YBCO Grain Boundary Research Articles

EBSD Characterization of Specific Microstructures in RE-BCO Superconductors

Using the electron backscatter diffraction (EBSD) technique, we analyze in this contribution the microstructure of YBa2Cu3O x (YBCO) thick films with embedded a -axis oriented grains. The YBCO films were prepared by liquid phase epitaxy in air using NdGaO3 substrates. Using this technique, a / c -grain boundaries (GBs) with a well-defined facet of the YBCO film characterized by a single crystalline nature are obtained. This type of GB is very important for the basic physics of GBs in high- T c superconductor materials, and will contribute to a better understanding of GBs also in bulk materials. The application of EBSD to the analysis of a / c -GBs in YBCO requires a high Kikuchi pattern quality (i.e., image quality) to enable a proper distinction of the orientation, as the a -axis corresponds to about one-third of the c -axis, which can cause a misinterpretation of the resulting data. Having solved this problem, the EBSD-based determination of the GB misorientation angles directly proof the specific grain orientation achieved in the film growth.

Flux cutting in superconductors

This paper describes experiments and theories of flux cutting in superconductors. The useof the flux line picture in free space is discussed. In superconductors cutting can either beby means of flux at an angle to other layers of flux, as in longitudinal current experiments,or due to shearing of the vortex lattice as in grain boundaries in YBCO. Experiments onlongitudinal currents can be interpreted in terms of flux rings penetrating axial lines. Morephysical models of flux cutting are discussed but all predict much larger fluxcutting forces than are observed. Also, cutting is occurring at angles betweenvortices of about one millidegree which is hard to explain. The double criticalstate model and its developments are discussed in relation to experiments oncrossed and rotating fields. A new experiment suggested by Clem gives moredirect information. It shows that an elliptical yield surface of the critical stateworks well, but none of the theoretical proposals for determining the direction ofE are universally applicable. It appears that, as soon as any flux flow takes place, cutting alsooccurs. The conclusion is that new theories are required.

Development of All-CSD Processes for Coated Conductors at Nexans: Limitations and Possible Solutions

Nexans experience in developing all CSD processes for coated conductors with the YBCO/CGO/LZO/Ni-5% W RABITS architecture is summarized. The chosen CC architecture has a principal disadvantage that the next layer only clones, but more often deteriorates the out-of-plane texture of the previous layer (in average by ~ 20% for LZO). Depending on the quality of RABITS, this may increase the amount of grain boundaries (GBs) not transparent for the supercurrent close to the percolation limit for the current-breaking path, which explains low and poorly reproducible Jc. Modifying the buffer architecture by introducing CGO seed layer suppresses deterioration of the out-of-plane texture. Another approach, extending the range of acceptable grain misorientations by selective doping YBCO GBs with Ca is shown not possible using the current MOD-TFA process because of decomposition of initial Y123 to disordered Y124 and Y247 during pyrolysis of the second MOD layer. Possible ways of further development are discussed.

High quality factor HTS Josephson junctions on low loss substrates

We have extended the off-axis biepitaxial technique to produce YBCO grainboundary junctions on low loss substrates. Excellent transport propertieshave been reproducibly found, with remarkable values of the quality factorIcRn (withIc the criticalcurrent and Rn the normal state resistance) above 10 mV, far higher than the values commonly reported inthe literature for high temperature superconductor (HTS) based Josephson junctions. Theoutcomes are consistent with a picture of a more uniform grain boundary region along thecurrent path. This work supports a possible implementation of grain boundary junctionsfor various applications including terahertz sensors and HTS quantum circuits in thepresence of microwaves.

Evidence for Extensive Grain Boundary Meander and Overgrowth of Substrate Grain Boundaries in High Critical Current Density ex Situ YBa2Cu3O7−x Coated Conductors

It has been generally accepted that YBa2Cu3O7−x (YBCO) films deposited on deformation textured polycrystalline metal tapes result in YBCO grain boundary (GB) networks that essentially replicate the GBs of the underlying substrate. Here we report that for thicker YBCO films produced by a BaF2 ex situ process, this is not true. Using electron backscatter diffraction combined with ion milling, we have been able to map the evolution of the YBCO grain structure and compare it to the underlying template in several coated conductors. For thin (≤0.5 μm) YBCO films deposited on rolling-assisted biaxially textured substrates (RABiTS), the YBCO GBs nearly directly overlap the substrate GBs. For 0.7–1.4 μm YBCO films, the GBs were found to meander along the substrate GBs and along the sample normal, with displacements several times the film thickness. In very thick films (2.5–2.9 μm), the YBCO grains can completely overgrow substrate grains and GBs, resulting in a substantial disconnection of the YBCO and substrate GB networks. Similar behavior is found for BaF2 ex situ YBCO films on ion-beam-assisted deposition-type templates. The ability of the YBCO to overgrow substrate grains and GBs is believed to be due to liquid-phase mediated laminar grain growth. Although the behavior of the YBCO GB networks changes with YBCO film thickness, the samples maintained high critical current density (Jc) values of >2 MA/cm2 for films up to 1.4 μm thick, and up to0.9 MA/cm2 for 2.5–2.9-μm-thick films.

