Correlation between superconducting properties and microstructure of and ceramics fabricated by the citrate gel process

The correlation between microstructure and superconducting properties of (deficient (D) samples) and (substituted (S) samples) fabricated by the citrate gel process has been studied. In both types of samples nonsuperconducting phases such as and CuO were observed for . EDX analysis of S samples by TEM and STEM revealed the Ag content in YBCO grains to be lower than the nominal one (x) for all x in a range . Ag segregated mainly as small precipitates in boundaries, triple junctions and grains. The dependence of the critical current density on the magnetic field B obtained showed that is controlled by weak links in both cases, D and S. The measurements of at temperatures close to indicated that the weak links have SIS character in D and SNS character in S samples. Ag doping leads to an increase in the proportion of clean boundaries in S samples compared with D samples. Specific Ag precipitations of 2-5 nm size on or near clean boundaries were found in S samples for x = 0.4. This is most likely to be the reason for the improvement of in these samples by a factor of 3. Some increase in with x in D samples was observed and was probably associated with an increase in the proportion of faceted boundaries among clean ones.