F. Sandiumenge

Tailoring of microstructure and critical currents in directionally solidified

Directionally solidified composites display a complex microstructure with several defects coexisting in the matrix and hence the identification of the flux-pinning mechanisms requires a detailed analysis of the critical currents. In this work we review the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with the refining strategies of (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic. The influence of these defects and post-processing treatments on the critical currents is then analysed focusing on their relative importance as flux-pinning centres at different temperatures and magnetic fields up to 20 T. A complete magnetic phase diagram is finally proposed as a basis for a systematic classification of the observed irreversible superconducting behaviour in single-grain melt-textured composites.

Thickness evolution of the twin structure and shear strain in LSMO films

X-ray diffraction analysis and orientation contrast scanning electron microscopy imaging of La0.7Sr0.3MnO3 epitaxial layers grown on (001)-SrTiO3 substrates have been used to track the shear strain and twin domain period as a function of the thickness of the films, t. To this end, the diffraction by a periodically modulated twinned structure is analyzed in detail. In contrast with current equilibrium models, here we demonstrate the occurrence of a critical thickness, tτ ∼ 2.0–2.5 nm, for twin formation in rhombohedral perovskite films. The absence of twinning below tτ is explained by the formation of a monoclinic interfacial phase presumably driven by electronic interactions between film and substrate not taken into account in theoretical models. Above tτ, twin domains develop concomitantly with the build-up of misfit shear strains associated with the formation of the rhombohedral structure. At a thickness ∼10 nm, the in-plane and out-of-plane shear strain components exhibit similar values, as imposed by the rhombohedral symmetry. However, upon increasing the film thickness, both strain components are found to follow divergent trajectories indicating a progressive perturbation of the octahedral framework which allows the in-plane lattice parameters to remain fully strained within the explored thickness range (up to 475 nm). Despite these structural perturbations, the twin size follows a t1/2 dependence as predicted for homogeneous films by equilibrium models.

Melt textured YBa2Cu3O7: fundamental properties and current limitation applications

Abstract Melt textured growth (MTG) YBa 2 Cu 3 O 7 (YBCO) is a unique material because it is the only one having practical power applications which can be considered as a single domain, not displaying magnetic granularity. We will first review how the single crystal character is disrupted by the existing subgrain boundaries. We will later stress the relevance of the composite structure of MTG YBCO as a driving force for the generation of defects and how the metastable character of the YBCO phase can be used to analyze the pinning mechanisms of this material. In the second part of the talk we will describe how the superconducting properties of bulk YBCO materials can be tuned to use them efficiently for current limitation. We will show that safe self-limitation can be obtained at 77 K in bulk YBCO materials, thus avoiding the destructive effect of hot spots. The test and integration of Bridgman MTG YBCO elements for resistive and hybrid fault current limiters will be demonstrated.

Microstructure and flux dynamics in YBa2Cu3O7 plastically deformed at room temperature

Abstract The influence of dislocations on pinning of vortices has been investigated in melt-textured YBa 2 Cu 3 O 7− x and YBa 2 Cu 3 O 7− x ceramics. The dislocations, introduced by plastic deformation at room temperature in the orthorhombic phase, are characterized using transmission electron microscopy. dc and ac magnetic measurements revealed that the plastic deformation of melt-textured samples induced a decrease of the activation energy for flux motion. This effect has been related in a preceding paper to the microcracks created in important density along the (001) planes during the deformation process. The comparison of these results with those obtained on a ceramic material confirms this interpretation.

Evolution of the microstructure during high-temperature creep and oxygenation in directionally solidified YBa2Cu3O7-x

Abstract The microstructure of YBa2Cu3O7−x −Y2BaCuO5 melt-textured composities deformed in the secondary and tertiary creep regimes has been investigated by transmission electron microscopy. The high density of Y2BaCuO5 precipitates plays an important role in the microstructural development as pinning sites for gliding dislocations. In the secondary regime, trapped dislocations are dissociated, leaving a stacking fault with displacement vector [½—δ 0 ⅓]. A second stacking fault, ⅙〈301〉, is typically associated with the former stacking fault. At this stage, the deformation microstructure is dominated by diffusive processes between precipitates interconnected by the trapped dislocations. In the tertiary stage, dislocation multiplication is the main factor controlling the microstructure, which is characterized by a dramatic increase in the density of perfect dislocations with Burgers vectors 〈100〉 and 〈110〉. Since deformation is performed above the orthohombic-to-tetragonal transition temperature, the sample...

