N. Sakai

Melt processing for obtaining NdBa2Cu3Oy superconductors with high Tc and large Jc

A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

Melt-processed light rare earth element-Ba-Cu-O

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

Flux Pinning in Melt-Grown NdBa2Cu3Oy and SmBa2Cu3Oy Superconductors

REBa 2 Cu 3 O y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T, due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa 2 Cu 3 O y with fine Y 2 BaCuO 5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba 1-x , RE x ) 2 Cu 3 O y phase in a good superconductive matrix

Enhanced Tc and Strong Flux Pinning in Melt-Processed NdBa2Cu3Oy Superconductors.

The effects of various oxygen partial pressures (P O2) during melt processing on the superconducting properties of fabricated NdBa2Cu3Oy (Nd123) superconductors have been systematically investigated. Unlike in air or O2, samples melt-processed in reduced oxygen atmospheres exhibited unprecedentedly high T cs of 95–96 K with a sharp superconducting transition. Especially, for a sample melt-processed in 0.1% O2 in Ar, the magnetization hysteresis ( M-H ) loop was kept open up to 7 T at 77 K and for the field parallel to the c-axis of the sample (H//c), implying that very effective flux pinning can be achieved in high magnetic fields.

Melt-processed Gd–Ba–Cu–O superconductor with trapped field of 3 T at 77 K

We fabricated a single-domain Gd?Ba?Cu?O bulk superconductor 65?mm in diameter and studied the microstructure, superconducting and field-trapping properties. Melt-processing was performed under a controlled oxygen partial pressure of 1.0% using a precursor containing Gd123 and Gd211 powders in a molar ratio of 2:1, with 0.5?wt% of Pt and 20?wt% of Ag2O added. The distribution of Ag and Gd211 particles was almost homogeneous. The addition of Ag was very effective in reducing the amount of cracking in the sample. The maximum trapped magnetic field recorded was 3.05?T at 77?K. We also measured the trapped field between two Gd?Ba?Cu?O bulk samples in order to minimize the demagnetizing effect and found that the trapped field reached 4.3?T at 77?K.

Influence of the size of Gd211 starting powder on the critical current density of Gd-Ba-Cu-O bulk superconductor

The relationship between the particle size of Gd211 powder in the precursor and the particle size of Gd211 inclusions in Gd-Ba-Cu-O bulk has been investigated. Gd211 starting powders with various diameters were prepared by the calcination of Gd2O3, BaO2 and CuO powders at different temperatures between 800 and 1000 °C. The particle size of Gd211 in the melt-grown bulk was proportional to the particle size of the initial Gd211 powder. In conclusion, the employment of fine Gd211 powder led to a size reduction of 211 particles in the bulk, while largely enhancing the Jc values in low magnetic fields. A large Gd-Ba-Cu-O/Ag bulk sample, 32 mm in diameter, could also be fabricated by the hot-seeding method. The maximum trapped field value revealed 1.5 T at 77 K.

High critical current density in Y-Ba-Cu-O bulk superconductors with very fine Y211 particles

We have succeeded in enhancing the critical current density (Jc) of melt-textured Y?Ba?Cu?O superconductors by employing ultra-fine Y211 powder as precursor. The Y211 starting powders were reduced down to 110?nm in diameter with ball-milling. Single-grain Y?Ba?Cu?O samples up?to?33?mm in diameter were fabricated from the precursors containing finely milled Y211 powders. The Y211 inclusions in the melt-textured samples could be refined by decreasing the particle size of the Y211 starting powders. The maximum value of Jc at 77?K was 1.1 ? 105?A?cm?2 in a self-field. However, the size reduction of Y211 led to an inhomogeneous distribution of Y211 particles, in which the volume fraction of the Y211 phase greatly decreased near a seed crystal. An Ag-added bulk sample 33?mm in diameter exhibited a high trapped magnetic field of 1.6?T at 77?K.

A new type of pinning center in melt grown Nd123 and Sm123

Abstract REBa2Cu3Oy (RE: Nd, Sm) superconductors exhibit relatively high Jc values even at 77K when melt-processed in a reduced oxygen atmosphere. The Jc values of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility field with H∥c axis was dramatically increased to 8T at 77K. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1−x, REx)2Cu3Oy phase in a good superconductive matrix.

Processing of high-performance Gd-Ba-Cu-O bulk superconductor with Ag addition

We fabricated large-grain c-axis-oriented Gd–Ba–Cu–O bulk superconductors with very fine 211 particles and studied their field-trapping properties. Single domain Gd–Ba–Cu–O/Ag bulk samples with different molar ratios of Gd-123:Gd-211 = 10:x (x = 2 to 6) were melt-textured under controlled oxygen partial pressure of 1.0% by employing Gd-211 starting powders with a particle size of 1.0 μm. The maximum trapped magnetic field of 2.0 T was recorded at 77 K in the bulk 48 mm in diameter with the composition of x = 5. It is notable that the trapped field in the space of two Gd–Ba–Cu–O bulks reached an extremely high value of 3.3 T at 77 K. Furthermore, the field-trapping performance of the bulk sample was improved by applying the ultra-fine 211 powders of a size of 0.2 μm. The single-bulk Gd–Ba–Cu–O 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K.

Optimization of matrix chemical ratio for high flux pinning in ternary (Nd–Eu–Gd)Ba2Cu3Oy

We prepared (Nd, Eu, Gd)Ba2Cu3Oy samples with various Nd: Eu: Gd ratios in the rare earth site. It was found that the three elements contributed to flux pinning in different ways. Nd mainly enhanced flux pinning at low magnetic fields, Eu controlled the second peak position and the irreversibility field, while Gd slightly enhanced intermediate and high-field Jc values. Scaling analyses for the pinning force density as a function of the reduced field h=Ha/Hirr (where Hirr denote the irreversibility field) showed that the highest peak was achieved at h=0.56. This value is even higher than the theoretically predicted highest value of h=0.5. We also show that a maximum flux pinning can be achieved in the whole magnetic field when very fine secondary phase particles are dispersed in a superconducting (Nd, Eu, Gd)Ba2Cu3Oy matrix with an optimum Nd: Eu: Gd ratio.