PERCOLATIVE BEHAVIOR OF AG-PHASE CLUSTERS IN SUPERCONDUCTING BI-PB-SR-CA-CU-O CERAMICS

Percolative behavior of the Ag-clusters in a sintered mixture of Ag-doped Bi-Pb-Sr-Ca-Cu-O (2223 phase) was investigated. The Ag-phase undergoes the percolation transition at its volume fraction f=0.25, whereas the ceramic phase undergoes it at the volume fraction f(s)=0.55. This implies that the growth rate of the Ag-clusters is much higher than that of the ceramic clusters. This means also that the degree of freedom of the interlinking between neighboring ceramic grains is lower than that between the Ag-grains. Owing to this structural difference, the superconducting network is easily damaged when the Ag-clusters grow. However, the observed critical-exponent t (when f>0.25) of the percolative conductivity at room temperature is much smaller than the universal value (t=2), whereas the observed critical-exponent s (when f<0.25) remains close to the universal one (s=0.7). This behavior of t and s is numerically estimated by using the scaling theory.