X.Y. Cai

Position-sensitive measurements of the local critical current density in Ag sheathed high-temperature superconductor (Bi, Pb)2Sr2Ca2Cu3Oy tapes: The importance of local micro- and macro-structure

Abstract Small sections cut from within individual superconducting filaments of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O y (BSCCO-2223) tapes have critical current densities ( J c ) which depend very much on the local microstructure and which can be as much as five times higher than the average J c . Tapes having average J c values of 12–15 000 A/cm 2 (77 K, 0 T) had local J c values up A/cm 2 , a value larger than hitherto reported for any bulk sample. Close to the Ag sheath the conversion to the 2223 phase is more complete and the grains are larger and better aligned and the J c is then much higjer and less field sensitive. Large variations are also found in more uniformly reacted tapes, perhaps due to variations of the local crack density. The microstructure of the highest J c regions is still far from ideal, thus showing that the limits of this technologically very important system are far from being achieved.

Weak-link-free behaviour of high-angle YBa2Cu3O7–δ grain boundaries in high magnetic fields

Nature 347, 167-169 (1990) THE expression used to calculate Jc values throughout the above paper was misquoted in the paper (bottom of left side, page 332) as /c = AM/(2r). The correct expression is /c = 3AM/r. The correct expression was used for all the calculations which appear in the text and figures.

Current-limiting mechanisms in individual filaments extracted from superconducting tapes

Many large-scale applications of high-temperature superconductors depend crucially on the ability to achieve high critical-current densities Jc (of the order of 105 A cm−2). Existing silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) tapes have Jc values that come within about 25% of this target, these values being limited by the fact that thesupercurrent flows percolatively around barriers that occur over many length scales. To elucidate the nature of these barriers, we have measured the transport properties of individual filaments extracted from very-high-Jc multifilament tapes. We find that Jc for individual filaments reaches at least 8× 104 A cm−2—about 50% higher than the average value over the whole cross-section of the wire. Although we injected the current to flow along the crystallographic a–b planes of the material, we found that all filaments possessed local characteristics of c-axis transport, indicating the presence of occasional nanometre- to micrometre-scale barriers at basal-plane-faced grain boundaries. An independent and much larger limiting influence on the critical current comes from unhealed cracks produced by deformation during the processing of the wires. These results provide direct evidence that better processing methods aimed at improving the c-axis alignment and at inhibiting residual cracks should raise the accessible Jc values towards those needed for applications.

Electrical transport across grain boundaries in bicrystals of YBa2Cu3O7δ

Abstract The superconducting properties of about 20 bulk-scale flux-grown bicrystals of YBa 2 Cu 3 O 7−δ have been measured in order to deduce the nature of the coupling across their grain boundaries. Bicrystals with two types of misorientation relationship were studied: (i) θ [001] (C-type) and (ii) 90° [010] plus an additional rotation about the a I | c II axis (P-type). A general trend for a transition from flux pinning (FP) to Josephson Junction (JJ) to resistive (R) behavior was observed as the misorientation angle increased. However, FP character was observed well into the high-angle regime, in particular, for P-type bicrystals, and JJ behavior was observed at as low as 10° [001]. The J α values of two bicrystals were increased by a second oxygen anneal. In one case (28° [001] boundary), extended oxygenation produced a change form JJ to mixed JJ/FP behavior, markedly raising the transport critical current density (J α ) at all fields. HRTEM has been performed on two of the same bicrystals which were electromagnetically characterized. A thin second phase layer unobservable by conventional TEM was observed in a resistive boundary, whereas no such phase was detected at a JJ boundary. We conclude that the character of the boundary cannot be predicted either from its misorientation angle or the magnitude of its J c .

