Vol 63, No 12 (2018)

The redistribution of impurity nanoparticles in a colloid mixture has been considered for two cases: (i) the mixture settles in a narrow channel first in the horizontal and then in the vertical position and (ii) the mixture is transferred from a vessel to a narrow channel. A flow that brings the system to stable equilibrium with a true vertical density gradient distorts the initially barometric distribution of impurity nanoparticles. It has been shown by numerical simulation that the initial inhomogeneity of the mixture to a great extent continues to persist in both cases.

The rotation of a spherical particle in a constant electric field (an effect found earlier) has been analyzed. The particle is illuminated to induce the electric dipole moment of the sphere. The dynamics of the rotation effect has been considered in general terms to refine conditions for adiabatic rotation. The features of the particle’s nonadiabatic rotation have been demonstrated with a sphere placed in a medium with an infinitesimal viscosity. It has been shown that the nonadiabatic rotation dynamics to a great extent depends on a relationship between the electrical and photoinduced dipole moments of the sphere. The rotation dynamics of a particle with a slightly nonspherical shape has been briefly analyzed.

The main problems of the development of 3D printers for machine engineering are related to low rate of printing and low quality of fabricated items. The problems can be solved using a new principle of high-efficiency 3D printing for steel melt based on the compression of the melt jet by magnetic field of flowing current and an increase in the crystallization temperature. Under certain conditions, the heat liberation of current can be compensated for using alternative processes. Ranges of parameters at which the effect can be implemented are determined.

We propose measuring the Bohm coefficient, thickness of the space-charge probe layer, and ion mass in Maxwellian low-pressure plasma based on the results of standard probe measurements using the Bohm effect, Boltzmann law, and “3/2” law in the case of their validity for the studied plasma. This possibility was implemented by comprehensive probe diagnostics of high-frequency xenon plasma of an induction discharge at a pressure of 2 mTorr. Analysis of the obtained results showed that these laws and the Bohm effect are valid with “engineering” accuracy, which made it possible to find Bohm coefficient C_BCyl ≈ 1.23 for a cylindrical probe, which served as the basis of the proposed method.

An extended interpretation of the Wolkenstein–Ptitsyn formula for temperature band δT_g, characterizing a range of transition from liquid to glass, is considered using the Razumovskaya–Bartenev concept. The derivation of this formula proposed by the authors does not depend on the specific type of temperature dependence of relaxation time. For silicate glasses, calculation δT_g by this formula agrees with the calculation by the Williams–Landel–Ferry relation and with the left side of glass transition equation qτ_g = δT_g, where q is the cooling rate and τ_g is relaxation time. Based on the experimental data, the calculation of parameters of the Razumovskaya–Bartenev equation for silicate glasses and amorphous polymers is carried out for the first time.

Chemical interactions in the Li₂O−B₂O₃−Nb₂O₅ system, as well as certain features of crystallization of LiNbO₃ crystals growing from melts containing nonmetal impurities, are considered. It is shown that boron changes the structure of the melt and has a significant impact on the structure and physical characteristics of LiNbO₃ : В crystals, practically not entering the lithium niobate structure. Notable changes and certain features were found in the Raman spectra of grown LiNbO₃ : В crystals, which indicates changing the sequence of the main cations and vacancies along a crystal polar axis and distortion of the oxygen octahedrons. Meanwhile, the distortion of the oxygen octahedrons is anisotropic. LiNbO₃ : В crystals have a higher structural homogeneity than congruent crystals, and are close in the number of NbLi defects to a crystal with a stoichiometric composition, differing from it by a substantially less photorefraction effect.

In this paper, we present the results of numerical simulation of explosive formation of a high-speed compact element from copper, steel, and aluminum cumulative liners combining the shape of a hemispherical segment smoothly converting into a cylindrical surface (“hemisphere–cylinder” liners). The problem is solved in a two-dimensional axisymmetric setting considering the limiting parameters of the dynamic stress-strain state causing plastic flow and destruction of the cumulative liner material. The original model of the functioning of cumulative shaped charge, which determines the effect of individual elements of the cumulative liners, including the difference in numerical characteristics of their physicomechanical properties and critical destruction conditions, on the final parameters of the high-speed compact element, was used. The plastic properties of the material and the critical conditions for its destruction were found not to affect the final velocity of the formed high-speed compact element, but they affect its shape, dimensions, and mass.

Electron microscopy is used to study samples of an aluminum matrix composite material with multiwalled carbon nanotubes (MWCNTs). The features of microstructure changes occurring during the preparation of a composite in a spark plasma sintering (SPS) setup are identified. The results of the simulation of the SPS process are presented taking into account the high electro- and heat-conducting properties of MWCNTs, which qualitatively explain the differences in the sintering processes of pure aluminum and composite samples.

