Author: norAdmin

K.V. Suliz¹, V.V. Shmakov¹, A.V. Pervikov¹, N.Yu.. Sdobnyakov^2
1 Institute of Strength Physics and Materials Science of Siberian Branch of RAS
² Tver State University

Abstract: In this work, preparation of powders of multicomponent (NiFeCoCuZn)_xO_y nanoparticles by the method of combined electrical explosion of wires in an atmosphere of Ar + 25 mol % O₂ gases is demonstrated. Heating of wires with a current pulse with a density of 3,15×10⁷ A/cm² at a buffer gas pressure of 0,15 MPa leads to the formation of nanoparticles of multicomponent oxide with an average size of about 54 nm. The particle size distribution is described by the normal-logarithmic law. The particles have the spherical shape. The synthesized sample contains two crystalline structures corresponding to rock salt (Fm3m, a = 4,213 Å) and spinel (Fd3m, a = 8,389 Å). The results of the study show that the crystal structure of (NiFeCoCuZn)_xO_y nanoparticles can be optimized by both changing the ratio of divalent and trivalent metals in the wire explosion products and changing the thermodynamic conditions for the formation of nanoparticles.
Keywords: high-entropy oxides, nanoparticles, exploding wires, transmission electron microscopy, energy dispersive analysis, X-ray phase analysis

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

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

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

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

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

G.P. Mikhailov
Ufa University of Science and Technology

Abstract: Subnanosized clusters containing paramagnetic titanium and oxygen ions can be considered as fragments of the surface of nanocrystalline titanium dioxide. The analysis of the temperature dependence of the magnetic susceptibility of titanium dioxide clusters makes it possible to study the manifestations of surface magnetic states and identify the type of magnetic ordering. Using the density functional theory method in the M06/6-31G(d, p) approximation, a quantum chemical calculation of the equilibrium geometry of (TiO₂)_n (n = 2-5, 13, and 15) clusters with a full or predominant proportion of surface atoms was performed. It is established that for all (TiO₂)_n clusters, the singlet electron configuration is the ground state. To assess the thermal stability of (TiO₂)_n cluster structures and study the effect of temperature in the range of 10-900 K in 50 K increments, calculations were performed using the ab initio method of molecular dynamics and the atom-centered density matrix propagation scheme. The length of each molecular dynamics trajectory was 1 ps with a time step of 1 fs. For each temperature, the magnetic susceptibility tensor was calculated using the coordinate invariant atomic orbitals (gauge including atomic orbitals) method in the approximation M06/6-31G(d, p). It is shown that the temperature dependence of the values of the isotropic magnetic susceptibility has a maximum at temperature T_max. At T > T_max, there is a decrease in the temperature dependence of the magnetic susceptibility of (TiO₂)_n clusters. It has been established that the main magnetic state of (TiO₂)_n clusters is an antiferromagnetic singlet. The importance of the anisotropy of magnetic susceptibility for the TiO₂ molecule and clusters of (TiO₂)_n is shown.
Keywords: subnanosized cluster, titanium dioxide, density functional theory, ab initio molecular dynamics, magnetic susceptibility

