Optimization of Tensile Strength Measurements on Glass-Ceramic Sealant Used for SOFC Stacks

Glass-ceramic sealants are commonly used as joining materials for planar Solid Oxide Fuel Cell (SOFC) stacks. Several requirements need to be fulfilled by these materials: besides electrical insulation and adequate thermal expansion, a good adhesion on the ceramic and metallic components of a SOFC stack is necessary to form a gas-tight joint [1]. A key requirement is the mechanical strength of the sealant to maintain the integrity of the stacks under operation conditions [2]. Due to the lack of standardized methods, tensile tests were developed in-house. Preliminary measurements achieved reproducible results with extremely low tensile strength values for circular butt joints made of the iron-chromium alloy Crofer22APU and a composite sealant based on a glass matrix of the system of BaO-CaO-SiO₂ reinforced with yttria-stabilized zirconia fibers [3, 4]. These results have been used for a qualitative ranking of different sealant materials.

The goal of our recent studies is to improve the characterization methods of mechanical strength in order to obtain quantitative results and to achieve the understanding needed to design more robust SOFC stacks. For this purpose, computer simulations using finite element analyses (FEA) have been performed. By modelling different joint configurations, it was possible to homogenize the stress distribution along the sealing layer and to optimize the geometries of the components. It was shown that bending of the metallic components as well as residual thermal stresses had a strong effect on the initial sample setup with thin cylinders. An optimized design for the tensile components allows a more uniform distribution of tensile stresses within the sample. Different joining conditions were investigated and the calculations were verified by experiments. Scanning electron microscopic analyses were carried out on cross sections and broken samples, which allow to evaluate the microstructure of the composite and its interfacial reactions. The mechanical strength values obtained by the improved characterization method represent a tenfold increase compared to the values published in [3].

[1] Gross et al., Fuel Cell Bulletin 9 (2006) 12-15

[2] Blum et al., J. Pow. Sources 196 (2011) 7175-7181

[3] Gross et al., J. Pow. Sources 196 (2011) 7338-7342

[4] Cela Greven et al., Fuel Cells 13, No.4 (2013) 565-57