Vol 42, No 4 (2016)
- Year: 2016
- Articles: 28
- URL: https://journals.rcsi.science/1063-7850/issue/view/12402
Article
Electron-stimulated reduction of the surface of graphite oxide
Abstract
Auger-electron spectroscopy has been used to study in situ the initial stage of graphite oxide (GO) reduction under the action of a low-intensity electron beam that does not lead to heating of the irradiated region. It was found that this stage evolves at a rate that is tens of times the rate of the subsequent reduction. It was shown that the fast stage is associated with the removal of oxygen groups from the GO surface. The effective cross sections of the initial and subsequent stages of GO reduction by 1500 eV electrons were found to be σin ~ 0.5 × 10–16 cm2 and σav ~ 1.2 × 10–18 cm2, respectively.
Photoluminescence of Ta2O5 films formed by the molecular layer deposition method
Abstract
Ta2O5 films of different thicknesses (20–100 nm) synthesized by the molecular layer deposition method on p-type silicon substrates and thermally oxidized silicon substrates have been studied by the methods of high-frequency capacitance–voltage characteristics and photoluminescence. A hole-conduction channel is found to form in the Si–Ta2O5–field electrode system. A model of the electronic structure of Ta2O5 films is proposed based on an analysis of the measured PL spectra and performed electrical investigations.
The influence of microwave radiation on the thermal stability of aluminum nanopowder
Abstract
The influence of microwave radiation with a power flux density of 80 W/cm2 and carrier frequency of 9.4 GHz on the thermal stability of aluminum nanopowder after irradiation in air is studied. It is established that, after irradiation, the chemical activity of aluminum nanopowder increases, the temperature for the beginning of its oxidation decreases by 40°, while the thermal effect of oxidation decreases by 13.5%.
Study of the high-coercivity material based on ε-Fe2O3 nanoparticles in the silica gel matrix
Abstract
We report the results of investigations of ε-Fe2O3 magnetic nanoparticles obtained by incipient wetness impregnation of silica gel. It was established that the obtained samples with an iron content of 12‒16% mass % containing ε-Fe2O3 nanoparticles with an average size of 10 nm on the silica gel surface exhibit a room-temperature coercivity of about 10 kOe. Along with fabrication simplicity, this fact makes the prepared samples promising for application as a magnetically hard material.
Optimizing the efficiency of dielectric barrier discharge for creating synthetic jets
Abstract
A study of synthetic jets created by dielectric barrier discharge of the symmetric actuator has been carried out. The parameter—specific thrust (thrust-to-power ratio)—is used for the first time. The current–voltage characteristics of the dielectric barrier discharge are obtained at different values of the ballast resistance. The dependence of the thrust-to-power ratio on the supply-voltage frequency is determined experimentally.
The influence of stepwise deformation of aluminum–magnesium alloy upon its electrical conduction
Abstract
The influence of stepwise deformation upon the electrical conduction of the AlMg6 aluminum–magnesium alloy is investigated experimentally. It is found that the nucleation and development of single deformation bands causing stepwise deformation increase the specific electric resistance of the alloy on average by 2–3%. It is supposed that the main mechanism of an increase in the electric resistance in the deformation band is growth of the deformation vacancy concentration, which is connected with intensive dislocation multiplication in the deformation band front.
Antireflection coatings based on fluoride formulations for organic solar cells
Abstract
An alloy of a mixture of fluorides MgF2 and AlF3 with CaF2 has been obtained in a 3-kW solar furnace. It was supposed that a minor CaF2 additive compensates for the tensile stresses appearing in thin MgF2 and AlF3 films, with their mechanical properties being thereby improved. The results of X-ray phase analysis demonstrated that both components of the mixture are present in the alloy, while the complex oxide CaAl4O7, the formation of which is attributed to the melting in air, is only identified in AlF3: CaF2 = 95: 5 (wt %). The increase in the transmittance of glass and polyethylene terephthalate upon deposition onto their surface of a thin film of the material synthesized in the study is due to the optical properties of AlF3 and MgF2.
