Corrosion of YBCO bulk superconductor in air

A corrosion process caused by reaction of surfaces of oxygenation cracks of YBCO bulk single‐grain superconductor with air moisture was studied. X‐ray photo‐emission spectroscopy, done on air exposed (001) surfaces, confirmed oxygen bonds related to barium hydroxide. Thermal analyses and mass spectrometry of exposed samples has shown a release of water caused by decomposition of barium hydroxide hydrates during sample heating. The sample weight increase versus time was related to formation of barium hydroxide phases at the surfaces of oxygenation cracks. The morphological changes of these phases at heating were observed with scanning electron microscope.     

Evolution of the Fishtail-Effect in Pure and Ag-doped MG-YBCO

We report on magnetic measurements carried out in a textured YBa₂Cu₃O_7?δ and YBa₂(Cu_1?x Ag _x )₃O_7?δ (at x≈ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4 <T/T _c < 0.8 for H ∥ c. The origin of the FE arises from the competition between surface barriers and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate R. The value H _max for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.