Volume 61, Nº 7 (2019)

Comparative experimental studies for elastic characteristics of commercial nitiline TiNi shape memory alloys subjected to various heat treatments have been performed nitinol in the temperature range 80–300 K, including the premartensitic and martensitic transitions. The distortion of the crystal structure during the premartensitic transition in the as-prepared sample is found to be accompanied by pronounced anomalies of the Young’s modulus and the internal friction near the phase transition temperature T_S ≈ 270 K both on h-eating and cooling. An additional heat treatment causes slight shifts of the premartensitic transition temperatures on heating T_Su and cooling T_Sd and an increase in the hysteresis, but the character of the elastic anomalies is changed only slightly. Conversely, anomalies of the elastic properties during the martensitic transition at T ~ 220 K are observed only on cooling, and the temperature and the character of the anomalies is significantly changed after a heat treatment.

Formation of high-temperature superconducting phases of some hydrides requires high pressure (several hundred gigapascals), which causes lattice compression and, correspondingly, exponential increase in the probability of hydrogen-atom tunneling between equivalent interstitial sites. At low temperatures, vacancies in the hydrogen sublattice (occupied under stoichiometric conditions) and hydrogen atoms in the interstitial sublattice (absolutely vacant under stoichiometric conditions) are quantum defects (defectons). The defecton contribution to the superconductivity of hydrides has been estimated. It is shown that this contribution can be large both for free defectons and defectons clusterized with formation of two-level systems.

Ferrite Fe₃O₄ nanoparticles have been formed in low-pressure arc discharge plasma. It has been shown that the obtained nanoparticles have an average size of 9.4 nm and a blocking temperature of 89 K, crystallize in the magnetite phase, and are superparamagnetic at room temperature. The features in the behavior of nanoparticles in a magnetic field related to their large specific surface are discussed.

Spontaneous and magnetic field-induced phase transitions in a hard domain structure of a uniaxial ferrite–garnet film are studied. The temperature and field ranges of the lattice stability of cylindrical magnetic domains are shown to be dependent on the domain-boundary structure.

The electrical properties of a system of nanogranular amorphous Fe–SiO films with a SiO concentration between 0 and 92 vol % have been investigated. The samples with a low SiO content are characterized by the metal-type conductivity. With an increase in the dielectric content x in the films, the concentration transition from the metal to tunneling conductivity occurs at x ≈ 0.6. At the same concentration, the ferromagnet–superparamagnet transition is observed, which was previously investigated by the magnetic method. The temperature dependences of the electrical resistivity ρ(T) for the compositions corresponding to the dielectric region obey the law ρ(T) ~ exp(2(C/kT)^1/2), which is typical of the tunneling conductivity. The estimation of the metal grain sizes from the tunneling activation energy C has shown good agreement with the sizes obtained previously by analyzing the magnetic properties. In the dielectric region of the compositions, the giant magnetoresistive effect attaining 25% at low temperatures has been obtained.

The variations of electrical conductivity and the primary pyroelectric coefficient with temperature for lithium niobate (LiNbO₃) crystals grown from melt containing K₂O flux are studied in the range of 292–450 K. These crystals are found to exhibit considerable anisotropy of electrical conductivity, and the proton conductivity is dominant in the studied temperature range.

The element composition modification is experimentally studied in PZT thin films obtained via the RF magnetron sputtering with variable working gas pressure. The experimental data were interpreted through the statistic simulation of thermalization and diffusion of sprayed Pb, Zr, and Ti atoms during the ion-plasma deposition of PZT thin films. The simulation data are shown to adequately describe (within the limits of 5%) the change in element composition at various pressures of the gas environment. The study conducted enables one to produce PZT thin films with a set element composition, which is essential for optimizing the electric and physical properties of solid solutions at the morphotropic phase boundary.

The effect of cyclic loading on the magnitude of coherent scattering regions (CSR) of X-ray radiation and microstresses of the II type in various crystalline materials: semiconductor (samarium sulfide) and metal (steel, duralumin) has been found. The loading was carried out by compressing the samples in various ways: all-round, uniaxial, bending compression. It is shown that with an increase in the number of compression cycles, the CSR values in all cases decrease, and the microstresses increase. These values can serve as parameters to assess the degree of mechanical fatigue of the material.

The paper presents an attempt to use the Hall–Petch relationship to relate the yield strength of copper and titanium in three different states (initial, annealed, and after equal-channel angular pressing) to the sizes of nano- and micrometer deformation jumps measured using a precision interferometric technique. It is shown that, upon a compression strain near the yield point, one can observe six levels of deformation with three nano- and three micrometer sizes of deformation jumps from 1–2 nm to 20–35 μm. Each of the six structural states of metals is characterized by its own set of deformation jump sizes. Dependences of the yield strengths of copper and titanium on the jump sizes L^–1/2 are constructed, and the general regularities and features of deformation jumps for each of the metals in different structural states are discussed.

The available literature data on the effect of the thickness of the epitaxial films and size of the crystallites in the microparticles of the Ni–Mn–Sn alloy powder on the parameters of the martensitic transitions in this alloy have been analyzed in the framework of the theory of defuse martensitic transitions (DMT), which is based on thermodynamic and kinetic ratios. The purpose of the analysis is to establish an explicit (functional) dependence of these parameters on the film thickness D and the size of nanocrystallites d in the film and microparticles. The results of the R-diffraction analysis show that internal elastic microdeformations and stresses arise in films and microparticles of the alloy powder due to the coherent coupling of the epitaxial film with a solid substrate or as a result of severe plastic deformation of the alloy during its grinding in a ball mill. The analysis shows that the local microstresses significantly affect the type of the dependence of the temperature interval (diffusity) ΔT of the transition on the size factors D or d. In the absence of microstresses, these dependences have the form ΔT ∼ 1/D² or ΔT ∼ 1/d². In the presence of microstresses, the temperature transition interval depends on the size factors as ΔT ∼ 1/D or ΔT ∼ 1/d.

