YBCO Bulk Samples Research Articles

Alumina nano-inclusions as effective flux pinning centers in Y–Ba–Cu–O superconductor fabricated by seeded infiltration and growth

The YBa 2Cu 3O 7− x (Y123) textured bulk superconductors with various amounts of nanometer alumina particles were fabricated by a seeded infiltration and growth process. The addition of nanometer alumina was found to be effective for an improvement of the superconducting properties. The critical current density ( J c) values were increased twice in self field with a slight addition amount of nanometer alumina particles (maximum J c at 0.01 wt.% alumina addition). The present work suggests that the use of insulating inclusions in the nanometer sub-scale can stabilize the flux-line lattice and greatly enhance the pinning capabilities of the infiltrated samples. No refinement of Y211 particles was observed with alumina addition. The J c improvement by nanometer alumina inclusions is likely rendered to the insulating nano-pinning centers that have been successfully embedded into superconducting Y123 matrix. On the other hand, we examined the effect of the pinning centers size on the superconducting properties of infiltrated YBCO bulk samples. To this effect insulating nano-pinning centers with two different size distributions has been successfully incorporated within YBCO matrix of bulk superconductor by slightly doping with nano-particle alumina dispersions. Two alumina nano-particle dispersions with mean size diameters of about 20 nm and 130 nm were used. It was shown that the size of the pinning centers can affect considerably the J c performances and the pinning mechanism.

Embedding of nanoparticles as flux pinning sites in superconducting samples

Crystallographic orientations of thin films and bulk, melt-textured YBa 2Cu 3O x high- T c superconductors with embedded Y 2Ba 4Cu MO x ( M = Nb, Zr, Ag) nanoparticles are studied by electron backscatter diffraction. The Y 2BaCuO 5 particles exhibit no preferred orientation but have a strong negative influence on the matrix orientation. In contrast, the nanoparticles are not disturbing the texture of the YBCO matrix. For the bulk samples, depending on the preparation route, a different particle orientation with respect to the matrix is obtained. Untextured M-2411 nanoparticles are formed by solid state reaction during the melt process by adding oxides (Nb 2O 5 or Y 2O 3) to the precursor powder. Pre-formed Y 2Ba 4Cu MO x particles added to the precursor in the form of pre-reacted nanopowder, exhibit a dominant single orientation related to the surrounding YBCO matrix. In the case of thin films prepared using laser ablation with a bulk YBCO sample with embedded nanoparticles as a target, the nanoparticles are transferred into the thin film, but the transferred M-2411 particles are found to be randomly oriented. Nevertheless, these films exhibit increased flux pinning properties.

Fiber Bragg grating based spatially resolved characterization of flux-pinning-induced strain of disk-shaped bulk YBCO samples

A new optical method for the characterization of magnetostrictive effects in bulk superconductors will be presented. Several of wavelength encoded fiber Bragg gratings can be inscribed in one fiber with small spatial distances, which gives the possibility to measure magnetostrictive effects spatially resolved. To demonstrate the performance, samples with Bragg grating arrays glued onto the surface of an YBCO disk where tested in a superconducting solenoid at temperatures of 60 K and magnetic inductances of 6.5 T. For the first time the radius position dependent strain and not only sum effects could be measured. It was stated that in the remnant state both, elongating and compressive strain, occur. This is in agreement with our simulations.

The Numerical Modeling and Measurement of Demagnetization Effect in Bulk YBCO Superconductors Subjected to Transverse Field

In this paper, the crossed field demagnetization effect of the bulk type-II superconducting materials in a variety of external magnetic field condition has been investigated both numerically and experimentally. A melt-textured YBCO bulk sample was magnetized along the axial direction and after that a transverse field perpendicular to the central axis applied. The collapse of the original trapped field was measured using a quantum design physical property measurement system. A numerical solver based on critical state model is proposed to simulate the measured results.

