Chemical analyses of grain boundaries in random superconductor/metal composites
Résumé
Chemical composition of interfaces in fractured superconductor/metal composites has been investigated using X-ray photoelectron spectroscopy (XPS). Superconducting phases under investigation were bismuth phases: Bi2Sr2CaCu2O8+δ (Bi-2212) and Bi1.7Pb0.3Sr2CaCu2O8+δ ((Bi,Pb)-2212). Such materials presented mainly intergranular fractures, which allowed chemical and statistical characterisation of the grain boundaries to be obtained. XPS analyses showed that the presence of silver does not change significantly the chemistry of grain boundaries of the superconducting phases. Comparison between Ca 2p3/2 and O 1s spectra of the two series suggested that (Bi,Pb)-2212 samples should exhibit better superconducting properties than Bi-2212 series (increase in hole doping). Such analyses were successfully compared to results obtained from resistivity measurements carried out on these composites. Distributions of the critical temperatures of junctions obtained from a phenomenological modelling based on the percolation theory actually support the above result of better superconducting grain boundaries in (Bi,Pb)-2212 as compared to Bi-2212 composites.