Flux-pinning Centers Research Articles

Role of defects in increasing the critical current density of reel-to-reel PLD (Eu,Er)Ba2Cu3Oy+BaHfO3-coated conductors

Given their excellent superconducting properties, REBa2Cu3Oy (REBCO)-coated conductors (CCs) are anticipated to be utilized in a variety of magnet applications. To further increase the critical current density J c of these materials to levels needed for commercial applications, this study employs reel-to-reel (RTR) pulsed laser deposition (PLD) to fabricate REBCO+BaHfO3 (BHO) CCs. PLD creates BHO nanorods, which serve as flux-pinning defects. The material is subjected to O2+ irradiation to introduce more defects. The irradiation-induced defects serve as flux-pinning centers to the REBCO+BHO-nanorod CCs, increasing J c along the c axis and over a wide range of magnetic-field angles compared with conventional REBCO+BHO-nanorod CCs. Both nanorods and irradiation-induced defects are demonstrated to be effective pinning centers in this material.

Reduced Critical Current Anisotropy and Improved Critical Current Performance in a Combined Pinning Landscape Created by Proton and Silver Irradiation

Particle irradiation using light ions and heavy ions is found to be an effective method to introduce flux-pinning centers into REBCO films and coated conductors. The degree of enhanced critical current at various conditions depends upon the size, morphology, and orientation of ion tracks. Proton irradiation to the optimised fluence results in greater isotropic enhancement at lower temperatures, the enhancement decreases as temperature increases. Silver ion irradiation on the other hand gives a greater enhancement at higher temperature but limited to particular angular ranges. We compare the results of these two types of irradiation and then produce a mixed pinning landscape with a combination of the two. We find a nearly isotropic enhancement in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> at lower temperatures and an enhancement about the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis direction, similar but broader than silver irradiation alone, at higher temperatures.

Superconductivity and magnetism in a novel La0.85Ga0.15FeAsO compound

We report magnetization measurements on samples of La0.85Ga0.15FeAsO in the wide temperature range of 10 K-400 K, up to the field of 90 kOe having been prepared by solid-state reaction. This novel compound crystallizes with the tetragonal ZrCuSiAs-type structure. The crystal structure was determined by X-ray powder diffraction (P4/nmm: a = 4.032 Å, and c = 8.741 Å).Temperature dependence of magnetization for this compound is measured with zero-field cooling (ZFC) and field cooling (FC) with an application of H = 50 Oe. The large bifurcation between the ZFC and FC curves has been observed starting at around 370 K. At lower temperatures below Tscset−on∼37 K, superconductivity and magnetism coexist. Intermediate temperature range (37 K< T < 300 K), the sample is in a mixed state of the ferromagnetic clusters with paramagnetic moments, described well by the Modified Curie Weiss expression. At higher temperatures above 360 K, it is a pure paramagnetic, described perfectly by the simple Curie-Weiss expression.The natures of the superconducting flux-pinning centers are revealed based on analysis of the pinning force, Fpin at T = 10 K. The normalized pinning force, Fp/Fp, max versus H indicates the filamentary character owing to the pinning parameters, p = 0.5, q = 2. These results point out that, in the superconductivity, the dominance of grain boundary with single-vortex pinning (μ =1/7) mechanismThe Curie temperature, Tcurie was estimated using the modified Arrot plots. The critical exponents, β, ϒ, and δ were estimated, and their values are in close agreement with those predicted for a mean-field-like behavior at a magnetic phase transition.

Effects of Pt and La doping on the superconductivity of CaFe2As2: Kondo effects

We report the magnetic properties of CaFe2As2, CaFe1·9Pt0·1As2, and Ca0·9La0·1Fe1·9Pt0·1As2 compounds in a wide temperature range of 5–300 K in the field up to 9 T. We have observed a well-defined anomaly in the resistivity around 165 K, which manifests clearly a well-known first order magneto structural phase transition from paramagnetic-tetragonal (T) state to antiferromagnetic orthorhombic (Ort) one. Pt doped and Pt and La co-doped CaFe2As2 samples exhibit a Kondo lattice behavior. The critical current density, Jc, and normalized pinning force Fp/Fp,max as a function of the applied field are obtained. Moreover, the normalized pinning force density, Fp/Fp,max curves versus h = H/Hirr were scaled using the Dew-Hughes model. Hence, the natures of the flux-pinning centers for these samples are revealed. Based on the results obtained from these analyses, we claim that the superconductivity in as-prepared CaFe2As2 occurs on the grain-boundary with the filamentary character.

