Melt-textured Bulk Research Articles

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.

Magnetization and magneto-resistance measurements of bulk YBa 2Cu 3O 7− x in pulsed magnetic fields up to 50 T

A melt-textured bulk YBa 2Cu 3O 7− x sample has been investigated by magnetization M( H) and magneto-resistance R( H) measurements in pulsed magnetic fields up to 50 T. We describe a technique which allowed us to determine the irreversibility fields H irr from M( H) and R( H) curves. The irreversibility line shows linear behavior in the fields above 20 T which cannot be explained by present models.

Improvement of superconducting properties by Y-211 particle size and oxygenation temperature control for bulk Y–Ba–Cu–O

The effects of CeO 2 adding on the reduction of Y-211 particle size and the oxygenation temperature on T c have been studied. It is shown that the decrease of pre-sintering temperature is more effective than adding CeO 2 for the Y-211 particle size reduction. For bulk YBCO, the oxygenation temperature affects not only the critical temperature T c and other superconducting property, but also its microstructure. Our results show the low oxygenation temperature is suitable for the oxygenation process of melt-textured bulk YBCO. If the oxygenation temperature is above 600 °C, the macrocrack will propagate.

Superconductivity deterioration of single-domain Nd-system melt-textured bulk doped with Pt

We fabricated single-domain Nd-system superconductors by a seeding and temperature gradient method in 0.1% oxygen atmosphere. Pt doping effect on the superconductivity was investigated. In the sample doping with 1.0 wt.% of Pt, the T c (on-set) and the T c (off-set) were 93 and 87 K, respectively, and the T c (off-set) was several degrees lower than that of undoped sample, and the J c value in the magnetic field up to 1.3 T decreased to one-third of the undoped sample. It was suggested that the J c lowering came from a broad superconducting transition with the lowering T c (off-set).

Effect of CeO2 and Pt doping on the microstructure of Nd-Sm123 bulk superconductor fabricated in air

We have succeeded in fabricating a (Nd, Sm)-Ba–Cu–O bulk superconductor with high Tc and sharp superconducting transition using Ba-rich (Nd0.8, Sm0.2)3.6Ba2.4Cu1.8O9.6 (NS422ss) powder in air. The magnetic property of this melt-textured bulk was similar in Tc and superconducting transition to the bulk with the same nominal composition melt-textured under low oxygen partial pressure (pO2). The samples also showed relatively high Jc values with the secondary peak effect, although Jc values were slightly lower than the samples melt-textured in low pO2. It was also found that CeO2 and Pt doping was very effective in improving Jc values even for Nd-Sm123 bulk melt textured in air.

Nd-422 content dependence of superconductivity in a large-sized single-domain Nd-system superconductor

We investigated the effect of the molar ratio of the Nd4Ba2Cu2O10 (Nd-422) phase in a NdBa2Cu3Ox (Nd-123) superconductor on the superconducting properties of a large-sized single-domain Nd-system melt-textured bulk fabricated using a seeding and temperature gradient method in 0.1% oxygen atmosphere. The single domain of Nd-system superconductors, about 18 mm in diameter and 5 mm in thickness, was fabricated reproducibly. The maximum Jc value was about 4.5 × 104 A cm−2 at 77.3 K and self-field, and the maximum trapped field was about 0.34 T when the molar ratio of Nd-123/Nd-422 was 1/0.25.

Experimental and theoretical investigations on the field trapping capabilities of HTSC-bulk material

Melt-textured bulk samples of YBa 2Cu 3O 7− δ which have a high potential for magnetic applications are usually characterized by trapped field measurements. In this paper we present a fitting procedure which allows to eliminate the influence of different geometric parameters of the sample and the set-up and to calculate an effective value of the critical current density. A more general fitting procedure allows to calculate the spatial distribution of the critical current density j c.

超電導バルク材料開発の現状

The critical current density and trapped field of large grain RE123 bulk superconductors have drastically increased in the last few years. High-quality bulk superconducting magnets can trap a very high magnetic field of 3T at 77K and above 15T at 20K. The recent progress in bulk superconducting materials has led to many applications such as superconducting flywheel systems, current leads, superconducting motors, magnetic separation systems, magnetron sputtering machines. In this paper, we review recent research and development of the processing, critical current density, trapped field and mechanical properties of melt-textured bulk superconducting materials.

