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.

REBa₂Cu₃O _y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T _c 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 YBa₂Cu₃O _y with fine Y₂BaCuO₅ 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 )₂Cu₃O _y phase in a good superconductive matrix.

We have studied the field and temperature dependence of the critical current density $J_c\mathrm{}(B,T)\mathrm{}$ for a flux-grown ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ single crystal (Nd123SC), melt-textured Nd-Ba-Cu-O (OCMG-Nd-Ba-Cu-O), and Y-Ba-Cu-O (MPMG-Y-Ba-Cu-O) bulks. In the $J_c-T$ curves, an exponential-like behavior in a low-temperature region could be explained in terms of the collective flux creep model, while $J_c-T$ curves in a high-temperature region could well be fitted to the Kim-Anderson creep model. For the scaling of $J_c-B$ curves, power-law relation of $J_c\propto B^{\gamma -1}$ is observed for all the samples in a low-field region in that the exponent γ is roughly $\frac{1}{2}$ for OCMG-Nd-Ba-Cu-O and MPMG-Y-Ba-Cu-O, and 1 for $\mathrm{Nd}123\mathrm{SC},$ corresponding to surface pinning and point pinning, respectively. The surface pinning is ascribed to the presence of second-phase particles. In an intermediate field region, the γ values become almost 2 for $\mathrm{Nd}123\mathrm{SC}$ and OCMG-Nd-Ba-Cu-O, showing that Δκ-point pinning is dominant, which presumably originates from the Nd-Ba substituted regions with depressed $T_c.$ On the basis of our $J_c\mathrm{}(B,T)\mathrm{}$ analyses, we could classify the $B-T$ phase diagram into several regimes of different dominant pinning mechanisms.

The effects of various oxygen partial pressures (P _O2) during melt processing on the superconducting properties of fabricated NdBa₂Cu₃O _y (Nd123) superconductors have been systematically investigated. Unlike in air or O₂, samples melt-processed in reduced oxygen atmospheres exhibited unprecedentedly high T _c's 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.