Using a four-point terminal method, measurements of critical current densities, Jcj(T, B), have been performed at magnetic fields mu 0Hext<or=30 mT in the temperature range 5 K<or=T<or=91 K on sintered YBa2Cu3O 7- delta samples. The mean grain sizes of these samples range from 4 mu m up to 14 mu m. Determinations of the lower critical field, H c1J (T), of the matrix phase and the corresponding Josephson penetration depth, lambda J(T), are presented. The dependence of Hc1J(T) and lambda J(T) on the mean grain size of the samples is shown. The grain size of sintered YBaCuO and the Josephson penetration depth turn out to be very important parameters for the attainable critical current density. A maximum current carrying capacity is reached if the mean grain size, (rG), and the Josephson penetration depth, lambda J(T) are similar in size. The results are discussed in terms of a weakly-coupled network of superconducting grains.
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