Vol 45, No 11 (2019)

The peculiarities of a convective regime aroused due to instability of mechanical equilibrium of a ternary helium–argon–nitrogen gas mixture at isothermal diffusion are experimentally investigated. The effect of the inclination angle of diffusion channel on the intensity of convective flows is considered. The intensity of the partial transport of components in the inclined channel was measured at increased pressures. The nonlinear dependence of the mixing rate of components on the inclination angle is discovered. It is established that the observed transport of components atypical for diffusion is possible at a certain composition of the gas mixture.

We studied optically induced effects in ultrafine Al₂O₃–BeO ceramics irradiated with a high-dose pulsed electron beam (130 keV). Upon optical excitation of ceramics at 470 nm, optically stimulated luminescence is observed due to the depletion of traps associated with thermoluminescence peaks at 380 and 530 K. These traps are characterized by a complex energy spectrum with a nonuniformly distributed photoionization cross section. The prospects of using optically stimulated luminescence of Al₂O₃–BeO ceramics in high-dose dosimetry (3–30 kGy) have been proved.

Silicon nanoparticles with sizes of 50–300 nm have been formed via picosecond laser ablation of porous silicon and silicon microparticles in water using various targets. Raman spectroscopy has revealed the presence of a low amount of the amorphous phase in the obtained particles (10–12%) prepared from micro- and mesoporous silicon layers and almost a zero degree of crystallinity in the case of laser fragmentation of silicon micropowders in water. The results are promising for further application of the nanoparticles in photonics and biomedicine.

A high-electron-mobility transistor (HEMT) based on InAlAs/InGaAs/InP heterostructures possessing higher breakdown characteristics is developed. An InGaAs composite channel structure, combined with completely selective forming of the double recess structure, is used in the devices. HEMTs with a T‑shaped gate 120 nm in length consist of four fingers, each 30 μm in width, exhibit a maximum transconductance of  810 mS/mm, 460-mA/mm maximum density of drain current and 8- to 10-V drain-to-gate breackdown voltage. The current-amplification cut-off frequency of transistors is over 115 GHz. Due to the enhanced breakdown voltage and the forming of a double recess structure by selective etching, the elaborated transistors are promising for application in the monolithic integrated circuits of the millimeter-wave medium power amplifiers.

An anomaly appearing in the photovoltaic characteristics of triple-junction GaInP/GaAs/Ge and of the corresponding double-junction GaInP/GaAs solar cells at ultrahigh concentrations (more than 2000 suns) of incident light has been studied. The light current–voltage characteristics at various concentration ratios of incident light and the dependence of the open-circuit voltage on the photogenerated current were analyzed. It was shown that the anomaly is observed due to the tunnel diode connected back-to-back between the GaInP and GaAs subcells. This diode absorbs photons that pass across the GaInP layers and creates a counter photovoltage.

The results of a study of toroidal Alfvén modes using the multifrequency method of Doppler backscattering at the Globus-M tokamak have been presented. The method for recording Alfvén modes in multichannel probing has been introduced. The possible causes of the observed oscillations of the poloidal velocity of plasma rotation at the frequencies of Alfvén waves have been discussed in detail. The data on the spatial distribution of Alfvén modes have been presented. The recommendations for further development of Doppler backscattering for a more detailed study of toroidal Alfvén modes at tokamaks have been defined.

The microhardness and crack resistance of two main gallium oxide polytypes: metastable α-Ga₂O₃ with a corundum structure and β-Ga₂O₃ (high-temperature phase) with a monoclinic crystal structure, have been studied using a Vickers method of diamond pyramid indentation. As far as we know, this is the first attempt to compare the mechanical properties of the two polytypes of gallium oxide.

The effect of spontaneous reverse transition of a stabilized high-temperature superconductor (HTSC) tape from the normal to resistive state during rapid (jumplike) voltage application and alternating current overload has been observed. It is established for the first time that, at a stabilizing layer thickness above 13 μm, the main heat evolution takes place in the copper stabilizer and this heat is effectively removed to the refrigerant, which significantly decreases the HTSC layer’s overheating. This is accompanied by a drop in the resistance of the secondary resistive state and increase in the duration of its existence. These data allow using the observed effect to increase the time of current limitation and improving reliability of HTSC current limiters.

Anodes manufactured from macroporous silicon have been studied. Galvanostatic cycling tests in half-cells with Li counterelectrode in a regime of charging capacity limited to Q₁ = 1000 mAh/g showed that the inclusion of a modifying cycle with increased time of electrochemical lithiation into the testing schedule allowed the charge/discharge rate to be increased upon return to the initial regime. The results are interpreted in terms of a two-phase model with a sharp a-Li_xSi/c-Si interface. The insertion of a large amount of lithium during the modifying cycle leads to an increase in the thickness of an amorphous layer, within which the insertion and extraction of lithium in subsequent cycles proceed at a higher rate.

Algorithms of cell segmentation on two-dimensional phase images and three-dimensional distributions of a refractive index obtained by means of digital holographic microscopy and tomography are developed. The proposed algorithms are optimized for determining cell morphology characteristics including the cell volume, projection area, and surface area. A comparative analysis of the error of cell volume determination by holographic methods using the proposed cell segmentation algorithms and the standard method of confocal fluorescence microscopy has been performed.

The internal aerodynamics of an isothermal laboratory model of a modified four-vortex furnace for a pulverized coal boiler have been experimentally studied. The digital tracer imaging (particle image velocimetry) technique was used to determine average flow velocity distributions and streamline projections in several sections of the model in various regimes of furnace operation determined by the velocity ratio of flows supplied via primary (main) and secondary nozzles. Conditions for the formation of a stable flow structure with four symmetric conjugated vortices in the entire volume of the furnace, as well as the domain of parameters leading to a nonregular spatial vortex structure, were determined.

Point damage to the surface of ZnS ceramics synthesized by chemical vapor deposition (CVD) was produced by means of either a sharp striker impact or slow indentation of a Vickers pyramid and monitored by measuring time series of strain-induced acoustic emission response. In the case of impact, the duration of acoustic emission was 0.3–0.5 ms, while the indentation was accompanied by a 3- to 5‑ms-long initial active phase of emission followed by the generation of weak sporadic signals over a time period of 80–100 ms. Statistical analysis of the recorded time series showed that the energy distribution in impact-induced acoustic pulses was always random (Poisson’s type), while the signals induced by indenter penetration obeyed a power law of the Gutenberg–Richter type. The different characters of energy release observed for the two types of load application are explained by specific features of the temporal regime of self-organized dislocations, the accumulations of which acted as the centers of microcrack nucleation.

Results of investigations of the influence of heat treatment at elevated temperatures on the elastic and microplastic properties of ultrafine-grained (UFG) titanium processed by severe plastic deformation are discussed. The samples were prepared from various batches of α-titanium (VT1-0, Grade 4, and PT3-V) with different initial polycrystalline structures and impurity compositions. Experiments showed that significant changes in the structure (grain size), elastic properties, and microplasticity characteristics only took place in the α-Ti grade of maximum purity (VT1-0). UFG titanium of Grade 4 and PT3-V showed much higher thermal stability of properties, which was probably related to the presence of large amounts of various impurities.