A.V. Tvardovskiy
Tver State Technical University

Abstract: Classical ideas about the adsorption process have always been based on the fact that the adsorbent remains inert and does not change its size when interacting with gases or vapors. Its role is limited to creating an adsorption field where the adsorbate molecules fall. It is on the basis of this principle that the well-known adsorption equations of Henry, Langmuir, Fowler-Guggenheim, Brunauer-Emmett-Teller and others were derived. However, modern experimental studies show that adsorbents are deformed in the adsorption process. This fact significantly changes the entire picture of the consideration of this phenomenon. For example, when the geometric dimensions of the pores of the adsorbent change during deformation of the latter, the adsorption field into which the adsorbate molecules fall changes significantly. And this affects the value of the calorimetric heat of adsorption registered during the studies. Thus, the adsorbent is an equal participant in the adsorption process along with the adsorptive, and the adsorption system should be considered as a two-component one. In this regard, when conducting adsorption studies, a comprehensive approach is needed, including taking isotherms, measuring calorimetric heats of adsorption, and conducting dilatometric experiments to study the adsorption deformation of adsorbents. Such a comprehensive approach was used for the Na-montmorillonite – methanol vapor system. The differential heat and adsorption isotherm at T = 293 K were obtained using a Calvet-type microcalorimeter and a McBain-Bakr microbalance. The adsorbent deformations were measured using a highly sensitive dilatometer. The main part of this dilatometer was a linear differential transformer, the core of which was connected to the adsorbent by means of a rod. Any changes in the geometric dimensions of the adsorbent changed the position of the core in the transformer, which affected the signal taken from the secondary winding of the transformer. Having calibrated the dilatometer, the adsorption deformation of the adsorbent was determined. Such a comprehensive approach allowed us to significantly detail the description of the adsorption process for the studied system.
Keywords: adsorption, adsorbent, adsorption isotherm, calorimetric heat of adsorption, adsorption deformation of the adsorbent, dilatometric method

A.V. Tvardovskiy
Tver State Technical University

Abstract: As is known, adsorption hysteresis is a phenomenon that occurs in physical adsorption processes. In this case, the amount of adsorbed substance differs when adding gas (or vapor) and when removing it, i.e. the adsorption branch does not coincide with the desorption branch during experimental measurement of the adsorption isotherm. The literature provides various reasons for this phenomenon: surface and volume phase transitions in the adsorbate; capillary effects in pores of a certain geometry; elastic and plastic deformation of the adsorbent; the presence of a potential energy barrier for the penetration of the adsorbate into the pores, etc. It is clear that in the case of capillary condensation in the pores of the adsorbent, irreversibility of adsorption is indeed characteristic, i.e. the adsorption and desorption curves do not coincide, and these curves form an adsorption hysteresis loop. It is assumed that in other cases the key role in this effect is played by the adsorption deformation of the adsorbent. The use of a modern highly sensitive dilatometer for recording the curves of adsorption and desorption deformation of the adsorbent, along with the traditional measurement of adsorption isotherms, allows us to study this problem in detail. The results of such a comprehensive approach are presented in this paper. Organo-substituted Pyzhevsky montmorillonite and organo-substituted synthetic fluorohectorite were used as adsorbents. Their interaction with hexane and benzene vapors was studied. A detailed analysis of the obtained data is carried out, allowing us to state that the adsorption deformation of the adsorbent plays a key role in the adsorption hysteresis.
Keywords: adsorption, adsorbent, adsorption isotherm, adsorption hysteresis, adsorption deformation of the adsorbent, dilatometric method

I.G. Shebzukhova¹, L.P. Aref`eva^2
1 Kabardino- Balkarian State University named after H.M. Berbekov
² Don State Technical University

Abstract: Technological processes and operational properties of materials largely depend on the surface properties of metals. Aluminum and lead are widely used in various industries, but there is still a wide range of experimental data on their surface energy and electron work function. Changes in these characteristics with increasing temperature have not been studied in practice. In this regard, the surface energy of the faces of aluminum and lead crystals was evaluated using the electronic statistical method, taking into account the contributions of dispersion, polarization, and oscillation corrections, as well as thermal effects. The analytical relationship between the surface energy and the electron work function of densely packed single crystal faces allowed us to evaluate the effect of temperature on the anisotropy of the properties under study. A comparison of changes in the tensile strength and surface energy of aluminum and lead polycrystals has shown the possibility of developing an analytical relationship between these values, which will make it possible to predict the behavior of metals and alloys, including materials with reduced dimensionality. However, in addition to the relationship between the surface energy of polycrystals, the cohesion energy, and the tensile strength, it is necessary to take into account other components of the fracture energy of materials.
Keywords: surface energy, electron output work, anisotropy, electron statistical method, tensile strength, aluminum, lead