Том 44, № 6 (2018)
- Год: 2018
- Статей: 28
- URL: https://journals.rcsi.science/1063-7850/issue/view/12893
Article
Instability of the Flow in a Spherical Layer under Torsional Oscillations of the Inner Boundary
Аннотация
The instability of viscous incompressible fluid flows caused in a thin spherical layer by torsional oscillations of the inner sphere in a thin spherical layer with respect to the sate of rest is studied numerically. It has been established that an increase in the frequency of torsional oscillations leads to a change in the mode of the instability, with a transition from secondary flows in the form of Taylor vortices to structures not observed earlier. The revealed instability is observed in the frequency range from 0.61 to 2.45 Hz or, if the wavelengths are taken relative to the layer thickness, from 0.67 to 1.33.
The Incredible Effect of Heat Load Increase Rate on the Specific Heat Flux at Boiling of Some Nanofluids
Аннотация
Boiling curves for different heat-load increase rates (HLIRs) are obtained for distilled water, tap water, and two nanofluids (NFs) based on a natural mixture of montmorillonite and attapulgite (AlSi-7) and titanium dioxide (NF-8). An unusually strong influence of the HLIR on the thermal parameters of NFs is detected upon boiling of AlSi-7 based on a mixture of highly anisometric nanoparticles of aluminosilicates of various morphologies capable of forming nanostructured openwork “architectures” on the heater surface. An explanation of the reasons for the observed effect is proposed. The data obtained indicate the existence of an optimal HLIR at which the structure of the deposit appearing on the heating surface during the boiling of an NF is most favorable for heat exchange.
Charge Carrier Transport and Deep Levels Recharge in Avalanche S-Diodes Based on GaAs
Аннотация
Carrier transport and deep-level recharging in semiconductor avalanche S-diode structures have been investigated. Gallium-arsenide n+–π–ν–n structures with the diffusion distribution of deep iron acceptors have been studied. It has been found by solving the continuity and Poisson equations with the use of a commercial software that the electron injection affects the avalanche breakdown voltage and the spacecharge region broadens due to capture of avalanche holes on negative iron ions in the π-region. It is demonstrated by comparing the results of numerical calculation with the experimental data that the S-shaped I–V characteristic of the diffusion avalanche S-diodes cannot be explained within the previously proposed mechanism of capture of avalanche holes on the deep iron levels.
The Structure, Dielectric, and Optical Properties of c-Oriented SBN-50 Films Grown on Pt/Al2O3 Substrate
Аннотация
Studies of the structure, dielectric and optical properties of thin films of a relaxor ferroelectric Ba0.5Sr0.5Nb2O6 grown on Pt(111)/Al2O3 (c-cut) substrate by high-frequency RF-sputtering in oxygen atmosphere, were performed. X-ray diffraction studies showed that Ba0.5Sr0.5Nb2O6 films are c-oriented. The unit cell parameters in the tetragonal approximation were c = 3.949(1) Å and a = 12.38(1) Å. It was established that the ferroelectric–paraelectric phase-transition temperature rises and optical anisotropy increases in the object in comparison with the bulk material. Reasons for the elucidated regularities are discussed.
An Experimental Study of the Influence of an External Signal on the Generation Mode of a Megawatt-Power Gyrotron
Аннотация
The frequency capture of radiation by an external signal was experimentally studied for the first time for a megawatt-power gyrotron. An external signal from the magnetron at a frequency of 35 GHz entered the working space of the gyrotron through a synthesized two-mirror quasi-optical converter that allowed transforming the input signal into a working mode. This converter has been recently developed and implemented in the Institute of Applied Physics, Russian Academy of Sciences. The radiation spectra were experimentally obtained in modes of frequency capture and frequency beats. The dependence of the frequency capture bandwidth on the power of the external signal was found. Comparison of experimental results with the data of numerical modelling of gyrotron equations with an external signal showed their good agreement.
Numerical Simulation of the Current Dependence of Emission Spectra of High-Power Pulsed Lasers Based on Separate-Confinement Double Heterostructures
Аннотация
A new physical model is proposed for describing the observed current dependence of the emission spectra of high-power pulse lasers based on double separate-confinement heterostructures. The model takes into account the presence of isotype n–n heterojunctions at the interface between the waveguide and the active region. A numerical simulation of the lasing processes was performed. The resulting analytical current dependence of the 2D concentration of holes in the quantum well made it possible to evaluate for the first time the current dependence of the short-wavelength limit of the lasing spectrum. A good agreement was reached between the calculated and experimental characteristics.
