Enhancement Of Critical Current Density Research Articles

Variation of pinning mechanism and enhancement of critical current density in MgB2 bulk containing self-generated coherent MgB4 impurity

Bulk MgB2, with self-generated MgB4 pinning centers, have experienced two-step sintering process, initially at 750 °C and then 900–1000 °C. On the contrary to the widely accepted point that MgB4 deteriorates superconductivity, it was found that MgB4 played a significant role in enhancing critical current density. The precipitation pattern of MgB4 was studied from the lattice scale images. It was observed that the initial coherent relation between the MgB4 and the matrix was destroyed to become semi-coherent and even incoherent as the second-step sintering temperature increased. Owing to the lattice distortion caused by the elastic accommodation of the coherent interface, the small-sized MgB4 particles controlled by the sintering temperature, and the fine grain connectivity affected by the porosity, the critical current density was improved over the entire magnetic field. Finally, the dominating pinning mechanism within the crystal was confirmed to be Δκ pinning in the two-step sintered MgB2 sample, where the κ is the Ginzburg-Landau parameter, while the mechanism of one-step sintered sample is surface pinning.

Significant enhancement of critical current density by effective carbon-doping in MgB2thin films

The pure and carbon (C)-doped <TEX>$MgB_2$</TEX> thin films were fabricated on <TEX>$Al_2O_3$</TEX> (0001) substrates at a temperature of <TEX>$650^{\circ}C$</TEX> by using hot-filament-assisted hybrid physical-chemical vapor deposition technique. The <TEX>$T_c$</TEX> value for pure <TEX>$MgB_2$</TEX> film is 38.5 K, while it is between 30 and 35 K for carbon-doped <TEX>$MgB_2$</TEX> films. Expansion in c-axis lattice parameter was observed with increase in carbon doping concentration which is in contrast to carbon-doped <TEX>$MgB_2$</TEX> single crystals. Significant enhancement in the critical current density was obtained for C-doped <TEX>$MgB_2$</TEX> films as compared to the undoped <TEX>$MgB_2$</TEX> film. This enhancement is most probably due to the incorporation of C into <TEX>$MgB_2$</TEX> and the high density of grain boundaries, both help in the pinning of vortices and result in improved superconducting performance.

Enhancement of critical current density in superconducting YBa2Cu3O7−x films by nanostructure development of substrate surface using sol–gel method

In this paper, a simple sol–gel route has been adopted in developing substrate surface for the first time. Only by adjusting the concentration of precursor solution, LaAlO3 (LAO) nanodot arrays were directly obtained on LAO single crystal substrate, without incorporating additional processing steps. Subsequently, YBa2Cu3O7−x (YBCO) superconducting films were prepared on the nanodot arrays using a low-fluorine solution process. In contrast, J c of YBCO films grown on the substrate developed with nanodot arrays is almost three times as large as that of YBCO films grown on undeveloped substrate in an applied magnetic field with the strength of 3 Tesla. The pinning force density (F p ) of YBCO film on the nanodot arrays developed substrate is 2.7 GN/m3 which exceeds that (1.8 GN/m3) of YBCO films on undeveloped substrate. The results of cross-section transmission electron microscope (TEM) images show that the extended defects along the a–b planes of YBCO, induced by the developed substrate with nanodot arrays, should be the origin of the J c enhancement.

The enhancement of critical current density of powder-in-tube processed MgB2 tapes by pre-heating of Mg and B mixed powder

MgB2 tapes were fabricated by a powder-in-tube method using a powder mixture of Mg and B pre-heated at 400–650 ° C for 1–10 h under low oxygen partial pressure. Application of this pre-heating drastically enhanced Jc values. Fine MgO particles existed in the core of MgB2 tape prepared from the pre-heated powder. These MgO particles were formed by the reaction of B2O3 on the surface of B particles with Mg powder. The excellent Jc values can be attributed to both the flux pinning of these fine MgO and the increase of MgB2 volume fraction in the tapes.