Carbon-Coating Layers on Boron Generated High Critical Current Density in MgB2 Superconductor.

Boron particles with a homogeneous carbon-coating layer were employed as the precursor to fabricate MgB2 superconductors to generate artificial two-dimensional (2D) flux-pinning centers. Systematic microstructure investigation reveals that the carbon layers are well-distributed in the MgB2 matrix without agglomeration. The thickness of the carbon layers is smaller than the MgB2 coherent length, which makes them transparent to supercurrent. The critical current density is increased because of the strong flux-pinning effects of the 2D carbon layers in the superconductor as highly efficient flux-pinning centers and the increased irreversibility field due to the carbon-doping effects.

Angular Dependence of the Critical Current Density of Nb Thin Films of Different Thicknesses with InAs Nanorods as Flux Pinning Centers

Angular Dependence of the Critical Current Density of Nb Thin Films of Different Thicknesses with InAs Nanorods as Flux Pinning Centers

Miniaturization of BaHfO3 nanoparticles in YBa2Cu3Oy-coated conductors using a two-step heating process in the TFA-MOD method

The critical current density (Jc) of YBa2Cu3Oy (YBCO)-coated conductors in a magnetic field can be enhanced by the doping of flux-pinning centers in a metal organic deposition (MOD) process with trifluoroacetates (TFA). The size of these flux-pinning centers should be less than 10 nm to achieve commercial use due to the coherence length of YBCO at 77 K. In this paper, BaHfO3 (BHO) nanoparticles were introduced into YBCO films using the TFA-MOD method. Microstructures and the Jc properties of the films prepared using a two-step heating process at crystallization were compared with film prepared using a conventional one-step heating process. The two-step heating process produced 15 nm average-sized BHO nanoparticles in the film compared to 19 nm nanoparticles in a film prepared using a one-step process. It was revealed that the size of nanoparticles in the films could be miniaturized by improving the heating processes in the MOD method, and the miniaturized nanoparticles could contribute to increased Jc in magnetic fields. The mechanism of miniaturization is also discussed based on microstructure observations of quenched films.

Effect of Tungsten (W) Substitution on the Physical Properties of Bi-(2223) Superconductors

The superconducting Bi1.6Pb0.4Sr2Ca2Cu3−x W x O10+y (x=0.00, 0.05, 0.10, 0.15) bulk samples were prepared by the solid-state reaction method. The effects of W substitution on the BSCCO system have been investigated by the electrical resistivity (ρ-T), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), magnetic hysteresis and critical current density measurements. It has been found that the Bi-(2212) low-T C phase is formed for all the substitution levels, together with the Bi-(2223) high-T C phase. The results obtained from the XRD data show that the Bi-(2223) phase gradually transforms into the Bi-(2212) phase with increasing W substituting for Cu. In addition, from the magnetization measurements at the temperatures below the zero resistance temperatures of the samples, we have observed that a decreasing in magnitude of |M| with the increasing measurement temperature and W concentration. Therefore, the decreasing of |M| related to superconducting volume seems to imply an existence of flux-pinning centers in our samples.

Flux-pinning properties of YBa2Cu3O7−x multilayered films with Y2O3 and ZnO nanoparticle inclusions

We investigated the flux-pinning properties of YBa2Cu3O7−x (YBCO) films in which Y2O3 and ZnO nanoparticles were uniformly embedded by using the pulsed laser deposition (PLD) technique. Y2O3 and ZnO nanoparticles with average sizes of 10–30 nm and with different densities by changing the number of nanoparticle layers were introduced in a YBCO matrix in the form of a multilayered structure. The total thickness of the YBCO superconducting layers was kept to be 250 nm. Dilute doping was found to be effective for both Y2O3 and ZnO nanoparticles. For Y2O3 doping, critical current density (Jc) enhancements over a wide range of fields were obtained in 1- and 5-layer doped samples, while weak Jc enhancements and a better field dependence of Jc were found in the YBCO film with 1 layer of ZnO. Based on an analysis of surface morphology, the transition temperature (Tc), and Jc, we discuss the difference between the two types of nanoparticles as flux-pinning centers.

Suppression of dissipative flux-motion in a high-Tc (Bi,Pb)-2212 superconductor by Dy-doping

The temperature and magnetic field dependences of the activation energy and flux-pinning properties of Dy-doped (Bi,Pb)-2212 superconductor are systematically measured for a broad doping range. The behaviour of vortex dynamics close to a vortex-liquid to glass transition [ U o ( B , T g ) = k B T g ] is analysed by the modified thermally activated flux-creep theory. The results show that the temperature and field dependence of U o ( B , T ) is related to the strong thermal fluctuation at elevated temperatures and fields which can be suppressed by the introduction of the point defects due to Dy-doping. These point defects act as additional flux-pinning centers and the superconductor remains in the glassy-vortex-state.

