Second generation, high-temperature superconducting wires are based on buffered, metallic tape substrates of near single crystal texture. Strong alignment of adjacent grains was found to be necessary from previous work that suggested large angle, YBa2Cu3O7−δ [001]-tilt boundaries reduce Jc exponentially with increasing misorientation angle (θ). We pursue the low-θ regime by evaluating single grain boundaries (GB) and biaxially aligned polycrystalline films utilizing both the rolling-assisted biaxially textured substrates and ion-beam assisted deposition coated conductor architectures. Analysis concludes that an exponential dependence on Jc is applicable for θ≳4°, where the spacing between the periodic disordered regions along the GB become smaller than a coherence length.
Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi, Pb)2Sr2Ca2Cu3Ox oxide-powder-in-tube conductors and (Bi, Pb)2Sr2CaCu2O8 (Bi-2212) and TlBa2Ca2Cu3Ox thick film conductors. It is demonstrated that in each of these conductors, critical current densities are dictated by the grain boundary misorientation distributions (GBMD's). Percolative networks of low-angle boundaries with fractions consistent with the active cross-sectional area of the conductor exist in each of these conductors. Further enhancements in the properties require increased numbers of small-angle grain boundaries. Given the processing methods used to fabricate these materials, no clear route employing a simple modification of the established processing method is apparent. To address this need, conductors with controlled or predetermined GBMD's are necessary. Development of biaxial texture appears to be the only possible way to increase the number of small-angle boundaries in a practical and controllable manner. We summarize in this paper recent results obtained on epitaxial superconducting films on rolling-assisted-biaxially-textured-substrates (RABiTS). This technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa2Cu3O7–δ films grown using laser ablation on such substrates have critical current densities exceeding 106 A/cm2 at 77 K in zero field and have a field dependence similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of the next generation high temperature superconducting (HTS) wire capable of carrying high currents in high magnetic fields and at elevated temperatures.
Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.
We investigated the dependence of critical current density (Jc) on thickness of Yba2Cu3O7−δ (YBCO) films grown by pulsed laser deposition on (100) SrTiO3 (STO) and on rolling-assisted biaxially textured substrates (RABiTS). The thickness of YBCO films varied from 0.19 to 3 μm. The highest Jcs of 5.3 and 2.6 MA/cm2 at 77 K, self-field were obtained for 0.19-μm YBCO films on STO and RABiTS, respectively. Jc was found to decrease exponentially with YBCO thickness on both substrates. However, the results suggest different mechanisms are responsible for the Jc reduction in the two cases. On STO, growth of a-axis grains within c-axis films and broadening of the in-plane texture were observed in thick films. On RABiTS, degradation in cube texture as well as development of a porous surface morphology were found to correlate with film thickness.
Coated conductors ͑CCs͒ using deformation textured substrates such as rolling assisted biaxially textured substrates ͑RABiTS™͒ are one of the most suitable conductor forms for applications at 77 K. These substrates are inherently polycrystalline and produce a network of grain boundaries ͑GBs͒ in the YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ layer. 1,2 Magnetooptical ͑MO͒ studies demonstrate that the vast majority of these GBs have a J c value less than that of the intragrain 1,2 regions. Under applied transport currents, MO imaging of the self-field shows a strong GB influence and a clear illustration of percolative current flow. 2,3 The self-field studies have shown that J c is usually limited in a local region by a small fraction of GBs, and that the majority of the sample is carrying a current much less than its local J c . Here we expand upon that earlier work by quantifying the inter-and intragrain J c values of several CC samples. This study shows that the intragrain J c is not the limiting factor in present CC technology, and the J c ( ) dependence of GBs in YBCO CCs fits well with previous studies of YBCO grown on ͓001͔ tilt SrTiO 3 ͑STO͒ bicrystals. 4,5 Standard optical photolithography and dry etching was used to pattern inter-and intragrain links in several CC samples. All samples consisted of a deformation textured Ni substrate with a Ni/CeO 2 or Y 2 O 3 /yttria-stabilized zirconia (YSZ)/CeO 2 buffer layer structure. The YBCO layers were grown by the barium fluoride 6 or trifluoroacetate 7 methods. All patterned links ͑inter-and intragrain͒ were 10 m wide and 30 m long. Extended voltage-current characteristics and J c (H) dependencies were measured. For intragrain links, J c values were determined using an electric field criterion of 1 V/cm. For a 30 m long link, this yields a voltage criterion of 3 nV, well above our typical measurement noise of 0.3-0.5 nV. For the intergrain links, a voltage criterion ͑3 nV͒ was used due to the uncertainty in the width of the GBs. Universal use of the 1 V/cm criterion is crucial to legitimate, quantitative comparisons of J c values. Also, it is well known that J c increases with decreasing film thickness, 8 and Zhao et al. have observed significant overestimates of J c for links less than 10 m wide due errors introduced in patterning. 9 All these factors need to be considered when comparing J c values. For intergrain links, electron backscatter Kikuchi pattern ͑EBKP͒ analysis was used to determine the GB misorientation angle . Where possible this was done in the YBCO layer; however, due to poor EBKP quality most angles were measured in the YSZ. We admit this may not be a true representation of the GB angle in the YBCO. X-ray diffraction and recent EBKP studies 10 have shown that in general, there is an improvement in the c-axis alignment of the YBCO relative to the YSZ layer, and this results in a reduction of . Table I lists several properties of the samples and patterned links investigated. At least two links were patterned per sample, but not all links were suitable for study due...
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