DTA STUDY FOR CLARIFYING THE ROLE OF SILVER IN THE FORMATION OF BI-2223 PHASE IN AG-SHEATHED TAPES

The role of silver in catalysing and promoting the textured growth of 2223 phase in Ag-sheathed (Powder In Tube) tapes, starting from a prec ursor ''bi-component'' powder, constituted by Bi-2212 and other mixed oxides, was investigated in detail by means of DTA and X-ray analyses at the different stages of the annealing of the tapes. By monitoring t he reaction it was possible to show how the presence of silver modifie s the thermal behaviour of the reactants. The reduction of the tempera ture at which a liquid phase is formed is believed to be the key mecha nism through which the presence of silver enhances the overall convers ion to 2223 phase. This temperature decrease was shown (for the first time, to our knowledge) to be active only as long as some precursor (u nreacted) powder was present, and not on fully reacted 2223 phase. Fur ther investigation on 2212 phase and the other mixed oxides, separatel y analysed by DTA, clarified that only the thermal behaviour of the fo rmer was modified by the presence of silver. An explanation based on o xygen unbalancing in the material possibly induced, at the annealing t emperature, by the presence of silver, is suggested: as a consequence of the very high solubility of oxygen in silver at high temperature, O -2 would be ''pumped off'' the superconducting 2212 phase, with the re sult of lowering its melting point.