Vol 43, No 7 (2017)

The technique of analytical reversal of a hypersingular equation is used to solve the equation of an impedance vibrator antenna. A numerical method for solving the equation is developed, and its efficiency is demonstrated.

An experimental technique was developed for reconstructing the surface profile of a drop of liquid on a horizontal solid substrate using refractive images obtained by means of layer-by-layer laser probing. Examples of typical images obtained by vertical probing of drops of distilled water with a volume of 10–15 μL using a collimated laser beam are given. The inverse refraction problem is solved, and the global shape of the drop surface is restored. It is noted that, to restore the surface parameters of a microrelief, it is effective to use probing with a wide collimated beam.

The characteristics of signal registration of nuclear-magnetic resonance in a weak magnetic field from a small volume of an investigated medium have been considered. The consideration of the characteristics revealed in a new developed design of a magnetic system for a compact nuclear-magnetic spectrometer, and also in the NMR-signal registration method allows significant extension of the possibility to use the phenomenon of nuclear-magnetic resonance for the study of condensed medium in an express mode. The experimental results of different media studies have been presented.

It is established that the critical shear stresses under tensile strain of Al_0.3CoCrFeNi high-entropy alloy single crystals are independent of crystal orientation. It is shown that the development of the planar dislocation structure in Al_0.3CoCrFeNi single crystals at T = 296 K leads to a decrease in the ratio between the strain-hardening coefficient and shear modulus relative to that the value for Cu single crystals, where the cellular dislocation structure is developed at the similar staking fault energy.

The stability of a supersonic near-wall flow above a finely corrugated plate in the incident flow inside an aerodynamic tube with a Mach number of 6 has been studied experimentally. We obtained the perturbation growth coefficients that show that a wavy wall stabilizes the flow in the boundary layer with respect to the perturbations of the second mode, but it destabilizes the perturbations of the first mode. A bispectral analysis of perturbations is carried out. It is shown that with decreasing pulsation amplitudes of the second mode, nonlinear processes in the boundary layer behind the wavy surface become strongly suppressed: there is no harmonic resonance of the second mode, and nonlinear processes in the low-frequency region of the spectrum have an interaction amplitude that is an order of magnitude smaller and a much more limited frequency range. Due to the stabilizing effect of a wavy surface on perturbations of the second mode, the onset of a laminar-turbulent transition is generally delayed.

The mechanisms of evaporation of silicon from the surface of silicon carbide (SiC) grown by atomic substitution are studied. It is assumed that the emergence of elastic deformations at the stage of cooling of the sample with a SiC film is one of the causes of Si evaporation. It is demonstrated theoretically that elastic stresses induce the mechanochemical Gorsky effect in the SiC layer. This effect initiates redistribution of Si and C atoms in the SiC layer, which results in violation of the stoichiometry of films and asymmetry of their composition over the SiC-layer thickness. A method for growing epitaxial SiC films with a homogeneous composition and a low density of silicon vacancies from a gas mixture of carbon monoxide and trichlorosilane is proposed.

We have experimentally studied subsonic laminar gas jets escaping from a flat nozzle with dimensions of 83.3 × 3600 μm. Reynolds numbers calculated for the given nozzle height and average flow velocity at the nozzle output edge ranged within 58–154. The working gas was air at room temperature. Distributions of the gas velocity and its pulsation along the jet axis were determined. It is established that the obtained characteristics of laminar subsonic microjets are fundamentally different from those of macroscopic turbulent jets. Based on the results of velocity pulsation measurements, it is shown that a laminar–turbulent flow transition is present and its position is determined.

Bistable magnetic states with two equiprobable orientations of the magnetization vector (corresponding to opposite polarities of a permanent magnet) exist in (NdSmDy)(FeCo)B magnetic alloy in the vicinity of a spin-reorientation transition. A critical value of the magnetic field strength ~1 kOe is determined, at which switching of these bistable magnetic states takes place. It is established that the polarity of polycrystalline sintered magnets of the Nd₂Fe₁₄B family in the vicinity of a spin-reorientation transition can be stabilized by a small external bias magnetic field, which opens up new possibilities for using these magnets in cryomagnetic systems.

We demonstrate the up-conversion of Tm:LiYF₄ infrared (IR) laser radiation with 1908-nm wavelength into visible light with a spectral maximum at 650 nm by ceramics with a composition of (100 − x)TeO₂−xBaF₂ − 1 wt % HoF₃−yYbF₃, where x = 20, 30, or 40 mol % and y = 0 or 0.5 wt %. The samples of 60TeO₂−40BaF₂ − 1 wt % HoF₃ − 0.5 wt % YbF₃ exhibited anti-Stokes luminescence at a threshold radiation power density of 1.0−1.5 W cm⁻².

The process of field-assisted sintering of nanostructured thermoelectrics for the formation of effective materials for alternative power engineering has been numerically simulated. Functionally graded thermoelectrics and segmented thermoelement branches have been sintered in a temperature-gradient field. Modified die tooling elements are proposed that allow the desired thermal conditions to be created for the sintering of inhomogeneous effective materials.

It is shown that molecular-beam-epitaxy technology can be used to fabricate heterostructures for quantum-cascade lasers of the wavelength range of 7–8 μm with an active region comprising 50 cascades based on a heterojunction of In_0.53Ga_0.47As/Al_0.48In_0.52As solid solutions. The optical emission is obtained using a quantum-cascade design operating on the principle of two-phonon resonance scattering. The properties of heterostructures were studied by the methods of X-ray diffraction and transmission electron microscopy, which showed their high quality with respect to the identical compositions and thicknesses of all 50 cascades. Stripe-geometry lasers made of these heterostructures exhibited lasing with a threshold current density below 1.6 kA/cm² at a temperature of 78 K.

We have studied the concentration dependences of the yield of electron-stimulated desorption (ESD) of Cs and Na atoms adsorbed on nonmetallic coatings grown on W substrates. It is established that ESD processes occur at the interface between the adlayer and underlying nonmetallic coating. The decay in the concentration dependence of the ESD yield of alkali-metal atoms observed after reaching their monolayer coverage is related to a decrease in the amount of exciting electrons attaining this interface.

We have studied the phase composition of thin polycrystalline tin oxide films deposited on silicon substrates by reactive RF magnetron sputtering. The films are sensitive to ethanol vapors at 38°C. They consist of SnO₂ crystallites with the average grain size of about 7 nm, covered with a nanodimensional layer of amorphous SnO phase. It was found that the volume fraction of the former phase is about 95% and that of the latter phase is about 5%. Calculations of the film texture indicate the predominant growth of SnO₂ crystals along 〈110〉, 〈211〉, and 〈310〉 crystal directions perpendicular to the substrate.