Numerical Studies of Axial and Radial Magnetic Forces Between High Temperature Superconductors and a Magnetic Rotor

The magnetic bearing is considered as one of the most prospective applications of high temperature superconductors (HTSs) which can realize stable levitation in a magnetic field generated by permanent magnet devices or coils. The exploration of this kind of HTS bearing through numerical investigation is usually made by assuming the induced current circulates only within the ab-plane and thus simplifying the actual three-dimensional problem to a two-dimensional one. In this paper, on the basis of the three-dimensional model of the HTS bulk established before, we further introduce the developed finite-element software to calculate the magnetic field generated by a magnetic rotor which is composed of permanent magnet (PM) rings and ferromagnet (FM) shims, and in this way, we can investigate the magnetic forces (radial force and axial force) of a simplified HTS bearing model, i.e., two symmetric HTS bulks and a magnetic rotor, at a three-dimensional level. The investigations performed in this paper lead to the observations: the favorable configuration to construct the HTS bearing is that the axial height of each HTS element should be equivalent to the axial heights of PM ring plus FM shim; the increase of the radial thickness of PM ring will improve both the radial force and the axial force considerably, but its margin decreases; the enhancement of critical current density of HTSs due to the decrease of operating temperature can result in a higher increase of both the radial and axial force with a lower nominal gap between the HTSs and the magnetic rotor.

Effect of the Indium Addition on the Superconducting Property and the Impurity Phase in Polycrystalline SmFeAsO1-xFx

We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the bringing together the superconducting grains. It means the total contact surfaces of grains are increased. We suggest that the enhancement of the magnetic critical current density is a direct effect of the indium addition.

Critical Current Density Enhancement in High Temperature Superconductors by Flux Pinning

Technological applications of High Temperature Superconductors (HTSC) are limited by Creep, an undesired magnetic vortices movement, creating a pseudo-resistance that depresses both critical current density and critical field. For practical applications (like energy storage systems, current limiters, magnetic bearings, etc.), it is desired to have high values of Critical Current Density (JC) even at higher applied field. Flux Pinning is an established mechanism of enhancing JC in HTSC by Swift Heavy Ion irradiation. This paper explains as to how JC can be enhanced in HTSC and thereby make HTSC technologically more useful. Some recent works and our initiatives in this direction are also discussed. Keywords - Superconductivity, High temperature, Transport properties, Critical Currents, Flux Pinning

Composites of MgB2 with Bi2O3, Bi, Sb2O3, or Sb Obtained by Ex-situ Spark Plasma Sintering

Mixtures of MgB2 and metal or oxide additions with starting compositions of (MgB2)(M2O3)x, x = 0.0025, 0.005, 0.015, and (MgB2)(M)y, y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (Jc) and irreversibility field (Hirr) enhancement viewpoints, optimum additions are oxides for x=0.0025, 0.005. Both oxides improve Jc at high fields, but Sb2O3 is effective up to 10 K, whereas Bi2O3 is effective up to 30 K. Metal additions decrease Jc and Hirr when compared with a pristine MgB2 sample.

Composites of MgB2with Bi2O3, Bi, Sb2O3, or Sb Obtained byEx-situSpark Plasma Sintering

Mixtures of MgB2 and metal or oxide additions with starting compositions of (MgB2)(M2O3)x, x = 0.0025, 0.005, 0.015, and (MgB2)(M)y, y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (Jc) and irreversibility field (Hirr) enhancement viewpoints, optimum additions are oxides for x=0.0025, 0.005. Both oxides improve Jc at high fields, but Sb2O3 is effective up to 10 K, whereas Bi2O3 is effective up to 30 K. Metal additions decrease Jc and Hirr when compared with a pristine MgB2 sample.

Enhancement of critical current density in superconducting/magnetic multilayers with slow magnetic relaxation dynamics and large magnetic susceptibility

We propose to use superconductor-magnet multi-layer structure to achieve high critical current density by invoking polaronic mechanism of pinning. The magnetic layers should have large magnetic susceptibility to enhance the coupling between vortices and magnetization in magnetic layers. The relaxation of the magnetization should be slow. When the velocity of vortices is low, they are dressed by nonuniform magnetization and move as polarons. In this case, the viscosity of vortices proportional to the magnetic relaxation time is enhanced significantly. As velocity increases, the polarons dissociate and the viscosity drops to the usual Bardeen-Stephen one, resulting in a jump in the I-V curve. Experimentally the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. For Nb and proper magnet multi-layer structure, we estimate the critical current density $J_c\sim 10^{9}\ \rm{A/m^2}$ at magnetic field $B\approx 1$ T.

