S.E. Babcock

Influence of nickel substrate grain structure on YBa2Cu3O7−x supercurrent connectivity in deformation-textured coated conductors

Coupled magneto-optical imaging and local misorientation angle mapping have been used to demonstrate the percolative nature of supercurrent flow in YBa2Cu3O7−x(YBCO) coated conductors grown on deformation-textured Ni substrates. Barriers to current flow occur at many YBCO grain boundaries (GBs) which have propagated through the buffer layers from the underlying Ni substrate, and all Ni GBs with misorientation angles >4° initiate percolative current flow. This type of current barrier is characteristic of the conductor form and has been found to exist in samples with Jc(0 T,77 K) values >2 MA/cm2. Sharpening of the local substrate texture or improving in low-angle GB properties should lead to higher Jc values.

Microstructural analysis of high critical current density Ag-clad BiSrCaCuO (2:2:1:2) tapes

Abstract The critical current density (Jc) values of variously processed Ag-clad Bi2Sr2Ca1Cu2Ox tapes are related to their microstructures as observed by TEM, SEM, and X-ray pole figure analysis. The coupled microstructure-Jc results confirm the general importance of a highly c-axis aligned microstructure for high critical current density. However, TEM experiments revealed a “colony” microstructure in all sample in which groups of several grains with nearly perfect c-axis alignment (colonies) are formed but where the c-axes of adjacent colonies are not parallel. SEM imaging of etched cross-sections showed that important features of this colony misalignment are not accurately reflected by stand alone X-ray pole-figure analysis or TEM imaging. Near-perfect alignment occurs within a few microns of the Ag cladding for all processing methods. However, the processing details control the macroscopic grain alignment within the bulk of the superconductor. In particular, second-phase particles disrupt the alignment of macroscopic colonies. These misalignments may control the transport critical current, yet they are not clearly evidenced by either macroscopic pole-figure analysis or by very local TEM examination.

Evidence for channel conduction in low misorientation angle [001] tilt YBa2Cu3O7−x bicrystal films

Extended voltage–current (V–I) characteristics of [001] tilt YBa2Cu3O7−x bicrystal films having misorientation angles, θ, of 3° to 20° showed a substantial and progressive change with θ, when measured in large fields. Transmission electron microscopy of the 10° bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10° and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°.

Evidence for local composition variations within YBa2Cu3O7−δ grain boundaries

The composition variations associated with grain boundaries in bulk sintered YBa2Cu3O7−δ have been investigated by means of energy‐dispersive x‐ray microanalysis in a high spatial resolution scanning transmission electron microscope. A series of composition profiles indicates that considerable variations in composition occur on a very local scale both parallel and perpendicular to these grain boundaries. The regular, oscillatory (period ∼25 nm) nature of the composition variations observed along a grain boundary suggests that these heterogeneities may be an intrinsic feature of the boundary. Some possible implications of these observations for percolative conduction mechanisms are discussed briefly.

Investigation of composition variations near grain boundaries in high-quality sintered samples of YBa2Cu3O7−δ

The composition at and near grain boundaries in sintered YBa2Cu3O7−δ has been investigated using (a) high-spatial-resolution scanning transmission electron microscopy in conjunction with energy dispersive X-ray analysis (STEM/EDAX), and (b) scanning Auger electron spectroscopy (SAM) of fracture surfaces. The preliminary results of these studies are reported here. The superconducting pellet used for this study is of high quality, as evidenced by its relatively low normal state resistivity of 250 μΩ cm at 100 K. Furthermore, the transport critical current of this sample is nearly magnetic field independent (50 A/cm2 at 2–20 T, 4 K and 77 K), indicating that tunnelling through thin normal barriers is not limiting the current flow in this case. The majority of the grain boundaries present in this sample are free of second-phase material. The STEM/EDAX data suggest that these boundaries are somewhat copper-rich and barium- and oxygen-defficient in the immediate vicinity ( < 5 nm) of the interface. A minority of the grain boundaries present ( < 25%) contain a thin ( ⋍ 15 nm) layer of a Cu-rich and yttrium- and barium-deficient oxide material. Short-range composition gradients of small magnitude were found in the grains abutting this type of boundary. Variations in composition were not apparent in the analysis of the SAM data. No significant difference in the concentrations of Y, Ba, Cu, O, or C was detected between the transgranular and intergranular fracture surfaces.

