Effects of Zn on the grain boundary properties of La2−xSrxCu1−yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-T_c cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La_2−xSr_xCu_1−yZn_yO₄ sintered samples with different hole concentrations, p (≡x), over a wide range of Zn contents (y) using field-dependent AC susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature T_c, and the temperature T_gcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of T_gcp(p, y), T_c(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both T_c and T_gcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article.     

The effects of C substitution and disorder on the field dependent critical current density in MgB2 with nano-SiC additions

In this work, nano sized SiC powders were mixed with Mg and B and reacted by either a one-step insitu or two-step method resulted in different level of C substitution. X-ray diffraction shows the presence of Mg₂Si signifying that the reaction between SiC and Mg occurred leading to the release of C in samples reacted in one-step method. Moreover, the much reduced value of a-axis indicates C substitution took place. Resistivity measurements showed higher intragrain scattering owing to a higher density of defects and/or impurities. These samples also show higher H_irr and H_c2 at 20 K in comparison to samples with mainly unreacted SiC (hence lower C substitution). More importantly, their J_c’s are more insensitive to high magnetic field (>4 T) at 6 K. However, at 20 K the effect of C content on J_c(H) is less pronounced. Finally, the order of magnitude of J_c(H) at both 6 K and 20 K is rather dominated by pinning.     

Critical current degradation behaviors of ReBCO coated conductor tapes under pressurized liquid nitrogen

In the case of 2G coated conductor (CC) tapes, it has been reported that thin–thick CC tapes with IBAD substrate showed a superior electromechanical property even at smaller bending radius compared with the cases of 1G BSCCO tapes. Considering the application of CC tapes it is significant to evaluate the transport property under operating environment, because CC tapes might experience a change in operating pressure that can affect its current carrying capacity due to temperature variation and deformation. This study was focused on the I_c degradation behavior in bent CC tapes under pressurized liquid nitrogen. Differently processed YBCO and SmBCO CC tapes with IBAD substrate are used as samples. The bending strain characteristics at elevated pressure levels were evaluated by using the ρ-shaped sample holder which can induce different bending strain values at pressured state. Depressurization and thermal cycling were performed to check the reversibility of I_c in CC tapes. Vacuuming tests were also carried out to investigate the characteristics of I_c at different LN₂ temperature levels.     

Influence of tensile damage on V–I curve and critical current of DyBCO coated conductor

The coated conductors are subjected to mechanical and electromagnetic stresses in preparation and service. When the stress is high, the coated layer is damaged, resulting in loss of superconducting property. The present work was carried out to reveal the influence of tensile damage on V–I curve, critical current and n-value of DyBa₂Cu₃O_7−δ coated conductor. The changes of the V–I curve, critical current and n-value with increasing applied tensile strain were measured experimentally. The features of the shift of the V–I curve to the lower current range and increase in its curvature, and accordingly the decrease in critical current and n-value, with increasing applied strain, were detected. For analysis of the experimental results, the model of Fang et al, which treats with the voltage generation due to the current shunting under existent crack, was applied. The experimentally observed features were described satisfactorily.     

Comparative study of type-II superconducting properties in polycrystalline NdFeAsO0.88F0.12 prepared by different methods

Polycrystalline NdFeAsO_0.88F_0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) methods were comparatively studied by magnetization and transport measurements. Upper critical field H_c2, irreversibility field H_irr and the anisotropy parameter Γ were estimated from resistance transition curves. The broadening of transition width was observed, and was ascribed to both H_c2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields indicate that there are weak links in both samples. Magnetization critical current density J_cm were derived from MHLs. Sample HP shows higher J_cm than sample AP. Direct transport I–V measurements show that the transport critical current density J_ct are very low but persist up to 9 T, suggesting intrinsic strong-link existing in both samples.     

Effects of pulse electrodeposition parameters on the properties of Ni-TiO2 nanocomposite coatings

The aim of this study is to investigate the effects of pulse electrodeposition parameters on the properties of nickel-titania composite coatings electrodeposited from a nickel Watts type bath. The effects of average current density, frequency and duty cycle on the surface morphology, crystal size, preferred orientation of the deposits and the amount of embedded nano-TiO₂ particles in the composite coatings were investigated. The results represented the optimum amount of average current density (e.g., 4 A dm⁻²) for obtaining the highest volume percentage of the incorporated titania particles and subsequently the maximum microhardness. Moreover, by increasing the frequency up to 10 Hz while reducing the duty cycle to 10% at constant peak current density, the volume percentage of particles increased to about 7% which is almost twice as much the volume percentage as deposited particles in direct current method. According to the results the composite coating exhibited obviously [1 0 0] + [2 1 1] as preferred orientation at low pulse frequency and the diffraction intensity of the [2 1 1] fiber orientation is increased, probably due to the pH increase adjacent to the electrolyte/cathode interface at higher frequencies. Also, it has been found that by reduction in the duty cycle, more titania particles were incorporated in the composite coatings and this promoted the nickel crystals growth on [1 0 0] planes and consequently the coatings preferred orientation changed from the [2 1 1] to [1 0 0] + [2 1 1] planes.     

