Critical Current Model for Polycrystalline Compounds with an Intrinsic Solid Solution of Two Mixed Superconductor Phases

We propose an S ₁ S ₂ I S ₂ S ₁ junction model to describe the intergrain junctions of compounds with intrinsic mixtures of two superconductor phases. Considering the polycrystalline compound Hg,Re-1223 as a representative case, fitting using this model of a junction in J _c as a function of applied magnetic field μ ₀ H proves to be better than a simple SIS junction model for μ ₀ H > 1.0 mT and T≈T _C .     

On the relationship between radiation-stimulated photoluminescence and nitrogen atoms in p-4H-SiC

Photoluminescence (PL) appearing in p-4H-SiC upon its electron irradiation has been studied. A model that accounts for the dependence of the PL intensity on the irradiation dose is suggested. The conclusion is drawn that nitrogen–radiation defect donor–acceptor pairs are PL activators.     

Quantum interference filters based on oxide superconductor junctions for microwave applications

We have studied superconducting quantum interference filters (SQIFs) based on bicrystal neodymium gallate substrates, which can be used in the microwave frequency range. The characteristics of a serial SQIF have been compared for the first time with those of a single superconducting quantum interference device (SQUID) and a chain of serially connected SQUIDs with equal areas of superconducting loops. The regime of SQIF operation with a voltage-flux (V-Φ) characteristic determined by the magnetic-field dependence of the critical current in the Josephson junction has been analyzed. It is shown that the output noise of a SQIF measured with a cooled amplifier in the 1–2 GHz range is determined by the slope of the V-Φ characteristic. The influence of a spread in the parameters of Josephson junctions in the SQIF on the integral V-Φ characteristic of the whole structure is considered.     

Flux Rule, <Emphasis Type="Italic">U</Emphasis>(1) Instanton,and Superconductivity

We examine some consequences of the duality that a U(1) phase factor added on a wave function describes a whole system motion and also plays the role of a U(1) gauge potential. First, we show that the duality solves a long-standing puzzling problem that the ‘flux rule’ (the Faraday’s induction formula) and the Lorentz force calculation for an emf emerging in an electron system moving in a magnetic field give the same result (Feynman et al. 1963). Next, we examine a U(1) phase factor induced on the wave function for an electron system due to the single-valuedness requirement of the wave function with respect to the electron coordinates, and its consequential appearance of a U(1) instanton. This instanton explains the Meissner effect, supercurrent generation, flux quantization in the units of \({{h} \over {2e}}\), and the voltage quantization in the units of \({{hf} \over {2e}}\) across the Josephson junction in the presence of a radiation field with frequency f. In the experiment, a radiation field must be present to have a finite voltage across the Josephson junction; but a clear explanation for it has been lacking. The present work provides an explanation for it, and also explains the high precision of the quantized voltage as due to a topological effect.     

Calculation of sublimation with allowance for impurity diffusion

We have calculated the impurity distribution in a spherical material during sublimation. The solutions found depend on two dimensionless parameters: separation factor β and diffusional Peclet number Pe = wR ₀/(ρ₁ D), where w is the mass vaporization rate, R ₀ is the initial sphere radius, ρ₁ is the density of the material, and D is the impurity diffusion coefficient. For the limiting cases Pe ? 1 and Pe < 1, we derived analytical expressions for the impurity distribution in the subliming material and for the average impurity concentrations in the residue and condensate as functions of time t or the degree of sublimation g(t). The present results can be used to optimize the dimensions of subliming materials and sublimation conditions.     

Mechanism of <Emphasis Type="Italic">p-n</Emphasis> junction breakdown at high reverse voltage ramps

It is shown that flat tunneling-assisted impact ionization fronts in p-n junctions operating at a high buildup rate (ramp) u of the reverse voltage can be obtained provided that the rate g_f of the tunneling generation of electron-hole pairs is much greater than the impact ionization rate g_i in the initial stage of breakdown. Usually (e.g., in silicon) g_f?g_i and, therefore, the multiplication of the first hundreds of electron-hole pairs due to the impact ionization leads to the avalanche-streamer transition much before the overlap of avalanches. In this case, the diode transition to the conducting state may be caused by the interelectrode gap shortage by a large number of streamers, the total resistance of which can be much smaller than the load resistance. A simple model of such a “multistreamer” breakdown is proposed according to which the switching time is virtually independent of the streamer parameters and is inversely proportional to u. The diode resistance in the conducting state significantly drops with increasing ramp u and grows with increasing velocity of streamers.     

