Thomas Koppitz

Joining properties of a composite glass-ceramic sealant

When assembling planar solid oxide fuel cell (SOFC) stacks, an electrically insulating and gas-tight sealing material is required. Glass-ceramic sealants have been shown to be an appropriate material for this application in the past. In the present study, the investigations are focused on a composite material consisting of zirconia in a glass matrix based on the system of BaO-CaO-SiO 2 (BCS). The joining behavior with ferritic stainless steel is macroscopically observed by so-called ‘sandwiched’ samples made out of two steel plates (size 50 × 50 mm 2 ) with the glass sealant in-between. Dilatometric measurements are carried out, and the coefficient of thermal expansion is taken for varying amounts of zirconia in the composite material. The crystallization behavior of the sealant is investigated by differential thermal analysis. The microstructure of joined samples, submitted to different scenarios of thermal treatment, is characterized by optical and scanning electron microscopy. The joining properties strongly depend on the amount of filler material. Additions of 20 wt% zirconia in the glass matrix prove to be the optimal composition. The glass matrix tends to crystallize very slowly, giving the prospect of an elastic seal during the initial operation of a stack.