Bulk YBCO growth monitored by in situ high-temperature magnetic susceptibility

In Situ high-temperature Susceptibility Measurement (ISSM) is a method to monitor the phase transformations in a material in real time depending on its susceptibility change. Examples are given where the effects of composition, heating rate and geometry on the growth of YBCO bulk sample have been investigated by ISSM. Differences in the behaviour during melting and solidification are shown for MTG and PMP samples because of their composition. The effect of processing condition is illustrated by higher heating rate increasing the temperature solidification onset, and the effect of geometry by a comparison of the thin-wall geometry of a drilled sample with a plain one, showing a faster growth rate.

Microstructure and superconductivity of highly orderedYBa2Cu3O7−δ nanowire arrays

In order to explore the fundamental properties of one-dimensionalnanostructured high-temperature superconductors and enhance their promisingapplications, a universal and general method for the synthesis of high-qualityYBa2Cu3O7−δ (YBCO) nanowire arrays is developed, which involves the combination of a novel sol–gelprocess to lower the crystallization temperature of YBCO, and porous anodic alumina(PAA) as an effective morphology-directing hard template. Field-emission scanning electronmicroscopy (FE-SEM) and transmission electron microscopy (TEM) results indicatethat the as-prepared YBCO nanowires have average diameters of about 50 nmand lengths up to several microns. The structures of the samples were analysedby x-ray diffraction (XRD), high-resolution transmission electron microscopy(HRTEM), energy-dispersive x-ray spectroscopy (EDX) and inductively coupledplasma (ICP) analysis, which indicate that the nanowires are well crystallizedwith orthorhombic YBCO-123 structure. The magnetization measurement underzero-field-cooled (ZFC) mode indicates that the superconducting transition temperature(Tc) of the nanowires is about 92 K, which is in agreement with that of a bulk YBCO sample.

Computation limits of Current Distribution in thick Superconducting Bulks from Magnetic Field Measurements

We present the latest progress of our computation method for critical currents in bulk YBCO samples from magnetic field measurements with Hall probe, based on discretization of the magnetization M and resolution of the linearized inverse Biot-Savart problem by QR inversion. While computationally harder than other linear/FFT methods, it is more robust in front of measurement errors, and allows the computation of detailed current maps without assuming any symmetry in the OXY plane. In particular, we are able to detect irregularities such as multiple domains, or the perturbation induced by a welding. If the current in the bulk sample is not homogenous along the OZ axis, we obtain the average along the OZ axis of the current density. Moreover, the inhomogeneity of the current along the vertical axis can be detected by the user by measuring the magnetic field on the side of the bulk sample and comparing it with that induced by the computed OZ-averaged current. Finally, we discuss what must be done in the measurement of the magnetic field and mathematical processing of the data to avoid numerical pitfalls, and discuss the limits to the computation of 3-dimensional current maps.

Influence of some metallic additives on the flux trapping ability of high temperature superconducting ceramics

By applying magnetic fields on high temperature superconducting (HTS) bulks, superconducting permanent magnets (SPMs) can be produced. Two relevant factors influencing the properties of the SPMs have been investigated: 1. The external magnetic field to be trapped by HTS bulks—that depends on the mode of the magnetization (static or pulse form). 2. The flux trapping ability of the HTS bulks (that is, which percentage of the external magnetic field can be trapped by the HTS materials)—that depends on and can be controlled by the composition and the preparation technique of the HTS bulks. YBaCuO base single domain bulks with CeSn and Ce + Ag additives were prepared aiming at the analyse of the influence of these additives on the flux trapping ability of Y 1Ba 2Cu 3O 7− x (Y123) superconducting bulks. In the reported stage of our long term R&D work we focused on the pulse magnetization of these superconducting bulks. A pulse generator equipped with set of magnetizing coils have been designed and built for supplying various pulse forms for magnetizing the bulks. Using this pulse magnetization equipment, a magnetic flux density as high as 4.0 T could be created by discharging a capacitor bank through ignitron switches into the magnetizing coil. The equipment is capable to generate variable pulse shapes. The YBCO bulk samples were magnetized in zero field cooled (ZFC mode). For cooling the samples, liquid nitrogen was used. The pulse magnetized bulks were measured and tested by field mapping.

