The ac magnetization and the microwave surface resistance (at 12 GHz) were measured on a series of Na-doped YBCO ceramic samples at 77 K. Their magnetic behavior was explained on the basis of the modified critical state model by taking into account the existence of a field-dependent component Jc1 (due to weakly linked grains) and a field-independent component Jc0 (due to perfectly linked grains) in the bulk critical current density of the samples. The Na and Cu remaining after the heat treatment of the samples changed the intergranular medium parameters and impeded the correlation between the grains. As a result, an increase of the Na concentration led to a decrease of the specimens critical current density and an increase of their surface resistance. At small Na concentrations, a certain increase of Jc1 was observed, which can be explained by taking into account the possibility of partial pinning of Josephson vortices in the "weakly seeded" places in the intergranular media.
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