Mechanisms of Ag penetration into the ceramics of Ag/Bi-2223 ribbon composites

Phenomena of silver penetration into ceramics of single-filament and multifilamentary (61 filament) Bi 1.6Pb 0.4Sr 2Ca 2Cu 3O x /Ag composite ribbons have been investigated by means of transmission electron microscopy (TEM), X-ray microanalysis, X-ray photoelectron spectroscopy and high-resolution Auger spectroscopy. Distribution curves of Ag in the ceramics have been plotted from the Ag-sheath farther inside the ceramics. A nonuniform distribution of Ag was revealed by high-resolution Auger spectroscopy in ceramics far from the Ag-sheath, which is in agreement with TEM results. Ag is found in the ceramics in form of isolated particles located along plane boundaries of crystallites (grains). Near the Ag-sheath there is a lot of particles, and in moving farther away from it their density decreases remarkably. Two mechanisms of Ag penetration into ceramics have been discovered: (a) conventional diffusion, shown by some other authors as well; (b) indentation of ductile Ag into brittle and porous ceramics in the process of plastic deformation of a composite. Ag-sheath strengthening caused by alloying with (Ni+Y) and (Zr+Al) drastically decreases the effect of the second mechanism. The previously described for unalloyed composites phenomenon of critical current instability with respect to thermal cycling in the temperature range of 77–293 K [1] has been understood and explained. It results from thermal stresses caused by the difference in coefficients of thermal expansion of Ag and the ceramics. These stresses arise not only at the Ag/ceramics interface, but also within the ceramics layer of about 10 μm thick. This can result in a fault of contact between grains. Ag-sheath strengthened by alloying drastically decreases the amount of silver in the ceramics and eliminates the composite instability.