Oxygen exchange in Bi(2223) tapes with Ag and alloyed AgMg sheaths monitored by a thermogravimetrical relaxation method

We investigated the kinetics of oxygen exchange in Ag-and AgMg-sheathed Bi(2223) tapes under the conditions of the reaction processes terminating PIT fabrication. For this purpose we applied a chemical relaxation method in which the reequilibration of the oxygen-dependent weight change was monitored with time following a rapid change of the oxygen partial pressure from pO(2)(1) to pO(2)(2). From the slope of the re-equilibration of weight with time one obtains a time constant describing the exchange kinetics of oxygen between the BSCCO phase and the surrounding gas phase. Under special assumptions the formal diffusion coefficients for the oxygen exchange can be obtained. Contrary to the situation for small pieces of tapes where oxygen exchanges through the ends at which the superconductor is in direct contact with the surrounding gas phase, we encounter a different situation in 'real' tapes with length up to several hundreds of metres. Simulating this case we investigated the exchange through the lateral faces of the tapes, i.e. through the sheath material itself. We developed an experimental technique which allows us to separate the diffusion component through the sheath and were thus able to obtain values for the time constant of oxygen exchange through the Ag sheath. We managed to determine the diffusion scaling with 1/d(2) with d the sample length which allows us to calculate diffusion coefficients for the in-and outdiffusion of oxygen into the Bi(2223) phase inside the Ag-sheathed tapes. The activation energy of the diffusion at the superconductor-gas phase interface was determined to be 0.39 eV. It is also shown that modifications of the sheath material with consequences for the achievable superconducting properties also strongly affect the kinetics of oxygen exchange in the tapes.