Etude de l’effet de l’addition et de la substitution dans les supraconducteurs a base de bismuth prépares par la méthode SOL-GEL

Abstract

Superconducting Bi2Sr2Ca1Cu2O8 ceramics samples have been prepared by Sol-Gel methods.The influences of the preparation conditions of compound oxide powder on structural and superconducting properties have been investigated. The effect of substation some of Y and Ce in Ca site, in matrix are performed and discuted by x ray diffraction (XRD), Scanning Electron Microscopy (SEM) equipped with EDS. The critical transition temperatures Tc have been determined by resistivity versus temperature measurements. Cell parameters samples were calculated from XRD patterns. The polyacrylamid gel makes the citrate process easier, more rapid and affords the possibility of synthesis of high quality oxide powders. In High Tc Superconductors (HTSC) the activation energy gives information about the pinning properties of the sample under applied magnetic field. Pinning of vortices determines the critical current density Jc which is of great importance for practical applications of HTSC. Instead of magnetic measurements, the activation energy may be calculated from resistivity measurements realized under magnetic field. This kind of measurement has been made in this work for yttrium doped samples of Bi2Sr2CaCu2O8+d (Bi2212) for different values of applied magnetic field. The measurements of resistivity were made using a classical four probe method and a DC current. The magnetic field was applied with constant amplitude of 0, 1, 2 and 3 T. The obtained results show that the activation energy decreases with introduction of yttrium, but has a relative maximum when x is equal 0.1. The decrease of the activation energy is explained by the granular nature of the samples which promote 3D transition to 2D of the vortex lattice.