Oxygenation and cracking in melt-textured YBCO bulk superconductors

Microstructural changes during the oxygenation of YBCO bulks were studied. It was shown that a lower temperature of oxygenation leads to the formation of a denser structure of a/b- and c-macrocracks and causes faster and more homogeneous oxygenation of the sample. The opening of created macrocracks is the way in which the macroscopic stresses induced by macroscopic 211 particle concentration inhomogeneity are released. This is very important, because it prevents the formation of fatal c-macrocracks, which divide the sample into more domains, during cooling from oxygenation temperature or during sample performance. Oxygenation with a multistage programme causes the oxygen concentration difference between the oxygenated layer and the tetragonal matrix to be smaller, and consequently fewer macrocracks are formed. This leads to the prolongation of oxygenation times for full oxygenation and to the insufficient release of macroscopic stresses. 211 low concentration regions and pores also enhance the oxygenation rate of YBCO bulks