No abstract

Single domain Y–Ba–Cu–O (YBCO) samples composed of a YBa2Cu3O7−δ (Y-123) matrix with Y2BaCuO5 (Y-211) inclusions were fabricated using the top-seed melt-textured growth (TSMTG) process. The influence of the Y-211 inclusions on the melt-textured growth mode has been studied for precursor pellets with a Y-123:Y-211 molar ratio of 1:0.2, 1:0.3 and 1:0.4. The surface morphologies obtained from these sintering pellets quenched from 1000 °C, 997 °C and 994 °C, respectively, have revealed that the initial peritectic recombination process near the seed crystal seems to take place earlier for the Y-123 matrix with low Y-211 inclusions than for that with a high level of Y-211 inclusions; however, the melt-textured growth rate of the single domain seems to be faster in the system with a high level of Y-211 inclusion than in that with a low level.

Experiments reveal that there is a non-ohmic dissipation behaviour in the Ca-doped polycrystalline Y 1−x Ca x Ba 2 Cu 3 O 7−δ even at temperature T = 20 K. This non-ohmic dissipation seems to come from the vortex excitation along the grain boundaries and appears in anomalous I -V characteristics with a non-power-law behaviour at temperatures below 60 K, and then with a power-law behaviour as the temperature approaches T c . This abnormal behaviour cannot be explained by the thermal flux creep model. We suppose that a giant flux motion along the grain boundaries might be responsible for this anomalous non-ohmic dissipation behaviour. A modified I -V relation was worked out based on Larkin's collective pinning model, and the analytical I -V curves were found to be coincident with the experimental curves quite well. This confirmed that the vortex excitation process along the grain boundaries in Ca-doped polycrystalline Y 1−x Ca x Ba 2 Cu 3 O 7−δ might be described with a modified collective pinning model.