Editor’s column

K.V. Patsuev, O.V. Malyshkina, A.I. Ivanova
Tver State University

Abstract: In this work, ceramic Li_0,1Na_0,9NbO₃ samples (pure and with modifying additives (5%) SrTiO₃ or LiTaO₃) were obtained by solid-phase synthesis. If the grain size of pure lithium sodium niobate and modified (5% of SrTiO₃) is practically the same, then the inclusion (5% of LiTaO₃) in the composition of Li_0,1Na_0,9NbO₃ ceramics leads to a significant spread in the grain size. In this case, grains of several times larger size are released. Based on analysis of the elemental composition, it was established that the inclusion of SrTiO₃ and LiTaO₃ modifiers in the composition of Li_0,1Na_0,9NbO₃ differs significantly. In the first case (for SrTiO₃), we can talk about the homogeneity of inclusion at the nanostructural level, whereas LiTaO₃, despite its presence throughout the entire volume of the sample, is included in the composition of Li_0,1Na_0,9NbO₃ in blocks. Measurement of pyroelectric properties showed that only the introduction of SrTiO₃ (5%) as a modifier leads to the possibility of obtaining a uniformly polarized state in the LNN ceramic sample, which is important for practical application.
Keywords: piezoelectric ceramics, lithium sodium niobate, lead-free materials, modifiers, grain structure, permittivity pyroelectric effect, polarization homogeneity

E.P. Neustroev, I.I. Kurkina
North-Eastern Federal University named after M.K. Ammosov

Abstract: Synthesis of nanometer-thick dielectric films is one of the key tasks in nano- and optoelectronics, which is associated with growing requirements for miniaturization and functionality of devices. This paper presents the results of a study of optical and electrical properties of graphene fluorinated by inductively coupled plasma CF₄. Fluorination was carried out in plasma with a power of 125 to 200 W for duration of up to 1 min. Two methods of arranging samples in the plasma chamber were used: 1) with a graphene film facing plasma and 2) with a substrate facing plasma. The methods of Raman spectroscopy, X-ray energy-dispersive spectroscopy, and current-voltage characteristics were used in the study. It was shown that the direct plasma action on the samples in position 1 results in graphene etching until the film is completely removed. In position 2, no intensive etching is observed during treatments lasting up to 1 min. In this case, fluorination of the graphene film occurs. As a result, the ratio of the number of fluorine atoms to carbon reaches a value of ~0,2. As a result of plasma treatment, there is a significant increase in the electrical resistance of graphene. The increase in surface resistance was from several kΩ/sq for the initial graphene to hundreds of GΩ/sq and tens of MΩ/sq for the samples subjected to plasma action in positions 1 and 2, respectively. The increase in the electrical resistance may be due to both the appearance of defects during plasma treatment and the fluorination process, which forms sp³-hybridized C-F bonds that distort the flat structure of graphene. Repeated conductivity assessments carried out after three weeks showed a decrease in resistance by two orders of magnitude for samples treated in plasma in position 2. This decrease may be due to the processes of defluorination and restoration of the flat structure of graphene.
Keywords: graphene, chemical vapor deposition, fluorination, plasma, carbon tetrafluoride, optical properties, electrical resistance

D.A. Kiselev^2,1, D.V. Stryukov³, A.V. Pavlenko^3
1 Fryazino Branch of V.A. Kotelnikov Institute of Radio Engineering and Electronics of RAS
² National University of Science and Technology «MISIS»
³ Federal Research Centre The Southern Scientific Centre of theRAS

Abstract: High-quality ferroelectric BaTiO₃/SrRuO₃/MgO(001) heterostructures with BaTiO₃ film thicknesses ranging from 36 to 360 nm were synthesized using high-frequency cathode sputtering in an oxygen atmosphere. A comprehensive study of their crystal structure, surface morphology, piezoelectric and ferroelectric properties was conducted using X-ray structural analysis and scanning probe microscopy. It was established that all the obtained films are single-phase and heteroepitaxial. X-ray diffraction measurements revealed a significant deformation of the unit cell, which reaches a maximum (~4,4%) for the thinnest films and decreases with increasing thickness. It was shown that the surface roughness of the films, estimated from scanning probe microscopy data, systematically increases with thickness, following a scaling law. Piezoresponse force microscopy demonstrated the possibility of local polarization switching in the films and revealed an increase in the magnitude of the residual piezoelectric response with the growth of the ferroelectric layer thickness. The surface potential of the films was measured by Kelvin probe force spectroscopy, and a tendency for it to decrease with increasing thickness was established. The paper discusses the reasons for the observed patterns. The obtained results are important for understanding size effects in nanoscale ferroelectric films and their applications in microelectronics and functional devices.
Keywords: ferroelectric, BaTiO3, dielectric characteristics, scanning probe microscopy, polarization

