Sol-gel is one of the effective techniques for preparing YBa2Cu3O7−δ (YBCO) coatings at relatively low cost. Its process from gel to oxide coatings, however, was not yet fully understood. This is one of the obstacles for achieving YBCO coatings with good superconducting properties. In this paper, we chose EDTA as the complexant to prepare gels with different kinds of M(EDTA)n−complexes, where M is any combination of Y, Ba, and Cu cations, in order to investigate the decomposition process of the gels with the help of Fourier transform infrared (FTIR) spectra. Comparing the FTIR spectra of the gel powders with those of the calcined powders, we found that the decomposed temperatures were different for different M(EDTA)n−complexes.We considered this was the cause of the Ba segregation (as BaCO3) in the coatings at low heating temperature, which in turn blocked the formation of high quality YBa2Cu3O7−δ.

Ultrafine SnO2 powder was made with grain size down to 20 nm. Such powder was introduced into the textured growth of Y1qx Ba2 Cu3 O7qd YBCO.superconductors. Crystalline inclusions, which were cubic in shape and from micrometer to submicrometer in size, were found trapped inside such grown YBCO superconductors. Determined by energy dispersive spectroscopy EDS., and further confirmed by X-ray diffraction, the composition of the inclusions was Y2Ba5 Sn3 yyyz Cuy Ptz.Ox. This Sn-based phase belongs to double perovskite structures. They were not trapped uniformly inside the YBa2Cu3O7yd 123., possibly caused by the agglomeration of the SnO2 powder at the very beginning. Nevertheless, it is possible for fine Sn-based inclusions down to a few hundred nanometers to be trapped inside the 123. The addition of the ultrafine SnO2 to YBCO textured superconductors without Pt did not seem to affect the size of Y2Ba Cu O5 211.particles. For comparison, Y2Sn2O7was also introduced to YBCO textured superconductor. In this case, the so-called f phase YBa3 Sn2 yx Cux O9yd was found. q1999 Elsevier Science B.V. All rights reserved.

Received 10 November 2000; received in revised form 24 January 2001; accepted 27 February 2001 By using top seeding melt-textured growth (TSMTG) process, large single domain YBaz Cu3 07 JY2 Ba Cu O5 (YBCO) samples were prepared with addition of nanometric SnO2 and normal CeOy. Polarized light microscope studies at each step of the TSMTG process showed that before YBa2 Cu3 07 δ (123) melting, the Sn-based reacted products were uniformly distributed in micrometer scale; after solidification, the trapped YyBaCuO5 (211) and Sn-based inclusions were more homogeneously distributed and with mean sizes of about one micrometer. Composition images of scanning electronic microscope for the textured samples showed that, on the whole, the Sn-based inclusions were smaller than the 211. They must be on average below one micrometer in size. Although evidence was found that very fine Sn-based inclusions were uniformly trapped in 123 matrix, the fraction of them was small in comparison to that of 211. No significant enhancement of superconducting critical current density (Jc) was evidenced for the nanometric SnOy-doped samples at a low magnetic field (B<1 T). However, values of Jc were decreased at intermediate magnetic field (1<B<3 T). 2001 Elsevier Science B.V. All rights reserved.

Weprepared precursors with fine BaZrO3 inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y1.5Ba2 Cu3 Oy-x BaZrO3 (x＝0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa2Cu3O7-/Y2BaCuO5 (YBCO). The BaZrO3 particles were about 100 nm in size in the textured YBa2 Cu3 O7-(Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO3 particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO3 fine particles exclude themselves as effective flux pinning centers in the textured YBCO.

The Ca-doped YBa2Cu3O7−δ/Y2BaCuO5 bulk-textured superconductors were prepared by using the top-seeding melt-textured growth process. Composition analyses were made for such prepared samples. We found that Ca concentration in the YBa2Cu3O7−δ grains decreased along the growing direction; Ca preferred to substitute Y in the YBa2Cu3O7−δ grains, rather than to stay at the grain boundary. The accumulation of extra CuO at the growth front was considered as one of the causes of the inhomogeneous distribution of Ca.