Application Research Of Electron Backscattered Diffraction (EBSD) On Texture Analysis Of REBCO Superconductors(RE=Y,Nd)

The quantitative determination and interpretation of crystallographic textures is of fundamental importance in oxide superconductive materials like YBCO in which the orientation character is aimed to be used in practical applications. Nowadays local orientation determinations have received increasing attention with the development of EBSD technique mounted on scanning electron microscopes. EBSD analysis provides a point-by-point measure of crystallographic orientation. Local characterization obtained by EBSD could in principle probe for structure determination, residual stresses and so on. EBSD technique can even map grain orientations and calculate misorientations on the sub-micron to millimeter scale, which provides microstructural insights, such as grain orientation mapping or misorientation distribution functions. EBSD provides complementary information to X-ray or neutron diffraction methods. Therefore, in recent years EBSD technique has also been introduced to determine orientations in REBCO samples.As a powerful tool for orientation and microstructure study, the EBSD technique was applied to characterise the microstructure and the local texture of bulk REBCO (RE=Y, Nd) superconductors. The main difficulties of EBSD technique in resolving the orientation of the pseudo-cubic structure in REBCO were investigated. In the work, different parameters related to the acquisition and indexing of Kikuchi patterns have been optimized to distinguish at best the a/b/c axis orientations. The choice of number of reflectors is important for correct indexing of REBCO Kikuchi patterns. Different number of reflectors leads to different simulated Kikuchi patterns. The appropriate number of reflectors has been determined. Polishing and conductivity of REBCO sample surface has been focused. Water polishing has been introduced to REBCO sample preparation, simplifying the polishing procedure. The orthorhombic REBCO presents good conductivity for EBSD scanning. The acquired Kikuchi patterns were indexed using either the tetragonal or the orthorhombic symmetry. REBCO c-axis orientation can be indexed by tetragonal symmetry, while for a/b-axis, there still have some difficulties to be discriminated.In this research, several texturation methods have been applied to improve grain alignment in YBa2Cu3O7-δ(YBCO) and NdBa2Cu3O7-δ(NBCO) samples. YBCO thick films have been textured by direct peritectic growth method in a temperature gradient. The low oxygen partial pressure reduced YBCO melting temperature below melting point of silver substrate. Cold rolling used before melt-textured growth (MTG) process enhanced texture growth. Magnetic melting process has also been introduced to enhance the texture development in YBCO superconductors. A static magnetic field of 12T during MTG process with low oxygen partial pressure has been applied. The results showed high magnetic field can effectively improve crystallographic texture of YBCO grains. Textured NBCO bulks have been obtained by oxygen-controlled melt growth process, combined with zone-melting method. Automated EBSD scans have been performed on these REBCO samples. The results were compared with X-ray diffraction measurements.