Direct observation of the Bi,Pb(2223) phase formation inside Ag-sheathed tapes and quantitative secondary phase analysis by means of in situ high-temperature neutron diffraction

Reactions occurring in Ag-sheathed Bi,Pb(2223) monofilamentary tapes have been studied in situ using high-temperature neutron diffraction. A neutron compatible furnace enabled us to anneal tapes under the same processing conditions as those used for standard high-performance monofilamentary tapes. Scattering data were collected all the time during the thermal treatment and different cooling procedures were compared. The data were analyzed using a full-pattern profile refinement: seven phases were simultaneously refined so that it was possible to carry out a full quantitative analysis of secondary phases during the reaction thermal treatment. The steady conversion of Bi,Pb(2212) into Bi,Pb(2223) was quantified, and the structural transformations occurring in the Bi(2212) prior to its conversion in to Bi(2223) were carefully analyzed. The role of different Pb-rich phases, such as Ca2PbO4 and Pb-3(Sr,Bi)(3)Ca2CuOy, at this stage of the reaction is discussed. Formation of SrgCa6Cu24O41 and Bi(2201) was observed at high temperature and quantitatively analyzed. During slow cooling, we observed a re-growth of Bi(2212) which did not correspond to any decomposition of Bi,Pb(2223). At the same time, the decomposition of Sr8Ca6Cu24O41 and Bi(2201) was observed and this phenomenon was likely to be related to the increase of the Bi(2212) amount on cooling. Thanks to the neutron diffraction technique, which allows a direct measurement of the absolute amount of crystalline matter inside the sample, it was possible to estimate for the first time the amount of the amorphous matter present. It was found to vary with time and temperature: its non-monotonic evaluation provides a proof of melting and recrystallization occurring over a wide time window at the early stages of the reaction. (C) 1999 Elsevier Science B.V. All rights reserved.