On the residual stress measurements in Y-based superconductor.

Abstract

XRD and residual stress (~ sin²ψ) measurements were carried out on Yba₂Cu₃Ox superconductor with varying oxygen stoichiometry x = 6.32 to 7. Using published data for c-axis, Tc and oxygen content it was found that the measured c-axis varies linearly with and is sensitive to variation of oxygen content and Tc. Variation of a and b parameters with oxygen content revealed a transition from orthorhombic to tetragonal at 6.48. Orthorhombicity and volume of the unit cell were plotted against oxygen content and an unexpected volume contraction of the unit cell was observed. This was discussed in terms of energetically favored filling of oxygen sites. Slopes of the strain versus sin² ψ which is proportional to the residual stress, were plotted against oxygen content for certain reflections (006) and (115), for example. Different behaviors were observed and are attributed to a c-axis compressional stress increasing with oxygen content and an overall tensile residual stress in the ab-plane is also increasing oxygen content. Both effects reach saturation near fully oxygenated state x = 7. This explains the anomalous very small hydrostatic pressure effect on Tc for fully oxygenated Yba₂Cu₃Ox (x = 7) compared to a relatively high effect for oxygen deficient material. Similar results were observed for the oxygen disorder effect. Changes in Tc for disordered material is attributed to changes in the residual stress induced by disorder phenomena.