ON THE DEGRADATION OF YBA2CU3O7-DELTA BY ZRO2

The complete reaction pathway describing the degradation of YBa2Cu3O7-delta by ZrO2 is determined. Initial dissociation of the superconductor into Y2BaCuO5, CuO and BaCuO2 is in accordance with the YBaCuO ternary phase diagram. Subsequent decomposition of Y2BaCuO5 in the presence of CuO and ZrO2 yields more BaCuO2 and the yttrium compound Y2Cu2O5. Ba loss through formation of the highly stable compound BaZrO3, occurs exclusively through a simple chemical reaction between BaCuO2 and ZrO2, in contrast to previous suggestions of out-diffusion of either Ba atoms, ions or BaO from the superconductor lattice. It is shown that certain one- and two-phase peritectic reactions take place at much lower temperatures when zirconia is present. The scheme presented here is consistent with earlier suggestions that de-stabilization of the superconductor lattice is promoted by an inward diffusion of Zr4+ ions.