  We investigate the pinning mechanism in high-critical-current polycrystalline
samples of Bi-2223 (Bi2Sr2Ca2Cu3O10) utilized in high current applications.
Using differential magneto-optical (DMO) imaging technique, we track the
magnetic field penetration in the sample. Our DMO imaging studies show circular
regions with an average diameter of 20 um with dark contrast. We identify these
as strong-pinning regions with a substantially higher local penetration field
than the surrounding regions. A unique feature of these strong-pinning centers
is that they survive upto high temperatures (near Tc) and produce a
non-Gaussian distribution of the penetration field strength. By analysing the
field dependence of the pinning force behaviour, we identify two distinct
pinning mechanisms: at low temperatures, well below Tc, it is predominantly
surface pinning mechanism and at higher temperatures near Tc, we see a
crossover into a purely delTc pinning mechanism. Our studies show that surface
pinning effects are related to grain alignment, grain boundary, and voids in
the sample. The effect of these diminishes near Tc, and the strong-pinning
regions here are related to local stoichiometric fluctuations. We investigate
the impact of these pinning centers on the current distribution in a
macroscopic Bi-2223 superconducting cylindrical tube. We map the current
distribution across the macroscopic cylindrical tube using an array of hall
sensors distributed around the cylinder. The map shows that the current
distribution is non-uniform across the tube at high currents. The
non-uniformity reveals an inhomogeneous distribution of strong-pinning centers
across large length scales in superconductors used for applications.
