V. Gomis

Bridgman growth and enhanced critical currents in textured YBa2Cu3O7 — Y2BaCuO5 composites

Abstract A new method of melt textured growth of monolithic YBa 2 Cu 3 O 7 superconductors has been developed based on a vertical Bridgman method. Long bar samples have been prepared having typical sizes of 100 × 3 × 2 mm 3 and the growth conditions were typically R = 1mm/h and G = 20 °C/cm. The microstructure of the samples was investigated by means of polarized optical microscopy. Single domain textured ceramics having different concentration and size of Y 2 BaCuO 5 (211) precipitates have been obtained. We show that our method allows preparation of monolithic conductors having a very homogeneous distribution of micronic or submicronic precipitates while the growth front is maintained along several cm. Critical currents of single domain samples have been measured with a SQUID magnetometer and we have found J c c values as high as 6 × 10 4 A/cm 2 at 77 K. We show also that the enhancement effect of critical currents by 211 precipitates is strongly field dependent, being effective only at low magnetic fields. This field dependent effect encompasses a saturation effect in the displacement of the irreversibility line towards higher fields. The mechanisms leading to the observed complex behavior are analyzed.

Hybrid superconducting fault current limiter based on bulk melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ ceramic composites

A hybrid fault current limiter (FCL) based on single domain bars of Y123/211 melt textured composites has been designed to operate at a 1 kV/400 A line. This consists of a conventional Cu primary coil inductively coupled to a secondary coil formed by the paralleling of single Cu turns each one shorted by a superconducting bar. The superconducting material has been prepared by the solidification techniques of Bridgman and top seeding in air which has allowed us to obtain bars with transport critical current densities of 20000 A/cm/sup 2/ at 77 K. Preliminary tests on the quench current, recovery time and AC-losses in normal operation have been performed. The differences and advantages of a hybrid FCL in comparison with a resistive and an inductive FCL is discussed.

Mechanical fragility and tri-dimensional crack structure in NdBaCuO top seeded and multi-seeded melt-textured samples

In large TS-MTG Nd123 single-domain samples, the main lifetime limiting problem is the appearing of macroscopic fissures in the bulk, that evolve into cracks and lead to sample destruction. To analyze this phenomenon we grew single-seeded pellets and multi-seeded bars, and characterized them by XRD, PLOM, SEM and EDS. We measured inductively their superconducting properties Tc and Jc. Fissures appear to belong to micro-fissures families, parallel to the crystallographic planes. We correlate their origin to local tetra-ortho structural transition, due to spontaneous oxygenation during growth process, and their evolution toward cracks to elastic relaxation of Nd422-rich quenched liquid deposits present in the growing front concavities. MS-TSG bars growth dynamics appear to differ from pellets one. In fact, with appropriate inter-seed spacing, the absence of quenched liquid deposits near the boundaries joining adjacent domains was observed. This fact reduces mechanical fragility perpendicularly to boundaries, but does not affect the fragility along the same. Further studies are in progress on this aspect.

Correlation between the formation of growth bands and Nd210 addition in Nd/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk samples

1 cm diameter single domain NdBaCuO pellets, obtained from powders of Nd/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (Nd123) with Nd/sub 2/BaO/sub 4/ (Nd210) addition, have been successfully prepared by top seeding melt textured growth technique. Distribution of Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ (Nd422) precipitates in the Nd123 matrix has been analyzed by means of polarized light optical microscopy and scanning electron microscopy. Relevant distribution inhomogeneity in the peripheral region of pellets with a high Nd210 addition have been revealed, as well as in regions of dendritic growth and around pores. Increasing difficulties in obtaining a complete growth of the single domain samples, in dependence of increasing Nd210 addition has been observed too. A qualitative growth model for high Nd concentration samples has been proposed to explain both these behaviors.

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.

Fabrication and microstructural control of high current ReBa/sub 2/Cu/sub 3/O/sub 7/ current leads (Re=Y, Nd, Sm)

The potential of YBCO superconductors for the development of current leads has been hindered by the difficulties in the fabrication of long single domain bars which exhibit much higher the critical current density than that for the BSCCO counterpart. In this work we report that very long single domain (10 cm/spl times/0.4 cm) self sustained vertical bars can be directionally solidified by using additives such as CeO/sub 2/. The influence of the processing rate has been investigated and we show that current leads which are able to carry a current higher than 2500 A can be fabricated. Quality factors of these bars such as homogeneity, critical current, thermal conductivity and resistance of ohmic contacts are evaluated. At the same time we report the growth conditions of long bars of ReBa/sub 2/Cu/sub 3/O/sub 7/ (Re=Nd, Sm) in air and we investigate the relationship between microstructure and superconducting properties.

Anisotropic magnetization and weak links in melt textured YBa2Cu3O7

Abstract Textured YBa 2 Cu 3 O 7 superconducting samples have been prepared by melt texturing growth (MTG) under variable conditions. We have investigated low field magnetic susceptibility (d.c. and a.c.), high field hysteresis loops and magnetic irreversibility through ZFC-FC temperature scans. We show that low field flux penetration effects, measured by screening capability and dissipative behaviour, may be convenient to detect residual weak links and to characterize the degree of texturation when their anisotropic behaviour is analysed. Critical currents up to 2 × 10 5 A cm −2 at 1 T and 25 K and for H | c have been obtained and their temperature and field dependence determined. We find that enhanced critical currents in the H | c configuration also lead to an increase in the magnetic anisotropy, which probably reflects reduced misorientation of the crystallites within a domain.

High pressure oxygenation in melt textured NdBa/sub 2/Cu/sub 3/O/sub 7/: identification of pinning mechanisms

A competing effect inducing both an enhancement of interface pinning and aging of the critical currents has been established in NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ (Nd123/422) melt-textured ceramic composites upon high oxygen pressure annealing. It is suggested that Nd123/422 interfaces are enriched in antisite defects, which are properly oxygenated only upon high PO/sub 2/ annealing. Additional evidence for the enhanced oxygenation is gained from the observation of a consequent decrease of the field induced anomaly at intermediate fields.

Persistent magnetic flux and mapping of intergranular junctions in melt textured YBa/sub 2/Cu/sub 3/O/sub 7/

Melt-textured superconductors have been prepared with a microstructure characterized by large domains having misaligned growth fronts. An RF mutual inductance microprobe has been developed to identify and characterize the weak-link behavior of the high-angle grain boundaries separating the domains. The mechanisms of flux penetration and trapping in a single-domain sample have been analyzed in a H/sub //c/ configuration at low temperature, 25 K, where J/sub c/ is essentially field independent. A consistent description of flux penetration, isothermal remanence and magnetic hysteresis is reached in the framework of the Bean model. The small value of the lower critical field deduced from this analysis suggests that the Bean-Livingston surface barriers are strongly reduced in melt-textured ceramics as compared to single crystals.<<ETX>>

Shaping of YBa2Cu3O7-Y2BaCuO5 bulk super-conducting composites

Bulk YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 textured ceramic composites of different shapes and sizes have been prepared through a directional solidification process. Green ceramic preforms for the directional solidification process have been obtained by using uniaxial pressures for the fabrication of pellets, ceramic moulds at low pressures for bars, cylinders and rings, and wet extrusion for the fabrication of fibres. We have studied different ratios of both phases, particle sizes, CeO 2 additions and temperature dependence. The directional solidification process allows to obtain quasi-single crystal superconductors with critical currently above 10 5 A/cm 2 at 77 K and zero field.