Enhancement of critical current density in glycine-doped MgB2 bulks

Samples of MgB2 + x wt% glycine (with x = 0, 2, 5 and 8) were sintered at 800 °C for 30 min. Obvious enhancement of critical current density (Jc) over the entire field was obtained in 5 wt% glycine-doped sample, indicating that grain connectivity and pinning effect were both improved. The improvement of Jc was attributed to the pinning effect by C substitution, MgO nano-particles, and relatively smaller MgB2 grains. Besides, 2 wt% glycine could only improve Jc at low field, while excessive glycine (above 5 wt%) would lead to unfavorable Jc performance.

Significant enhancement of critical current density in Gly-doped MgB2 bulk by tailoring the formation of MgO

A significant enhancement of critical current density ( J c ) was obtained in MgB 2 + 3 wt.% glycine samples with a large amount of MgO. The improvements in J c and in the irreversible magnetic field ( H irr ) are attributed to the pinning effects of high C-substitution level, nanoscale MgO, and to the even boundaries of small grains. This doping method overcomes the degradation of J c under a low field in previously reported SiC-doped samples. Moreover, the formation of MgO was demonstrated to be the result of the reactions between Mg, CO 2 and O 2 .

Enhancement of critical current density of liquid-infiltration-processed Y–Ba–Cu–O bulk superconductors using milled Y2BaCuO5 powder

The size effects of a precursor Y2BaCuO5 (Y211) powder on the microstructure and critical current density (Jc) of liquid infiltration growth (LIG)-processed YBa2Cu3O7−y (Y123) bulk superconductors were investigated in terms of milling time (t). Y211 powders were attrition-milled for 0–10 h in 2 h increments at a fixed rotation speed of 400 rpm. Y211 pre-forms were made by pelletizing the milled Y211 powders followed by subsequent sintering, after which an LIG process with top seeding was applied to the Y211/Ba3Cu5O8 (Y035) pre-forms. Spherical pores were observed in all LIG-processed Y123 samples and the pore density gradually decreased as t increased from 0 to 8 h. In addition to the reduced pore density, the Y211 particle size in the final Y123 products also decreased with increasing t. As t increased further to 10 h, unexpected Y211 coarsening and large pore evolutions were observed. The magnetic susceptibility–temperature curves showed that the onset superconducting transition temperature (Tc,onset) of all samples was the same (91.5 K), but the transition width became greater as t increased. The Jc of the Y123 bulk superconductors fabricated in this study was observed to correlate well with t of the Y211 precursor powder. The maximum Jc of 1.0 × 105 A cm−2 (at 77 K, 0 T) was achieved at t = 8 h, which is attributed to the reduction in pore density and Y211 particle size. The prolonged milling time of t = 10 h decreased the Jc of the LIG-processed Y123 superconductor owing to the evolution of large pores and exaggerated Y211 growth.

Properties of High-Energy Ball-Milled Fe–Se Based Superconductors

We have synthesized FeSe0.5Te0.5 superconductors by high-energy ball-milling and subsequent annealing. High-energy ball-milling of elemental powder mixtures resulted in the formation of metastable and/or nanocrystalline phases. Both XRD and DSC results show that the ball-milled powers were completely transformed to FeSe0.5Te0.5 with the grain size of a few nanometers during sintering at low temperatures. The resulting materials exhibited superconducting transition at 14 K. The enhancement of critical current density was observed for the high-energy ball-milled powder, compared with the un-milled powders.

