Enhancement of mechanical strength in Y–Ba–Cu–O bulk superconductor through liquid binder addition

We have studied the effects of the liquid binder (polyvinyl alcohol) addition (0–10 wt%) on the mechanical properties of the green compacts and also on the superconducting properties of bulk Y–Ba–Cu–O superconductors of 20 mm diameter produced with the top-seeded melt growth (TSMG) process. The mechanical properties of the green compacts with binder addition were characterized with the compression tests, which revealed that mechanical strength increased dramatically with increasing the amount of the binder addition. The binder-added green compacts were then subjected to the TSMG process and oxygen annealing. The trapped field measurements showed that we could produce single-grain bulk Y–Ba–Cu–O samples with binder additions up to 8 wt% without any deterioration in the superconducting properties.     

Conceptual design of a novel insertion device using bulk superconducting magnet

An undulator or a wiggler with a strong magnetic field will play an important role in future synchrotron light sources, free electron lasers, and linear colliders. We proposed the bulk high critical temperature superconductor staggered array undulator (Bulk HTSC SAU) in order to generate a strong periodic field. The Bulk HTSC SAU consists of stacked bulk high-Tc superconductors (HTSs) and a solenoid magnet which is used to magnetize the bulk HTSs. A periodic magnetic field was produced and controlled using a prototype of the Bulk HTSC SAU using 11 pairs of REBaCuO bulk HTSs at 77 K. The expected performance at low temperatures around 20 K is calculated using a loop current model.     

Fast joining of melt textured Y–Ba–Cu–O bulks with high quality

Y–Ba–Cu–O bulk superconductors were fast joined using Ag-doped Y–Ba–Cu–O solder (metallic Ag powder additive). The joining process was relatively shorter (around 24 h) comparing with the traditional joining method. The microstructures and the superconducting properties of the joints were evaluated carefully. Microstructure analysis revealed that the crystal pattern in the joint was almost the same as that of the base material. This indicated that the bonding zone grew along the growth direction of the base material. The trapped-field distribution of the joined bulk was almost uniform and only single peak was found. This demonstrated that strong bonding was achieved in the joining process. The ratio of joined bulk’s levitation force to that of original base material was up to 93.5%.     

Large spontaneous Hall angle in [Co,CoFe/Pt] multilayer

We have quantitatively investigated the Hall effect in [Co, CoFe/Pt] multilayer films. The [Co, CoFe/Pt] multilayers exhibit large spontaneous Hall resistivity (ρ_H) and Hall angle (ρ_H/ρ). Even though the Hall resistivity in [Co, CoFe/Pt] multilayer films (2.7–4 × 10⁻⁷ Ω cm) is smaller than that of amorphous RE–TM alloy films which show large spontaneous Hall resistivity (<2 × 10⁻⁶ Ω cm), the Hall angle of multilayer (6–8%) is almost twice than that in amorphous rare earth–transition metal alloy films (∼3%). The Hall angle provides evidence of the effects of the exchange interaction of the Hall scattering. The exchange is between conduction electron spins and the localized spins of the transition metal. The large Hall angle of [Co, CoFe/Pt] multilayer can be considered due to the high spin polarization and high Curie temperature of Co and CoFe transition metal layers. Even though the role of interfaces and surfaces in the magnetic properties of multilayer films may dominate that of the bulk, the Hall effects in [Co, CoFe/Pt] multilayer may be mainly dominated by the bulk effect.     

Fabrication of single domain GdBCO bulks with different new kind of liquid sources by TSIG technique

Single domain GdBCO bulk superconductors have been fabricated with different new kinds of liquid sources by Top Seeded Infiltration and Growth process. A conventional liquid source is a mixture of REBa₂Cu₃O_7?x and Ba₃Cu₅O₈ in a ratio of 1:1; new liquid sources are made of a mixture of RE₂BaCuO₅ + 9BaCuO₂ + 6CuO or RE₂O₃ + 10BaCuO₂ + 6CuO. The microstructures and levitation forces of the GdBCO bulks have also been investigated. The properties of the samples fabricated are nearly the same as those of samples fabricated with conventional liquid sources, which indicates that the new liquid sources are very important to reduce the cost and improve the working efficient on the fabrication of single-domain GdBCO bulk superconductors.     