Adsorption of indigo carmine and methylene blue dye: Taguchi’s design of experiment to optimize removal efficiency

In the present study, Taguchi’s experimental methodology has been applied to find optimum level (value) of adsorption parameters (factors) for the removal of indigo carmine dye (ICD) and methylene blue dye (MBD) using activated carbon derived from Acacia Nilotica sawdust (ACSA). The effect of significant adsorption parameters, viz. adsorbent dose (m), initial concentration (C₀), temperature (T) and contact time (t), on the adsorption capacity (q_t) of ACSA for each dye has been discussed. Average values and S/N ratio for each parameter at three different levels have been estimated using L₉ orthogonal array (OA). Analysis of variance (ANOVA) has been used to identify the significant parameters and the most favourable optimal conditions for both raw data and S/N data. The study revealed that, for ICD, initial dye concentration is found to be the most significant parameter with 55.8% contribution followed by ACSA dose, temperature and contact time with 35.7%, 5% and 3.4% contribution, respectively. For MBD, the ACSA dose (m) is found to be the most significant parameter with 46.4% contribution followed by initial dye concentration, temperature and contact time with 44%, 5.3% and 4.4% contribution, respectively. The contact time (t) is found to be the least significant parameter in the overall sorption process for both ICD and MBD. The optimized levels of parameters for both dyes are found to be A₁, B₃, C₃ and D₃. The predicted and average confirmatory values of total dye adsorbed (q_t) on ACSA at optimized levels were found to be 31.02 and 31.01 mg/g, respectively, for ICD and 57.35 and 57.36 mg/g, respectively, for MBD. The percentage removal of ICD and MBD at optimized levels was found to be 77.5% and 95.4%, respectively.     

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

We investigate the iron-based superconductor Ba(Fe _1?x Co _x ) ₂As ₂ (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I _c R _n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I _c R _n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I _c R _n to 90 μV for planar TiO _x barrier junctions.     

Photocarrier Radiometry for Non-contact Evaluation of Monocrystalline Silicon Solar Cell Under Low-Energy (&lt; 200 keV) Proton Irradiation

A three-layer theoretical model is developed for the characterization of the electronic transport properties (lifetime τ, diffusion coefficient D, and surface recombination velocity s) with energetic particle irradiation on solar cells using non-contact photocarrier radiometry. Monte Carlo (MC) simulation is carried out to obtain the depth profiles of the proton irradiation layer at different low energies (<?200 keV). The monocrystalline silicon (c-Si) solar cells are investigated under different low-energy proton irradiation, and the carrier transport parameters of the three layers are obtained by best-fitting of the experimental results. The results show that the low-energy protons have little influence on the transport parameters of the non-irradiated layer, but high influences on both of the p and n-region irradiation layers which are consisted of MC simulation.     

Tailoring of optical and electrical properties of PMMA by incorporation of Ag nanoparticles

Silver–poly(methyl methacrylate) (Ag–PMMA) nanocomposite films were prepared via ex situ chemical route by employing sodium borohydride (\(\hbox {NaBH}_{4}\)) as a reducing agent. In this study, PVP-stabilized Ag nanoparticles were prepared and mixed with PMMA solution. Optical and structural characterizations of resulting nanocomposite films were performed using UV–visible spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Characteristic surface plasmon resonance (SPR) peak of Ag nanoparticles was observed at about 3.04 eV (408 nm) in absorption spectra of Ag–PMMA nanocomposite films. TEM micrograph revealed that the spherical Ag nanoparticles with an average diameter of 5.4\(\,\pm \,\)2.5 nm are embedded in PMMA. In Raman spectra, besides shifting of vibrational bands, enhancement in intensity of Raman signal with incorporation of Ag nanoparticles was observed. Current (I)–voltage (V) measurements revealed that conductivity of PMMA increased with increasing concentration of Ag nanoparticles. Analysis of I–V data further disclosed that at voltage <2 V, ohmic conduction mechanism is the dominant mechanism, while at voltage >2 V Poole–Frenkel is the dominant conduction mechanism. Urbach’s energy, the measure of disorder, increased from 0.40 eV for PMMA to 1.11 eV for Ag–PMMA nanocomposite films containing 0.039 wt% of Ag nanoparticles.     

