Abstract: YBa2Cu3O7-x artificial grain boundary Josephson junctions have been fabricated, employing a recently implemented biepitaxial technique. The grain boundaries can be obtained by controlling the orientation of the MgO seed layer and are characterized by a misalignment of the c-axes (45 degrees a-axis tilt or 45 degrees a-axis twist). These types of grain boundaries are still mostly unexplored. We carried out a complete characterization of their transport properties and microstructure. Junctions and de SQUIDs associated with these grain boundaries exhibit an excellent Josephson phenomenology and high values of the ICRN product and of the magnetic flux-to-voltage transfer parameter respectively. Remarkable differences in the transport parameters of tilt and twist junctions have been observed, which can be of interest for several applications. A maximum speed of Josephson vortices as calculated from the voltage step values of the order of 2 x 10(6) m s(-1) is obtained. These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries.

Abstract: Nd1+xBa2-xCu3O7-delta (123) bulk superconductors have been synthesised by the oxygen controlled melt growth method. Unlike the YBaCuO system, platinum doping or Nd4Ba2Cu2O10 (422) rich compositions do not refine the peritectic '422' secondary phase, but the latter improves the microstructural quality. Low oxygen partial pressure and high purity precursors are necessary to achieve in a reproducible manner high T-c and J(c) (up to 56 000 A/cm(2) in 0T and 30 000 A/cm(2) under 1.5T). The fishtail effect observed at 77 K is compared with the one observed in the YBCO system, and is discussed in terms of oxygen deficiency, Nd-Ba substitution, defects and vortex lattice. Our TEM observations did not evidence any Nd-Ba substituted clusters in the Nd123 matrix; however, some diffuse streaks, observed in the [100] zone ED pattern, support the fact that the source of the peak effect in this system is mainly due to oxygen disorder and low stability of the orthorhombic phase. (C) 1999 Elsevier Science S.A. All rights reserved.

Abstract: Nanocrystalline titanium dioxide was prepared via aerosol pyrolysis of titanium alkoxide precursor at 200-580 degrees C in air and in nitrogen atmospheres. Powders were characterized by x-ray diffraction, thermogravimetric analysis, Brunauer-Emmett-Teller analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, x-ray fluorescence, Raman and infrared spectroscopy, and Berner-type low-pressure impactor. The anatase phase transition was initiated at 500 degrees C in nitrogen and at 580 degrees C in air. Under other conditions amorphous powders were observed and transformed to nanocrystalline TiO2 via thermal postannealing. In air, smooth and spherical particles with 2-4-mu m diameter were formed with an as-expected tendency to convert to rutile in the thermal postannealings. In nitrogen, a fraction of the titanium tetrabutoxide precursor evaporated and formed ultrafine particles via the gas-to-particle conversion. At 500 degrees C thermally stable anatase phase was formed in nitrogen. A specific surface area as high as 280 m(2) g(-1) was observed for an as-prepared powder.