The Mechanism of Generation of Singlet Oxygen in the Presence of Excited Nanoporous Silicon
Аннотация
A theoretical analysis of the mechanism of generation of singlet oxygen in the presence of photoexcited nanoporous silicon is presented. It is demonstrated that the mechanism of generation of singlet oxygen is based on nonradiative energy transfer from nanoporous silicon to an oxygen molecule by the exchange mechanism. An analytical expression of the probability of energy transfer from nanoporous silicon to an oxygen molecule is obtained, and a numerical estimate of this process is given. The numerical estimation is of the order of 103–104 s–1, a value that agrees rather well with the experiment.
Synthesis of Porous Tungsten from Tungsten–Cadmium Film Coatings
Аннотация
Solid solutions (alloys) with a Cd concentration of 50.3–76.3 at % were synthesized for the first time in the form of coatings by ion–plasma sputtering and codeposition of ultrafine W and Cd particles. When coatings were formed by tungsten and cadmium nanolayers, the components dissolved mutually to produce solid solutions of one metal in the other. A solid solution of cadmium in tungsten was synthesized at Cd concentrations up to 60.9 at %. At a cadmium concentration of 68.6 at % in the coating, the crystalline structure of cadmium with an admixture of amorphous tungsten was produced. At 800°C, tungsten evaporated from tungsten–cadmium coatings to form porous tungsten. The results of examination of materials fabricated on the basis of porous tungsten are planned to be used in practice.
Galvanomagnetic Properties of Bi95Sb5 Thin Films on Substrates with Different Thermal Expansions
Аннотация
Results of an investigation of galvanomagnetic properties of Bi95Sb5 block thin films on substrates with different coefficients of thermal expansion covered with polyimide are presented. The difference between thermal expansions of the film material and the substrate was found to have a strong effect on the films’ galvanomagnetic properties. Analysis of the properties of the films using the two-band model showed that the concentration and mobility of the charge carriers in the Bi95Sb5 films are related to the coefficient of thermal expansion of the substrate material.
Solving the Electrodynamic Problem for a Microstrip Radiating Structure with a Chiral Substrate
Аннотация
We present a solution of the electrodynamic problem for a microstrip radiating structure with a substrate of a chiral metamaterial using the singular integral representation of the field, which in turn is reduced to a singular integral equation with the Cauchy-type singularity relative to the longitudinal component of the surface current density. Graphs of the current distribution for different types of substrates and the chirality parameters of a substrate are given.
Electric Currents Induced in Plasma by Ion-Acoustic Solitons: Allowance for Trapped Electrons
Аннотация
The currents induced by ion-acoustic solitons have been studied in a two-component magnetic–hydrodynamic approximation (MHD model) taking into account the trapped electrons. The solitons have been shown to excite unipolar nonpolar pulses of the ion and electron currents, and the established mechanisms of their excitation are. The spatiotemporal characteristics of the current pulses have been calculated, and the requirements for the spatiotemporal resolution of the experimental equipment necessary for detection of plasma currents induced by solitons have been determined. It has been shown that the solitons are the effective mechanism of plasma current generation.
The Effect of Constant Magnetic Fields on the Dynamics of Molten Zones in the Field of Structural Inhomogeneity of Silicon
Аннотация
The effect of constant magnetic fields on the formation and dynamics of molten Al–Si inclusions in silicon in the field of structural inhomogeneity of a crystal (the dislocation density-gradient field) is considered. The migration rates of liquid inclusions in crystals have been found experimentally under conditions of prevailing melting–crystallization mechanisms at the phase interfaces between the melt and matrix before and after magnetic exposure. It has been found that a preliminarily exposure of dislocation silicon samples in a constant magnetic field leads to an increase in the displacement speed of molten zones in the density-gradient field.
Kinetic Model of the Formation of Protein Crystals in Capillaries by Counterdiffusion
Аннотация
A kinetic model of crystallization of lysozyme is proposed, and computer calculations of this process are carried out. The conditions for the formation of such crystals are determined. Under these conditions, individual crystals were grown that were suitable for X-ray examination. The developed model enables prediction of the quantity, size, and place of crystal nucleation inside the capillary.