Study by Hall probe mapping of the trapped flux modificationproduced by local heating in YBCO HTS bulks for differentsurface/volume ratios

The aim of this report is to compare the trapped field distribution under a local heatingcreated at the sample edge for different sample morphologies. Hall probe mappings of themagnetic induction trapped in YBCO bulk samples maintained out of thermal equilibriumwere performed on YBCO bulk single domains, YBCO single domains with regularlyspaced hole arrays, and YBCO superconducting foams. The capability of heatdraining was quantified by two criteria: the average induction decay and the size of the thermally affected zone caused by a local heating of the sample.Among the three investigated sample shapes, the drilled single domain displays a trappedinduction which is weakly affected by the local heating while displaying a high trappedfield. Finally, a simple numerical modelling of the heat flux spreading into a drilled sampleis used to suggest some design rules about the hole configuration and their size.

THE GROWTH TRACE OF SINGLE-DOMAIN YBCO BULK SUPERCONDUCTORS WITH C-AXIS NORMAL TO THE TOP SURFACE

The grown cross traces on the top surface of single-domain YBCO bulk sample with c-axis normal to the top surface has been clearly revealed. It is found that there are many fish-scale like YBCO layers, which are well connected each other. The c-axis is normal to the top surface of the sample, which indicates that YBCO crystal starts to grow from the center to the edge along a(or b) axis in all the four regions divided by across traces, and forms a 90° angle when the four regions meet each other, The clear traces observed is just because of the slightly difference in height between adjacent regions. A simple model has been afforded to explain the growth mechanism of single-domain YBCO bulks with c-axis normal to the top surface.

THE GROWTH TRACES OF SINGLE-DOMAIN YBCO BULK SUPERCONDUCTORS WITH C-AXIS PARALLEL TO THE TOP SURFACE

The morphology of the top surface and the vertical section of single-domain YBCO bulk samples with c-axis parallel to the top surface have been clearly revealed. The sample with c-axis parallel to the top surface grows epitaxial from the seed (in the center) to the edge in all the four regions divided by grown cross traces on the top surface, but the growth style is different in the adjacent regions. If YBCO crystal grows along a (or b) axis in one region, it must be grow along c-axis in the adjacent regions. The grown traces on the top surface forms a 78° angle when the four regions meet each other. The clear traces are formed by the different growth rates, styles along c-axis and a (or b) axis and also the slightly height difference of YBCO crystal between adjacent regions.

YBCO bulk superconductors prepared by top-seed PMP process

Top-seeded powder melting process (TSPMP) is an effective melting processing technique for the preparation of high J c bulk YBCO. The remarkable advantage of TSPMP is that it is very convenient to control the size and distribution of 211 particles within bulk YBCO. We observed the microstructure of TSPMP-YBCO sample and the mean size of 211 particles is about 500 nm and well distributed. The thickness of 123 planes is about 2 μm. The density of twins in line is about (1.2–1.6)×10 4 cm −1. We fabricated the YBCO bulk sample with dimensions of ∅30 mm×15 mm in batch by the TSPMP process. The maximum levitation force was measured to be 85 N at 77 K in 0.5 T, by using ∅28 mm×30 mm NdFeB magnet.

