The transport current characteristics of the textured Bi-2223 thick film were studied by the measurement of the voltage–current curves for different temperatures and various magnetic fields. The results show that the transitions can be fitted approximately by a power law with n-values at the low electrical field region. The n-value is observed to fall with increased applied magnetic field and temperature, and exhibits the thermally activated flux flow behavior. The activation energies U(T, H, J) derived from the V-I curves were found to follow a (n − 1)k B T × ln (J0/J). The critical temperature T c was found to follow the irreversibility line described by the function of H ∝ (1− T CH /T CO )1.47 for H//ab-plane. The temperature and magnetic field dependences of the transport critical current density follow a power law J c ~ (1−T COH /T COO )m and J c ~ H−α for H//ab-plane. The exponent m increases with increasing magnetic field, and α decreases with decreasing temperature. These indicate that the flux motion may be caused by a weak pinning potential energy.
We present typical results of experiments carried out in (YBaCuO)1-xAgxtubular samples, for different values of the silver concentration x, having in mind the possible use of this material as a magnetic shield. We measured the times governing the dynamics of magnetic flux penetration, on a microsecond time-scale, and tried to correlate them with d.c. measurements of the resistivity and the critical transport current density, using a simple theoretical model based on the flux-flow régime.
Magnetic Shielding With High-Tc(YBa2Cu3O7-δ)1-xAgxSuperconducting Tubes