Abstract. Over the past ten years the perception of grain boundaries in YBa 2 Cu 3 O 7−δ conductors has changed greatly. They are no longer a problem to be eliminated but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high angle boundaries. The time is therefore right for a targetted review of research into low-angle YBa 2 Cu 3 O 7−δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa 2 Cu 3 O 7−δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (Rev. Mod. Phys., 74, 485, 2002). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa 2 Cu 3 O 7−δ coated conductors both individually and as a collective system.

Surface and bulk texture measurements have been carried out on highly aligned NiFe tapes, suitable for use as coated conductor substrates. Data from small-area electron backscatter diffraction measurements are compared with those from bulk x-ray analysis in the development of a two-dimensional percolation model, and the two are shown to give very similar results. No evidence of grain-to-grain correlation is found. The model is then developed to assess how the properties of a superconducting layer grown epitaxially on buffered tapes will depend on parameters such as sample size, grain size and the extent of grain alignment.

The preparation of single grain, Y-Ba-Cu-O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa 2 Cu 3 O 7−δ (Y-123) and Y 2 BaCuO 5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, T c and J c , of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

The change in microstructure associated with the decrease in critical current density (Jc) of Yba2Cu3O7−δ (YBCO) films with increasing thickness was examined. Samples of pulse laser deposited YBCO films varying in thickness from 0.19 to 3.0 μm on rolling-assisted biaxially textured substrates with an architecture of CeO2/YSZ/CeO2/Ni were prepared by tripod polishing for cross-sectional electron microscopy. More randomly oriented grains in the upper portion of the YBCO film surface were observed with increasing film thickness, resulting in less cube texture. In addition, increases in mismatch across the boundaries of the c-axis grains with increasing time during deposition, along with the development of BaCeO3 and Y2BaCuO5 phases at the YBCO/CeO2 interface, contributed to the degradation of film properties. Surface outgrowths of the YBCO film were examined as well as the defect structures and second-phase formations within the films.

Superconducting YBa 2 Cu 3 O 7−␦ (YBCO) films with critical transition temperature T c (0) of 90 K were fabricated via a fluorine-free, metal trimethylacetate based sol-gel route. Precursor films were spin-coated onto single-crystal (001) LaAlO 3 (LAO) and (001) SrTiO 3 (STO) substrates. Optimization of the burnout process resulted in films with excellent out-of-plane and in-plane texture. Transport critical current densities, J c , of 1.6 and 1.1 MA/cm 2 were obtained at 77 K in self-field on films grown on LAO and STO substrates, respectively. This is the first demonstration of high-J c YBCO film fabrication using a fluorine-free, ex-situ process.