Fabrication and Superconducting Properties of Alternately-layered MgB2/Ni Thin Films with Different Ni-layer Spacing

We will show the superconducting properties of alternately-layered MgB2/Ni thin films inserted as very thin (1 nm) nickel layers between MgB2 layers a few tens of nanometers thick. The MgB2/Ni thin films were prepared on silicon (100) substrates by sequentially switching electron-beam evaporation and coaxial vacuum arc evaporation techniques without post-annealing. In this study, we prepared alternately layered MgB2/Ni thin films with varying Ni-layer spacings. The Ni layer spacings were set to 32, 23 and 16 nm, respectively. The MgB2/Ni thin films were neither inter-diffusion nor chemical reactions between MgB2 and Ni. Clear enhancements of the Jc were observed in the MgB2/Ni thin films when the magnetic fields were applied parallel to the inserted Ni layers, and the peak positions in the Fp–B curves shifted to higher magnetic fields with the decrease of the Ni-layer spacing. These results clearly indicate that the Ni layers inserted in alternately-layered MgB2/Ni thin films work as very effective flux-pinning centers.

Material modifications induced by swift heavy ions in NbTi superconducting wires

The Facility for Antiproton and Ion Research (FAIR) to be built at GSI in Darmstadt will be equipped with superconducting magnets. Due to the high ion beam intensities and related beam losses, radiation damage of the Cu/NbTi superconducting wires used in the magnet coils has to be considered. Here we report first experimental results on NbTi multifilament wires exposed to 2.6 GeV U ions at room temperature. Radiation-induced effects were examined using X-ray diffraction and transmission electron microscopy. With increasing ion fluence the amount of the α-Ti phase decreases. This is crucial because α-Ti precipitates are the main flux-pinning centres in the Nb–Ti superconducting alloy. The equilibrium hcp α-Ti transforms into the hexagonal ω-Ti phase.

Critical current density and flux pinning in aBi1.7Pb0.4Sr2−xLaxCa1.1Cu2.1Oy system

The critical current density of Bi2Sr2CaCu2O8+δ (Bi-2212) superconductors can be enhanced either by changing the charge carrierconcentration (holes) to the optimum value, from the overdoped state, or by creatingflux pinning centres to inhibit the flux flow in magnetic fields. In this work wesubstitute the rare earth La at the Sr site of Pb-doped Bi-2212 [(Bi, Pb)-2212].La3+ ionsat the Sr2+ site decrease the hole concentration to the optimum value and enhance the superconductingproperties. Also La at the Sr site creates point defects which act as flux pinningcentres. These dual effects of La at the Sr site highly enhance the transportJC in self and applied magnetic fields.

Strongly enhanced critical current density inMgB2/Fe tapes by stearic acid and stearate doping

Fe-sheathed MgB2 tapes with cheap stearic acid, Mg stearate and Zn stearate doping wereprepared through the in situ powder-in-tube method. It is found thatJc,Hirr and Hc2 were significantly enhanced by doping. Compared to the pure tapes,Jc for allthe doped samples was improved by more than an order of magnitude in a high-field regime. At 4.2 K, thetransport Jc reached 2.02 × 104 A cm−2 at 10 T for the Zn stearate doped tapes and3.72 × 103 A cm−2 at 14 T for the stearic acid doped samples, respectively. Moreover, at 20 K,Hirr for the Zn stearate doped tape achieved 10 T, which is comparableto that of the commercial NbTi at 4.2 K. The improvement ofJc–H properties in doped samples can be attributed to the increase ofHc2 resulting from the incorporation of C atoms into theMgB2 crystal lattice as well as a high density of flux-pinning centres.

Superconducting properties of single-crystal Nb sphere formed by large-undercooling solidification process

Abstract An electrostatic levitation (ESL) system has been used for investigating undercooling effects on superconducting materials. In this report, preliminary experiments on Nb (melting temperature: T m =2477 °C) have been performed by melting Nb in levitation using 150 and 250 W Nd-YAG lasers. Since molten Nb is solidified without any contact in a high vacuum condition, a significantly undercooled state up to 400 °C is maintained before recalescence followed by solidification. Spherical single crystals of Nb are formed by the ESL process due to the suppression of heterogeneous nucleation. The field dependence of magnetization of Nb shows a reversible behavior as an ideal type II superconductor, implying that it contains almost no flux-pinning centers.