High-Resolution Raman Spectra of Metformin
Аннотация
High-resolution spectra of molecular vibrations in metformin, one of the most widely used antidiabetic drugs, which also possesses antiaging and antitumor properties, have been obtained by a high-sensitivity Raman-scattering technique. It is established that rather narrow (4–10 cm–1) spectral lines observed in a 300–1200 cm–1 range can be used for separating spectral components corresponding to the vibrations of particular functional groups. This circumstance makes metformin an interesting model for studying molecular biosensors.
Regimes of Intensified Heat Transfer during Evaporation of Thin Horizontal Liquid Layers at Reduced Pressures
Аннотация
The influence of structures formed during the evaporation and boiling of a liquid (n-dodecane) in thin horizontal layers on heat transfer has been analyzed. Structures shaped with the shapes of funnels and craters are formed at low pressures in liquid layers with thicknesses above the capillary constant under the action of a vapor recoil force. Increasing pressure leads to the onset of bubble boiling. It is established that the formation of these structures in the regime of intense evaporation at low reduced pressures leads to an about 70% increase in the heat-transfer coefficient at analogous layer thicknesses in comparison to the case of bubble boiling.
The Effect of Boron on the Structure and Conductivity of Thin Films Obtained by Laser Ablation of Diamond with Deposition at 700°C
Аннотация
Structural features of CBx films obtained by pulsed laser ablation of targets made of pressed diamond powder with boron-powder additions at B/C atomic ratio of x = 0.33 have been studied. The films were deposited on heated substrates, so that diffusion processes involving C and B atoms on the surface and in the volume of films were possible. Selected conditions of film deposition ensured their effective doping with boron (0.4 ≤ x ≤ 0.6). The incorporation of B atoms was accompanied by the formation of B–C chemical bonds, whereas the formation of sp2 graphite bonds and their ordering in clusters with laminar packing was suppressed. The films possessed very low resistivity (~1.4 mΩ cm) at room temperature and exhibited metallic type of conductance on decreasing the temperature to 77 K.
A Singular Integral Equation for the Current Density on the Surface of a Strip Dipole in the Free Space
Аннотация
The problem of current density distribution over the surface of a strip dipole antenna occurring in the free space is reduced to a singular integral equation with the Cauchy singularity. Patterns of current distribution on the antenna surface for various values of the dipole length are presented.
The Formation of Hollow Lead Structures on the Surface of PbSe Films Treated in Argon Plasma
Аннотация
Conditions for ion sputtering of a PbSe/CaF2/Si(111) epitaxial system in high-density inductively coupled plasma of high-frequency low-pressure discharge in argon have been established that ensure the formation of submicron-sized hollow lead structures on a lead-selenide surface. The surface was plasma-treated for time periods within 60–240 s at low energy (20–30 eV) of Ar+ ions, which is close to their sputtering threshold energy. The properties of the obtained material were studied by the techniques of scanning electron microscopy and energy-dispersive X-ray microanalysis. It is shown that the characteristic size, shape, and density of surface structures can be varied within broad limits depending on the time of plasma treatment and temperature of the material surface. Physical processes responsible for the formation of hollow lead structures under the proposed conditions of plasma sputtering are considered.
Graphene As a Sensitizing Additive to High-Energy Cobalt Salt for Enhanced Initiation by a High-Current Electron Beam
Аннотация
We have studied the influence of graphene additives on the initiation of high-energy cobalt salt by a high-current electron beam of nanosecond duration. The high-energy composite was prepared by ultrasonic mixing of 80 wt% cobalt amminate (NCP) and 20 wt % graphene. It is established that the use of graphene as a sensitizing additive in a high-energy composite decreases the necessary energy parameters of the initiating electron beam and increases the energy characteristics and combustion rate of high-energy materials.
Semipolar GaN Layers Grown on Nanostructured Si(100) Substrate
Аннотация
We propose a new method for growing semipolar GaN films on a Si(100) substrate with an array of sub-100-nm-sized V-grooves formed on the surface. It is shown that, using such a nanostructured substrate for metalorganic hydride vapor-phase epitaxy, it is possible to obtain GaN (101̅1̇) epilayers deviating by an angle of about 62° from the polar direction and having an X-ray rocking curve with a minimum FWHM value of ωθ ~ 60 arcmin.
Synthesis of Highly Oriented Zinc-Oxide Films on Amorphous Substrates by the Method of Direct-Current Magnetron Sputtering
Аннотация
We describe the technology of obtaining highly oriented zinc-oxide (ZnO) films on amorphous substrates at high growth rates (up to 7 nm/s) by means of direct-current magnetron sputtering. It is suggested to optimize the substrate position with respect to magnetron and consider the floating potential to which the substrate is charged in magnetron discharge plasma as one of the main technological parameters. Electrondiffraction study of the structural characteristics of the obtained ZnO films showed that increase in the substrate temperature was accompanied by transformation of the crystallite shape from platelike to columnar.
