S. Meslin

The effect of Y-211 precursor particle size on the microstructure and properties of Y?Ba?Cu?O bulk superconductors fabricated by seeded infiltration and growth

Large, single grains of Y?Ba?Cu?O (YBCO) bulk superconductor composed of a YBa2Cu3O7?? (Y-123) matrix with Y2BaCuO5 (Y-211) inclusions have been fabricated using a seeded infiltration and growth (IG) process. Y-211 precursor pellets prepared from powders containing different Y-211 particle sizes were processed by the seeded IG technique to investigate the effect of Y-211 size on the single grain microstructure. The superconducting properties of the single grains fabricated in this study are observed to correlate well with the variation in Y-211 particle size in the fully textured Y-123 phase matrix. The critical current density, Jc, in particular, has been observed to increase from 68?000?A?cm?2 for a reference sample to 94?000?A?cm?2 at 77?K in self-field for a sample prepared using refined Y-211 precursor powder with added platinum oxide.

EBSD study on YBCO textured bulk samples: correlation between crystal growth and 'microtexture'

This work describes an electron backscattered diffraction (EBSD) study of the perovskite-derived structures YBa2Cu3O7−δ .A fter having pointed out the difficulties of EBSD analyses in resolving the orientations of these pseudo-cubic structures, various YBaCuO bulk samples are analysed and the correlation between the microstructure, crystal growth and global texture, determined by neutron diffraction, is carried out. Homogeneous ‘microtexture’ with small subdomain misorientation of 12 ◦ are measured for YBCO top seeding melt textured growth (TSMTG) samples. YBCO perforated samples also exhibit misoriented subdomains, giving rise to a heterogeneous ‘microtexture’ correlated to the YBCO growth front and to the pattern used for the perforating. (Some figures in this article are in colour only in the electronic version)

Hall probe mapping of melt processed superconductors with artificial holes

Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were investigated by means of Hall scans of the trapped field distribution and the magnetoscan technique. We show that the remnant flux profiles are affected by the perforation in different ways. The location of the holes can be resolved by field mapping at activation fields below that for complete penetration of the bulk. The magnetoscan technique allows one to spatially localize the array of holes.

Study by Hall probe mapping of the trapped flux modification produced by local heating in YBCO HTS bulks for different surface/volume ratios

The aim of this report is to compare the trapped field distribution under a local heating created at the sample edge for different sample morphologies. Hall probe mappings of the magnetic induction trapped in YBCO bulk samples maintained out of thermal equilibrium were performed on YBCO bulk single domains, YBCO single domains with regularly spaced hole arrays, and YBCO superconducting foams. The capability of heat draining was quantified by two criteria: the average induction decay and the size of the thermally affected zone caused by a local heating of the sample. Among the three investigated sample shapes, the drilled single domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field. Finally, a simple numerical modelling of the heat flux spreading into a drilled sample is used to suggest some design rules about the hole configuration and their size.

Measurements of Thermal Effects in a Bulk YBCO Single Domain Superconductor Submitted to a Variable Magnetic Field

When YBCO single domains are subjected to a variable magnetic field, the motion of vortices may give rise to a significant temperature increase and a degradation of the superconducting properties. We have experimentally investigated such effects in bulk melt-processed YBCO single domains. Several temperature sensors, a pick-up coil and two Hall probes were placed against the surface of a monolithic bulk YBCO pellet. Data were simultaneously recorded during the application of ac magnetic fields of various amplitudes, either with or without a pre-existing dc trapped flux. The measurement results agree well with to those obtained by numerical modeling. It was also found that a superimposed dc magnetic moment reduces the temperature increase caused by the ac magnetic field.

Shaping of melt textured samples for fault current limiters

Resistive fault current limiters require long length of superconducting materials with relatively low sections. Meander is now a frequently used geometry for this type of application. Nevertheless, the shaping of a textured sample is a quite delicate operation because of its brittle character. Cracks are often observed in the final meander and reduce the life time of these samples. Recently, we have presented a new process that allows overcoming this problem. Sintered pellets are drilled to adopt the chosen geometry. Then, the texturing process is performed. By this way, no large mechanical stresses are applied to the fragile textured samples. Here, the idea is to reproduce the meander pattern. Holes performed do not cross the whole sample thickness but alternatively emerge from top or bottom of the pellet. Slices extracted from the textured samples face like meanders. Different geometries have been tested with holes emerging perpendicular to the (ab) planes or parallel to the (ab) planes. The annealing time under oxygen flow has been studied to fit the required critical current densities values, which were evaluated through transport measurements at 77 K. SEM studies allow comparing both configurations and in particular the crack existence. The quality of the textured material around the holes was found to be not deteriorated compared to the bulk microstructure. Finally, the influence of the holes on the texture quality was studied by neutron diffraction.

Study of thermal effects in bulk RE-BCO superconductors submitted to a variable magnetic field

When bulk RE-BCO superconductors are used as permanent magnets in engineering applications, they are likely to experience transient variations of the applied magnetic field. The resulting vortex motion may cause a significant temperature increase. As a consequence the initial trapped flux is reduced. In the present work, we first focus on the cause of a temperature increase. The temperature distribution within a superconducting finite cylinder subjected to an alternating magnetic field is theoretically predicted. Results are compared to experimental data obtained by two temperature sensors attached to a bulk YBCO pellet. Second, we consider curative methods for reducing the effect of heat flux on the temperature increase. Hall-probe mappings on YBCO samples maintained out of the thermal equilibrium are performed for two different morphologies : a plain single domain and a single domain with a regularly spaced hole array. The drilled single-domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field.

Shaping and reinforcement of melt textured YBa2Cu3O7−δ superconductors

From porous Y2BaCuO5 (Y211) with various grain sizes, single domain ceramic composites YBa2Cu3Oy/Y2BaCuO5 have been prepared by combination of the infiltration and top seed growth (ITSG) process. In addition, perforated Y123 has been prepared from Y211 by the ITSG method in order to magnify the specific surface and then increase oxygen diffusion into the core of the material. Magnetic and electrical properties were determined and correlate well with the microstructure of the composites and were compared to the conventional doped or undoped YBa2Cu3Oy (Y123). From magnetic measurements, high critical current densities, Jc, of 86 000 A cm−2 have been measured. Transport Jc values higher than 10 600 A cm−2 are reached at 77 K and 0 T, corresponding to the nominal critical currents of 325 A injected reproducibly through sections less than 3.082 mm2. This confirms the high quality of single domains obtained with a well controlled ITSG process. On the other hand, the perforated samples were reinforced using resin impregnation and the flux mapping has been investigated.