Stability and evolution of stacking faults in melt textured YBa2Cu3O7−x

Abstract

The annihilation process of large 16〈301〉 stacking faults in melt textured YBa2Cu3O7−x is in situ monitored in the transmission electron microscope. The process is activated at the onset of the orthorhombic to tetragonal transition induced in the electron microscope, thus evidencing the instability of such defects in the deoxygenated YBa2Cu3O7−x matrix. The observations suggest that the annihilation (and growth) process is limited by surface diffusion along Y2BaCuO5/YBa2Cu3O7−x interfaces. The propagation mechanism of such stacking faults is addressed in the light of the present findings.

Tailoring of microstructure and critical currents in directionally solidified

“…Moreover such interface defects also provide high-diffusivity paths for copper and oxygen (see also [8]). In practice, in melt-textured 123 materials having a high density of 211 precipitates, 211-123 interfaces also provide heterogeneous nucleation sites for such 1 6 301 faulted loops [7,8,65]. Such faulted loops propagate by a non-stoichiometric partial dislocation climb mechanism, preferentially along the direction of the in-plane shear component of their Burgers vector [8,65].…”

“…In practice, in melt-textured 123 materials having a high density of 211 precipitates, 211-123 interfaces also provide heterogeneous nucleation sites for such 1 6 301 faulted loops [7,8,65]. Such faulted loops propagate by a non-stoichiometric partial dislocation climb mechanism, preferentially along the direction of the in-plane shear component of their Burgers vector [8,65]. Such a preferred direction for their propagation leads to a 'dendritic' or 'finger-like' shape of the final configuration.…”

“…On the other hand, it has been proposed that SFs can be annealed out by heating the sample at 800-900 • C [56,57,66]. Further insight into the stability range and kinetics of this local transformation has recently been provided by in situ monitoring of the dissolution process of 1 6 301 faulted loops nucleated at a 211-123 interface in the TEM [65]. This study indicated that the dissolution process is activated at the onset of the orthorhombic-to-tetragonal phase transition, thus evidencing the instability of such defects in the deoxygenated 123 matrix, in agreement with the previous computer simulation results [64] and TEM examinations of the microstructure of non-oxygenated as-solidified samples.…”

Directional solidification of (Re = Y, Nd): microstructure and superconducting properties

“…Under oxygenation conditions dislocation loops enclosing a CuO x layer nucleated at inclusion interfaces propagate rapidly along the [100]-[010] directions leading to a dendriticlike morphology, and their size may reach a few microns in well-oxygenated materials [25]. Recent in situ TEM observations showed that, on heating of the samples, such stacking faults annihilate above the orthorhombicto-tetragonal transition temperature (∼350 • C under the vacuum condition of the TEM instrument) by a backwards migration to the interface [27].…”

Optimization of Flux Pinning in Bulk Melt Textured 1-2-3 Superconductors: Bringing Dislocations under Control

“…Accordingly, annealing at higher temperatures (900±930 C) in 0.2±1 atm oxygen results in the annihilation of the stacking faults. [20,21] Two driving forces are required in the material for the nucleation and growth of stacking faults: 1) a thermodynamic driving force, Ñm, where m is the chemical potential associated to the intercalation of a double chain CuO x layer in the structure (in fact, the large splitting widths observed in transmission electron microscopy (TEM) studies are relevant to a negative stacking fault energy that can be associated to a local phase transformation to the Y124 phase [22] ), and 2) a mechanical driving force, s (s is the re- [17], crosses: tetragonal-to-orthorhombic transition line for Y123 [18].…”