Peak effect in Jc and sub-micron structures of Y-Ba-Cu-O superconducting bulks melt-grown under varying oxygen pressure

Abstract

Abstract A novel melt-growth technique, or a melt-growth under varying oxygen pressure (MG-VOP) method in which the oxygen partial pressure is increased to solidify the initial melt, has been employed to obtain textured bulk samples consisting of a superconducting phase, YBa2Cu3O7−δ (Y-123), and a secondary (non-superconducting) phase, Y2BaCuO5 (Y-211). The Jc values of such samples reached as high as 40,000 A/cm2 (at 65 K) even under a high magnetic field of H=3 T, to exhibit a peak in the Jc- vs -H relation, i.e. a peak effect. To understand the mechanism of such a peak effect and the high values of Jc at high magnetic fields, sub-micron structures of the MG-VOP processed samples were examined by means of transmission electron microscopy (TEM), and the distributions of both Y-211 particles and twin boundaries were studied. In order to collect microstructural data for statistical analyses, TEM specimens were prepared using a focused ion-beam (FIB) thinning technique. TEM-observable area of such a specimen was as wide as 20×20 (μm)2. A statistical analysis of the TE micrographs yielded the fraction of the Y-211 phase, the average size of Y-211 grains and the inter-twin-boundary distance for each sample. It is concluded that a sample that has the higher twin-boundary density possesses the higher Jc value under a high magnetic field in the vicinity of the peak, while fine Y-211 particles are likely to contribute to the Jc value only at very low fields.