A comparison of [0 0 1] low-angle tilt grain boundaries of (1 0 0) and (1 1 0) grain boundary planes in YBa 2Cu 3O 7− δ coated conductors

The microstructure and grain boundary structure in YBa 2Cu 3O 7− δ (YBCO) thick film coated conductors are characterized by transmission electron microscopy. The films contain low-angle [0 0 1] tilt grain boundaries with periodic arrays of edge dislocations parallel to the c-axis. A majority of the grain boundary planes are of either the (1 0 0) or the (1 1 0) type. Grain boundary dislocations (GBDs) with a [1 0 0] Burgers vector were observed in tilt boundaries with (1 0 0) boundary planes. However, partial dislocations, separated by stacking faults, were found at boundaries with near (1 1 0) grain boundary planes. Extensive g · b and g · R analyses confirmed the partials to be of 1/2[1 1 0] type. These results suggest that the characteristics of dislocation structures, which have been proposed in various grain boundary-based flux-pinning models, depend not only on the misorientation angle θ, but also on the types of grain boundary plane. The effect of dissociated GBDs on the transport properties of low angle grain boundaries in YBCO coated conductors is discussed.

Grain boundaries in bulk YBCO

The problem of grain boundaries in high temperature superconductors is very important in determining the ability of these materials to carry supercurrent. The success of fabricating polycrystalline bulk YBCO with high-angle grain boundaries carrying high critical currents initiated microstructural studies of these boundaries in order to determine mechanism of current flow across them. TEM studies have been performed on grain boundaries with known J c, followed by simulation of their atomic and electron structures as well as estimations of their superconducting order parameter. Using a model developed to correlate grain boundary microstructure and superconducting properties, we have shown that coupling behavior of grain boundaries does not depend on misorientation characteristics alone. The grain boundary plane as well as the atomic ordering in the vicinity of grain boundary play an important role in the current carrying capabilities of these grain boundaries.

Critical current of YBCO grain boundaries in large magnetic fields

Superconducting electrical transport across single grain boundaries (GBs) in YBCO shows interesting behavior in substantial magnetic fields. Measurements on [001] tilt bicrystal YBCO thin films in strong magnetic fields are consistent with the following: Low-angle GBs (<10/spl deg/) are composed of well-ordered bridges that act as strong superconducting inter-granular links; large-angle GBs exhibit weak coupling behavior that retains a well defined critical current even at high field.

Investigating the Relationship Between Atomic Structure and Carrier Depletion at [001] tilt Grain Boundaries in Ybco

AbstractRecent theoretical analyses [1] have suggested that the origin of the reduced Jc at grain boundaries in high-Tc superconductors may be band bending, which results in the depletion of mobile charge carriers at the boundaries. For this to occur in these p-type superconductors there must, by definition, be a high density of localized donor states in the boundary plane. Here we report a “structural unit” analysis of [0011 tilt grain boundaries in YBa2Cu 3O7-δ (YBCO) that indicates there may be a simple structural origin for such localized donor states.

Superconducting properties of natural and artificial grain boundaries in bulk melt-textured YBCO

A range of experimental techniques have been used to characterize melt-processed YBa 2Cu 3O 7− δ samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained by joining two single domains, have been investigated. Electrical resistivity, current–voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate the boundaries. Results are compared with the properties of single domain material (i.e., containing no grain boundary) for which T c≈89 K and J c‖ ab (77 K, 1 T)>10 4 A/cm 2. Resistance measurements across all the grain boundaries show a stronger dependence on current and magnetic field than that measured within the grains and exhibit a pronounced resistive `tail'. The I– V curves obtained for the high-angle natural grain boundary are sharp and differ from the rounded I– V curves which are characteristic of single-grains. Field mapping measurements used to evaluate the critical current anisotropy are in agreement with magnetisation measurements. The limitations of this technique for investigating boundaries are discussed. It was found that current anisotropy can conceal a weak link between two grains, leading to a false indication of single-grain behaviour. Artificially engineered boundaries are shown to have significant potential for applications in high fields at 77 K.

Analysis and prediction of the critical current density across

A qualitative analysis of the expected dilatation strain field in the vicinity of an array of grain-boundary (GB) dislocations is presented. The analysis provides a basis for the prediction of the critical current densities (jc) across low-angle YBa2Cu3O7- (YBCO) GBs as a function of their energy. The introduction of the GB energy allows the extension of the analysis to high-angle GBs using established models which predict the GB energy as a function of misorientation angle. The results are compared to published data for jc across [001]-tilt YBCO GBs for the full range of misorientations, showing a good fit. Since the GB energy is directly related to the GB structure, the analysis may allow a generalization of the scaling behavior of jc with the GB energy. © 1995 The American Physical Society.