Role of the twin boundary for the occurrence of peak effect in Y-Ba-Cu-O superconducting bulks melt grown under varied oxygen pressure

Abstract

A novel melt-growth technique, i.e. melt growth under varied oxygen pressure (MGVOP) in which the oxygen partial pressure is increased to solidify the initial melt, has been employed to prepare melt-textured superconducting bulks of the Y-Ba-Cu-O system. The obtained textured bulk samples consisted of two phases, i.e. superconducting and non-superconducting (Y-211). The critical current density of the samples reached as high as 40 kA (at 65 K) at a high magnetic field of T to exhibit a peak in the versus relation, i.e. a peak effect. To elucidate the causes of the peak effect and the high values of at high magnetic fields, submicron structures of the MGVOP-processed samples were examined by transmission electron microscopy (TEM), and the distributions of both Y-211 particles and twin boundaries were studied. In order to collect sufficient data for statistical analyses, specimens for `large-area TEM observations' were prepared using a focused ion beam thinning technique such that the TEM-observable area of the specimen was as wide as . Statistical analyses of the TEM images yielded the volume fraction of Y-211 phase, the average size of Y-211 grains and the inter-twin-boundary distance for each sample. It was concluded that a sample that had a higher twin-boundary density possessed a higher value at the peak in the versus characteristics, while fine Y-211 particles were likely to contribute to the value only at low magnetic fields.