We investigated the chemical reactivity and percolation characteristics of insulating nanocrystalline YBa2ZrO5.5 prepared by modified combustion route and the YBa2Cu3O7？δ superconductor composite system. Structural analysis was done by using X-ray diffraction technique, surface morphology of the samples was studied using scanning electron microscopy, and electrical transport measurements like critical transition temperatures ( ) and self-field transport critical current ( ) were done by using standard four-probe technique. It is found that, in YBa2Cu3O7？δ-nano-YBa2ZrO5.5 composite system, the superconductor and insulator materials coexist as separate phases without any noticeable chemical reaction even after sintering at high temperatures. Furthermore, percolation threshold and critical exponent are found to be , and . And the analysis of the current flow in the polycrystalline samples reveals weak link behavior in the majority of grain connections. 1. Introduction High temperature superconductors with transition temperatures above 77？K in ceramic materials have received tremendous responsiveness because of their scientific and practical potential. The study of superconducting small aggregates, clusters, or particles is very important from both the fundamental and the technological standpoint [1–3]. It will be significant to study the percolation and superconductivity of composites involving superconductor inserted in an insulator medium. Granular nature along with short coherence length [4] and large penetration depth [5] of high temperature superconductors allows us to investigate the percolation behavior, fractal properties, quantum size effects, thermal fluctuations, and size effects on superconductivity. The percolation concept was first employed to describe superconductors by Davidson and Tinkham [6], who analysed resistivity data of composite Nb3Sn/Cu wires. For the growth of high quality films, the choice of substrate is vital. A high superconductor-insulator system is very difficult to obtain without compromising the superconducting properties. The chemical nonreactivity of the substrate materials with superconductors indicates their potential as substrates for film deposition. The chemical compatibility of materials with the superconductor at the processing temperature is crucial. Also superconductor-insulator percolation studies are a medium to understand the fundamental mechanism behind high temperature superconductivity. A percolation model can be regarded as a collection of points or occupied sites distributed in a space; certain pairs of
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