High-Efficiency AlGaAs/GaAs Photovoltaic Converters with Edge Input of Laser Light

High-efficiency photovoltaic converters (PVCs) have been developed and fabricated by liquidphase epitaxy in the AlGaAs–GaAs system with laser light (λ = 850 nm) introduced through the edge surface in parallel to the plane of the p–n junction of the device structure. To raise the efficiency of light “capture” by the p–n junction, an Al_xGa_1–xAs waveguide layer is formed, in which the content of aluminum gradually varies from x = 0.55 to 0.15 so that the refractive index gradient is created in this layer and light beams are diverted toward the p–n junction. When a PVC (having no antireflection coating) is exposed to 0.1- to 0.2-W laser light, an efficiency of 41.5% is obtained. Depositing an antireflection coating on the edge surface of a PVC raises its efficiency to 55%.     

Kinetics of Competitive Sorption in Decontamination of Materials from Radionuclides

kinetic model of the mass transfer of a microcomponent in the simplest competitive system from the sorbed state (A) into a solution (B) and then into a sorbent (C) in accordance with the scheme A ? B ? C was formulated within the framework of competitive sorption statics. The kinetic equations were solved numerically. The influence exerted by the weight of competing sorbents А and С and by the degree of reversibility of linear reactions on the nonequilibrium decontamination factor K_dec(t) was determined. The time in which the equilibrium decontamination factor is attained for the model of chemical sorption kinetics was estimated from the experimental data on the rate constants of direct and reverse heterogeneous reactions and on the distribution coefficients of Cs(I) in the SiO₂ (A)–CsCl solution (B)–Prussian Blue (C) system.     

Assessing abrasion performance of self-consolidating concrete containing synthetic fibers using acoustic emission analysis

The key objective of this investigation was to evaluate the abrasion resistance of self-consolidating concrete (SCC) with and without synthetic fibers (SynFs). The abrasion resistance of normal concrete was also investigated in this study for comparison. The abrasion test was performed on concrete specimens according to the rotating-cutter method along with continuous monitoring of acoustic emission (AE) using attached AE sensors. The effects of changing concrete type and incorporating various types (flexible and semi-rigid) and lengths of SynFs on the abrasion behaviour were investigated with the aid of AE analysis. AE signal characteristics such as amplitude, signal strength, number of hits, and duration were gathered during testing. Furthermore, the collected AE data was used to complete b-value analysis as well as intensity analysis resulting in three additional parameters: b-value, severity (S_r), and historic index (H(t)). The results showed that the AE parameters were directly correlated with the abrasion damage in all tested mixtures. Adding SynFs to all SCC mixtures enhanced their abrasion resistance. The flexible fibers variety exhibited better abrasion performance on average than the semi-rigid fibers. Meanwhile, longer fibers showed lower abrasion resistance than the shorter ones with the same type. The results also indicated that AE intensity analysis was able to determine the ranges for H(t) and S_r that identify the extent of damage due to abrasion of SynF-reinforced SCC.     

Electrical conductivity of undoped and rare-earth-doped high-resistivity GaSe crystals

The time-dependent conductivity of nominally undoped and rare-earth-doped (N _R?10^?5 to 10^?1 at % Gd, Dy, or Ho) high-resistivity gallium selenide crystals has been measured under various conditions. At a relatively low applied voltage and T ≤ 150 K, the conductivity of the crystals reaches a steady-state value rather slowly. When a voltage above a certain threshold is applied for a long time at T ≤ 300 K, the material exhibits electric fatigue. An energy-band model is proposed which provides qualitative explanation of the results.     

Electrochemical etching of <Emphasis Type="Italic">p–n</Emphasis>-GaN/AlGaN photoelectrodes

Specific features of etching of GaN/AlGaN p–n structures in a KOH-based electrolyte have been studied. It was found that the corrosion process first passes across p layers through vertical channels associated with threading structural defects. Then, the corrosion process occurs in the lateral direction along n layers of the structure, with local hollows and voids thereby formed. The lateral etching is due to the presence of positive piezoelectric charges at boundaries of n-AlGaN and n-GaN layers and positively charged ionized donors in the space-charge region of the p–n junction.     

