CHEMICAL-STABILITY AT NOBLE-METAL M/YBA2CU3O6.8 INTERFACES (M = PT, AG, AU)

The chemical compatibility between YBa2Cu3O6.8 (Y123) and Pt, Ag or Au was studied using quasi-infinite diffusion couples which were encapsulated and annealed at 650 to 800-degrees-C for 5 to 80 h. The phase formation at the interface was analyzed in cross sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. In addition, bulk powder mixtures of Y123 with Pt, Ag or Au were annealed at 800-degrees-C for 100 h and phase analysis was performed using X-ray diffraction. At the Pt/Y123 interface a reaction zone grows slowly but decisively at 800-degrees-C. Its microstructure is multiphase with Y-Cu-oxides and a fine-grained dispersion Of Y2BaCuO5 (Y211) with Ba-Cu-oxides. Additional Ba-Pt oxides and other phases are seen in powder mixtures annealed for longer times. Barium is suspected to diffuse out from the superconductor along grain boundaries from as deep as 1 mm, causing the decomposition of Y123 into Y-Cu-oxides in the depleted regions. Ag and Au form a stable contact at the interface to Y123 with no reaction zone or new phases. Interdiffusion at the Ag/Y123 interface at 800-degrees-C was too low to be clearly detected. In contrast, Au diffuses very fast into Y123 and at 800-degrees-C the solubility is 4.2 mass% Au. Yttrium and barium diffuse much slower into the (Au) phase, Cu diffusion was not detected. Weak traces of decomposition products, mostly Y211 and Ba-Cu-oxides, were observed in bulk powder mixtures of Y123 with Ag or Au annealed at 800-degrees-C for 100 h in closed capsules. These decomposition products are considered to be due to the high oxygen pressure in the closed capsule, exceeding the stability limit of Y123, and not due to the reduction of Y123. Both Ag and Au are virtually non-reactive with Y123.