Inter- and intragrain transport measurements in YBa2Cu3O7−x deformation textured coated conductors

Abstract

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...

Critical Current Densities of Isolated Grain Boundaries in Coated Conductors

“…Most of this work has been done using model systems consisting of YBCO thin films grown on bicrystal substrates [1], [2], [6], [8], [11] and only recently data obtained on isolated CC grain boundaries has been published [4], [12], [13]. Manuscript Whereas useful and important insights have been made using bicrystals, studying GBs on coated conductors is essential for two reasons.…”

EBSD study on YBCO textured bulk samples: correlation between crystal growth and ‘microtexture’

“…The practical applications of the bulk superconductors YBa 2 Cu 3 O 7−δ depend crucially on the quality of the texture in the material and, more precisely, on the grain boundary misorientations [6][7][8], which are known to influence detrimentally the transport critical current density [9][10][11][12][13][14][15]. As a ceramic, Y123 specimens can encounter typical difficulties in the sample preparation, like cracking or oxygen understoichiometry, and the achievement of good quality EBSD patterns suited for a correct interpretation is difficult, because surface damage, polishing distortion and charging under observation can occur [16].…”

Effect of Strain on Grain and Grain-Boundary Critical Currents of YBCO Coated Conductors

“…Therefore the final percolative current crossing the sample, would be affected by the return field of structural grains and grain colonies. By performing simulations based on the critical state model [11] we have obtained that 35% of isolated colonies of 10 in a matrix of small grains (2 ) are able produce the effect observed on the hysteresis loops for . Comparing the values of obtained for both type of CCs after deformation tests (Fig.…”