Variation of the yield stress and latent strain energy in the course of plastic deformation of a neutron-irradiated chromium-nickel steel

The laws of variation of the flow stress (σ) and latent strain energy (E_s) in the course of plastic deformation have been studied in the samples of a titanium-doped chromium-nickel steel (12Kh18N10T grade) before and after neutron irradiation to a dose of 5×10²²m^?2 (neutron energy E>0.1 MeV). The E_s versus σ curve can be divided into several characteristic regions, the first of which is described by a relation of the type E_s～E₀+kσ² (E₀ and k being certain coefficients). It is established that neutron irradiation shifts the E_s versus σ curve toward smaller latent energies.     

The influence of photoelectron processes in a semiconductor substrate on the adsorption of polycationic and polyanionic molecules

White-light illumination during the adsorption of polyanionic molecules of glucose oxidase (GO _x ) enzyme on the surface of p-Si/SiO₂/polyethylenimine structure leads to a threefold decrease in the surface concentration of GO _x molecules. Same illumination during the GO _x adsorption on the n-Si/SiO₂/PEI structure leads to a sevenfold increase in the surface concentration of enzyme molecules. Changes in the amount of adsorbed GO _x molecules depending on the intensity of irradiation are explained by electron transfer processes and recharging of electronic states at the Si/SiO₂ interface and within SiO₂ layer.     

Electric field induced by collective vortex creep in superconductors with fractal clusters of normal phase

The influence of the collective creep of magnetic flux on the electric field induced in a superconducting composite with fractal cluster structure is considered. Current–voltage (I–V) characteristics of these superconductors are determined with allowance for the influence of the fractal dimensionality of boundaries of the normal phase clusters and the height of pinning barriers on the nonlinearity of I–V characteristics at small transport currents. A relationship is established between the collective pinning and vortex glass state that is formed in superconductors with a fractal cluster structure. It is shown that the intensity of an electric field induced in the case of collective creep is smaller than that for Anderson–Kim creep.     

Multiscale structural characterization of methyltriethoxysilane-based silica aerogels

A series of methyltriethoxysilane-based silica aerogel monoliths were prepared by ambient pressure drying with various volume ratios of water to ethanol (R). The pore volumes and average pore sizes of silica aerogels were obtained by Barrett–Joyner–Halenda (BJH) method from nitrogen adsorption–desorption isotherms. The stress–strain curves of the cylindrical aerogel specimens were measured by performing uniaxial compressive tests. The particle size distributions and the average particle sizes of silica aerogels were also evaluated based on scanning electron microscopic observations. The experimental data revealed that the average particles size increased from 0.115 to 3.08 μm as R varied from 0.7 to 1.5, and that the silica aerogels exhibited two characteristic types of the compressive stress–strain responses. By proposing a multiscale structural model to describe microstructures of silica aerogels, a structural parameter, defined as the slenderness L/D of the cube column length L and the average particle diameter D, was related to the specific volume and the BJH volume of the silica aerogel monoliths, as well as the specific volume of silica. Accordingly, the two types of the compressive stress–strain responses may be distinguished by the critical value (L/D)_c.     

A direct method of thermal time constant measurement for lithium tantalate based terahertz pryroelectric detectors

Thermal time constant (τ _th ) is one of the most important parameters of pyroelectric detectors (PEDs). However, unlike other parameters such as voltage responsivity (R _v ), noise equivalent power and specific detectivity (D*), measurement of τ _th of lithium tantalate (LT) based PEDs was seldom reported in literatures. In this work, a convenient direct method for the measurement of τ _th was presented according to theoretical analysis and simulation result. Afterwards, this method was successfully applied to practical measurement of a homemade current mode terahertz (THz) LT based PED, of which τ _th was measured to be about 882 ms. To verify the measured result, τ _th was also calculated from the measured voltage-time curves according to its theoretical expression, which agrees well with the measured one. Compared with indirect measurement of τ _th through frequency response curves of PEDs, the direct method is simpler and more accurate. Furthermore, this method suggested an easy way for the measurement of pyroelectric coefficient of PEDs.