The traps of charge carriers in thermal films of silicon dioxide and silicon dioxide with a nanocomposite layer consisting of silicon oxide and silicon nanocrystallites have been studied using the methods of Kelvin-probe microscopy and cathode-luminescence. Based on experimental studies, the presence of electrons and holes in the trap samples is established. The effect of the charge state of electron traps on the luminescent properties of films is demonstrated. It is shown that the number of traps in nanocomposite layers is greater than in thermal oxides, but their activation energies are close in their values. This suggests that the nature of the traps in such layers is the same.

Bismuth-doped ytterbium and holmium titanates have been synthesized and their magnetic properties have been investigated. The magnetization curves and temperature dependences of the susceptibility in fields of up to 30 kOe at temperatures from 2 to 300 K have been obtained. The properties of the doped and undoped titanates have been compared. It has been established that the temperature dependences of the product of susceptibility and temperature contains a linear portion and the slope sign in these dependences changes upon doping for both the Ho₂Ti₂O₇ and Yb₂Ti₂O₇ titanates. Based on the investigated temperature dependences of the susceptibility, the magnetic dipole and exchange couplings in the titanates have been analyzed.

Dependences of the magnetization M and the resistance R on the temperature T in the range from 2 to 300 K and on the magnetic field H up to 14 T were investigated in polycrystalline samples of terbium antimonide TbSb. A peculiarity (bending) was observed on the M(H) dependences at each temperature below the transition point of TbSb into the antiferromagnetic phase (T_N = 14.8 K), which is due to a metamagnetic phase transition. Positive giant magnetoresistance was found in TbSb near the antiferromagnetic phase region.

The formation and evolution of the domain structure in the vicinity of the Curie point is studied in triglycine sulfate crystals subjected to fast cooling T_C. We derive an analytical expression for time dependence of the mean characteristic domain size. This expression enable us to investigate the process of enlargement of domain structure through all stages of its relaxation into a state of thermodynamic equilibrium. The model predicts a root-square behavior of the mean domain size, which agrees well with available experimental data. The rate of enlargement of the domain structure is shown to depend on the degree of overcooling. The time dependence of the mean domain size obtained in this study enables us to determine the radius of intermolecular interactions in the crystals under study.

Some results of studying the dielectric and electromechanical properties of ceramic 16BiScO₃–42PbMg_1/3Nb_2/3O₃–42PbTiO₃ relaxor ferroelectric samples in electrical fields (0 < E < 20 kV/cm) are presented. An appreciable decrease in the dielectric permittivity with time, and the disappearance of hysteresis in the dependence of the longitudinal strain on the electrical field intensity were revealed in the fields exceeding the coercive field (E > 10 kV/cm) and explained by the induced ferroelectric phase transition. The field-induced phase (presumably, tetragonal) was shown to be unstable and partially ordered. A nonmonotonical character demonstrated by the dielectric permittivity–time dependences of the studied ceramics unlike the other relaxor ferroelectrics was explained by the coexistence of glassy and ferroelectric phases.

It has been shown that almost every theoretical model known to date allows for correctly describing the properties of polymer nanocomposites on the condition that the real not nominal characteristics of their components are used. These real characteristics are always related to the formation of the interfacial regions in the mentioned nanomaterials. This means that the efficiency of the filler in polymer nanocomposites is controlled by its ability to generate interfacial regions.

The features of forming section and projection topography images of an edge dislocation normal to the crystal surface are considered for the case of X-ray anomalous transmission. Experimental images are analyzed using numerical simulation of the diffraction experiment. A new mechanism for image formation of defects arranged near the output crystal surface has been proposed. Differences between topography images of dislocations and rosettes of local misorientations of reflecting planes have been established.

The main objective of this paper is to reveal the mechanism of enhanced excitation light absorption in nano-pores structure BaMgAl₁₀O₁₇:Eu²⁺ (BAM) phosphor by optical analysis. The optical refractive index of the BAM was calculated from the reflectance spectra by Kramers–Kronig dispersion relation. And based on the effective medium theory, the anisotropic optical properties of porous BAM layer and its relations of absorption enhancement with porosity and thickness were investigated. A finite element simulation model was used for study the influence of pores size on optical properties. All the numerically evaluated results were match the experimental data.

