EFFECTS OF MICROSTRUCTURAL AND ORTHORHOMBICITY CHANGES ON THE SUPERCONDUCTING TRANSITION BEHAVIOR OF YBa2(Cu1-x Znx)3O7

Abstract

The effects of Zn substitution on the superconducting transition behavior of YBa₂(Cu_{1 x}Zn_x)₃O_7-δ for x = 0 to 0.03 were investigated. The samples were prepared using the solid-state reaction method and characterized by using X-ray diffraction method (XRD), scanning electron microscope (SEM) and DC electrical resistance measurements. The XRD patterns showed that all samples consisted of the YBa₂Cu₃O_7-δ phase. The grain size did not change systematically with Zn substitution (~1.2 µm). However, the grain shape and boundaries were dramatically changed. The resistance (R) versus temperature (T) curves showed metallic normal state behavior for x = 0 and semiconductor-like behavior for all Zn substituted samples. The superconducting transition behavior was further investigated by plotting dR/dT versus T. The results showed that as Zn was substituted, two peaks were observed in the dR/dT curves where T_p1 marks superconductivity within grains with the grain boundaries not yet superconducting and T_p2 indicates the temperature where supercurrent flows between grains through proximity effects. All Zn substituted samples showed lowering of T_p2 and broadening of DT_p (T_p1 – T_p2). The changes in the grain shape, grain boundaries and orthorhombicity of the unit cell due to Zn substitution may have affected the superconducting transition behavior. The grains may have different T_cs due to orthorhombic-tetragonal transition and the samples became superconducting due to proximity effects between the inhomogeneous grains as the temperature was lowered to T_p2.