Joining of Co coated ferritic stainless steel to ceramic solid oxide cells by a novel Ag-SiO_(2) braze

A novel Ag-SiO_(2) braze for Solid Oxide Cells was successfully developed and applied to join Co coated AISI 441 ferritic stainless steel to YSZ-NiO half cells.The braze showed an improved wettability on the coated steel and the YSZ compared to pure Ag.A defect-free steel/YSZ joint was obtained using an Ag-1SiO_(2) braze jointed at 1000℃for 30 min in air.The microstructure of the joints was characterized by scanning electron microscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy.Furthermore,a Co-Si-O_(2)phase diagram was calculated and used to explain the experimental results and formulate a reaction mechanism.It was found,that during the air brazing process,Co_(3)O_(4)and(Co,Fe)_(3)O_(4)are formed by the oxidation of the prefabricated Co coating.The growth of Cr_(2)O_(3)was effectively impeded by the formed Co-based spinels.In a second step,Co_(2)O_(3)reacts with the SiOpresent in braze.The CoSiOformed by the reaction ensures a firm bonding between the steel and Ag-SiObraze.