Elastic properties and chemical bonding in ternary arsenide SrFe2As2 and quaternary oxyarsenide LaFeAsO – Basic phases for new 38–55 K superconductors from first principles

Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related layered phases, namely, the ternary arsenide SrFe₂As₂ and the quaternary oxyarsenide LaFeAsO – basic phases for the newly discovered “1 2 2” and “1 1 1 1” 38–55 K superconductors. The independent elastic constants (C_ij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. The numerical estimates of the elastic parameters of the polycrystalline SrFe₂As₂ and LaFeAsO ceramics are performed for the first time. Additionally, the peculiarities of chemical bonding in these phases are discussed.     

Improvement of microstructure and mechanical properties of large single domain GdBaCuO superconductor with low void density

We fabricated large single domain of GdBaCuO (Gd-123) densified superconductor with a seeding and temperature gradient method in oxygen at the melt stage then in air at the growth stage for future applications such as a superconducting flywheel for energy storage system, magnetic separation, motor, current lead, and a bulk magnet. The innovative melt-textured single domain of Gd-123 system superconductor with low void density was about 46 mm in diameter and 15 mm in thickness. Gd-123 densified superconductors are expected to have high J_c and trapped magnetic field properties with higher mechanical strength. In this paper, we discuss processing, superconducting and mechanical properties of the densified Gd-123 bulk for improving the microstructure and strength of the RE-123 bulk. The relation between the microstructure and the flexural strength of the Gd-123 bulk was clearly shown.     

Gap-related trapped magnetic flux dependence between single and combined bulk superconductors

Aiming at examining the trapped-flux dependence between single and combined bulk superconductors for field-pole applications, three rectangular Y_1.65Ba₂Cu₃O_7?x (YBCO) bulks with a possibly compact combination were employed to investigate the trapped-flux characteristics of single and combined bulks with a field-cooling magnetization (FCM) method. A gap-related dependence was found between them. At lower gaps of 1 mm and 5 mm, the peak trapped fields and total magnetic flux of combined bulks are both smaller than the additive values of each single bulk, which can be ascribed to the demagnetization influences of the field around the bulk generated by the adjacent ones. While, at larger gaps like 10 mm, the situation becomes reversed. The combined bulks can attain bigger peak trapped fields as well as total magnetic flux, which indicates that the magnetic field by the bulk combination can reach higher gaps, thanks to the bigger magnetic energy compared with the single bulk. The presented results show that, on one hand, it is possible to estimate the total trapped magnetic flux of combined bulks by an approximate additive method of each single bulk while considering a demagnetization factor; on the other hand, it also means that the performance of combined bulks will be superior to the addition of each single bulk at larger gaps, thus preferable for large-scaled magnet applications.     

Superconductivity in CeO1−xFxFeAs with upper critical field of 94 T

We have successfully synthesized Ce based oxypnictide superconductors with fluorine doping (CeO_1?xF_xFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high H_c2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in H_c2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg–Landau coherence length is estimated to be ～27 Å at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron–electron correlations in this multiband superconductor.     

Synthesis under ambient pressure and tri-axial magnetic orientation in REBa2Cu4O8 (RE = Y,Sm, Eu,Gd, Dy,Ho, Er)

We report the rare-earth (RE)-dependent magnetization axes of REBa₂Cu₄O₈, which was synthesized by a flux method under ambient pressure, using powder samples tri-axially oriented in a modulated rotating magnetic field of 10 T. By optimizing the growth temperature and cooling rate, RE124 crystals were successfully grown for RE = Y, Sm, Eu, Gd, Dy, Ho, and Er. From the X-ray diffraction measurement, the magnetically oriented directions were largely dependent on the type of RE ions of RE124. However, the tri-axial magnetic anisotropies of RE124 could be qualitatively understood in terms of the magnitude relation between the single-ion magnetic anisotropy of RE³⁺ ions and the magnetic anisotropy generated by the CuO₂ plane and Cu–O chain. For the practical use of this magneto-scientific process, the control of magnetization axes and tri-axial magnetic anisotropies through crystallochemical control is indispensable.     

Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni₂X₂ (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi₂As₂. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T_c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.     

Magnetic tri-axial orientation in (Y1−xErx)2Ba4Cu7O15−y superconductors

We report the tri-axial grain-orientation effects under a modulated rotation magnetic field for (Y_1?xEr_x)₂Ba₄Cu₇O_y [(Y, Er)247]. The magnetic easy axis at room temperature was drastically changed around x ～ 0.1; however, the Er-doping levels for the conversion of magnetic easy axes from the c-axis to the ab-direction and from the a- to b-axes were quite different. Tri-axial single-ion magnetic anisotropy of Er³⁺ was roughly 10 times greater than tri-axial magnetic anisotropy generated by both the superconducting CuO₂ plane and the blocking Cu–O chain layer. An appropriate choice of rare-earth (RE) ions in RE-based cuprate superconductors enables the reduction of the required magnetic field for the production of bulks and thick films based on the magnetic orientation technique.     

Synthesis of carbon nanotubes via Fe-catalyzed pyrolysis of phenolic resin

Carbon nanotubes (CNTs) with 40–100 nm in diameter and tens of micrometers in length were prepared via catalytic pyrolysis of phenol resin in Ar at 673–1273 K using ferric nitrate as a catalyst precursor. Structure and morphology of pyrolyzed resin were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Ferric nitrate was transformed to Fe₃O₄ at 673 K, and to metallic Fe and Fe_xC carbide at 873–1273 K. The optimal weight ratio of Fe catalyst to phenol resin for growing CNTs was 1.00 wt%, and the optimal temperature was 1073 K. In addition, use of a high pressure increased the yield of CNTs. Density functional theory (DFT) calculations suggest that Fe catalysts facilitate the CNTs growth by increasing the bond length and weakening the bond strength in C₂H₄ via donating electrons to the C atoms in it.     

Band-gap engineering of ZnSe quantum dots via a non-TOP green synthesis by use of organometallic selenium compound

Single-step green synthesis of ZnSe quantum dots (QDs) from 1,2,3-selenadiazole and zinc acetate resulted in formation of high-quality mono-disperse ZnSe with engineered band-gap. The present method is a non-TOP green route where oleic acid is used as a surfactant. The size quantization effect can be monitored by UV–visible spectroscopy which shows in the range 370–387 nm (3.20–3.35 eV), a blue shift of about 70–90 nm with respect bulk ZnSe. Photoluminescence measurement revealed band-gap emission at ∼390 nm (3.18 eV, Stokes shift of <10 nm with FWHM <30 nm). Broadened XRD pattern indicated formation of cubic ZnSe and the estimation of particle size from the line broadening at 〈1 1 1〉 matched well the TEM analysis.     

Characterization of nanoporous carbon fibrous materials obtained by chemical activation of plane tree seed under ultrasonic irradiation

An ultrasonic irradiation was applied for the impregnation by chemical agents in the chemical activation process of new type of active carbon precursor. Plane tree seed, due to the unique fibrous structure and low cost is a promising eco-friendly raw material for the preparation of activated carbon materials. Ultrasonic irradiation was used for the impregnation step allowing the chemical activation by different agents: potassium or sodium hydroxide, hydrogen peroxide and pyrogallol. The porous structures were examined by nitrogen adsorption/desorption isotherms at 77 K and electrochemically by cyclic voltammetry. The textures of these materials were observed by scanning electron microscopy. The application of ultrasonic irradiation in the impregnation step increased surface area of the final material more than two times in comparison to the material which impregnation in the activation process was by conventional stirring. Ultrasonic irradiation enhances the chemical activation process and the activated carbon fibrous materials with nanoporous structure were obtained by impregnation of seeds with alkaline hydroxides. Total surface areas of these samples were 976 m² g⁻¹ and 1130 m² g⁻¹. These fibers have total specific capacitance as high as 125 F g⁻¹ and 53 F g⁻¹ which major fraction in both cases originate from internal micropores structure.     

