Thomas Pfeifer

Ceramics for Energy Conversion, Storage, and Distribution Systems: Ceramic Transactions

MgAl2O4 coatings, used for electrical insulation along the metallic seals in high temperature fuel cell stacks, tend to fail prematurely due to generated stresses during on-off thermal cycles. To enhance stress bearing capability and the durability of these coatings, selfhealing was investigated by introducing additives consisting of SiC as primary additive along with a secondary additive material. Secondary additives were one of the following compounds: BaO, CaO, ZnO, Y2O3, Al2O3, La2O3, TiO2, GeO2, Ce0.9Gd0.1O1.95 (GDC) or Ta2O5. Crack healing can be attained due to reaction between SiC, secondary additive and oxygen that was transported to the additive due to a crack. Such reaction would be associated to a phase formation in the additive material linked with mass and volume increase assisting in closure of the advancing crack. Using TGA/DSC reaction temperatures and mass gains of SiC with or without secondary additives were identified. SiC+Y2O3 and SiC+ZnO were opted as promising additive materials. 20 wt% (SiC+Y2O3) containing MgAl2O4 coatings were produced by plasma spraying. In these developed coatings, healing was demonstrated after heat treatment at 1050°C in air for 10 hour. Defect healing in spinel coating with SiC+ZnO is under investigation. INTRODUCTION Defects in the sealing in high temperature solid oxide fuel cells (SOFC) has been reported as the foremost cause of failure in the fuel cell stacks1,2. The sealing, traditionally made of glass or glass-ceramic composites, ensures flow of fuel gas and air in designated compartments of SOFC stack. A leakage across the seal leads to catastrophic loss in cell potential and power output. Glassbased seals exhibit limited reliability which suffers further when the stack should undergo thermal transients such as during intermittent operation. In our earlier work3, an alternative approach was proposed in which Ag-based filler material is used for sealing of two consecutive cells. Despite mismatch of coefficient of thermal expansion (CTE) between filler alloy and neighboring components of stack, the high ductility and creep of the filer alloy compensate for stresses. However, as the filler alloy is electronically conductive, short circuiting between the cells is avoided by introducing an Mg-spinel (MgAl2O4) insulating coating in between. The spinel deposit is produced by plasma spraying. The schematic of sealing the approach is given in Figure 1. In spite of enhanced reliability compared to glass-based seals, the coating-braze based seals suffer from defects and cracks in the coating. These defects, associated to the manufacturing process or arise due to thermal cycling, give site for further crack nucleation and propagation, decrease the elastic modulus, yield strength and fracture energy of the coatings. At elevated temperatures crack initiation and propagation mechanism changes and failure may occur at the featureless zones, as suggested by Lowrie and Rawlings4. Overall, the increase in temperature from room temperature to 800°C caused a 23–30% reduction in flexural strength of bulk 8YSZ. Ansar et al5 have reported that the elastic modulus of plasma sprayed 8YSZ reduces from 35±2 GPa at room temperature (instead of 120 GPa for bulk material) to 16±1 GPa at 800°C. The decrease in elastic properties of such a coating was almost twice to bulk material and this was associated to the intrinsic elastic modulus of the YSZ but also to the structure of the splat boundaries.