An Experimental Study of the Influence of Added Nanoparticles on the Filtration of Microsuspensions via Porous Media
Аннотация
We have experimentally studied the filtration of microsuspensions with added silica (Aerosil) nanoparticles via porous media of variable permeability. The concentration of added nanoparticles in solution was changed from 0.25 to 4 mass % and their average size was varied from 5 to 100 nm. Dependences of the filtration losses of microsuspension on the concentration and size of nanoparticles and the permeability of porous medium variable have been determined.
Obtaining Iron-Oxide Films on Sapphire Substrates
Аннотация
Iron-oxide films on sapphire substrates were obtained using laser radiation with 1064-nm wavelength and studied by methods of scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission Mössbauer spectroscopy. The obtained Fe2O3 films probably occur in a superparamagnetic state and may find application in gas sensors and various magnetic devices.
Femtosecond Laser Writing of Waveguide Microstructures in Pb(Zr,Ti)O3 Films and Their Characterization by the Nonlinear Optical Method
Аннотация
Femtosecond laser writing of ferroelectric one-dimensional microstructures possessing waveguide and nonlinear optical properties in Pb(Zr,Ti)O3 films is considered, and the passage of light beams via obtained waveguide structures is traced. The optimum speed and power density of the writing laser beam are within 50–500 μm/s and 1–1.5 MW/cm2, respectively. A two-stage method of writing is proposed, which provides for improved nonlinear optical properties of obtained waveguide microstructures.
Short-Range Order and Charge Transport in SiOx: Experiment and Numerical Simulation
Аннотация
The structure of nonstoichiometric silicon oxide (SiOx) has been studied by the methods of highresolution X-ray photoelectron spectroscopy and fundamental optical-absorption spectroscopy. The conductivity of SiOx (x = 1.4 and 1.6) films has been measured in a wide range of electric fields and temperatures. Experimental data are described in terms of the proposed SiOx structure model based on the concept of fluctuating chemical composition leading to nanoscale fluctuations in the electric potential. The maximum amplitude of potential fluctuations amounts to 2.6 eV for electrons and 3.8 eV for holes. In the framework of this model, the observed conductivity of SiOx is described by the Shklovskii–Efros theory of percolation in inhomogeneous media. The characteristic spatial scale of potential fluctuations in SiOx films is about 3 nm. The electron-percolation energy in SiO1.4 and SiO1.6 films is estimated to be 0.5 and 0.8 eV, respectively.
Reducing Power Losses in Corona Discharge under Rainfall Conditions
Аннотация
Aluminum wire of AS300/39 grade with coatings synthesized by micro-arc oxidation in an electrolyte containing additives of graphene oxide and carbon nanotubes was tested under conditions of high-voltage corona discharge in the presence of a model rainfall. It is established that hydrophilic properties of the proposed coating provide for up to 40% reduction in the level of corona power losses.
Reciprocal-Space Maps of X-Ray Diffraction Intensity Distribution and Their Relation to the Dislocation Structure of Epitaxial Layers
Аннотация
X-ray diffraction (XRD) in asymmetric Bragg geometry was measured and XRD intensity distribution maps in the reciprocal space were constructed for GaN epitaxial layers with various degrees of structural perfection grown on c-sapphire substrates. It is established that equal-intensity lines (isolines) related to a regular system of perpendicular rectilinear threading dislocations are extended in a direction parallel to the surface. For a more chaotic distribution of dislocations with a large fraction of horizontal fragments, these isolines are rotated toward a direction perpendicular to the reciprocal lattice vector, although they still not attain a limiting position characteristic of the ideal mosaic crystal.
Electron-Stimulated Desorption of Cesium Atoms from Graphene-Covered Iridium
Аннотация
Electron-stimulated desorption (ESD) of cesium atoms from graphene on a metal (iridium) substrate has been directly observed for the first time using the time-of-flight technique with a surface-ionization detector. The appearance threshold of the yield of Cs atoms from the sample surface at 160 K was 20 eV. An additional ESD peak of Cs atoms was observed at an electron energy of 56 eV. The observed ESD of Cs atoms from graphene is related to the nonmetal nature of this carbon film on the metal substrate surface.