A noncontact temperature measurement method in polymerase chain reaction reactors
Abstract
A new noncontact method for measuring temperatures of liquids, which is based on the fluorescent probes, is proposed. The method is intended for measuring temperatures of reaction media in reactors of devices for polymerase chain reactions in real time and can be used for determining dynamic temperature parameters.
Organic–inorganic nanostructures for luminescent indication in the near-infrared range
Abstract
Amplifying and quenching of IR luminescence of colloidal Ag2S quantum dots were revealed to take place when they couple to organic dye molecules of 3,3′-di-(γ-sulfopropyl)-9-ethyl-4,5,4′,5′-dibenzothiacarbocyanine betaine and erytrosine pyridinium salts, respectively. The observed effects are explained as due to the formation of organic–inorganic heterostructures with different mutual arrangement of electronic states of the dyes and the quantum dots.
The effect of pulsed vacuum-arc discharge on the surface of elements of a discharger
Abstract
Regularities of erosion and transport of the material of discharger elements have been studied by the methods of surface structure visualization and X-ray fluorescence elemental microanalysis. Microrelief and oxide film on the surface of cathode material are found to affect the development of arc discharge in a commutator.
Discharge development and minimum switching time in a kivotron
Abstract
Experimental studies of the breakdown stage in a kivotron, a high-voltage switching device based on an open discharge with counterpropagating electron beams, are performed. It is demonstrated that a fast stage of the breakdown is provided by photoemission of resonance radiation of fast atoms with large Doppler shift with respect to the line center of thermal atoms. For working pressure pHe ≈ 20 Torr, switching time τs ≈ 80 ps is achieved. The estimated minimum switching time is ~35 ps.
Structural and phase transformations and properties of TiNi–TiCu quasi-binary alloys
Abstract
The influence of copper doping (25 at. %) upon the structural and phase transformations of triple alloys of the TiNi–TiCu quasi-binary cut is studied by transmission and scanning electron microscopy and electron and X-ray diffraction. A generalized complete diagram of B2 ↔ B19′, B2 ↔ B19 ↔ B19′, and B2 ↔ B19 thermoelastic transformations proceeding upon cooling as the copper content increases in the intervals of 0–8, 8–15, and 15–25 at. %, respectively, is constructed. Specificities of changes in the mechanical properties and microstructure of B19 and B19′ martensites in relation to copper doping are studied.
A microwave cryogenic low-noise amplifier based on sige heterostructures
Abstract
A low-noise cryogenic amplifier for the measurement of weak microwave signals at sub-Kelvin temperatures is constructed. The amplifier has five stages based on SiGe bipolar heterostructure transistors and has a gain factor of 35 dB in the frequency band from 100 MHz to 4 GHz at an operating temperature of 800 mK. The parameters of a superconducting quantum bit measured with this amplifier in the ultralow-power mode are presented as an application example. The amplitude–frequency response of the “supercon-ducting qubit–coplanar cavity” structure is demonstrated. The ground state of the qubit is characterized in the quasi-dispersive measurement mode.
Focusing of charged particles by electric field lenses with a pulsed voltage source
Abstract
Focusing properties of a single electric field lens with a pulsed voltage source are studied by means of numerical simulation. The possibility of reducing the waist diameter of the focused particle beam by more than an order of magnitude compared to the static regime is demonstrated.
The nonlinear influence of an electric field on phase transitions in ferromagnetic semiconductors: Lanthanum manganite
Abstract
We investigate nonequilibrium processes of self-heating induced by electric current in ferromagnetic semiconductors exhibiting colossal magnetoresistance (CMR) in the vicinity of the Curie temperature. The heat balance equation is solved taking into consideration localized states that appear as a result of scattering from magnetic inhomogeneities and are characterized by a percolation threshold proportional to the amplitude of spin fluctuations. The appearance of N-shaped current-voltage characteristics and hysteresis in the dependence of magnetization on electric potential difference, which are caused by the emergence of a “hot” (with respect to internal temperature) semiconductor paramagnetic phase, is revealed in the steady-state regime. The possibility of suppression of the effect of colossal magnetoresistance with increasing potential difference is indicated. The onset of self-oscillation of current and magnetization with decreasing transverse dimensions of the sample is demonstrated.
