Phase equilibria, superconductivity, and flux creep in Nd1+[subscript x]Ba2-[subscript x]Cu3O7+[subscript delta] superconductors

Abstract

<p>NdBa[subscript]2Cu[subscript]3O[subscript]7 (Nd123) is one of REBa[subscript]2Cu[subscript]3O[subscript]7 (RE123) superconductors (RE = Y and rare earths), having the superconducting transition temperature (T[subscript] c) of about 90 K. From a processing standpoint, Nd123 is an interesting alternative to Y123 for applications because it exhibits both a higher peritectic decomposition temperature and a broader primary solidification range than Y123, and unlike Y123, the low temperature orthorhombic phase exists over a range of Nd compositions in Nd[subscript]1+xBa[subscript]2-xCu[subscript]3O[subscript]7+[delta] solid solution (Nd123ss) with little effect on T[subscript] c;In this study, the phase diagram in the Nd-Ba-Cu-O system, superconductivity in Nd123ss, and the role of Nd substitution for Ba sites as the magnetic flux pinning sites were investigated. In the phase diagram study, the discrepancies in the published phase diagrams were clarified. Significant results contain the solubility limits of 0.04 ≤ x ≤ 0.6 in Nd123ss, the existence of Nd[subscript]2BaCuO[subscript]10 having a solid solution, and a discovery of a new phase NdBa[subscript]6Cu[subscript]3O[subscript]y;Superconductivity in Nd[subscript]1+xBa[subscript]2-xCu[subscript]3O[subscript]7+[delta] was studied for well characterized samples with sharp transitions. With the substitution of Nd[superscript]+3 for Ba[superscript]+2, there is an increase in the O(5) sites surrounding this substitution. While for x > 0.1 there is a significant depression of T[subscript] c, for x < 0.1 there is no substantial decrease in T[subscript] c within experimental error. T[subscript] c vs x is characterized by two plateaus analogous to T[subscript] c vs d in YBa[subscript]2Cu[subscript]3O[subscript]7-d. Analysis of oxygen contents of Nd123ss suggests the analogy with Y123 is valid. The transition width and Meissner fraction for all Nd123ss samples are strongly dependent on the sintering temperature. Hysteresis measurements show good intragranular critical currents (J[subscript] c) at high fields but the intergranular J[subscript] c's are still low;Since the magnetic fields penetrate a Type II superconductor at high field, flux lines must be pinned to increase the J[subscript] c. While T[subscript] c is not affected over the range 0 < x < 0.1, the oxygens surrounding the substituted Ba sites are expected to locally suppress the superconducting order parameter. Such a local suppression of the order parameter may act as a flux pinning site. Hysteresis measurements exhibit a small increase in the magnetic J[subscript] c, but flux creep measurements show no apparent effects of this substitution on the flux pinning energy.</p>