A.D. Bradley

Processing, microstructure and characterization of artificial joins in top seeded melt grown Y–Ba–Cu–O

A technique based on the presence of an unreacted barium-cuprate phase within the melt-processed microstructure (predominantly at the platelet boundaries), which melts below the peritectic temperature of the YBa2Cu3O7–δ (Y123) phase has been developed to join large Y–Ba–Cu–O (YBCO) grains. On heating under an applied pressure of 0.1 MPa, the Ba–Cu–O liquid phase is released to the grain interface. This technique has been investigated for YBCO a–b-plane-aligned grain boundaries with misorientation angles of up to 45° and an aligned c-axis grain boundary. Optical and transmission electron microscopy studies indicate that the low-angle a–b plane grain boundaries are epitaxial in nature and are generally of high microstructural quality. The electrical and magnetic properties of the grain boundaries fabricated by this technique have been characterized by I–V and transport current measurements as a function of magnitude and orientation of the applied field. The temperature dependence of the irreversibility field of low-angle a–b grain boundaries is similar to that of the grains, providing direct evidence for the quality of the join. A join of nominally zero misorientation between the a–b planes was found to support a critical current density, Jc, exceeding 2000 A cm−2 at 77 K in fields of up to 4 T.

Development of non-weak link bulk YBCO grain boundaries for high magnetic field engineering applications

A non-weak link joining technique has been developed for YBCO pseudo-crystals fabricated by seeded peritectic solidification based on the formation of a liquid phase which segregates from the platelet boundaries at temperatures above /spl ap/920/spl deg/C. Electrical and magnetic measurements on these boundaries suggest that their irreversibility field can be as high as 7 T at 77 K in fully oxygenated pseudo-crystals joined along their crystallographic ab-planes which is comparable to the irreversibility behaviour of the adjacent YBCO grains.

High field behavior of artificially engineered boundaries in melt-processed YBa2Cu3O7−δ

Artificial bulk “zero-angle” boundaries parallel to the c axis have been engineered between large melt-processed YBa2Cu3O7−δ (YBCO) grains and observed to carry a transport supercurrent at fields up to at least 5 T at 77 K. The temperature and angular dependencies of the boundary resistance have exactly the same form as those of the grains, which is evidence that the grains are intimately coupled. The limiting mechanism for current transfer across these boundaries is, therefore, not a simple weak link or Josephson effect. This joining technique is extremely promising for production of macroscopic engineering artifacts.

Magnetic field and current distributions at an artificial grain boundary in YBa2Cu3O7-x

Using a magneto-optical technique, magnetic field distributions have been measured in melt-textured YBa2Cu3O7-x, joined to form an artificial grain boundary, in an external magnetic field perpendicular to the sample surface. An inversion scheme using the inverse-matrix method has been applied to obtain the current distribution from the magnetic field distribution. Both magnetic field and current distributions around the grain boundary show different behaviour between the field increasing up to 150 mT and decreasing down to 0 T after zero-field cooling. The magnetic field at a weakly coupled section of the grain boundary in the increasing field is higher than that in the decreasing field, even in the same external field. This study supports a model in which such differences in the magnetic field at the weak-link grain boundaries give rise to the hysteresis behaviour in the field dependence of transport critical current density in polycrystalline samples.

Transport properties of bulk-bicrystal grain boundaries in artificially joined large-grain YBCO

A technique for joining large-grain YBCO has been used to produce bulk-bicrystal grain boundaries of different orientations. The behaviour of boundaries nominally of 0/spl deg/[100], 0/spl deg/[001] and asymmetric 15/spl deg/[100]-tilt misorientation have been investigated using low frequency transport measurements in fields up to 7 T. The boundaries exhibit a metallic normal state and a superconducting transition which broadens with increasing field having an irreversibility line which closely matches that of the adjoining grains. This same qualitative behaviour is seen in all the samples measured for field applied parallel and perpendicular to the c-axis and up to the highest current densities employed (20 Acm/sup -2/). Variation between the different samples due to microstructural differences are discussed. We interpret these results as strong suggestive evidence that this joining technique can be used to produce strongly-coupled large-grains for bulk-scale engineering applications.

Microstructure and growth of joins in melt-textured YBa2Cu3O7-δ

Joining of melt-textured YBa 2 Cu 3 O 7-δ (Y123) grains has been achieved without use of an external agent. The technique uses barium-cuprate liquid phase released from platelet boundaries to mediate the growth of Y123 at the interface between two grains. The epitaxial nature and high quality of the growth was determined by optical and transmission electron microscopy. The composition of Ba–Cu–O phases found in some parts of the joins was determined by electron probe microanalysis. A clean low-angle join was found to consist of a grain boundary with dislocation networks and facets. Transport critical current measurements on this type of join revealed strongly coupled behavior. The technique shows promise for the joining of melt-textured material for power engineering applications.

Magnetic field and current distributions in melt-textured YBa/sub 2/Cu/sub 3/O/sub 7-x/ with an artificial grain boundary

Using a magneto-optical (MO) technique, magnetic field distributions have been measured in a melt-textured YBa/sub 2/Cu/sub 3/O/sub 7-x/ bulk superconductor, joined to form an artificial grain boundary (GB), in an external magnetic field perpendicular to the sample surface. The magnetic field at a weak section of the GB shows different values between the field increasing up to 150 mT and decreasing down to 0 T after zero-field-cooling. Namely, the magnetic field in increasing field is higher than that in decreasing field, even in the same external field. This result supports a model in which such differences in magnetic field at the weak-link GB give rise to the hysteresis behavior in the field dependence of transport critical current density in polycrystalline samples. The field distributions across a well-joined region of the GB behave similarly to the adjoining bulk material and this result indicates the possibility of creating useful artifacts provided that the strongly coupled sections can be reproduced on a larger scale.