FIZIKA A 1 (1992) 1, 67-84

HOMOGENEITY AND PERCOLATION IN CERAMIC HIGH TEMPERATURE SUPERCONDUCTORS

EMIL BABIĆ, MLADEN PRESTER, DINKO BABIĆ, ŽELJKO MAROHNIĆ and ĐURO DROBAC
Institute of Physics of the University, POB 304, 41001 Zagreb, Croatia
Department of Physics, Faculty of Science, POB 162, 41001 Zagreb, Croatia

Received 15 June 1991

The main results of systematic investigation of the electric transport properties and ac susceptibility of ceramic high temperature superconductors (HTS) are presented. An effort has been made to deduce a suitable description for these properties. Due to the presence of the weak links at the grain boundaries the superconducting and the normal state properties of these systems deviate strongly from those of the corresponding homogeneous compounds. Because of this, ceramic HTS can be modeled as the two phase systems consisting of grains and 3D disordered array of intergrain links. The variation in the strength of the coupling between the grains determines the bulk critical currents (I_c) of these systems. A more detailed analysis shows that properties of ceramic HTS depend strongly on density and the ratio between the dimensions of the sample and the grain size. Therefore only some properties of fine-grained dense samples can probably be described by averaging the local properties over the volume of the sample. In contrast to that, the resistivity at lower densities, the diamagnetic transition below T_c and the transition to the dissipative state above I_c all exhibit the percolation effects.

UDC 538.93

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