High Magnetic Fields in Superconducting Permanent Magnets

We report on the temperature dependence of the maximum trapped field B o(T) in the high-T c superconductor YBa2Cu3O7−x (YBCO). Trapped fields of bulk melt-textured YBCO samples are limited by their pinning behaviour and mechanical properties. The mechanical properties of bulk YBCO samples were improved by the addition of silver. Furthermore, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process. High trapped fields up to 13.3 T were measured at 33 K on the surface of a single YBCO disk containing Ag additions and reinforced with steel tubes. The flux pinning properties of this YBCO disk were improved by neutron irradiation and Zn doping resulting in a significant shift of the trapped field curve B o(T) to higher temperatures. In YBCO mini-magnets, consisting of two YBCO disks, maximum trapped fields up to 16.0 T were achieved at 24 K by a combination of Zn-doping and Ag addition. The improved reinforcement, which was used in this case, was found to withstand the maximum trapped field of 16 T without cracking.

Improvements in the mechanical properties of bulk YBCO superconductors with carbon fibre fabrics

We have measured the tensile strengths of bulk YBCO samples using a tensile test machine. We have found that the tensile strength was dramatically enhanced by wrapping the bulk with carbon fibre fabric prior to resin impregnation. We have also characterized the mechanical properties of large-grain YBCO with a diameter of 46 mm using the strain gauges. The electromagnetic stress was imposed on the bulk samples by reducing the external field from 7 T to 0 T at 65 K. The sample without resin was broken during the field-decreasing process with the maximum stress reaching 150 MPa. In contrast, the maximum stress was only 40 MPa for the resin-impregnated sample covered with the carbon fibre fabrics. These results show that wrapping the bulk YBCO sample with the carbon fibre fabrics may have a beneficial effect in stabilizing the mechanical properties in addition to enhancing the tensile strength.

Refinement mechanism of CeO 2 addition on Y 2BaCuO 5 particles in PMP YBCO bulks

YBCO bulk samples with a nominal composition of YBa 2Cu 3O 6+ x +15 wt.% Y 2BaCuO 5+0.5 wt.% CeO 2 have been prepared by the powder melting process technique and the microstructure has been investigated by using scanning electron microscopy and transmission electron microscopy. An obvious refinement effect resulting in Y 2BaCuO 5 particles with a mean diameter about 500 nm in textured YBCO bulks has been achieved by CeO 2 addition. About one-fourth of the Y 2BaCuO 5 particles were observed to contain at least one nanometric core which was proved to be Y 2O 3 by selected area diffraction. In addition, a BaCeO 3 phase with a square shape was found. Our experimental results provide further evidence for the heterogeneous nucleation mechanism of Y 2BaCuO 5 particles on Y 2O 3 previously generated through a reaction between CeO 2 and precursors, which was first proposed by Piñol et al. [4] for the melt texture growth process.

Synthesis of ultrafine Y 2BaCuO 5 powder and its incorporation into YBCO bulk by powder melting process

A solid state sintering process for the synthesis of ultrafine Y 2BaCuO 5 precursor has been developed and YBCO bulk samples containing Y 2BaCuO 5 particles less than 500 nm in diameter have been prepared successfully by powder melting process (PMP). The solid state sintering process consists of two steps: firstly, BaCuO 2 powder was synthesized by calcining the mixture of BaCO 3 and CuO powders; secondly, BaCuO 2 powder was well mixed with commercial Y 2O 3 in a molar ratio of 1:1, and sintered at 900°C for 14 h in flowing oxygen. After ball milling, ultrafine Y 2BaCuO 5 powder with an average diameter below 0.5 μm was produced. This kind of ultrafine Y 2BaCuO 5 powder was used as precursor and quasi-single crystal YBCO samples with a nominal composition of YBa 2Cu 3O 6+ δ +15 wt.% excess Y 2BaCuO 5+0.5 wt.% CeO 2 were prepared by PMP. Y 2BaCuO 5 particles less than 500 nm in diameter were found within textured YBCO matrix. A high density of twins with an average spacing of about 60–80 nm was also observed. Magnetization measurements have been performed on the sample and critical current densities ( J c) have been calculated based on the extended Bean critical state model. The sample exhibits J c values of 4.57×10 6 and 1.85×10 5 A/cm 2 in 1 T magnetic field at 20 and 77 K, respectively. Such high J c values are attributed to the fine 211 particles and related defects.