Effect of microstructure on the superconducting properties of Gd–Ba–Cu–O bulk fabricated in air

Bulk Gd–Ba–Cu–O superconductors were melt processed in air. The Gd 2BaCuO 5 (Gd211) particles dispersed in the melt-textured bulk could be refined by employing fine Gd211 starting powders. Since T c of the bulk samples was lowered with decreasing the size of Gd211 particle, J c was not improved. However, we found that a depression of T c was prevented by Ag addition, leading to higher J c values. The J c of Gd–Ba–Cu–O/Ag with Gd211 particles about 0.7 μ m in size exhibited 60 000 A/cm 2 at 77 K and self-field.

Grain growth and microstructure of large-domain RE-systems prepared by mixture of Y-123 and RE-211 or 422 powders

We prepared Y-123 melt-textured bulk with Sm-211, Gd-211 and Nd-422 powders. The grain growth and the microstructure of single-domain ( Y-123)+( RE-211 or 422 ) bulks were studied through scanning electron microscope and electron probe microanalyzer observations. Large-domains 10 mm in size were formed in all the ( Y-123)+( RE-211 or 422 ) systems using a seeding and temperature gradient method. The molar ratio of Y/RE changed at the interface between a single-domain and poly-grains. The molar ratio value of Y/Sm was similar to that of Y/Gd, while the molar ratio of Y/Nd was different from those of Y/Sm and Y/Gd. It was found that the substitution mechanism of RE elements in the single-domain was dependent on the kind of RE elements for ( Y-123)+( RE-211 or 422 ) system superconductors.

Enhancement of critical current density of Dy–Ba–Cu–O bulk superconductor by the refinement of Dy211 particles

We have studied the relationship between the particle size of Dy 2BaCuO 5 (Dy211) starting powder in the precursor and the particle size of Dy211 inclusions trapped in the melt-textured bulk Dy–Ba–Cu–O. The mean diameter of Dy211 particles trapped in the DyBa 2Cu 3O y could be reduced with decreasing the particle size of Dy211 starting powder. The J c was increased with reducing the size of Dy211 secondary phase. The J c of the bulk Dy–Ba–Cu–O sample with fine Dy211 inclusions 0.6 μm in size exhibited about 70 000 A/cm 2 at 77 K and self-field. Large c-axis oriented Dy–Ba–Cu–O/Ag bulk sample could also be fabricated with the top-seeded melt-growth process in air. The trapped magnetic fields of the bulks with the diameters of 32 and 48 mm reached high values of 1.4 and 1.7 T at 77 K, respectively.

Non-steady state solidification of the Nd-123 superconducting oxides

Crystal growth of the melt-textured bulk Nd 1+ x Ba 2− x Cu 3O 6+ d (Nd123) superconducting oxides was investigated by employing isothermal undercooling solidification with hot-seeding in air. From the relationship between growth length and holding time, the Nd123 crystal was found to have almost stopped growing after a certain growth period, while the growth length increased proportionally to the holding time at an early stage of the crystal growth. As a result of quantitative analysis for the Nd123 crystal of which solidification was terminated, the distribution of the Nd/Ba substitution was observed to decrease in the Nd123 single crystal matrix from the seed crystal to the edge of the Nd123 crystals. Also, the substitution content at the edge of the Nd123 crystal, which corresponds to that at the final stage of the crystal growth, was found to be in good agreement with the minimum substitution of the Nd123 solid solution phase in the equilibrium phase diagram at this process temperature. These compositional changes could be explained using the equilibrium phase diagram as associated with the solid solution formation, which is responsible for the non-steady state solidification of the Nd123 crystals even for the isothermal undercooling processing.

High current transport test of a YBCO bulk conductor up to 25 kA

A melt-textured YBCO bulk material was employed to obtain high current transport characteristics of a bulk high temperature superconductor. A disk-shaped YBCO bulk material 65 mm in diameter and 15 mm thick was fabricated by a modified quench and melt growth (QMG) process. An "H" shaped bulk conductor was cut from the YBCO disk. The cross sectional area and length of the conductor were 7 mm/spl times/7 mm and 20 mm, respectively. The bulk conductor was mounted in a high current bus line with a conductor-holder which allows high current feed and mechanical support against the thermal stress and electromagnetic force. Both ends of the conductor were soldered to electrodes of the conductor-holder. At 4.2 K, high current transport experiments were carried out up to 25 kA.

