Reduced twinning efficiency and tri-dimensional crack structure in melt-textured NdBa2Cu3O7−δ bulk samples fragmentation process

Abstract

Here we report on a detailed investigation on NdBa2Cu3O7−δ (Nd123) crack generation achieved by an extensive morphological and structural study on 1 cm diameter top-seeded melt-textured grown pellets, where crack development stops at an earlier stage, and on the parallelepiped fragments resulting from the spontaneous breaking of larger (3 and 5 cm diameter) pellets. In this way complementary information on the process is obtained. Sample morphology was investigated by polarized light optical microscopy, scanning electron microscopy, and atomic force microscopy, confirming that microscopic fissures and macroscopic cracks act as preferential oxygenating channels. This fact suggests that mechanical stress, generated locally by an oxygenation induced tetragonal–orthorhombic transition, is the driving mechanism for fissure evolution into cracks and their further growth. Our observations reveal that Nd123 fissure bundles have a tri-dimensional character, lying not only along the a–b plane, but also along two a–b out-of-plane perpendicular directions. Twin and fissure bundle orientations appear to differ by 45°, thus suggesting that fissures lie along a–c and/or b–c crystallographic planes. This observation was confirmed by θ–2θ x-ray diffraction and x-ray pole figure analysis (XPFA) performed on the parallelepiped fragments, which revealed that samples are single domain, with edges oriented along the principal crystallographic planes. XPFA detected a–b axes twinning in the domain on the global scale. A remarkable result achieved by electron backscattered diffraction (EBSD) is the evidence that cracking along b–c plane is preferred with respect to a–c, acting in this way as a second preferential cleavage direction. Indeed EBSD measurements show long-range ordering of a and b axes, suggesting that twinning is less effective in releasing in-plane mechanical stress in Nd123 than in parent compounds. Results are discussed on the basis of the identification of Nd–Ba cationic substitutions as the main competing relaxation mechanism. The superconducting properties of the analysed samples were studied by measuring the fragments superconducting parameters by vibrating sample magnetometry.