Volume 42, Nº 7 (2016)
- Ano: 2016
- Artigos: 29
- URL: https://journals.rcsi.science/1063-7850/issue/view/12467
Article
A new permanent-magnet trap for ultracold neutron storage
Resumo
The design of a new permanent-magnet trap for ultracold neutron storage in an experiment on determining the neutron lifetime is presented. Particular attention is paid in this new trap design to the neutron confinement system. Optimization of this system provides an opportunity to raise the strength of the confinement magnetic field and thus increase considerably the number of neutrons stored within the trap and the statistical accuracy of neutron lifetime measurements.
The state of vortex glass induced by creep of vortices in percolation superconductors
Resumo
The influence of magnetic flux creep on the dynamics of vortices in percolation superconductors containing fractal clusters of the normal phase has been considered. Dependences of the resistance of these superconductors on the transport current are obtained for different fractal dimensions of cluster boundaries. It is established that the vortex-glass state is implemented in percolation superconductors with a fractal cluster structure under collective creep of vortices.
Field effect in a graphene oxide transistor for proton and electron–hole conductivities
Resumo
Proton (wet atmosphere) and electron (reduced graphene oxide) conductivities can be observed in graphene oxide films. The field effect in a graphene oxide transistor for different conductivity types has been discovered and investigated.
A magneto-optical scheme of a proton microscope based on the U-70 synchrotron radiograph complex
Resumo
A magneto-optical scheme of a proton microscope based on an active U-70 accelerator radiography complex is presented. The microscope is created in the magnetic structure of the radiography complex only by changing strengths of quadrupole lenses without changing their position. The achieved magnification ratio of the image of the studied object is no smaller than 10 at E = 60 GeV. This is sufficient for a spatial resolution of images of 10–20 μm.
Nanofluids for power engineering: Emergency cooling of overheated heat transfer surfaces
Resumo
The possibility of emergency cooling of an overheated heat transfer surface using nanofluids in the case of a boiling crisis is explored by means of synchronous recording of changes of main heat transfer parameters of boiling water over time. Two nanofluids are tested, which are derived from a mixture of natural aluminosilicates (AlSi-7) and titanium dioxide (NF-8). It is found that the introduction of a small portions of nanofluid into a boiling coolant (distilled water) in a state of film boiling (theater > 500°C) can dramatically decrease the heat transfer surface temperature to 130–150°C, which corresponds to a transition to a safe nucleate boiling regime without affecting the specific heat flux. The fact that this regime is kept for a long time at a specific heat load exceeding the critical heat flux for water and theater = 125–130°C is particularly important. This makes it possible to prevent a potential accident emergency (heater burnout and failure of the heat exchanger) and to ensure the smooth operation of the equipment.
The length distribution function of semiconductor filamentary nanocrystals
Resumo
The length distribution function of semiconductor filamentary nanocrystals is analyzed based on the adsorption–diffusion growth model. It is demonstrated that the asymptotic distribution has a Gaussian shape. If the diffusion flux to the apex comes from the entire lateral surface, the average length increases exponentially with time, and the mean-square deviation is proportional to the average length (exponential growth regime). If the diffusion collection of adatoms is limited to the top of the crystal, the average length increases linearly and the mean-square deviation equals the square root of average length (linear Poisson growth regime). In real-world systems, transition from exponential to Poisson growth occurs at lengths of the order of the diffusion length of adatoms. The dispersion of the distribution is actually defined at the exponential stage. The general classification of length distributions of various crystals is given. It is demonstrated that self-induced GaN- and Ga-catalytic III–V filamentary nanocrystals should be more homogeneous than Au-catalytic ones.
Spectral peculiarities of turbulent pulsations of submerged water jets
Resumo
The spectra of turbulent jet temperature pulsations at 1–40 Hz frequencies have been experimentally studied based on high-speed thermography of the water boundary layer: the region where an impact jet interacts with a surface transparent to IR radiation, as well as the near-wall region where two submerged jets interact in a disc-shaped tee-joint. It has been indicated that the slopes of the spectra of impact submerged jet turbulent pulsations are close to–5/3 and a double inertial interval exists in a quasi-2D turbulent flow that is formed when two jets mix.
High-voltage thermionic polaron emission in the presence of adsorbed nonmetal nanofilm on a cathode
Resumo
The equation of thermionic emission has been derived, which takes into account the presence of thin films of adsorbed gas-environment molecules on a cathode under conditions of high-voltage gas discharge. It is shown that the consideration of electron polaron tunneling mechanism leads to a significant (by more than an order of magnitude) decrease in the emission-current density. A comparison with the classical Richardson–Schottky equation is performed. The role of the polaron effect is shown to strongly increase with an increase in the applied electric field strength and a decrease in temperature, which is due to an increase in the electron work function because of polaron nature.
