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Publication# Percolation and electronic properties of superconducting (YBa2Cu3O7-δ)1-xAgx ceramics and thick films

Abstract

The authors present the percolation and electronic properties of (Y1Ba2Cu3O7-δ)1-xAgx compounds in which silver fills the intergranular space without reducing Tc, which remains at 92±1 K. Normal-state resistivity is decreased by up to two orders of magnitude when adding up to 50 wt.% Ag (Tc=87 K), and samples exhibit improved contact resistance, better mechanical properties, and resistance to water. They analyzed the percolation properties of these compounds and found that the critical indices t, s are in agreement with percolation theory, but pc is higher than expected, probably due to the effect of holes. The Jc estimated from magnetization reaches 5·104 A/cm2 (at T=4.2 K, H=0) and shows enhancement of 15-50% by addition of ~10 wt.% Ag, which exists also in samples having a higher Jc due to preparation conditions (temperature). They present preliminary results on the 2D percolation problem in (Y1Ba2Cu3O7-δ)1-xAgx samples, obtained by preparing Y1Ba2Cu3O7-δ thick films using the spin-on technique. Preliminary results show good adhesion, but a reduced Tc of Y1Ba2Cu3O7-δ films compared with bulk samples

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Percolation theory

In statistical physics and mathematics, percolation theory describes the behavior of a network when nodes or links are added. This is a geometric type of phase transition, since at a critical fraction of addition the network of small, disconnected clusters merge into significantly larger connected, so-called spanning clusters. The applications of percolation theory to materials science and in many other disciplines are discussed here and in the articles Network theory and Percolation (cognitive psychology).

Percolation

In physics, chemistry, and materials science, percolation () refers to the movement and filtering of fluids through porous materials. It is described by Darcy's law. Broader applications have since been developed that cover connectivity of many systems modeled as lattices or graphs, analogous to connectivity of lattice components in the filtration problem that modulates capacity for percolation.

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The percolation threshold is a mathematical concept in percolation theory that describes the formation of long-range connectivity in random systems. Below the threshold a giant connected component does not exist; while above it, there exists a giant component of the order of system size. In engineering and coffee making, percolation represents the flow of fluids through porous media, but in the mathematics and physics worlds it generally refers to simplified lattice models of random systems or networks (graphs), and the nature of the connectivity in them.

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