Influence of low magnetic fields on the transport properties of sintered YBa₂Cu₃O_{7- delta} with different grain sizes

Using a four-point terminal method, measurements of critical current densities, J_c^j(T, B), have been performed at magnetic fields mu ₀H_ext<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 _c1^J (T), of the matrix phase and the corresponding Josephson penetration depth, lambda _J(T), are presented. The dependence of H_c1^J(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, (r_G), 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.