Cracking caused by thermal and transformation stresses in YBCO composite superconductors

The reasons for cracking in YBCO composite superconductors are mechanical stresses, arising in the sample during its fabrication. Two main sources of stresses appearing during fabrication were identified: different thermal expansion coefficient of YBCO composite components and dependence of YBa2Cu3Ox phase lattice parameters on oxygen stoichiometry. The formation mechanisms of three basic types of cracks are here characterised. The most typical are a/b-microcracks, which are observed as dense lines parallel to the a/b plane. Their length does not exceed a few 211 interparticle distances. The a/b-microcracks are formed at 211 particles due to tangential tensile stress developed around each 211 particle during cooling from the crystallisation temperature and during oxygen uptake. Further frequently observed cracks are so-called a/b- and c-cracks. They are larger than the previous ones and are formed by a combined action of tensile stresses developed during sample oxygenation and stresses induced by a macroscopic concentration macroinhomogeneity of 211 particles. Some possibilities of cracking suppression are suggested.