N. Vilalta

Aging of critical currents and irreversibility line in melt textured YBa2Cu3O7

The critical currents and the irreversibility line of bulk melt textured YBa2Cu3O7 have been found to evolve with the increase of the oxygenation time. A strong field‐dependent reduction of the critical currents is observed which finally leads to a downward shift of the irreversibility line. Transmission electron microscopy micrographs of aged samples display a huge increase in the concentration of stacking faults near the microcracks and at the YBa2Cu3O7‐Y2BaCuO5 interfaces. Low field ac susceptibility measured with H∥ab shows a strong reduction of the screening capability thus meaning that nonsuperconducting planar defects parallel to the ab planes have been created. We suggest that an enhanced two‐dimensional behavior of the vortex flux lattice occurs when H∥c as a consequence of this layered defective structure.

Interface pinning in high Tc-high Jc Nd1+xBa2−xCu3Oy directionally solidified in air

Single domain bars of NdBa2Cu3O7/Nd4Ba2Cu2O10 (Nd123/422) superconducting composites have been fabricated in air by using a directional solidification technique. Tc values of 96.5 K can be obtained after an Ar heat treatment and further oxygenation thus showing that reduced oxygen partial pressure during solidification is not a fundamental requirement to prepare high-Tc Nd123 bulk superconductors. The temperature dependence of the critical currents point to a limited efficiency of the Nd123/422 interfaces to pin vortices. High resolution electron microscopy observations clearly reveal smooth Nd123/422 interfaces thus suggesting that interface pinning may not be as relevant as compared to Y123/211 composites.

Polygonization of directionally solidified high critical current YBa2Cu3O6+x

The mosaic structure of single domain YBa2Cu3O6+x/Y2BaCuO5 (123/211) directionally solidified composites is investigated by x‐ray diffraction and transmission electron microscopy. This mosaic structure results from a polygonization process before the oxygenation step. A topological model is given based on the observed distribution of preferred subgrain boundary planes. This polygonization, characteristic of melt textured 123, is proposed to be an intrinsic critical current density limitation for these materials.

Three dimensional dislocation substructure in NdBa2Cu3Oy

A three dimensional dislocation substructure is reported by transmission electron microscopy in an as-grown directionally solidified (rare earth) Ba2Cu3Oy superconductor. Dislocations with a near or perfect [001] orientation and Burgers vectors lying on the basal plane are found to glide on (010) and {110}. A model is proposed in which cubic-like glide is achieved through the formation of a track of Nd→Ba antisites along the path swept by the dislocations. This finding can be correlated with the enhanced flux pinning under H‖[001] observed in these materials.

Modified growth mechanism in directionally solidified YBa/sub 2/Cu/sub 3/O/sub 7/

Minute amounts of CeO/sub 2/ additions have a strong effect on the solidification path, physicochemical parameters and final size distribution of Y/sub 2/BaCuO/sub 5/ (211) precipitates of directionally solidified YBa/sub 2/Cu/sub 3/O/sub 7/ (123). Inductive and transport critical currents density (J/sub c/) measurements indicate a clear increase with CeO/sub 2/ addition.<<ETX>>

Microstructure and pinning in high--high- directionally solidified in air

Single-domain bars of (Nd123/422) superconducting composites have been fabricated in air by using a directional solidification technique. values of 96 K can be obtained after an Ar heat treatment and further oxygenation. The microstructure of Nd123 shows distinctive features compared with Y123, which affect the dislocation configuration, Nd422-Nd123 interfaces and local strain fields within the matrix, which correlate with the particular behaviour.

Effect of melt-processing temperature on the microstructure and the levitation force of YBCO melt-textured superconductors

We have studied the growth of cylinders of large-grain (YBCO) with diameters of 30.0 mm in the presence of a temperature gradient. The microstructure and the levitation force were found to depend on the maximum holding temperature. Increasing the holding temperatures above the peritectic temperature has been observed to be beneficial in enhancing the levitation force of the samples. The levitation force measurements have been correlated with the change of crystalline orientation of the dominant domains after melt-processing. Based on a Y-diffusion-controlled growth mechanism, the crystalline configuration of YBCO cylinders in relation to the intrinsic growth rate, which is closely related to the maximum processing temperature, has been analysed.

Subboundary mesostructures and variable dislocation networks in single domain melt textured YBa2Cu3O7: implications on critical currents

Abstract The microstructure of subboundaries (SBs) occurring in single domain YBa 2 Cu 3 O 7 (Y123) melt textured composites is studied by transmission electron microscopy. The form of SB dislocation networks is controlled by the properties of constituting dislocations. For arbitrary SB configurations, it can be parametrised using the generalised Franks formula, allowing the calculation of dislocation densities, and an estimation of the SB critical current. Besides the underlying dislocation networks, SBs may develop mesostructures like faceting and stepped interfaces accommodating the deviation from low index planes. Implications of the observed microstructures on the bulk critical currents are addressed.

Plastic deformation in high critical current melt‐textured YBa2Cu3O7

The effect of high‐temperature creep under different conditions in melt‐textured YBa2Cu3O7–Y2BaCuO5 composites, with high critical currents before deformation, is investigated. Transmission electron microscopy studies show that the low‐temperature oxygenation process causes a strong modification of the as‐deformed microstructure and leads to a clear degradation of the superconducting performances. This behavior of the superconducting properties is well correlated with the formation of extended planar defects such as large stacking faults and microcracks that severely diminish the vortex cutting length, thus enhancing the thermal activation effects on the critical currents and the irreversibility line.

High-pressure-high-temperature effect on the structure of YBaCuO- and NdBaCuO-based superconductive ceramics

The structure of dense (99%) high-pressure (2-5 GPa)-high-temperature treated single-phase and melt-textured YBaCuO and NdBaCuO ceramics has been investigated. High-pressure-high-temperature (HP-HT) treatment under 5 GPa in contact with annealed zirconia allows us to prevent oxygen losses from Nd123 and Y123 structures even when heated up to 1100 and , respectively, and thus, keep the superconductive characteristics. For MT-YBaCuO ceramics, a more than twofold decrease in the anisotropy of critical current density occurs because of doubling critical current density in the direction of the c-axis of Y123 crystallites (in the perpendicular direction it remains almost unchanged) may be explained by the increase in dislocation density in the (001) plane (from up to ) and by narrowing of twin domains (from 150 down to 20 nm). For MT-NdBaCuO, narrowing of spacing between the twins has been observed in the polarized optical microscope.