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In the self-field approximation, the influence of the grain radius on the current–voltage characteristics and associated critical current density Jc are investigated by considering the conventional flux flow motion of vortices. It is shown that the direct summation theory of Campbell et al. could be adopted to the granular systems [A. M. Campbell, J. E. Evetts, and D. Dew-Hughes, Philos. Mag. 10, 333 (1964)]. We found that Jc is enhanced as the size of the grains is decreased. The case of the critical current density being greater for smaller grains, which is consistent with both transport and low-field magnetization measurements is discussed by the weak links between the superconducting grains and also with the percolation model.

The temperature and sample size (width, thickness, and cross-sectional area) dependences of the zero-field current-voltage characteristics and transport critical current density Jc of YBa2Cus07-d have been studied by using the motion of the flux lines which depends on the pinning force Fp and the Lorentz forces FL. We have obtained analytical expressions for the sample size and temperature dependences of the critical current density and current-voltage characteristics, which are in reasonable agreement with most of the experimental data published previously. It has been shown that the viscous drag coefficient I) of the medium changes with temperature as I) = q d l -(T/T#]. We have also found that the pinning force Fp is proportional to [ 5 ( T P which implies that the diameter of the spherical pinning centres is larger than the coherence length as in the case of conventional hard superconductors.

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