Electrical transport and percolation behaviour of superconductor- insulator composite system

The electrical transport properties and percolation behaviour of superconducting Bi(2223)-DyBa2SnO5.5 and YBCO-DyBa2SnO5.5 composite systems have been studied by x-ray diffraction and temperature resistivity measurements. In both the superconductor-insulator systems, the normal state percolation threshold values are found to be ∼ 20 vol% of the superconductor in the composite. However, the superconducting percolation threshold value is found to be 20 vol% of Bi(2223) in the Bi(2223)-DyBa2 SnO5.5 system and 30 vol% of YBCO in the YBCO-DyBa2SnO5.5 system. The values obtained for the critical exponents describing the normal state transport behaviour of the system matched with the theoretically expected values for an idealized metal-insulator composite system. No detectable chemical reaction was observed between the superconductor and the ceramic insulator DyBa2SnO5.5 (within the precision of XRD technique), even under severe heat treatment at processing temperature. The implications are discussed. (author)