Melt-textured Bulk Superconductors Research Articles

Effect of Gd211 Addition on the Vertical Levitation Force of Gd123 Superconductors

We investigated the microstructure and magnetic properties of Gd123:Gd211 = 1-y : y (y = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) bulk superconductors with the diameter of 13 mm and thickness of 5 mm fabricated by melt-textured method. The vertical levitation force between melt-textured bulk superconductors and a permanent magnet is measured at 30, 40, 50, 60, and 77 K under zero-field-cooling (ZFC) and field-cooling (FC) regimes by Magnetic Levitation Force Measurement System (MLFMS). The phase and microstructures properties of all samples are determined by SEM (Scanning Electron Microscope). The measurements of superconducting characteristics showed that the maximum vertical levitation forces of the Gd01 sample were noted as 3.04, 2.30, 1.53, 0.91 and 0.16 N at the temperatures of 30, 40, 50, 60 and 77 K, respectively. The present results perform that a high-performance good-quality GdBCO can improve from laboratory conditions to technological applications for large scale levitated system.

Enhanced Mechanical Properties of Single-Domain YBCO Bulk Superconductors Processed With Artificial Holes

Single domain YBCO bulk superconductors were prepared using a conventional top-seeded melt growth technique. Artificial holes were introduced to the green sample prior to thermal processing using a bespoke “spiked” mould. Mechanical properties such as elastic modulus, Vickers hardness, compressive strength, and tensile strength were measured and compared to the properties of a standard bulk. The presence of the holes the bulk microstructure was observed to limit porosity and lower the concentration of macro-cracks in the bulk microstructure, resulting in significantly enhanced mechanical properties of the bulk single grains. The elastic modulus of the perforated bulks was observed to exhibit an increase of more than 45% compared to the standard samples. Compressive and tensile strengths were also improved significantly in the samples containing artificial holes. Observed differences in Vickers hardness, on the other hand, were negligible. This could be because of the fact that the hardness is measured on a small surface area of the single grain sample, where the effect of lower porosity and lower concentration of macro-cracks is less relevant. The introduction of artificial holes to the bulk, single grain microstructure appears to be a very promising technology for the production of melt-textured bulk superconductors with enhanced mechanical properties.

Effects of densification of precursor pellets on microstructures and critical current properties of YBCO melt-textured bulks

Effects of densification of precursor disks on the density of residual voids and critical current properties for YBCO melt-textured bulk superconductors were systematically investigated. Six YBCO bulks were prepared from precursor pellets with different initial particle sizes of YBa2Cu3Oy (Y123) powder and applied pressures for pelletization. It was revealed that use of finer Y123 powder and consolidation using cold-isostatic-pressing (CIP) with higher pressures result in reduction of residual voids at inner regions of bulks and enhance Jc especially under low fields below the second peak.

Field trapping property of HTS bulk magnet with reduced voids in pulsed field magnetizing process

The field trapping ability of melt-textured bulk superconductors is usually restricted by the mechanical properties because of the magnetic stress induced in the samples during the activation processes which give them intense magnetic fields of several T Therefore it is of crucial importance to enhance the toughness of the materials by means of reinforcing techniques. In this experiment, we adopted a dense Dy-based HTS bulk sample fabricated by means of controlling the concentration of voids inside during the heat treatment. A single magnetic pulse fields up to 7.66 T have been applied in the temperature range down to 30.6 K with use of GM refrigerator and the profiles of pulsed fields and the resultant trapped fields are discussed with respect to their performances. It was found that the trapped field grows as elongating rise time of the pulsed field and lowering initial temperature. The densely fabricated Dy123 sample was activated up to 1.98 T at 31 K in the same manner with no substantial difference from the porous Gd123 samples when the magnetic pulse field of 7.66 T was applied. The temperature rises up to 20 K are not substancial in comparison to those of the Gd123 compounds which exhibit the temperature rises more than 30 K at 40 K by applying 4.64 T This is attributed to the fact that the Jc value of the dense Dy123 are inferior to that of Gd123, which reflectes the field trapping ability.

