Transport Of Charge Carriers Across The Normal-Superconducting Interfaces In Bi1.65pb0.35sr2ca2cu3o10+Delta Nanoceramics

Samples of superconducting Bi1.65Pb0.35Sr2Ca2 Cu3O10+delta (Bi-2223) with nanoparticle-sized grains were produced with the help of high-energy ball milling, followed by pelletizing, and flash sintering. X-ray powder diffraction patterns suggest that the grain size was not altered by flash sintering. The temperature dependence of the magnetic susceptibility chi(T), electrical resistivity rho(T), and thermoelectric potential S(T), all revealed features consistent with superconductivity below T-c approximate to 108 K, though these features were progressively suppressed with the milling time. These data indicate that milling generates nanoparticle-sized grains with the Bi-2223 phase at the core, surrounded by a severely damaged and disordered outer shell. The thickness of this outer layer increases with milling time. The extent of the damage surrounding the grain cores has severe implications on the properties of the pellets. While the onset of superconductivity is clearly marked by the diamagnetic response in chi(T), and a drop in S(T), the signature in rho(T) is more subtle and difficult to resolve, as it is superimposed on a semiconducting-like resistivity background. Measurements of the real (epsilon') and imaginary (epsilon '') components of the dielectric permittivity in the low frequency limit reveal two distinct interfaces of charge accumulation, one at the margins between the crystalline grain cores and the damaged outer layers, and the second between the grain boundaries. The behaviors of rho(T) and epsilon '' near room temperature follow closely an Arrhenius dependence, yielding activation energies in the range between 50 and 200 meV. The transport properties can be explained by a hopping model.