ORIENTED CRYSTALLINE LAYERS ON THE SURFACE OF BISMUTH SUPERCONDUCTING GLASS CERAMICS. S. Simon1, Gh. Borodi2, T. Farcas1, 1Babes-Bolyai University, Faculty of Physics, 3400 Cluj-Napoca, Romania, 2Institute of Isotope and Molecular Technologies, 3400 Cluj-Napoca, Romania
X-ray diffraction, scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) results on undoped and Gd-doped Bi1.8Pb0.2Sr2Ca1Cu2O and Bi1.8Pb0.2Sr2Ca2Cu3O systems are presented. The samples were prepared by quenching of melted oxides mixture corresponding to the desired compositions on different metallic supports like steel, copper and silver.
The glass-ceramic route is a very attractive preparation technique for the fabrication of dense superconductors with desired shapes like unsupported and metal-supported fibres. For a large superconducting current flow it is very important to have the crystalline grains on the surface parallel oriented to each other and with Cu-O planes parallel to the surface.The type and concentration of initial crystalline grains that appear at the metal-glass interfaces and their orientation are function on metallic support temperature, its nature and temperature gradient between support and molten. In order to improve the crystallinity and to have a preferentially oriented superconducting microcrystals with c axis perpendicular to the surface, we followed different heat treatment procedures in which we changed the heating rate, the final heat treatment temperature and its duration and the atmosphere nature: air or oxygen. After heat treatments spectacular changes of preferential orientation and grains shape took place as were evidenced by X-ray diffraction and SEM. We found that the presence of gadolinium in the samples in which calcium was partially substituted by this affect not only the type of final crystalline phases [1] but also the stability of their precursor glasses. The crystalline data obtained by X-ray diffraction and SEM on these samples were correlated with EPR data.
1. S. Simon, E. Burzo, O. Cozar, I. Barbur, V .Simon, I. Ardelean, V. Pop, G. Borodi, Physica C, 185 - 189 (1991) 899