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.

Weak links and the poor transport critical currents of the 123 compounds

Abstract The results of recent transport measurements, DC and AC magnetization measurements, and investigations of the grain boundary morphology and composition of 123 compounds which have been conducted by our group are described. The samples studied have low normal state resistivities (200–600 μΩcm at 100 K) and largely magnetic-field-independent transport critical current densities (J ct ) in the range 2–10 T at temperatures of 4–77 K, indicating that they are of relatively high quality. Nevertheless, the J ct values measured on these samples are still very low (4–100 A/cm 2 , 2–20 T, 4.2 K), and the large values of J cm /J ct (J cm is the magnetization critical current density) determined for these specimens indicate that weak links still limit J ct . Critical current measurements have been made under different relative orientations of applied field, crystal axes, and measuring current on sintered textured samples (c axes of all of the grains nearly parallel). Anisotropy of J cm similar to that observed in single crystals was found. By comparing the ratio J cm /J ct for different orientations of B relative to the c axis, it is concluded that local composition variations must act as efficient pinning centers at 4.2 K. A very low and strongly-field-dependent J cm for B ⊥ c at 77 K suggests, however, that these 4.2 K pinning centers become weak links at 77 K. Evidence is presented for the occurrence of weak links at regions internal to the grains. Detailed microstructural investigations using conventional and analytical transmission electron microscopy and scanning Auger microanalysis on fracture surfaces did not provide any definitive information about the nature of the weak links.

Microstructure of electron-beam-evaporated epitaxial yttria-stabilized zirconia/CeO2 bilayers on biaxially textured Ni tape

Abstract Transmission and scanning electron microscopy, atomic force microscopy, X-ray pole figure analysis and Auger electron spectroscopy were used to characterize the microstructure and surface topography of epitaxial yttria-stabilized zirconia (YSZ) and CeO 2 thin films deposited by electron beam evaporation on rolling-assisted biaxially textured Ni substrates (RABiTS™). The as-deposited YSZ layer is composed of highly crystallographically aligned, slab-shaped columnar grains with sharply defined, rectangular cross sections and average dimensions of 10 nm by 50 nm by the film thickness. The faces of the YSZ slabs lie on the {110} planes that contain the surface normal. Their caps are roof-shaped with a peak-to-valley height of about 10 nm and a RMS roughness, measured by atomic force microscopy, of 1.3 nm. The resultant surface morphology is rough, but shows a regular, cross-hatched pattern on the length scale of about 10 nm. The length scale and crystallographic directionality of the YSZ microstructure is retained when YBa 2 Cu 3 O 7− δ is pulsed laser deposited on it, but the YSZ columns appear to have sintered into a less angular, more distinctly porous microstructure. The CeO 2 layer also is columnar, but appears to be denser, with a flatter, less directional surface topography. Auger sputtering-depth profiling experiments revealed that the compositions of both films are constant through the film thickness and that interdiffusion along the surface normal is not extensive.

Microstructure of lateral epitaxial overgrown InAs on (100) GaAs substrates

Substantial defect reduction was achieved in InAs/GaAs by lateral epitaxial overgrowth in which InAs was grown on mask-patterned (100) GaAs with stripe-shaped windows of various widths by metalorganic chemical vapor deposition. The InAs growth morphology, crystal quality, and microstructure were evaluated using double-crystal x-ray rocking curves and scanning and transmission electron microscopy. The microstructure of the InAs grown on mask-free control samples was comprised of micron-scale misoriented grains and dislocations at a density of 1011 cm−2. As the width of the mask openings decreased to 0.8 μm, the rocking curves narrowed, grain boundaries disappeared and the dislocation density decreased to <107 cm−2. The distribution of the remaining defects suggests substantial changes in microstructural development when the window width is ≲1 μm.