A TEM-EELS study of hole concentrations near strongly and weakly coupled grain boundaries in electromagnetically characterized YBa2Cu3O7−δ bicrystals

Abstract High-spatial-resolution electron energy loss spectroscopy in a transmission electron microscope (TEM-EELS) was used to obtain hole-concentration profiles near grain boundaries in flux-grown YBa 2 Cu 3 O 7−δ bicrystals with misorientation relationships of 7°, 12°, 28°, and 31° about their common [001] rotation axes. A key aspect of the work is that the electromagnetic properties and coupling character of each of the bicrystals also were determined explicitly. The 28° and 31° bicrystals showed Josephson-junction-like weak coupling, and evidence for severe hole depletion at the boundary was present in the oxygen K near-edge fine structure (ELNES) of the EELS spectra obtained from them. The hole-deficient zone extended 20 to 40 nm into each crystal. In striking contrast, the 7° and 12° bicrystals were strongly coupled and dramatically less hole depleted at the grain boundary. These results show that the boundary region within which the electronic structure differs from that of the bulk is substantially wider than both the YBa 2 Cu 3 O 7−δ coherence length and the grain-boundary structural and compositional width suggested by high-resolution TEM images and cation non-stoichiometry studies. They also suggest a continuous transition from strongly to weakly coupled character.

Critical current limiting factors in post annealed (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes

Processing (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) tape by the oxide-powder-in-tube technique normally includes two heat treatments and one intermediate rolling. A three-step heat treatment was applied for the second heat treatment (HT2). Experimental procedures were designed for understanding the roles of each processing step in HT2 and the post anneal. It was found that post annealing improved the critical temperature T/sub c/, critical current density J/sub c/ and flux pinning characteristic field H/sub p/. The midpoint T/sub c/ was raised from 103 to 109 K by post anneal, while H/sub p/ was increased from 126 to 183 mT. Post anneal at about 790/spl deg/C precipitated Pb from Bi-2223 phase to form a Pb-rich phase, thus the microstructure looked worse but T/sub c/, J/sub c/ and H/sub p/ were raised. J/sub c/ was raised as the Bi-2212 phase signature in the T/sub c/ trace decreased. The most important observation is that raising T/sub c/ and minimizing residual Bi-2212 are decisive factors for increasing J/sub c/ at 77 K in Bi-2223 tapes.

Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

The connectivity and flux pinning components of the critical current density Jc in a high-Jc Ag-clad, 19-filament (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though Jc more than tripled to reach ~50 kA cm-2 (77 K, 0 T) after final heat treatment. Filament resistance and Jc data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable Jc of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.

Electromagnetic coupling character of [001] twist boundaries in sintered Bi2Sr2CaCu2O8+x bicrystals

Abstract The electromagnetic characteristics of [001] twist grain boundaries in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO-2212) have been deduced from measurements of the resistive transitions, voltage-current ( V - I ) characteristics, and field-dependent transport critical current densities ( J ct ( H )) of single- and bicrystalline samples. The bicrystals were prepared by solid-state sintering of (001) faces of freshly-cleaved, bulk-scale single crystals placed at a pre-determined misorientation angle, θ. A low-angle 2° bicrystal was strongly coupled with V - I and J ct ( H ) characteristics that were indistinguishable from those of the single crystals, indicating that the sintering process itself does not produce weak coupling. 30° and 36° [001] bicrystals were weakly coupled. 23° and 88° twist bicrystals were strongly coupled, even though their boundary T c values were reduced. This behavior contrasts starkly with that of [001] tilt boundaries in YBa 2 Cu 3 O 7 − δ , where a reduced boundary T c is always associated with weak coupling.

Local measurement of current density by magneto-optical current reconstruction in normally and overpressure processed Bi-2223 tapes

Magneto-optical current reconstruction has been used for detailed analysis of the local critical current density (J/sub c/) variation in monocore Bi-2223 tapes. We find, even in high quality tapes with bulk transport j/sub c//spl sim/40 kA/cm/sup 2/ (77 K, 0 T), that there exist local regions which possess current densities of more than 200 kA/cm/sup 2/. Overpressure processing at 148 bar significantly improved J/sub c/ to 48 kA/cm/sup 2/ by improving the connectivity. For the overpressure-processed sample we find that the current distribution is more uniform and that the maximum local current density at 77 K is increased almost to 300 kA/cm/sup 2/.

Examination of current limiting mechanisms in monocore Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape with high critical current density

Magneto-optical imaging, electromagnetic measurements and SEM have been used for the characterization of a monocore BSCCO tape made by oxide powder-in-tube process which had a transport J/sub c/ (77 K, 0 T) of 37 kA/cm/sup 2/. Our study evaluates the connectivity between superconducting grains and reconstructs the local current distribution at <10 /spl mu/m scale. Some local regions are found to carry up to 180 kA/cm/sup 2/ at 77 K, a new record for BSCCO wires.