The impact of sintering additive Al₂O₃ on the electrical conductivity of calcium zirconate-based polycrystalline proton-conducting electrolyte CaZr_0.95Sc_0.05O_3 – δ (CZS) is studied. With the sintering additive Al₂O₃, ceramic samples synthesized by a combustion technique are denser, and relatively low synthesis temperatures can be used (1470°C). Adding 0.1–0.5 wt % Al₂O₃ leads to an increase in the grain size from 100 nm to 1–2 μm. Increasing the Al₂O₃ content up to 0.3 wt % favors a growth in the electrolyte conductivity. For samples containing Al₂O₃, calcium aluminates are detected at grain boundaries. In samples exposed to humid air, charge transfer both in the grain bulk and at intergrain boundaries is found to be mediated by protons.

Molecular relaxation processes in lithium sulfate (Li₂SO₄), sodium sulfate (Na₂SO₄), and potassium sulfate (K₂SO₄) are studied using Raman spectroscopy (RS). The order parameter decreases in the low-temperature phase as it approaches the phase transition. For example, this is typical of a first-order phase transition close to a second-order phase transition. The existence of the pretransition region in the investigated sulfates Li₂SO₄, Na₂SO₄, and K₂SO₄ is proven.

The optical properties of the Cd_0.4Mn_0.4Te/Cd_0.5Mg_0.5Te heterostructure with quantum wells of 7 monolayers in thickness are studied. The temperature dependences of the integral intensity of luminescence of the barrier layer and quantum well and intracenter luminescence of manganese ions are determined, and the luminescence excitation spectra are measured. Numerous components corresponding to LO phonons of the CdTe, MgTe, and MnTe types are observed in the Raman scattering spectra.

The processes of electron-stimulated desorption of neutral cesium atoms from graphene to iridium are studied in the cases in which the graphene layer is or is not intercalated with cesium. It was found that there are two peaks in the kinetic energy distributions of desorbed neutral cesium atoms: a high-energy peak at 0.36 eV and a low-energy one at 0.13 eV. The high-energy peak is observed in both cases and is associated with the excitation of the 2s core levels of carbon. The low-energy peak is observed only in the absence of intercalation and is associated with the excitation of the 4f and 5p core levels of iridium. A model describing the processes of electron-stimulated desorption of cesium atoms from a graphene surface on iridium is proposed, and a conclusion is made that graphene on iridium behaves in these processes as a dielectric.

The adsorption of atomic and molecular nitrogen and ammonia on silicon carbide is considered within two physically different (solid-state and quantum-chemical) approaches. In the solid-state approach, the Haldane–Anderson model is used for the density of states of the SiC 4H and 6H polytypes to demonstrate that the energy of binding to the substrate is 6 and 3 eV for N atoms and N₂ molecule, respectively. In the quantum-chemical approach, the model of a surface diatomic molecule is used to find that the binding energy of atomic nitrogen is 6 and 4 eV for adsorption on the C- and Si-edges, respectively. It has been established that the charge transfer between an adsorbate and the substrate may be neglected in all the considered cases. It has been hypothesized that the dissociation of a molecule with the further passivation of its dangling sp³-orbitals with hydrogen atoms takes place for silicon carbide as in the case of ammonia adsorption on Si(100).

The heterocyclic polymer network nanocomposites obtained from bisphtalonitrile and various (from 0.03 to 5 wt %) modified silicate montmorillonite (MMT) nanolayers are studied. The nanostructure together with thermal, relaxation, and elastic properties of the composites are characterized with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). DMA and TGA measurements were performed in air and nitrogen mediums in the temperature range from 20 to 600–900°C. There was a various degree of exfoliation of MMT in the matrix to form single nanolayers, as well as thin and “thick” packs of MMT nanolayers, depending on its amount. We showed that there are strong constraining dynamics effects of the matrix by nanoparticles and sharp dynamic heterogeneity in the glass transition. We found that there are possibilities of complete suppression of the latter and of preservation of elastic characteristics of the composites unchanged at the temperatures from 20 to 600°C. The nanocomposites exhibit uniquely high thermal properties. A glass transition temperature of 570°C and satisfactory thermal stability are achieved with preservation of integrity of the material up to ~500°C in air and up to ∼900°C in nitrogen.

Dispersion properties of surface plasmon polaritons in a semiconductor film with graphene plates in the far-IR range and the possibility of controlling propagation modes due to the change in the chemical potential on one or both plates have been investigated. Dispersion relations for the TE- and TM-polarized waves have been obtained and analyzed, and distributions of the wave field and energy fluxes over the structure have been constructed. Spectral ranges where the surface-wave group velocity is negative are found.

The temperature dependence of the specific heat of the Bi_0.95Sm_0.05FeO₃ multiferroic has been studied in the temperature range 130–780 K. The substitution of samarium ions for bismuth ions in the bismuth ferrite is found to lead to the appearance of additional specific heat component that is due to the manifestation of the Schottky effect for three-level states and multiplet structure of 4f electrons of samarium ions.