It is reported that Ca-doping can improve the weak links between grain boundaries and increase the critical current density Jc of YBa2 Cu3 O7d polycrystalline films [Phys. Rev. B 58 (1998) 9543; Nature 407 (2000) 162]. In this paper, we investigated the preparation and superconducting properties of YBa2Cu3O7 d (YBCO) and Y1x Cax Ba2 Cu3 O7d (x=0.05, 0.1, and 0.2) (YCBCO) coatings on yttrium stabilized zirconia (YSZ) single crystal substrates by powder melting growth process. The X-ray diffraction (XRD) indicated that: (1) the coatings were of YBa2 Cu3 O7 d (Y123) phase with c-rientation; (2) the Y2 Ba Cu O5 (Y211) phase remained clearly in the YBCO coating, but much less in the YCBCO coatings. The effect of Ca-doping on the formation of the Y211 phase was analyzed with the help of the differential thermal analysis (DTA). The results of R-T and I-V relationship at 77K demonstrated that the zero resistance temperature Tc0 decreased with the Ca content, while the critical current density Jc (77K) was improved for x=0.05.

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.

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.

The recent observation of superconductivity above 160 K under high pressures has prompted us to investigate the possibility of retaining such high-temperature superconductivity at ambient pressure by substituting Sr for Ba in Hg Ba2 Caz Cu3 08 (Hg-1223). High-purity (≥ 90%) Hg-1223 phase samples were synthesized with a nominal composition of Hgz Srx Ba2_x Caz Cu3 O8+& (where z=0.75 and x=0.0.5 or 1). By adding a second element, Pb or Mo, supposedly to the Hg site, (Hg, Pb/Mo)Sr2 Caz Cu3 O8+& with purity up to 90% was achieved. Unfortunately, Tc decreases with Sr substitution. The roles of Sr in the phase stability, and the synthesis conditions of the Sr based Hg-1223, and the Sr substitution effect on the Tc of these samples will be discussed.

idth at half-height of powder X-ray diffraction, the MgO powder was about 20nm in size on average. A supersonic vibration was introduced to destroy the accumulation of the ultrafme powder and to mix the powder with the YBa2 Cu3 07-δ precursor. The melt-process was employed to grow textured samples. From the Scanning Electron Microscope (SEM) graphs, some particles were found in the textured samples, which were about several nanometer in size. The composition of these particles, however, leaves to be identified. The magnetic critical current density (Jcm) at 77K dropped rapidly with the increase of Mg content.