On the sensitivity of running-fluid NMR magnetometers
Resumo
A new procedure for determining the sensitivity of running-fluid NMR magnetometers is considered. The procedure is based on mathematical processing of experimental data that are related to measuring the gradient of a nutation-line slope at the point at which an inverted NMR signal crosses zero. The procedure allows one to determine the sensitivity of running-fluid NMR magnetometers for resonance frequencies of magnetic-field measurements within a range of 0.5 Hz to 840 MHz.
A numerical study of non-equilibrium flows with different vibrational relaxation models
Resumo
Comparative analysis of a widely used Landau–Teller formula for small deviations from thermal equilibrium and its generalized form, derived from the kinetic theory of gaseous, for an arbitrary deviation from the thermal equilibrium is performed by numerical simulation. Thermally non-equilibrium flows of carbon dioxide near a sharp-edged plate, pure nitrogen flows between two symmetrically located wedges, and the N2/N mixture flow with vibrational relaxation and dissociation over a cone have been considered. A comparison has been performed with the available experimental data.
The relationship between the reliability of transistors with 2D AlGaN/GaN channel and organization type of nanomaterial
Resumo
The first experimental results demonstrating that the carrier mobility in the AlGaN/GaN 2D channel of transistor structures (AlGaN/GaN-HEMT) is correlated with the manner in which the nanomaterial is organized and also with the operation reliability of transistor parameters are presented. It is shown that improving the nature of organization of the nanomaterials in AlGaN/GaN-HEMT structures, evaluated by the multifractal parameter characterizing the extent to which a nanomaterial is disordered (local symmetry breaking) is accompanied by a significant, several-fold increase in the electron mobility in the 2D channel and in the reliability of parameters of transistors fabricated from these structures.
Track membranes based on a 20-μm-thick polyethylene terephthalate film obtained with a beam of argon ions having a range shorter than the film thickness
Resumo
The possibility of performing an energy-efficient variant of irradiation of 20-μm-thick polyethylene terephthalate films to obtain track membranes was considered. Irradiation was done on both sides of a film with a beam of 53.4-MeV Ar+8 ions having energy insufficient for a through track to be formed. The characteristics of the resulting track membrane samples were studied. It was found that these membranes can be used in some cases as a basis for fabrication of composite gas-separating membranes.
Allowing for hysteresis in the calculation of fields in the elements of accelerator magnetic systems
Resumo
Iron magnetic circuit residual magnetization may contribute as much as several Gs to the magnetic field in charged-particle accelerators. This contribution depends on the magnetization “history.” It is not taken into account in most of the existing software that uses the main magnetization curve. Therefore, an error in field calculations usually exceeds 1%, which is unacceptable for accelerators. In this article, a simple phenomenological magnetic-hysteresis model that is suitable for numerical computations is suggested. Approximations based on the proposed model are compared to the results of measurements on partial hysteresis cycles in a steel ring.
Electron beam focusing features in a plasma electron source under forevacuum pressures
Resumo
Features of focusing electron beams generated by a forevacuum plasma source under pressures of 10–30 Pa are investigated and the principle possibility of obtaining submillimeter beams is demonstrated. A beam power density of 105 W/cm2 is reached at an electron beam diameter of 0.6 mm. The obtained beam parameters offer opportunities for precision electron beam processing of dielectric materials, including hightemperature alumina ceramics.
Production technology and optical absorption characteristics of CuIn0.95Ga0.05Se2 semiconductor solid solution films
Resumo
A production technology of thin CuIn0.95Ga0.05Se2 films has been developed based on the method of two-stage selenization of CuIn0.95Ga0.05 precursor by a reactive component (selenium) in a carrier gas (nitrogen) flow. The morphology and structure of obtained films were studied by electron microscopy and X-ray diffraction techniques. The spectral dependence of the optical absorption coefficient was measured.
Statistical analysis of excitation–emission matrices for laser-induced fluorescence spectroscopy
Resumo
An algorithm for statistical processing of the set of multicomponent excitation–emission matrices for laser-induced fluorescence spectroscopy is proposed that is based on principal component analysis. It is shown for the first time that the fluorescence emission and excitation spectra of unknown fluorophores in optically thin samples can be calculated. Using the proposed algorithm, it is possible to pass from principal components with alternating signs to positive quantities corresponding to the spectra of real substances. The method is applied to a mixture of three fluorescent dyes, and it is demonstrated that the obtained spectra of principal components well reproduce the spectra of initial dyes.
A neural network method for restoring the initial impurity concentration distribution from data of ion sputter depth profiling
Resumo
A new approach to solving the problem of restoring the initial impurity concentration distribution from data of ion sputter depth profiling is proposed. The algorithm of impurity profile restoration is based on using an artificial neural network with the input signals representing surface concentrations of impurity determined at sequential moments of sputter depth profiling. The artificial neural network is trained for various depths and thicknesses of the impurity-containing layer and various values of parameters of the adopted model equation of diffusion-like ion mixing.
