Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-d during reactive air brazing

Reactive air brazing of the ceramic oxygen transport membrane material BSCF causes microstructural modifi-cations. The trend towards minimizing of membrane thicknesses requires a physical understanding of these modifications since they may influence both oxygen permeation and mechanical properties. To this purpose, wetting samples with variations in the Ag-xCuO braze alloy (1 < x < 25 at.-%) and the brazing time (0 < t < 120 min) were characterized by quantitative image analysis. We found that the pore size in reactive air brazed BSCF increases with increased brazing time as well as CuO-content in the braze. A local porosity minimum is always observed at the end of the reaction zone close to the unaffected bulk BSCF. Additionally, a zone with unidi-rectionally elongated grains at the end of which silver residues were found is observed. The observed coarsening effects are explained by liquid melt film penetration along the grain boundaries which increases the grain boundary mobility. Phase field simulations qualitatively confirmed the experimentally observed elongated grain structure.