Sonja M. Gross

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.

A Comparative Study Between Resistance Measurements in Model Experiments and Solid Oxide Fuel Cell Stack Performance Tests

Several combinations of glass-ceramic and steel compositions with excellent chemical and physical properties have been tested in the past in solid oxide fuel cell (SOFC) stacks, but there have also been some combinations exhibiting pronounced chemical interactions causing severe stack degradation. Parallel to the examination of these degradation and short-circuiting phenomena in stack tests, recently less complex model experiments have been developed to study the interaction of glass-ceramic sealants and interconnect steels. The sealants and steels were tested in the model experiments at operation temperature using a dual air/hydrogen atmosphere similar to stack conditions. The present work compares electrochemical performance under constant current load of SOFC stack tests with the resistance changes in model experiments. In addition, microstructural results of post-operation inspection of various sealant-steel combinations are presented. The model experiments have shown that under the chosen experimental conditions, distinct changes of the specific resistance of the specimens correlate well with the changes of the electrochemical performance of SOFC stacks, indicating that this method can be considered as an excellent comparative method to provide useful information on the physical and chemical interactions between glass-ceramic sealants and ferritic steels.