Transmission Electron Microscopy and Atomic Force Microscopy Observation of Air-Processed GdBa2Cu3O7-δ Superconductors Doped with Metal Oxide Nanoparticles (Metal = Zr, Zn, and Sn)

Single-domain, c-axis-oriented 30-mm-diameter GdBa2Cu3O7-δ (Gd123) melt-textured bulk superconductors have been successfully grown by the top-seeded melt growth method from precursors of Gd123, Gd2BaCuO5, and Ag2O in air with doping of nanosized ZrO2, ZnO, or SnO2. Transmission electron microscopy (TEM) unveils a large amount of BaZrO3 or BaSnO3 particles with an average diameter of approximately 50 nm respectively embedded in ZrO2 or SnO2 doped samples, while no Zn-rich nanoparticles are observed in the ZnO-doped samples. The critical temperature Tc is almost unchanged up to a doping amount of 10 mol % for ZrO2- or SnO2-doped Gd123 melt-textured bulks, while ZnO-doped Gd123 becomes non-superconductive at this doping level. By atomic force microscopy (AFM), nanostripes with a wavelength of 15 nm are observed in the ZnO-doped Gd123 sample. Nanoscale particles in the grown Gd123 single domain, together with the micro-defects induced by nanoparticle doping, can account for the enhanced superconducting properties.

Near net shape processing of RE–Ba–Cu–O bulk superconductor by machining binder-added precursors

Several practical applications of melt-textured bulk superconductors require the complex-shaped products such as curved, ring-shaped, and drilled blocks rather than simple shaped pellets. In general, complex-shaped bulk products are prepared by machining melt-grown massive blocks. However, melt-textured bulks are often damaged when they are mechanically machined. With the aim of reducing such damages, we have investigated the preparation of the complex-shaped bulk superconductors by previously machining the precursors compacted with organic binders. The mixed powders of Y123 and Y211 were pressed into pellets, for which the organic binder was added are machined into complex-shaped bodies. After removal of the organic components, melt-processing was performed in air, which led to a reproducible fabrication of complex-shaped bulk products. The superconducting properties were not degraded with this process.

Design of microstructure in melt-textured bulk superconductors with the employment of granules containing ultra-fine Y 2BaCuO 5 particles

In melt-textured RE-Ba–Cu–O bulk superconductors, an excessive size reduction of RE211 inclusions results in inhomogeneous distribution of RE211 particles. To reduce such inhomogeneity, we attempted to combine granules containing fine RE211 into the melt-textured RE123 materials. First, we prepared Y–Ba–Cu–O granules containing Y123 and ultra-fine Y211 (0.1 μm in size) powders with organic binder. The size of the granules was about 1.0 mm. Next, Dy–Ba–Cu–O powders containing Dy123 and Dy211 (2 μm in size) were prepared. The Y–Ba–Cu–O granules and Dy–Ba–Cu–O powder were compacted together and melt-processed in air. We found that fine RE211 inclusions were uniformly distributed in this sample, which led to improved spatial homogeneity in critical currents.

Trapped field measurements on melt textured bulk superconductors in liquid nitrogen

The technique of measurng trapped fields B T by Hall probe scanning was carefully analyzed and several aspects were identified that influence the results in a significant way. Firstly, the influence of the active area of the Hall probes and of varying distances between the probes and the sample surface were investigated. Secondly, we find that the temperature of liquid nitrogen can vary by several degrees and needs to be monitored during the entire process, in order to reproducibly determine B T at 77.4 K. Thirdly, relaxation effects have to be taken into account. Finally, the method of establishing the full critical state (dc versus pulsed field activation, field cooling (fc) versus zero field cooling (zfc) plays an important role.

Influence of Y2BaCuO5 particles on the microstructure of YBa2Cu3O7-x (123)-Y2BaCuO5 (211) melt-textured superconductors.

The paper is devoted to the microstructural defects in melt-processed YBaCuO which can be active either as magnetic-flux-pinning centers or weak links. The influence of 211 particles in 123-211 melt-textured bulk superconductors on the microstructural features such as microcracking, subgrains, twins, and twin complexes was studied by polarized light microscopy. The observed linear dependence between a-b microcrack spacing, and ${\mathit{l}}_{\mathit{c}}$, and ${\mathit{d}}_{211}$\ensuremath{\cdot}${\mathit{V}}_{123}$/${\mathit{V}}_{211}$ (${\mathit{d}}_{211}$ means 211 particle size, ${\mathit{V}}_{123}$ means 123 volume fraction, ${\mathit{V}}_{211}$ means 211 volume fraction) proves that the microcracking process can be described under the framework of a model devised for the multiple failure of a unidirectional composite under uniaxial tensile loading. Shorter mean-free distance between 211 particles depresses the subgrain thickness, the twin complex size, and twin spacing. The level of residual tensile stress in the 123 a-b plane was estimated from detwinning observed around 211 particles.