Chemical Degradation in Thermally Treated Ferrite/Superconductor Multiphase Materials: Modeling Parameters

Solid state chemical evolutions are studied in the case of superconductor/ferrite composites as a function of time and temperature. Pellets have been fabricated from ferrite NiFe2O4 and superconducting cuprate Bi1.6Pb0.4Sr2Ca2Cu3O10+X (noted as Bi-2223). Two types of experimental approaches are presented: high-temperature electrical complex impedance spectroscopy, and EDAX analyses performed from scanning electron microscopy. From the in situ electrical analyses, two steps in the solid state chemical evolutions have been evidenced for the first time. They can be associated with two types of solid state reactions: (i) direct reactions between the ferrite phase and the superconducting matrix and (ii) a self-degradation of the superconducting phase probably associated with a homogenization of elements. The electrical analyses are modeled using two types of kinetics parameters. From the EDAX analyses, the local distribution of each element is determined. The concentration profiles found for the various elements (Ni, Fe, Bi, Sr, Ca, Cu) are interpreted in terms of a virtual diffusion law involving virtual D* coefficients. These coefficients are found to be of about 10-11 (cm2/s) in the range 800–830°C.