Effect of spin paramagnetism on upper critical field of disordered nanocrystalline PbMo6S8 superconductor

Ignoring the paramagnetic effect Niu and Hampshire [H.J. Niu, D.P. Hampshire, Phys. Rev. Lett. 91 (2003) 027002] have claimed that a disordered nanocrystalline sample of the Chevrel phase superconductor PbMo₆S₈, having a grain size of 20 nm, corresponds to an upper critical field of 110 T. Whether the paramagnetic effect is indeed negligible in this sample or not has been investigated here by using the Werthamer–Helfand–Hohenberg theory of upper critical field of dirty superconductors. The extent of the paramagnetic effect is obtained by matching the experimental values of the upper critical field for various temperatures with those calculated from the Werthamer–Helfand–Hohenberg theory. It has been found that the 20 nm superconductor involves sufficient paramagnetic effect such that the value of the upper critical field cannot be larger than ～70 T.     

Sonochemical degradation of ethyl paraben in environmental samples: Statistically important parameters determining kinetics,by-products and pathways

The sonochemical degradation of ethyl paraben (EP), a representative of the parabens family, was investigated. Experiments were conducted at constant ultrasound frequency of 20 kHz and liquid bulk temperature of 30 °C in the following range of experimental conditions: EP concentration 250–1250 μg/L, ultrasound (US) density 20–60 W/L, reaction time up to 120 min, initial pH 3–8 and sodium persulfate 0–100 mg/L, either in ultrapure water or secondary treated wastewater.A factorial design methodology was adopted to elucidate the statistically important effects and their interactions and a full empirical model comprising seventeen terms was originally developed. Omitting several terms of lower significance, a reduced model that can reliably simulate the process was finally proposed; this includes EP concentration, reaction time, power density and initial pH, as well as the interactions (EP concentration) × (US density), (EP concentration) × (pH_o) and (EP concentration) × (time).Experiments at an increased EP concentration of 3.5 mg/L were also performed to identify degradation by-products. LC–TOF–MS analysis revealed that EP sonochemical degradation occurs through dealkylation of the ethyl chain to form methyl paraben, while successive hydroxylation of the aromatic ring yields 4-hydroxybenzoic, 2,4-dihydroxybenzoic and 3,4-dihydroxybenzoic acids. By-products are less toxic to bacterium V. fischeri than the parent compound.     

Optical and Raman spectroscopy studies on Fe-based superconductors

A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re = rare-earth element) and AFe₂As₂ (A = alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe₂As₂. Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron–phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the T-dependent mid-infrared peak at 0.6–0.7 eV, electronic correlation, and the similarities/differences between high-T_c cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.     

Ferrofluidic masking of solid state nuclear track detectors during etching

This work presents a novel method for determining bulk etch rate of CR-39 during prolonged etching by masking the surface with a ferrofluidic film held in position by magnetostatic forces. The CR-39 etching conditions were 6.25 M NaOH solution for 24 h at temperatures ranging from 50 to 80 °C. After etching, the heights of the resulting un-etched plateaus were measured using a Talyscan 150 profilometer. The removed layer thicknesses ranged from 12 to 85 μm, giving corresponding bulk etch rates in the range 0.5–3.54 μm/h.     

Vortex structure in Sr2RuO4

The vortex structure in p-wave superconductors is investigated by the Bogoliubov–de Gennes theory on a tight-binding model. We calculate the temperature dependence of the electronic state at each site in the vortex lattice state, and show the difference between sin p_x+i sin p_y-wave and sin p_x−i sin p_y-wave superconducting state. Furthermore the relation of the electronic structure and the site-dependence of the nuclear magnetic relaxation time is also discussed.