The influence of three-dimensional capillary-porous coatings on heat transfer at liquid boiling
Abstract
The process of heat transfer at pool boiling of liquid (Freon R21) on tubes with three-dimensional plasma-deposited capillary-porous coatings of various thicknesses has been experimentally studied. Comparative analysis of experimental data showed that the heat transfer coefficient for a heater tube with a 500-μm-thick porous coating is more than twice as large as that in liquid boiling on an otherwise similar uncoated tube. At the same time, no intensification of heat exchange in the regime of bubble boiling is observed on a tube with a 100-μm-thick porous coating.
Studying energy evolution in the discharge chamber of a multichamber lightning protection system
Abstract
We present experimental data on the distribution of energy deposition along the discharge chamber of a multichamber lightning protection system at the initial stage of a discharge process modeling a lightning current pulse with 10 kA amplitude. The multichamber system comprised serially connected gas-discharge chambers. The breakdown between electrodes situated on the bottom of a channel in each chamber induces the formation of a shock wave. Subsequent energy evolution during the development of discharge proceeds in the entire volume bounded by the shock wave.
Influence of partial shape memory deformation on the burst character of its recovery in heated Ni–Fe–Ga–Co alloy crystals
Abstract
Room-temperature stress–strain curves of Ni49Fe18Ga27Co6 alloy single crystals possessing shape memory (SM) have been studied. Specific features of these diagrams are revealed upon compressive loading of these single crystals in the [110]A direction. The influence of preliminary SM deformation on the process of its recovery during the reverse martensite transformation has been studied. It is established that SM deformation above 4.2% leads to a sharp increase in the shape recovery on heating and the process exhibits a burst character, involving motion of the entire crystal. The experimental data are analyzed and stress–strain curves are simulated in the framework of the theory of diffuse martensitic transitions.
A microwave photonic generator of chaotic and noise signals
Abstract
The transition to chaos in a microwave photonic generator has been experimentally studied for the first time, and the generated broadband chaotic microwave signal has been analyzed. The generator represented a ring circuit with the microwave tract containing a low-pass filter and a microwave amplifier. The optical tract comprised a fiber delay line. The possibility of generating chaotic oscillations with uniform spectral power density in a 3–8 GHz range is demonstrated.
Development of an algorithm of hydrodynamic analysis of quantum system evolution in expanding space
Abstract
A method for calculating the spectrum of ionized electrons is developed based on a solution of the equation of quantum system evolution in expanding space in the hydrodynamic representation. Using one-dimensional test problems, it is shown that the proposed numerical algorithm provides results with satisfactory accuracy for smooth initial distributions, but needs improvement in cases when the system wave function has zeros.
Dynamics of pulsed laser ablation of gold in vacuum in the regime of nanostructured film synthesis
Abstract
The dynamics of heating and vaporization of gold in vacuum under the action of laser pulses of low intensity, when the main ablation products are neutral particles, is investigated by mass spectrometry and numerical modeling. The laser-ablation plume is found to consist of gold atoms and dimers with kinetic energies considerably higher than their thermal energy upon evaporation. Nanostructured gold films are synthesized by depositing the ablation products onto a substrate. Formation mechanisms of the gold nanostructures and possibilities of controlling their sizes are discussed.
Low-temperature diffusion of implanted sodium in silicon
Abstract
We have studied the low-temperature diffusion of sodium atoms implanted (at primary ion energy E = 300 keV to total doses within Φ = 5 × 1014–3 × 1015 cm–2) in single-crystalline silicon grown by the method of float-zone melting (fz-Si) with low oxygen concentration NO and by the Czochralski method in the presence of magnetic field (mCz-n-Si and mCz-p-Si) with NO ≈ 5 × 1017 cm–3. The diffusion was studied at annealing temperatures within Tann = 500–420°C for periods of time tann = 72–1000 h. It is established that the temperature dependence of the diffusion coefficient D(103/T) of sodium in fz-Si in a broad range of Tann = 900–420°C obeys the Arrhenius law with Efz = 1.28 eV and D0 = 1.4 × 10–2 cm2/s. The same parameters are valid for the implanted sodium diffusion in mCz-Si in the interval of Tann = 900–700°C. However, at lower temperatures, the values of D in mCz-Si are lower than to those in fz-Si, which is related to the formation of more complicated Na–On (n > 1) complexes in the former case. Estimation of the diffusion activation energy of these complexes yields ΔE ≈ 2.3 eV.
