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

Abstract

Coupled magneto-optical imaging and local misorientation angle mapping have been used to demonstrate the percolative nature of supercurrent flow in YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ coated conductors grown on deformation-textured Ni substrates. Barriers to current flow occur at many YBCO grain boundaries ͑GBs͒ which have propagated through the buffer layers from the underlying Ni substrate, and all Ni GBs with misorientation angles Ͼ4°initiate percolative current flow. This type of current barrier is characteristic of the conductor form and has been found to exist in samples with J c (0 T,77 K) values Ͼ2 MA/cm 2 . Sharpening of the local substrate texture or improving in low-angle GB properties should lead to higher J c values. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00341-7͔High critical current density (J c ) conductors capable of operating in fields of several tesla at liquid-nitrogen temperature are critical to large-scale applications of hightemperature superconductors. Coated conductors ͑CCs͒ with biaxially textured YBa 2 Cu 3 O x ͑YBCO͒ respond to this need. [1][2][3] One widely employed approach today uses deformation texturing of a metal substrate, generally pure Ni, on which buffer layers and YBCO are grown. 1,2 Such architectures permit J c (0 T,77 K) values 1,2 Ͼ1 MA/cm 2 , but many samples have lower values. Here we couple magneto-optical imaging and local misorientation angle mapping to show that many such barriers to current flow occur at YBCO grain boundaries ͑GBs͒ which have propagated through the buffer layers from the Ni GBs in the underlying substrate. All Ni GBs with misorientation angles Ͼ4°were found to initiate percolative current flow. Since typical deformation-textured substrates have many GBs misoriented in the range of 5°-10°, this study shows that it will be very valuable for CC technology to further enhance substrate texture and/or to improve low-angle GB properties.Magneto-optical ͑MO͒ imaging, light microscopy, and backscattered electron Kikuchi pattern ͑BEKP͒ analysis of the local texture were conducted on a series of four CC samples grown on deformation-textured Ni substratres with in-plane and out-of-plane full width at half maxima of 6.6°-7.4°and 5.8°-8.7°, respectively, as measured by x-ray pole figures. The buffer and YBCO layers were deposited by pulsed-laser deposition ͑PLD͒ with architecture YBCO/CeO 2 /yttria-stabilized zirconia ͑YSZ͒/CeO 2 /Ni and thickness of 300-1200/100/500/100 nm for the respective oxide layers. The thickness of the YBCO layer varied from sample to sample without obvious differences in the properties measured by MO imaging and ac susceptibility. A 0.6-m-thick YBCO sample had a high transport J c (0 T,77 K) of 1.2 MA/cm 2 . The remaining samples were taken directly to other characterizations.A representative MO image of the granular fluxpenetration network obtained using standard imaging procedures 4,5 is shown in Fig. 1. This network is common to CCs with varying constructions from multiple sources. Among the variations are deformation-textured s...

Importance of low-angle grain boundaries in YBa2Cu3O7−δcoated conductors

“…Visualisation of areas of magnetic flux penetration indicate that dissipation occurs at regions which match up to the grain boundaries in the substrate (identified by etching the YBa 2 Cu 3 O 7−δ film), again indicating that the boundaries in the YBa 2 Cu 3 O 7−δ match up with those in the substrate and buffers [24].…”

“…EBSD studies generally indicate that the grain structure of the underlying layer is reproduced in a YBa 2 Cu 3 O 7−δ overlayer deposited by PLD [30,31,24,27].…”

“…The fact that the grain boundary network is important in determining the electromagnetic properties of coated conductor films has been well-demonstrated using both Magneto-Optical (MO) imaging measurements and light microscopy [32,24,1]. Visualisation of areas of magnetic flux penetration indicate that dissipation occurs at regions which match up to the grain boundaries in the substrate (identified by etching the YBa 2 Cu 3 O 7−δ film), again indicating that the boundaries in the YBa 2 Cu 3 O 7−δ match up with those in the substrate and buffers [24].…”

“…Detailed spatial and angular information regarding the relationship between the grain networks in substrates and buffer layers has been gathered using Electron Back-Scatter Diffraction (EBSD) [24,17,21] and microdiffraction [25]. EBSD measurements have not generally shown significant angular correlation between adjacent grains in the substrate tapes and hence grain boundary angles are approximately as would be predicted from the macroscopic texture [26].…”

Resolution of two-dimensional currents in superconductors from a two-dimensional magnetic field measurement by the method of regularization

“…Of particular interest is YBa 2 Cu 3 O 7 coatedconductors (CCs) and BiSrCaCuO (BSCCO) tapes, where current percolates and transport J c values are frequently a macroscopic average of large local variations in J c (Ref. [1][2][3][4][5][6][7][8][9]. Probing the local J c may be done directly with transport measurements, but such measurements are destructive and provide information only in localized regions 3,5 .…”