A study of the effects of oxygen environment on the stoichiometry, phase assemblage and stability of BiSCCO 2212 and 2201 using EPMA
A method of performing accurate oxygen analysis on cuprate based superconducting materials was established using electron probe micro analysis (EPMA). A range of YBa2Cu3Oδ ceramics with varying oxygen concentration were used to test the method. Using YBCO as a reference material, a suitable standard for oxygen analysis of Bi2Sr2CaCu2Oδ (BiSCCO-2212) materials was obtained. This standard was used to perform full elemental analysis of a range of BiSCCO-2212 crystals, post annealed in pO2's between 10-5-2atm. When the average Cu valence of each crystal was calculated and plotted as a function of the critical temperature (Tc) for each crystal, it was shown that BiSCCO-2212 materials conformed to the 'universal' trend illustrated by most other HTS and did not exhibit anomalous behaviour as had been previously believed. The phase assemblage and superconducting properties of BiSCCO-2212 Ag-clad multifilamental wires, prepared using the powder-in-tube (PIT) method by BICC, were studied as a function of a time/temperature profile. pO2 of the processing atmosphere was found to be the predominant factor in determining the stoichiometry of the 2212 phase within wires. The phase assemblage is not simply a function of pO2 as previously believed and can be controlled, in part, by the post annealing temperature. Homogenisation of the phase assemblage in BiSCCO-2212 Ag-clad wires can be achieved by prolonged heating (96hrs) at an appropriate temperature. An investigation into the 10K superconducting BiSCCO phase has shown the Sr-rich solid solution to extend towards the ideal stoichiometry of 2:2:1 (Bi:Sr:Cu) with increasing pO2. Using a combination of high pO2 (60atm) to achieve the appropriate Bi:Sr stoichiometry followed by post annealing in N2 to adjust the oxygen content, it was possible to prepare single-phase ceramics of stoichiometry Bi2.11(2)Sr1.90(2)Cu0.99(2)Oδ with a Tc=10.5K(5).