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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.

The correlation between microstructure and superconducting properties of copper-deficient sintered DyBa2Cu3-xOy (0 x0.4) ceramics doped by 1 wt% Pt has been studied. For non-stoichiometric compositions with x0.2 magnetoresistance measurements showed a large widening of the resistance-temperature transition in low magnetic fields in the region corresponding to the transition of intergranular material. Such an enhancement of the effect of granularity in these samples is in good agreement with the estimation of their normal state resistivity at working temperatures and the observed refinement of grains. Doping by Pt was found to assist the formation of submicron size secondary phases homogeneously located in 123 grains. In addition, in the compositions with x0.2, Pt was also incorporated into intergranular BaCuO2 secondary phases. The addition of Pt had no influence on the size and distribution of Y2BaCuO5. Traces of Pt in the 123 grains were revealed in all compositions doped with Pt. Platinum doping only slightly increased the critical current Ic in stoichiometric composition, but it resulted in a substantial increase in Ic and pronounced `fish-tail' effect in magnetic field at T77 K in non-stoichiometric composition with x = 0.2. Better behaviour of Ic in magnetic fields in these samples may be associated with the substantial increase in the proportion of clean boundaries and the decrease of twin spacing. The measurements of Ic(T) at temperatures close to Tc indicated the SIS character of the grain boundary weak links in all compositions with x0.2 whether they were doped by Pt or not.

The effect of ZrO 2 addition (1 wt% and 5 wt%) on the superconducting properties of sintered DyBa 2 Cu 3 O y ceramics was studied. The doping was found to assist the formation of secondary phase inclusions such as Dy 2 BaCuO 5 (211), BaZrO 3 and CuO. No traces of Zr in the 123 grains (matrix) were revealed. Doping by ZrO 2 favoured the formation of a fine-grain microstructure, the amount of clean boundaries strongly increasing compared with the undoped composition. Doping by 1 wt% ZrO 2 resulted in a substantial increase in the critical current density J c . The samples with the addition of 5 wt% ZrO 2 showed the best flux pinning in a magnetic field. Such behaviour seems to be associated with a substantial increase in proportion of clean boundaries and the presence of fine facets and regularly distributed dislocations in these boundaries in the doped samples. The measured dependence of the critical current I c on temperature close to T c indicated that weak links at grain boundaries behave as a network of superconductor-insulator-superconductor (SIS) junctions in all compositions, except for the composition with 1 wt% ZrO 2 which showed superconductor-normal metal-superconductor (SNS) behaviour.