A.I. Ivanova¹, A.Yu. Karpenkov¹, E.M. Semenova¹, I.I. Musabirov^2
1 Tver State University
² Institute for Metals Superplasticity Problems of the RAS

Abstract: The paper presents the results of studying the microstructure, the magnetic domain structure and some magnetic properties of NiMnGaCu alloy samples in initial and deformed states. It has been shown that carrying out deformation-thermal treatment, including homogenizing annealing, extrusion with subsequent vacuum annealing, contributes to a change in the microstructure of the initial alloy (reduction in the size of crystallites and martensitic plates). Magnetic domains on the surface of the samples were visualized by magnetic force microscopy, and a substructure of secondary microtwins with their own magnetic domain structure was revealed. A study of the field dependences of the magnetization of the initial and deformation-heat-treated samples allows us to conclude that both samples have the same saturation magnetization value. Magnetometric measurements demonstrate an insignificant shift in the phase transition temperature towards lower temperatures for the deformed sample. The maximum values in the temperature dependences of the magnetocaloric effect for the original and deformed samples strictly correspond to phase transitions.
Keywords: Heusler alloys, martensitic relief, deformation-thermal treatment, extrusion, micro- and nanostructure, magnetic domain structure, magnetostructural transition

V.I. Ivanov, A.V. Myagotin
Far Eastern State Transport University

Abstract: Self-focusing of radiation is usually considered as a nonlinear effect in a medium with a positive coefficient of the cubic nonlinearity. Nanosuspensions have such nonlinearity due to electrostrictive flows of nanoparticles in a non-uniform light field. The description of this type of nonlinearity is traditionally limited to the consideration of the mode of weak intensities of the light field. In this case, the quasi-stationary change in the concentration of nanoparticles is directly proportional to the radiation intensity. Therefore, the analysis of the self-action of radiation corresponds to the classical case. In this paper, we analyze the self-focusing mode of a Gaussian beam in a transparent nanosuspension at high radiation intensities, when a response of the medium no longer corresponds to the cubic nonlinearity. A solution is given to the nonlinear stationary problem of light-induced transfer of nanoparticles in a liquid medium under the action of electrostrictive forces at high radiation intensities. The obtained result demonstrates an exponential dependence of the change in the concentration of nanoparticles on the radiation intensity. This is fundamentally different from the mode of weak intensities, where the change in concentration is linearly dependent on the intensity. In the classical case of the cubic nonlinearity, the self-focusing mode is determined by the total beam power. The considered model demonstrates a significant decrease in the critical self-focusing power at high radiation intensities, the use of which is more preferable for the experimental implementation of the self-focusing mode.
Keywords: self-action of the radiation, cubic nonlinearity, electrostriction, nanosuspension, self- focusing, critical power, optical diagnostics

R.A. Magomedov, E.N. Akhmedov
Institute for Geothermal Research and Renewable Energy of the Joint Institute for High Temperatures of the RAS

Abstract: The paper presents calculations of the state equation (isobars) of helium-4 in the pressure range from 10 to 100 MPa and the temperature range from 600 to 1500 K using the fractal state equation and the Fract EOS software. It was discovered that the temperature dependence of the fitting parameter α for the temperatures above 400 K weakens sharply and practically disappears approaching to 600 K. For helium-4, this allowed to approximate the dependence of α on density by a polynomial and to use it in calculations at temperatures above 600 K. For calculation of isobars, a set of isotherms with a small temperature step was calculated. Then, a point with the required pressure was selected on each isotherm. Obtained results show good agreement with literature data. In addition, for the specified temperatures and pressures, the isobars of the classical equation of state and the Redlich-Kwong equation of state were calculated. A comparison of the relative calculation error showed a significant advantage of the fractal equation of state.
Keywords: mathematical modeling, software for substance properties calculation, fractal state equation, Redlich-Kwong state equation, integral-differentiation of fractional order, Maxwell relations, Helmholtz potential, partition function, helium-4, isobar, thermophysical properties

V.G. Kul’kov, D.Sh. Norov
Branch of the National Research University «Moscow Power Engineering Institute» in Volzhsky