X-ray diffractometry of AlN/c-sapphire templates obtained by plasma-activated molecular beam epitaxy
Abstract
The structure of AlN/c-sapphire templates obtained by plasma-activated molecular beam epitaxy (PAMBE) has been studied by X-ray diffractometry techniques. The results show the advantages of using coarse-grained AlN nucleation layers prepared by high-temperature (780°C) adatom-migration-enhanced epitaxy. Using 3.5-nm-thick GaN inserts (obtained by three-dimensional growth under N-rich conditions), it is possible to obtain templates with insignificant residual macrostresses and relatively narrow widths (FWHM) of 0002 and 10\(\bar 1\)5 diffraction reflections.
Ferroelectric films of barium strontium titanate on semi-insulating silicon carbide substrates
Abstract
Thin ferroelectric BaxSr1–xTiO3 (BST) layers have been grown for the first time on semi-insulating silicon carbide substrates by RF magnetron sputtering of a ceramic target without using buffer sublayers. Results of investigation of the structure of obtained BST films and the electrical properties of related planar capacitors are presented. The obtained structures are characterized by high nonlinearity and low dielectric losses at microwave frequencies.
An electron-impact ionization study of molecular selenium beams
Abstract
We describe the method and presents results of a mass-spectrometric investigation of the yield of positive ions formed as a result of the dissociative ionization of a molecular selenium beam by electron impact. Appearance energies of fragment ions have been determined from the ionization efficiency curves. The dynamics of formation of the molecular ions of selenium in a temperature interval of 420–495 K has been studied. The energy dependences of the efficiency of formation of singly charged Sen+ (n = 1–3) and doubly charged Se++ ions are analyzed.
The influence of growth conditions on the surface morphology and development of mechanical stresses in Al(Ga)N layers during metalorganic vapor phase epitaxy
Abstract
We have studied the influence of technological parameters on the surface morphology and development of mechanical stresses in Al(Ga)N layers during their growth by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates. Minimization of tensile stresses under conditions of a retained atomically smooth surface can be achieved by using a combination of factors including (i) nitridation of substrate in ammonia flow, (ii) formation of two-layer AlN–Al(Ga)N structures by introducing a small amount (several percent) of Ga after growth of a thin AlN layer, and (iii) reduction of ammonia flow during growth of an Al(Ga)N layer.
A random telegraph signal in tunneling silicon p–n junctions with GeSi nanoislands
Abstract
We have experimentally discovered random telegraph signal generation in tunneling silicon p+–n+ junctions with embedded self-assembled GeSi nanoislands. The observed phenomenon is related to blocking of the electron tunneling via individual GeSi nanoislands due to the generation of holes in, and their thermal emission from, the nanoislands.
Graphene suspensions for 2D printing
Abstract
It is shown that, by processing a graphite suspension in ethanol or water by ultrasound and centrifuging, it is possible to obtain particles with thicknesses within 1–6 nm and, in the most interesting cases, 1–1.5 nm. Analogous treatment of a graphite suspension in organic solvent yields eventually thicker particles (up to 6–10 nm thick) even upon long-term treatment. Using the proposed ink based on graphene and aqueous ethanol with ethylcellulose and terpineol additives for 2D printing, thin (~5 nm thick) films with sheet resistance upon annealing ~30 MΩ/□ were obtained. With the ink based on aqueous graphene suspension, the sheet resistance was ~5–12 kΩ/□ for 6- to 15-nm-thick layers with a carrier mobility of ~30–50 cm2/(V s).