Résumé:
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.
