  The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox
(Bi2223) have been measured at temperature range from 10 to 90 K. The
experimental dependences have been analyzed within the model allowing for
pinning by clusters of a normal phase with fractal boundaries, as well as the
model taking into account phase transformations of vortex matter. It has been
found that the electrical resistance of the superconductor material
significantly increases at temperatures of 60-70 K over the entire range of
magnetic fields under consideration without changing in the sign of the
curvature of the R(I) dependence. The melting of the vortex structure occurs at
these temperatures. It has been assumed that this behavior is associated with
the specific feature of the pinning in a highly porous high-temperature
superconductor, which lies in the fractal distribution of pinning centers in a
wide range of self-similarity scales.