Pinning properties of quenched and melt growth method-YBCO bulk samples

A comparison between two different irradiation effects was accomplished on bulk YBCO samples synthesized with the quenched and melt growth method (QMG-YBCO) to investigate strong pinning properties. High-energy proton-irradiation introduces small defects comparable to unit cell size into the sample, and they act as effective pinning centers for all temperature range in low field around 1 T. No enhancement, however, can be seen in higher field range. The defects introduced with the irradiation reinforce the pinning properties of preexisting pinning centers randomly distributing in the sample. Column-like defects with 3.5 GeV Xe-irradiation were introduced but the pinning properties show no significant enhancement except higher temperature region. This indicates that pre-existing pinning centers are strong enough than the columnar defects. In higher field region, the contribution of columnar defects for pinning becomes relatively large. For both irradiation cases, almost pinning properties are determined by the pre-existing pinning centers.

Design aspects of a differential magnetic susceptometer forhigh temperature superconductors

The design of an ac magnetic induction susceptometer is describedand results given for bulk YBCO samples in parallel and perpendicular appliedfields and thin film YBCO samples in perpendicular applied fields. Thesusceptometer employs a double-differential technique designed to accept bulkor thin film samples mounted parallel or perpendicular to the applied field.Theoretical details are given regarding the particular methods used tocalculate the complex external and internal susceptibility, flux density andmagnetization and the validity of the quadrature detection technique.Performance details of the susceptometer are provided regarding the systemsensitivity for frequencies 20, 50 and 100 Hz. Complex susceptibility resultsare given as a function of applied field magnitude and frequency for thevarious specimens analysed. The results obtained are compared with Bean platetheory and the critical currents for intergranular and intragranular regionsare determined.

New permanent magnets

Permanent magnets play an important role and are widely spread in daily-life applications. Due to their very low costs, large availability of the row materials and their high chemical stability, hard ferrites are still dominant in the permanent magnet market although their relatively poor magnetic properties are a distinct disadvantage. Today's high-performance magnets are mostly made from Nd 2Fe 14B. The aim of research is to combine the large spontaneous magnetization of 3d metals with strong anisotropy fields known from rare-earth transition-metal compounds and, at the same time, to maintain a high value of the Curie temperature. However, the number of iron-rich rare-earth intermetallics is very limited and, consequently, not much success can be noted in this field for the last 10 years. One alternative concept is to use magnetic fields trapped in type II superconductors where much higher fields can be achieved compared to conventional rare-earth magnets. Very recently, we obtained a trapped field as high as 14.4 T in a melt-textured YBCO bulk sample of a few centimeters in diameter. This is the highest value ever achieved in a bulk superconductor. The trapped field of a superconductor is not governed by the Laplace equation and, therefore, levitation works without any additional (active) stabilization. The disadvantage of these magnets is their low working temperature (of liquid nitrogen and below).

Nd partial substitution in YBCO bulk samples with Y211 additions

In this study, cylindrical samples of YBa2Cu3O6+x/Y2BaCuO5 (Y123/Y211) composite with differentneodymium oxide additions (⩽5 wt%) were textured using the partialmelting zone method to investigate how these additions affected themorphology, structure and superconducting properties of the samples. Thedetermination of the melting and crystallization temperatures showed that theNd increases the melting temperature but maintains the crystallizationtemperature. Morphological analysis indicated that adding Nd2O3 up to 2 wt% does not destroy the principal features of YBCOsuperconductor samples (there is a homogeneous distribution of small Y211precipitates and large single domains are generated). Electron probemicroanalysis applied to the samples revealed that, within measurement error,the Nd substitutes the Y. Jc measurements showed that additions of up to2 wt% maintained the superconducting properties of the YBCO samples and evenimproved the Jc values.