The crystal structure features and light-emitting properties of 3C–SiC island films grown at decreased temperatures on the Si(100) surface by vacuum chemical epitaxy with the use of hydrogen-containing compounds are studied. The nucleation character and growth mechanisms of the nanocrystalline texture of microislands and the effect of elastic stresses accumulated on the surface of a growing carbide film on the shape of nucleating islands are traced by the methods of microscopy. The cathodoluminescence spectra from the surface carbidized Si layer and different areas of an individual 3C–SiC island are compared. The possible mechanisms of the appearance of additional spectral lines shifted with respect to the major peak towards the red and ultraviolet spectral regions in the observed spectra of epitaxial structures are discussed. These emission bands were earlier revealed only in the luminescence spectra of SiC nanocrystallites embedded into different (most often SiO₂) matrices. The comparative analysis of the behavior of lines in the observed luminescent spectra has not revealed any appreciable size effect of formed surface nanocrystallites on their positions, but demonstrated their evident dependence on the oxygen content at the 3C–SiC layer/silicon substrate interface.

The influence of nanoinclusions on the macroscopic stiffness of amorphous systems was studied in the context of the random matrix model with translation symmetry. The numerical analysis of nanoinclusions, whose radius R is large enough, admits the use of the macroscopic theory of elasticity, defining the addition to the Young’s modulus as ΔE ~ R³. Nevertheless, a decrease in nanoinclusion radius makes this dependence quadratic, i.e., ΔE ~ R². Reducing the energy of the whole system to a sum of quadratic forms enables the Young’s modulus to be evaluated via the Gauss—Markov theorem. As follows, the stiffness of a medium depends on the difference between the number of bonds and the number of degrees of freedom of a system, which is proportional to the nanoparticle surface area. Furthermore, the scale of heterogeneity of the amorphous solids corresponds to a certain nanoinclusion radius, which determines the lowest characteristic nanoparticle size and the applicability of the macroscopic theory of elasticity.

The Potts model on a Kagome lattice is considered. The Monte Carlo method is used to obtain the temperature dependences of the thermodynamic parameters: heat capacity C, order parameter m, and susceptibility χ. The calculations were performed for systems with periodic boundary conditions. Systems with linear dimensions L × L = N, L = 20–90, were considered. Based on the fourth-order Binder cumulant method, the critical temperature (T_c) was calculated for the three-vertex Potts model on a Kagome lattice. It is shown that the obtained value of T_c, within the statistical error, is in good agreement with the results obtained by the transfer matrix and polynomial approximation methods.

The effect of the concentration of alloying components on the crystallization of amorphous (Fe₇₃Si₁₃B₉)_{1 – x – y}Nb_xCu_y and (Co₇₀Si₁₂B₉)_{1 – x – y}Fe_xNb_y alloys has been studied by X-ray diffraction and transmission electron microscopy in a wide composition region. The formation of the bcc structures in both the alloy groups is shown to be substantially dependent on the alloying component concentrations. The bcc phase is found to form in the cobalt-based alloys in the concentration region, where it was not observed before. In the cobalt-based alloys, the bcc phase appears at a niobium concentration higher than 1 at % and the average bcc nanocrystal size varies from 40 nm (at 1 at % Nb) to 14 nm (at 5 at % Nb). In the Fe-based alloys, nanocrystals with the bcc lattice form at the copper concentrations of 0.45–1 at %, and the average nanocrystal size is dependent on the alloy composition and varies in the range 16–24 nm. The causes of the concentration dependence of the formation of nanostructures in these alloys are discussed.

The low-temperature (T = 2 K) light reflectance spectra of organic semiconductor structures produced by depositing Langmuir–Blodgett films on a cadmium sulfide (CdS) crystal surface are studied. The spectra were studied in the region of the resonant frequency of the exciton state A_n = 1 in CdS. The spectra were analyzed within a multilayer medium model with allowance for the spatial dispersion and an exciton-free “dead” layer near the crystal surface contacting with the film. A conclusion is made that, as a result of the deposition of an organic film on a semiconductor crystal surface, the Wanier–Mott exciton is spatially localized near the film–crystal interface.

In this paper, we studied cobalt intercalation of single-layer graphene grown on the 4H-SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

We perform a comparative study of melting of polytetrafluoroethylene (PTFE) in its unmodified form, after irradiation, and after adding plant-derived silicon dioxide as an additive (content, 1%). Calculation of peak shapes on thermal capacity profiles within the theory of smeared phase transitions enable us to identify the structural changes and their features resulting from irradiation of PTFE and incorporation of the additive in it. α-relaxation at the glass transition in our samples is studied by dynamic mechanical analysis.

We obtained Pb_10 – xNd_x(GeO₄)_2 + x(VO₄)_4 – x (x = 0–3) compounds with an apatite structure by solid-phase synthesis from initial oxides PbO, Nd₂O₃, GeO₂, and V₂O₅ with successive annealing at 773–1073 K in the air. Their high-temperature heat capacity was measured by differential scanning calorimetry. The thermodynamic functions (changes in enthalpy, entropy, and reduced Gibbs energy) are calculated using the experimental dependences of C_p = f(T).