Ideal Type II Superconductivity of Single-Crystal Niobium Spheres Solidified from the Large Undercooled State

We have developed an electrostatic levitation furnace for performing a containerless solidification process on superconductors. Spherical niobium is solidified from the large undercooled state at approximately around 400°C below the melting temperature. The morphologies show homogeneous cellular structures over the entire surface. X-ray Laue photographs indicate that the spherical niobium is composed of a single grain. Magnetization below the superconducting transition temperature shows perfect diamagnetism in the low magnetic field independent of whether loading is carried out under field cooling or zero-field cooling conditions. The field dependence of magnetization shows almost reversible behavior as that of an ideal type II superconductor. These results suggest that the spherical niobium formed from the large undercooled state in this study has a very small number of flux-pinning centers compared to that formed by conventional arc melting.

Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2 pellets, filaments and thin films

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO3 substrate. Thick plate-shaped grains with an aspect ratio of ∼3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (∼0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (∼10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 103 times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high-Tc superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB2 and indeed act as flux-pinning centres, which enhance the critical current density.

リソグラフィーによりＮｂ薄膜に導入した周期的な人工磁束ピンニングセンターの効果

Artificial flux-pinning centers have been introduced into NbTi filaments through the drawing process for the achievement of high critical current density. We discussed periodic patterns fabricated by a lithography instead of the usual drawing process as effective artificial flux-pinning centers for practical superconductors. Periodic grooves and also lattice like holes with various depths were formed on Nb films for evaluation of the flux-pinning effect. The Nb films with 0.22μm thickness were developed on MgO (100) by electron-beam evaporation. The flux-pinning properties of the fabricated Nb films were measured by using a SQUID magnetometer. It is found that the magnetization of the fabricated Nb films increased; thus the patterns work effectually as artificial flux-pinning centers. The increase of magnetization of the film with a groove pattern was two times as large as that of the film with a hole pattern. It is explained from the effectiveness of the groove pattern for the Lorentz force. The magnetization of films with both patterns increased until fabrication depth reach half the film thickness. It is considered that this increase of magnetization is caused from an increase of elementary pinning force for each flux line.

Natural strong pinning sites in laser-ablatedYBa2Cu3O7−δthin films

At low temperatures dislocations are the dominant flux-pinning centers in thin films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ deposited on (100) ${\mathrm{SrTiO}}_{3}$ substrates [B. Dam et al., Nature (London) 399, 439 (1999)]. Using a wet-chemical etching technique in combination with atomic force microscopy, both the length and the lateral dislocation distribution are determined in laser ablated ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. We find that (i) dislocations are induced in the first stages of film growth, i.e., close to the substrate-film interface, and persist all the way up to the film surface parallel to the c axis, resulting in a uniform length distribution, and (ii) the radial dislocation distribution function exhibits a universal behavior: it approaches zero at small distances, indicating short-range ordering of the defects. This self-organization of the growth-induced correlated disorder makes epitaxial films completely different from single crystals with randomly distributed columnar defects created by means of heavy-ion irradiation. Since the substrate temperature can be used to tune the dislocation density ${n}_{\mathrm{disl}}$ over almost two orders of magnitude (\ensuremath{\sim}1--100/\ensuremath{\mu}${\mathrm{m}}^{2}$), the mechanism by which dislocations are induced is closely related to the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.

Flux-pinning enhancement in Ag-sheathed Bi-2223 tapes by nanometer-SiC addition

We have studied the influence of nanometer SiC particles on the flux pinning and the magnetization critical current density J cm of Ag-sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x superconducting tapes. The results show that below 40 K SiC particles up to 0.5 wt.% introduce additional flux-pinning centres, and enhance J cm to different extent dependent on the temperature and applied magnetic field. For the tapes with SiC addition up to 0.5 wt.%, the magnetization critical current density J cm at 5 K in applied fields up to 50 kOe is twice that of the tape without SiC, for both H∥ c and H∥ ab. A study of the magnetization decay shows that tape with 0.15 wt.% nanometer SiC particles has a reduced flux-creep rate and increased pinning potential compared with the tape without SiC. The enhanced pinning properties can be attributed to the small SiC particles dispersed in the superconducting matrix and to the secondary defects at the interface between SiC particles and the superconducting matrix created by the introduction of the SiC particles.