Abstract: The physical processes leading to the formation of nonequilibrium grain boundaries in nanocrystalline and ultrafine-grained materials are considered. The problem is solved for a two-dimensional diffusion equation on a boundary segment exposed to variable compressive stresses. The vacancy distribution and the corresponding normal tension in the segment are found. From the consideration of vacancy dynamics, the rate of mutual displacement of grains in the normal to the boundary direction and the amount of internal friction are determined. Internal friction has the character of a high-temperature background. The effect of stress adjustment is taken into account. The process of atomic relaxation of the boundary structure over time is discussed. The change in relaxation energy is shown with a change in a complex parameter, including frequency, grain size, activation energy, and temperature. From the graph of the dependence of the logarithm of the product of internal friction on temperature on the reverse temperature, the activation energies on the high- and low-temperature parts of the process are found. It is shown that at pre-melting temperatures, areas with the highest activation energy may appear. A method for determining the activation energy of internal friction at equilibrium and nonequilibrium boundaries is considered. The method of grain size estimation is discussed. The relaxation time of the atomic structure of the boundary can be determined from the change in the amount of internal friction over time.
Keywords: nanocrystalline and ultrafine-grained materials, diffusion, internal friction, activation energy, relaxation time

D.V. Zhigunov, A.A. Romanov, V.M. Samsonov
Tver State University

Abstract: A method has been developed and tested for thermodynamic simulation of the surface segregation in binary nanoalloys. The method is based on the numerical solution of a system of two equations: the Langmuir-McLean equation and the mass balance equation for a two-cell system represented by the central region (core) of a nanoparticle and its surface layer (shell). Apparently, for the first time, when applying the Langmuir-McLean equation, the dependence of the segregation heat on the composition of the core and shell of the two-cell nanoparticle model has been taken into account. The developed approach has been applied to predict the surface segregation in binary Ag-Cu and Ni-Cr nanoparticles. The obtained results predict the surface segregation of Ag for the Ag-Cu nanoalloy and the surface segregation of Cr for Ni-Cr nanoparticles. We have shown that the surface segregation decreases with decreasing the nanoparticle size (the effect of the core depletion as a source for the segregating component) and with increasing temperature. The results of thermodynamic prediction of the surface segregation based on the Langmuir-McLean equation are consistent with both the results of thermodynamic simulation based on the solution of the Butler equation and with our results of atomistic simulations obtained earlier.
Keywords: binary Ag-Cu and Ni-Cr nanoparticles, surface segregation, thermodynamic simulation, Langmuir–McLean equation, heat of segregation

N.P. Uglev¹, S.N. Uglev^2
1 Perm National Research Polytechnic University
² LLC «Information Technologies-Volga region»

Abstract: The results of several different experiments on the study of the process of stratification and diffusion of components of a binary tin-lead metal melt located in «capillaries» of a non-wettable material and complex shape are presented. The diffusion process was studied with the direct mutual dissolution of pure initial components from the «lead below, tin above» state in cells that allow lead atoms to spread both upward and downward in the volume of liquid tin. The «stratification» experiment was carried out in a flat capillary with an internal defect included, which made it possible to create a transverse cavity up to 0,5 mm deep in the lower part of the wide face of the sample, completely overlapping the width of one of the sample planes. The quantitative analysis of the composition of the samples was carried out by the X-ray-fluorescence method according to a pre- constructed calibration scale. In the metal melt, experimental results made it possible to reveal the existence of a second mass transfer mechanism, in addition to diffusion, consisting in the flow of lead along the boundary between the liquid sample and the non-wettable wall of the «capillary». The results of the diffusion experiments fully correspond to the data on stratification in a planar capillary, as well as to previous studies.
Keywords: metal melt, stratification, diffusion, second mechanism of mass transfer in metal melts, interfacial flow

K.V. Suliz, A.V. Pervikov
Institute of Strength Physics and Materials Science of Siberian Branch of RAS

Abstract: Nanoparticles of multicomponent NiFeCoCrCuAlMo and NiFeCoCrCuAlMoW alloys were synthesized by combined electrical explosion of wires of various metals/alloys in an argon atmosphere. Transmission electron microscopy and X-ray diffraction analysis were used to determine the structural characteristics of the nanoparticles. The average particle size is about 50 nm (with the ratio of the energy E introduced into the wires to the total sublimation energy of the wires ΣE_c of the order of 1,6), and the crystal structure is represented by the bcc and fcc phases of substitution solid solutions and the bcc phase corresponding to a substitution solid solution based on a refractory metal. It has been suggested that greater homogeneity of the elemental and phase composition of nanoparticles in multicomponent NiFeCoCrCuAlMo and NiFeCoCrCuAlMoW alloys can be achieved by varying the energy parameters of the combined electrical explosion of wires. Thus, the research results indicate the need to optimize the synthesis parameters to obtain nanoparticles with a desired elemental composition and crystal structure.
Keywords: high-entropy alloy, nanoparticles, exploding wires, transmission electron microscopy, energy dispersive analysis, X-ray phase analysis