A technique is developed to determine the freezing time, thermal resistance, heat transfer coefficient, and cooling power of water droplet on the surface of studied materials by means of video recording in the long-wavelength infrared range. These parameters are found for water droplets on a hydrophobic coating based on organosilicon polymer filled with carbon nanotubes as well as on polyethylene terephthalate film. On hydrophobic coatings the water droplets froze 1.5–4 times slower than on polyethylene terephthalate film, which is explained by an increase in the contact angle and thermal resistance between the droplet and surface of the coating. Correlation between the freezing time and thermal resistance reveals that the heat flux from the droplet towards the material surface is crucial for the freezing time. In tests, partial adiabatic droplet freezing with the following complete isothermal freezing were observed during a single freezing process. The studied hydrophobic coatings may potentially be used as anti-icing coatings.

Bilayer CoSi₂/Si/CoSi₂/Si system has been obtained by the method of ion implantation, and optimal conditions for implantation and postimplantation annealing have been found. It has been shown that this system forms when the high and low ion energies differ by no less than 15–20 keV. The structures have smooth surface and high crystallinity.

Various methods of binding rare-earth metal ions with nanoparticles make it possible to obtain materials with new properties. The “green” synthesis method has been used for obtaining silver nanoparticles functionalized with Dy³⁺ ions. The absorption and photoluminescence spectra of colloidal solutions have been measured, and the characteristics of resultant Ag nanoparticles have been analyzed using an electron microscope. The results of observations are in good agreement with the estimates obtained from the absorption spectra using the classical model. The nanoparticle shape (mainly spherical), size (d = 70 nm), and volume fraction (f = 6 × 10^–4) of silver in the colloidal solutions have been determined. Functionalizing silver nanoparticles with dysprosium ions with the help of the technique developed in this study can be extended to other rare-earth elements.

It is shown that damages accumulating in a rotor steel after more than 20 years in service under creep conditions at high temperatures reveal themselves in the change of acoustic emission (AE) parameters as compared with the initial structural state. The formation of clusters of AE activity is observed in zones 10²–10⁴ μm in size inducing a similar process by the shear mechanism in a neighbor zone with a larger effective elastic modulus. Hence, AE revealing of clusters of plastic deformation takes place.

The necessity of recording nuclear magnetic resonance (NMR) spectra in weak magnetic fields during the express control of liquid media has been substantiated, and the conditions in which such recording is possible have been determined. A new scheme of a compact NMR spectrometer intended for such measurements has been proposed. The results of experimental investigation of some substances based on the developed technologies are reported.

The persistence of molecules to destruction under the action of low-energy (0–15 eV) electrons is investigated for a representative series of aromatic compounds of interest to organic electronics and photonics. The energy regions of the most effective interaction of free electrons with isolated molecules, which leads to the dissociative decay of molecules due to the formation of short-lived negative molecular ions unstable to fragmentation, are determined using resonance electron capture negative ion mass spectrometry. The characteristic fragmentation channels for molecular ions are revealed for some groups of compounds, and the threshold energies of the most significant fragmentation processes are estimated. Polycyclic aromatic hydrocarbons and oligophenyls are stable to the action of electrons in the electron energy range ~0–3 eV, above which these compounds are decomposed into the single channel of hydrogen atom detachment. For compounds with heterocycles (oxadiazole and maleimide derivatives) in their structure, the stability range narrows down to ~0–1 eV. Electrons with energies exceeding this range initiate the decay of molecules (anions) via various channels among which the detachment of a cyanate anion from a heterocyclic nucleus is the most intense and destructive process.

Electron-optical systems are synthesized to form a converging sheet electron beam with a compression of 15 and 20, a cross section of 0.05 × 2 mm, and a current density of 100 A/cm² in the presence of complete magnetic shielding of field-emission cathode. Deformation of low-perveance flux in the presence of magnetic field in the drift tunnel of slow-wave structure is analyzed with the aid of 3D computer simulation of the electron-optical system with the sheet electron beam.

Transition of field-emission electrons to the runaway regime in the region of enhanced electric field determined by the configuration of a microtip on a cathode is studied at several pressures of gas medium. The problem is solved using simulation of electron motion in the presence of nonuniform electric field with the aid of the Monte Carlo procedure in the 2D configuration. Nitrogen is used as a working gas. Passage through a relatively small region of the enhanced field in the vicinity of the microtip may substantially facilitate electron escape to the runaway regime, especially, at pressures of greater than 10 atm. In our opinion, the resulting runaway electrons may provide preionization of gas medium and formation of the initial stage of a 3D discharge.