Evaluations on the Durability of Magnesia Magnesium Aluminate (MMA) Solid Oxide Fuel Cell Materials
Stark, Alexander James
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Solid oxide fuel cells (SOFCs) are one of the most promising candidates for generating clean energy. Also known as “all ceramic” fuel cells, SOFCs use a range of advanced ceramic components to achieve the functionality that is required to attain high levels of efficiency, which exceed most other types of fuel cell systems. However, SOFC systems operate at high temperatures (500-1000°C). Therefore, the mechanical reliability and service life of advanced ceramic components has become “the” important area of research in this industry. Specifically, the static and dynamic mechanical integrity of these components under fuel environments at room and operating temperatures need to be investigated. To design viable commercial fuel cell assemblies, a comprehensive understanding of the mechanical behavior of the cells under the operating conditions is critical, and a qualification of the vendor components is essential. Flaws resulting from the manufacturing process, or that are introduced during handling and assembly, can cause a reduction in the strength of the cells as well as degrade their reliability. That requires a statistical approach for characterizing the mechanical properties of fuel cell materials and an understanding of the strength distribution rather than a deterministic definition. In this investigation, the strength distributions of dense and porous Magnesia Magnesium Aluminate (MMA) candidate cell materials are evaluated under a range of environments, including exposure to hydrogen, nitrogen and moisture under elevated temperatures. Using this data, the slow crack growth behavior of the materials is characterized, and a fractographic analysis was performed on the component fracture surfaces to identify the origins of failure. The Mode I fracture toughness of the MMA is also evaluated at low (50°C) and elevated temperature (850°C) within a moist environment. It was found that the MMA exhibits a Weibull modulus of approximately 20, with limited differences between room temperature and 850°C, and that the strength exhibits very limited rate dependence, indicating low susceptibility to failures attributed to slow crack growth at these two temperature conditions. However, evaluations of the dense MMA at 50°C showed much greater SCG dependence, with an exponent of 29. Failures often initiated at surface pores that were introduced during the manufacturing process, which could serve as the limiting factor in commercial systems. The fracture toughness was not dependent on temperature, with the overall average value of 1.78 ±0.06 MPa•m1/2. Results of this investigation contribute to the development of a comprehensive understanding of the structural behavior of MMA fuel cell materials, and will be instrumental in the choice of materials and manufacturing of reliable commercial systems.