Cracking in melt-grown RE-Ba-Cu-O single-grain bulk superconductors

In this review paper the results on the cracking phenomena in melt-grown 123 bulk superconductors are summarized. The reasons for cracking in this material are mechanical stresses, which arise in the sample during its fabrication. Two main sources of stresses appearing during fabrication were identified: the different thermal expansion coefficient of 123 and 211 phases and the dependence of 123 phase lattice parameters on the oxygen stoichiometry. The formation mechanisms of three basic types of cracks are characterized. The most typical are a/b-microcracks, which are observed as dense lines parallel to the a/b plane, and their length does not exceed some 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 crystallization temperature and during oxygen uptake. The second and the third type of observed cracks are so-called a/b- and c-macrocracks. They extend along larger sample areas and are formed under the combined influence of tensile stresses developed during sample oxygenation and stresses induced by 211 concentration macroinhomogeneity. Some possibilities of suppression of cracking are considered. According to our analysis it is possible to find oxygenation conditions under which the formation of the c-macrocracks or both c- and a/b-macrocracks is suppressed. Other possibilities Of Suppression of cracking, such as oxygenation under uniaxial pressure, sample bandage and sample impregnation are discussed.