The thermovoltaic effect in variband solid solution Si1–xGex (0 ≤ x ≤ 1)
Resumo
The thermovoltaic effect in films of variband solid solution Si1–xGex (0 ≤ x ≤ 1) has been observed for the first time. The samples comprised n-Si–p-Si1–xGex (0 ≤ x ≤ 1) heterostructures grown by liquid phase epitaxy. An electromotive force within 0.05–0.3 mV and a current of 0.0025–0.0035 μA appeared on heating samples in a temperature range from 40 to 250°C.
Graphene-based biosensors
Resumo
Results of developing and testing graphene-based sensors capable of detecting protein molecules are presented. The biosensor operation was checked using an immunochemical system comprising fluorescein dye and monoclonal antifluorescein antibodies. The sensor detects fluorescein concentration on a level of 1–10 ng/mL and bovine serum albumin–fluorescein conjugate on a level of 1–5 ng/mL. The proposed device has good prospects for use for early diagnostics of various diseases.
The influence of substrate slope on the rate of evaporation of gas-driven liquid in a locally heated channel
Resumo
Numerical simulations reveal the influence of substrate slope on the rate of evaporation and the extremal thicknesses of gas-driven liquid film in a locally heated microchannel. It is established that the most intense evaporation takes place at small angles of channel sloping relative to the horizon.
Noise-induced binary synchronization in nonlinear systems
Resumo
The phenomenon of noise-induced binary synchronization has been discovered in two independent dynamical systems generating aperiodic binary signals under the action of a common noise source. The presence of a synchronous regime was confirmed by the calculation of Lyapunov exponents for the two systems. The mechanism of development of the noise-induced binary synchronization regime has been found. A relation of the observed regime to binary generalized synchronization is established.
Laser-pulse-induced generation of attosecond electron bunches on crossing the vacuum–plasma boundary
Resumo
A physical mechanism responsible for the self-injection of plasma electrons and the formation of ultrashort electron bunches in the wake wave generated by laser pulse crossing the boundary of inhomogeneous plasma has been studied.
Growing InAs/GaAs quantum dots by droplet epitaxy under MOVPE conditions
Resumo
Results of studying the formation of InAs quantum dots (QDs) on GaAs(100) substrates by droplet epitaxy using trimethylindium and arsine (AsH3) as precursors are presented. The growth process was carried out at temperatures within 230–400°C in a horizontal reactor for metalorganic vapor phase epitaxy (MOVPE) using high-purity hydrogen as the carrier gas. Data on the influence of process temperature on the QD size and the density of QD array and results of investigation of the low-temperature photoluminescence of obtained samples are presented.
Normally off transistors based on in situ passivated AlN/GaN heterostructures
Resumo
A molecular beam epitaxy technology of in situ passivated SiN/AlN/GaN heterostructures with an ultrathin AlN barrier has been developed. Based on these structures, normally off transistors with maximum current density of about 1 A/mm, saturation voltage of about 1 V, transconductance up to 350 mS/mm, and breakdown voltage above 60 V have been fabricated, in which the drain and gate current collapse phenomena are virtually absent.
Determination of the critical ionization distance and ionization zone during high-temperature field evaporation of molybdenum
Resumo
Steady-state field evaporation of molybdenum at high emitter temperatures (T ∼ 2000 K) has been studied using a magnetic mass spectrometer equipped with a field ion source. Only low-charge ions (Mo+2 and Mo+) have been observed in the course of evaporation. The measured ion energies and evaporating field strengths (Fev) were used to determine the critical ionization distances (xcr) and ionization zones (Δ) for singly and doubly charged ions. The obtained xcr and Δ values show that the formation of ions takes place at a certain distance from the emitter surface.
Ion plasma deposition of oxide films with graded-stoichiometry composition: Experiment and simulation
Resumo
A method of ion plasma deposition is proposed for obtaining thin multicomponent films with continuously graded composition in depth of the film. The desired composition–depth profile is obtained by varying the working gas pressure during deposition in the presence of an additional adsorbing screen in the drift space between a sputtered target and substrate. Efficiency of the proposed method is confirmed by Monte Carlo simulation of the deposition of thin films of BaxSr1–xTiO3 (BSTO) solid solution. It is demonstrated that, during sputtering of a Ba0.3Sr0.7TiO3 target, the parameter of composition stoichiometry in the growing BSTO film varies in the interval of x = 0.3–0.65 when the gas pressure is changed within 2–60 Pa.
Size dependence of the shape of a silicon nanocrystal during melting
Resumo
Based on the experimentally established dependence of the melting temperature of a silicon nanocrystal on its size, it is shown that, as the size of a melting nanocrystal decreases, its shape also exhibits variation. Greater nanocrystals possess either rodlike or platelike shapes. As the nanocrystal size decreases, its shape transforms and approaches the shape of a cube.
Amplitude and phase chimeras in an ensemble of chaotic oscillators
Resumo
The transition from coherence to incoherence in an ensemble of nonlocally coupled logistic maps is considered. Chimera states of two types (amplitude and phase) are found. The mechanism and conditions of their appearance are determined.