CORRELATION BETWEEN SUPERCONDUCTING PROPERTIES AND MICROSTRUCTURE OF YBA2CU(3-X)O(Y) AND YBA2CU3-XOY-AG-X CERAMICS FABRICATED BY THE CITRATE GEL PROCESS

The correlation between microstructure and superconducting properties of YBa2Cu3-xOy (deficient (D) samples) and YBa2Cu3-xOy:Ag-x (substitut ed (S) samples) (0 less than or equal to x less than or equal to 0.4) fabricated by the citrate gel process has been studied. In both types of samples nonsuperconducting phases such as Y2BaCuO5, BaCuO2 and CuO were observed for x greater than or equal to 0.2. EDX analysis of S sa mples by TEM and STEM revealed the Ag content in YBCO grains to be low er than the nominal one (x) for all x in a range 0 < x less than or eq ual to 0.4. Ag segregated mainly as small precipitates in boundaries, triple junctions and grains. The dependence of the critical current de nsity J(c) on the magnetic field B obtained showed that J(c) is contro lled by weak links in both cases, D and S. The measurements of I-c(T) at temperatures close to T-c indicated that the weak links have SNS ch aracter in D and SNS character in S samples. Ag doping leads to an inc rease 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 t o be the reason for the improvement of J(c) in these samples by a fact or of 3. Some increase in J(c) with x in D samples was observed and wa s probably associated with an increase in the proportion of faceted bo undaries among clean ones.