Effect of Bi0.75Y0.25O1.5 electrolyte additive in collector layer to properties of bilayer composite cathodes of solid oxide fuel cells based on La(Sr)MnO3 and La(Sr)Fe(Co)O3 compounds

The sintering features, electroconductivity, and electrochemical characteristics of bilayer electrodes with functional composite layers based on La(Sr)MnO₃ (LSM) and La(Sr)Fe(Co)O₃ with LSM collector layer and Bi(Y)O_1.5 (YDB) electrolyte additive in contact with Ce (Sm)O₂(SDC), La(Sr)Ga(Mg)O₃, and Zr(Sc)O₂ electrolytes were studied. YDB additive to the electrode collector layer was shown to produce a positive effect to the properties of the studied electrode systems. The maximum electrochemical activity and electroconductivity was observed for the electrodes with 5 wt % of YDB electrolyte additive in the collector layer. Thus, electroconductivity of electrodes is almost doubled and 100 mV cathode overvoltage current density is increased by 30% at the temperatures of 800 to 900°C and up to 10-fold at 650 to 700°C. The collector layer sintering temperature of bilayer electrodes can be reduced from 1150 to 1000°C without loss of electrochemical activity. The service life tests (about 1200 h) of composite electrodes with LSM2-SDC functional layer and 90% LSM2 + 10% YDB collector layer in contact with SDC electrolyte showed the time dependences of polarization resistance tending to saturation and described with damped exponent. Original Russian Text ? N.M. Bogdanovich, D.I. Bronin, G.K. Vdovin, I.Yu. Yaroslavtsev, B.L. Kuzin, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 4, pp. 486–494.     

Theoretical and Experimental Study of the Homogeneous Catalytic Oxidation of Nicotinamide Adenine Dinucleotide (NADH) at Spherical Gold Electrodes Using Linear Sweep Voltammetry and Chronopotentiometry

The homogeneous catalytic oxidation of coenzyme nicotinamide adenine dinucleotide (NADH) with dopamine at a thiolactic acid modified gold spherical electrode of radius 0.0128 cm has been studied in aqueous phosphate media (pH 7.0) using linear sweep voltammetry and chronopotentiometry with power time and exponential time currents. We have proved that, under the conditions of this study, the steady state is reached, in such a way that the expressions for the I?E and dI/dE?E responses are simplified and become independent of time and the electrochemical technique employed. Therefore, the experimental I?E and dI/dE?E curves obtained in chronopotentiometry and linear sweep voltammetry are practically superimposable and, from easy expressions for the peak parameters of the dI/dE?E curves deduced in this paper, values of the chemical and electrochemical rate constants have been obtained.     

Properties of Zinc alloy electrodeposits produced from acid and alkaline electrolytes

Zinc–cobalt (Zn–Co) and zinc–nickel (Zn–Ni) alloy electrodeposits each prepared from acid and alkaline formulations were compared for their properties. Compared to alkaline baths, acid baths offer higher metal percent of the alloying element and higher current efficiency. In alkaline baths, the variation of metal percent in deposit with current density is less significant, but that of current efficiency with current density is more. Electrolyte pH does not change significantly in alkaline solutions compared to acid solutions. X-ray diffraction evaluation of Zn–Co deposits from both electrolytes indicated their presence in the η-phase, while Zn–Ni shows pure γ-phase for deposits obtained from alkaline solutions and the existence of γ-phase with traces of η-phase of zinc for deposits obtained from the acid electrolytes. Scanning electron microscope examination shows finer grain structure for deposits obtained from alkaline solutions, and atomic force microscope studies confirm their nanostructure with reduced surface roughness. Deposits obtained from the alkaline baths exhibited higher corrosion resistance probably due to their nanostructure.     

Learning Electricity with NIELS: Thinking with Electrons and Thinking in Levels

Electricity is regarded as one of the most challenging topics for students of all ages. Several researchers have suggested that na?ve misconceptions about electricity stem from a deep incommensurability (Slotta and Chi 2006; Chi 2005) or incompatibility (Chi et al. 1994) between na?ve and expert knowledge structures. In this paper we argue that adopting an emergent levels-based perspective as proposed by Wilensky and Resnick (1999), allows us to reconceive commonly noted misconceptions in electricity as behavioral evidences of “slippage between levels,” i.e., these misconceptions appear when otherwise productive knowledge elements are sometimes activated inappropriately due to certain macro-level phenomenological cues only. We then introduce NIELS (NetLogo Investigations In Electromagnetism), a curriculum of emergent multi-agent-based computational models. NIELS models represent phenomena such as electric current and resistance as emergent from simple, body-syntonic interactions between electrons and other charges in a circuit. We discuss results from a pilot implementation of NIELS in an undergraduate physics course, that highlight the ability of an emergent levels-based approach to provide students with a deep, expert-like understanding of the relevant phenomena by bootstrapping, rather than discarding their existing repertoire of intuitive knowledge.