Enhancement of critical current density and irreversibility field by Te or TeO2 addition to MgB2 bulk processed by spark plasma sintering

Te metal or TeO2 were added to MgB2 ((MgB2) + (A)x, x = 0.005, 0.01, 0.03, A = addition). The mixtures were processed by spark plasma sintering. The critical current density, Jc, of the samples containing the Te/TeO2 additions is enhanced at high fields and at temperatures up to 20 K. The curve of Jc(5 K, H) for the sample with optimum xTe = 0.01 shows no accelerated decrease up to 8 T (Jc(5 K, 8 T) = 1.8 × 103 A cm−2). The irreversibility field of the samples containing the Te/TeO2 additions is also enhanced.

Enhancement of in-field critical current density by irradiation of MeV-energy ions in YBCO films prepared by fluorine-free metal-organic deposition

Abstract We performed irradiation of epitaxial YBa2Cu3O7 films grown on CeO2-buffered SrTiO3 substrates by fluorine-free metal organic deposition with 3-MeV Fe2+ ions to study their effects on critical current density (Jc). We observed an increase of in-field Jc by up to 70%, which did not show a significant field-angle dependence. Since the ion energy used in this study is two orders of magnitude lower than those in majority of the previous works with a similar concept, our results indicate that the enhancement of Jc by ion irradiation in high-temperature superconducting films has wider processing window than previously recognized.

Superconducting composites of MgB2 with Additions obtained by Spark Plasma Sintering

ABSTRACTSb2O3, Sb-metal, Bi2O3 or Bi-metal, powders were mixed with MgB2 powder. Starting compositions were ((MgB2)(M2O3)x, x = 0.0025, 0.005, 0.015, and (MgB2)(M)y, y = 0.01, M = Sb, Bi. Mixtures were processed by Spark Plasma Sintering (SPS) technique. As obtained composite samples show high density, above 94% of the theoretical density. While the secondary phases indicate on similar reactions, samples show different behavior vs. addition type and amount. This does not directly correlate with the melting temperature of the addition. From the critical current density (Jc) and irreversibility field (Hirr) enhancement viewpoints, optimum additions are oxides for x=0.025, 0.005. Both oxides are improving Jc at high fields, but Sb2O3 is effective up to 10 K, while Bi2O3 is up to 30 K. Metal additions are decreasing Jc and Hirr when compared to pristine MgB2sample.

Enhancement of Intragrain Critical Current Density in Bi-Based Superconductor by Substitutional Structural Defects

We synthesized Ca-rich Bi-based superconducting whiskers by an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -seeded glassy quenched platelet method. The grown whiskers were precisely characterized by synchrotron radiation X-ray photoemission spectroscopy and high-resolution transmission electron microscopy. The Ca-rich Bi-based superconducting whiskers show a high critical current density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 40 K in self-field. We found that excess Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions substitute for the Sr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> sites and cause nano domains with shorter-period modulation within the conventional 2212 crystal structure. The embedded nano crystalline domains can result in structural distorted defects which work as strong pinning center.

Critical current density enhancement in strongly reactive ex situ MgB2 bulk and tapes prepared by high energy milling

The ex situ technique is promising for its ease of preparation, lowmaterial shrinkage during annealing and the fact that it is alreadysuperconducting without annealing. However the critical current density (Jc) needs to be enhanced.To improve the Jc values of commercial powder high energy milling was used—a method commonly use inindustry and easy to apply. It was found that high energy milling is successful in improvingJc inbulk and tapes. The reason for the improvement is reduced crystallite size and the large fraction offresh grain surfaces. In this paper we also experimentally investigate the problem of decompositionof MgB2 ex situ during heat treatment.

In-field Jc improvement by oxygen-free pyrene gas diffusion into highly dense MgB2 superconductor

Oxygen-free pyrene gas as a carbon (C) dopant was delinked and incorporated into highly dense MgB2 structure via a gas phase diffusion method. The technique offers the advantages that molecular C is homogeneously distributed into MgB2 and substituted at the boron sites without any severe deterioration of grain connectivity. The C substitution causes a significant shrinkage of the a-lattice parameter and an increase in the lattice strain, resulting in high disorder. The introduction of structural disorder as a result of C doping leads to a considerable enhancement of the in-field critical current density (Jc) and upper critical field.