Interface reactions between YBa2Cu3O7−x and W, Si or WSi2
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The chemical compatibility between YBa2Cu3O7−x (Y123) and W, Si or WSi2 was studied using quasi-infinite diffusion couples which were encapsulated and annealed at 650 to 800‡C for 5–80 h. The phases formed at the interface and the morphology and growth kinetics of the reaction zone were analysed in cross-sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. A Si/Y123 sample was also studied using secondary ion mass spectroscopy (SIMS). In addition, bulk powder mixtures of Y123 with W, Si or WSi2 were equilibrated at 800‡C for 100–300 h and phase analysis was performed using X-ray diffraction.
The following primary reactions are identified, for tungsten: 6YBa2Cu3O7+7W→ 2BaWO4+5Ba2WO5+3Y2O3+18Cu and for silicon: 2YBa2Cu3O6.5 +3Si→2BaSiO3+Ba2SiO4+Y2O3+6Cu. The reactivity of WSi2 is not much smaller compared to that of elemental W and Si. The results of diffusion couples and bulk samples are consistent and complement each other, the dominant product phases are observed in both experiments. A diffusion controlled mechanism with a parabolic growth law and a relatively small activation energy (Q = 44 kJ mol−1) is verified for the W/Y123 reaction. In both the W/Y123 and Si/Y123 couples the dominantly diffusing species are barium and oxygen. Barium diffuses far through the reaction zone to form double oxides, leaving a fine grained Y-Cu-O layer behind (mostly Y2O3+Cu), and eventually the entire superconductor phase Y123, being depleted of Ba, turns into this Y-Cu-O mixture.
KeywordsBarium Y2O3 Reaction Zone Interface Reaction Powder Mixture
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