Superconducting properties of (Y _1-xGd _x)Ba ₂Cu ₃O _7-δ composites processed in partial oxygen atmospheres

The effect of Gd substitution for Y on the superconducting properties of (Y _1-xGd _x)Ba ₂Cu ₃O _7-δ {(Y,Gd)123} composites was studied in powders processed by solid-state reaction and equilibrated at 910°C in air and finally in 1% O ₂ atmosphere. Powders were subsequently annealed in 100% O ₂ atmosphere at temperatures < 500°C to maximize T _c. After processing, the powders were nearly pure single-phase as determined by X-ray diffraction. With increasing Gd substitution, the T _cs as measured by ac susceptibility showed a steady increase from 92 K to 95 K. The flux pinning J _c(H) properties of (Y,Gd)123 compared to Y123 at 65 K and 77 K and magnetic fields up to 9T were consistently increased up to 10 times with Gd substitution, but only the Gd - 0.6 composition showed an increase compared to Gd123 for H > 7 T. A "fish-tail" effect was measured for the Gd-substituted powders at 65K, with H _peak, ∼ 3.5 T and ∼ 6 T for Gd = 0.2 and Gd = 0.6 compositions, respectively.