Observation of a novel stacking-fault nucleation mechanism in melt-textured YBa2Cu3O7

The dislocation substructure of melt-textured YBa2Cu3O7-Y2BaCuO5 composites deformed under uniaxial compressive stress and a superposed gaseous confining pressure has been investigated by transmission electron microscopy. Observations reveal the generation of perfect dislocations in (001) as well as out of this plane. Screw segments of the latter lying on (001) are seen to act as Frank-Read sources for ⅙ ⟨301⟩ partials bounding YBa2Cu4O8-type stacking faults, according to a mechanism previously anticipated from theoretical considerations by one of the present authors.

Critical currents in air processed NdBa2Cu3O7 melt-textured superconductors

Abstract Single domain ceramics of NdBa2Cu3O7 with high Tc and high critical currents have been prepared in air by directional solidification techniques (Bridgman growth and top seeding growth). The temperature and field dependence of the critical currents have been investigated with the magnetic field oriented along the c-axis and within the ab-planes to analyze the microstructural features influencing the superconducting properties. The effect of Nd4Ba2Cu2O10 and Nd2BaO4 additives on the transition temperature and the critical currents has been studied in order to examine if the concentration of antisite defects may be modified with this procedure. It has been found that no noticeable difference is generated when the samples are grown in a directional solidification procedure. We discuss as well the relevance of post-processing heat treatments in Ar and O2 atmospheres on the critical currents. Finally, some clues on the general features of the magnetic phase diagram of these materials are given.

Analytical and structural transmiission electron microscopy study of normal- superconductor interfaces in melt-textured Nd1+xBa2−xCu3Oy

Abstract The microstructure and cation stoichiometry associated with normal - superconductor Nd 4−2x Ba 2+2x Cu 2−x O 10−2x -Nd 1+x Ba 2−x Cu 3 O y interfaces in melt-textured Nd 1+x Ba 2−x Cu 3 O y submitted to various oxygen pressures, has been investigated by transmission electron microscopy using high resolution lattice imaging and energy dispersive x-ray analysis. The interfaces processed at 1 bar and 90 bar oxygen pressure display a variety of misfit accommodation mechanisms and appear, as a general trend, structurally sharp. Plots of the Ba:Nd ratio across the interfaces show a fairly constant ration (typically ∼1.85) in the matrix up to close to the interface, whereas in the precipitate there appeared to be a gradual increase in the Ba:Nd ratio toward the interface over about 0.1−0.5 μm. The reported dependence of the interface pinning efficiency on the oxygenation might be due to a local Nd enrichment (a few percent) of the matrix near the interface (

Engineering stacking fault configurations for high critical current melt textured YBa2Cu3O7

Abstract High oxygen pressure processing is successfully applied to enhance the critical currents in melt textured YBa 2 Cu 3 O 7−x . Electron microscopy observations indicate the formation of short dendritic-like stacking faults at the interface of Y 2 BaCuO 5 particles. Partial dislocations bounding such structures act as efficient flux pinning centers, allowing a J c enhancement of 180% at 5 K relative to the virgin sample after processing at 350°C under P(O 2 ) = 100 bar.

Identification of Vortex Pinning Centers in Melt Textured ReBa2Cu3O7 (Re = Y,Nd)

Melt textured single domain ReBa2Cu3O7 (Re = Y,Nd) samples were post-processed, either under high oxygen pressure (PO2 ≤180 bar) or under cold isostatic pressing (CIP) conditions. We show that CIP processing can lead to critical current enhancements up to 100% at 77K. The analysis of the evolution of the microstructure by TEM allows us to attribute the enhanced pinning to 1/6[031] partial dislocations which are distributed anisotropically within the (001) plane. In Nd123/422 composites we demonstrate that high oxygen pressure annealing enhances interface pinning in parallel with a reduction of the fishtail anomaly due to the presence of Nd-Ba antisite defects.