A controlled introduction of second-phase Y 2 BaCuO 5 (211) nanoparticles into YBa 2 Cu 3 O 7−␦ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >10 11 cm −2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211 y nanoparticles/123 z )×N were deposited by pulsed laser deposition on LaAlO 3 substrates, with N bilayers ‫ס‬ 24 to 100, y thickness ‫ס‬ 1 to 2 nm, and z thickness ‫ס‬ 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼1 m).

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...

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