High-quality Tl 2 Ba 2 CaCu 2 O 8 films for microwave filter application are prepared on both sides of 2 inch lanthanum aluminate substrates as well as on double-sided CeO 2 coated r-cut sapphire substrates. The films have thicknesses as high as 1 µm in the case of lanthanum aluminate substrates. On CeO 2 buffered sapphire substrates it is possible to prepare crack-free superconducting films as thick as 400 nm. For both substrate materials we reach high critical current densities of J c > 2 × 10 6 A cm −2 at 77 K. The microwave surface resistances R s were measured to fall short of 100 µ at 5.6 GHz and 77 K in the low-field case, comparable to the best films made of YBa 2 Cu 3 O 7−x material. The films were able to tolerate high microwave power levels corresponding to magnetic field amplitudes of B RF > 2 mT. The critical temperature exceeds 102 K. This excellent performance is maintained even after several thermal cycles. The films were used to produce four-pole band-pass filters for the C-band which were operated without any detectable loss in their transfer characteristics up to temperatures of about 85 K. This is the first time that large area (2 inch) double-sided Tl 2 Ba 2 CaCu 2 O 8 films have been prepared on sapphire with excellent critical current density in conjunction with low surface resistance.

Modifications of a recent model by Izumi et al. 1 on diffusion controlled growth of YBa 2 Cu 3 07x (123) superconductors are proposed, taking into account especially the engulfment process of the Y 2 BaCuOs (211) particles into the solidifying 123 interface. The proposed modifications are evidenced by experimental results and applied to explain microstructural features of the 123 superconducting material. In particular, the 1:1 correlation between 123 platelet thickness (planar defect spacing) and 211 particle size as described by Jin et al. 2 is explained by an observed bridge growth resulting in a zipper-like mechanism. By this mechanism the platelets grow together in an oriented way leading to a quasi single crystalline material.

No abstract

No abstract

No abstract