Improvement of critical current densities by fission tracks in U-doped high temperature superconductors

Results on the enhancement of critical current densities in melt-textured bulk (Y-123) superconductors, which were doped with and exposed to thermal neutron irradiation to create fission tracks, are presented. These tracks, which all originate from U-containing clusters of a stable UPtYBaO compound, form isotropic `starlike' defects with diameters of approximately 10 nm and lengths of . However, if we consider projections onto the symmetry plane of the flux line lattice and allow for a certain tilt modulus , their pinning-effective size is mostly (90%) between 10 and 20 nm, i.e. only twice as large as the characteristic defect structure (amorphous collision cascades with diameters of nm) introduced into undoped superconductors by fast neutron irradiation. We report on studies of and the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope . In the best case (0.3 wt% U, thermal neutrons), we find (77 K) to be around (0 T) and (5 T). Control experiments on samples containing (nominally) only and exposure to thermal or to thermal plus fast neutrons confirm the above flux pinning considerations. As reported before for undoped melt-textured Y-123, the irreversibility lines remain practically unchanged due to the presence of strongly pinning Y-211 precipitates.

Effect of initial composition on distribution of RE211 (422) particles in RE123 superconductors

Microstructure in melt-textured bulk RE1Ba2Cu3O6+d crystals (RE123; RE = Sm, Nd) was investigated, changing the initial composition from the tie-line composition of RE123–Sm2Ba1Cu1O5 (Sm211)/Nd4Ba2Cu2O10 (Nd422) to the Ba-enriched side. It was found that the Sm211/Nd422 particle size decreased in the liquid with increasing the Ba/Cu ratio of the initial composition, and this tendency was also found in the grown Sm123 crystals. Composition of the Sm123 grown crystal could be controlled by selecting the Ba-enriched initial composition to obtain an almost stoichiometric compound, which resulted in higher Tc values. Furthermore, the Jc values also increased under low magnetic fields due to the significant decrease of Sm211 particle size. Therefore, changing the initial composition toward the Ba-enriched side was found to be a new process to enhance both Jc and Tc values simultaneously.

Vortex confinement by top and bottom channeling in ion irradiated YBCO melt textured bulk

The effects of gold ion implantation either on both surfaces of melt-textured bulk samples or just on one surface are compared. The aim of the work is to rule out the role played by vortex channeling inside surface layers in to changing bulk properties. It is shown that characteristic behaviors related to the vortex localization do not depend linearly on the irradiated volume.

Enhanced vortex pinning in melt-textured YBCO by means of gold-ion-induced defects

We have studied the effects of 235 MeV197Au ion implantation on critical current density, creep rate and irreversibility line of YBCO melt-textured bulk samples. The fluence was 1.25·1011 ions/cm2, corresponding to a dose equivalent field of 2.5 T. We created two surface layers with correlated defects on the opposite sides of the sample, in such a way that the irradiated part was about 13% of the total volume. Such a surface irradiation caused a quite large widening of the irreversibility region in theB-T phase diagram. Critical current enhancements —having different behaviours as a function of field in different temperature ranges and reaching more pronounced values at higher temperatures and fields—were found.

Levitation force of a stack of epitaxial YBa 2Cu 3O 7−δ films

The levitation force of three epitaxial YBa 2Cu 3O 7−δ films and a stack made of these films was measured. The films show a very high force density of about 100 N cm −3 and a large hysteresis of the force versus distance relation. The filmstack provides a levitation force which is nearly as high as the sum of the forces of the single films. The magnetic stiffness of the stack is comparable to that of melt-textured bulk samples and shows a crossover from a B 1.5 to a B 2 dependence as observed on single films and melt-textured samples. The use of filmstacks for levitation devices may offer several specific advantages.

The influence of initial composition and oxygen partial pressure on the properties of melt-textured NdBaCuO

The influence of several admixtures on the microstructure formation and the superconducting properties is explained on the base of phase-equilibrium considerations. Conditions are derived which permit the stabilization of Ba rich Nd-123 phase to occur which is a precondition for preparing textured NdBaCuO materials with Tc > 90 K and a sufficient high jc. A melt-textured bulk material with Tc = 91 K and jc(77 K, 0 T, B ∥c) = 21.8 kA/cm2 was produced at p(O2) ≈ 100 Pa using an initial mixture consisting of NdBa2Cu3O7−δ and Nd2BaO4.

Hysteretic current-voltage characteristics of platelet boundaries in melt-textured bulk Ag-doped YBa 2Cu 3O 7,δ and heating effects

The current-voltage characteristics (CVCs) of isolated platelet boundaries in melt-textured bulk Ag-doped YBa 2Cu 3O 7,δ have been measured. The CVCs showed pronounced hysteresis. However, the critical current did not show any field dependence. Such hysteresis can be explained by a self-heating model.