Thesis (Master's)--University of Washington, 2019In the pursuit of economic sources of “green energy”, Solid Oxide Fuel Cells (SOFCs) are of substantial interest due to their high efficiency, long-term stability, low emissions, and relatively low costs. With a capability of achieving an efficiency of 80%, SOFCs are the most efficient of all the hydrogen fuel cells. These systems operate at high temperatures, often ranging between 500°C and 1000°C. Due to such high operating temperatures, the components of SOFCs are fabricated from ceramic or cermet materials, which exhibit high sensitivity to pores and flaws. Hence, it is important to characterize the reliability and service life of advanced ceramic components at extreme operational environments. Due to their relatively high flaw sensitivity, the mechanical properties of ceramics should be described in a probabilistic manner, to enable a complete description of component reliability. Magnesia Magnesium Aluminate (MMA) is a candidate ceramic material for high-temperature applications including SOFCs. In the present investigation, the strength distribution and Slow Crack Growth (SCG) behavior of a dense MMA manifold material and porous MMA tube material were evaluated via 4-point flexural testing at temperatures of 20°C, 50°C, and 850°C under wide range of environmental conditions involving exposure to moisture and a mixture of hydrogen and nitrogen gas. A fractographic analysis was performed to identify the origins of failure and as a function of the environment and temperature. It was found that the flexure strength distribution for the dense MMA (proposed as a manifold material) exhibited a Weibull modulus of approximately 11. In addition, at 50°C and 3.5% moisture by volume, the strength displayed significant rate dependence with an SCG exponent of 17, indicating relatively high susceptibility of the material to slow crack growth failures. The flexure strength distribution of the porous MMA (proposed as a tube material) exhibited a Weibull modulus of approximately 10, with large difference between the ambient conditions and high temperature response. At 850°C and within a reformulated fuel environment (50% moisture, 45% nitrogen and 5% hydrogen by volume) the strength displayed significant rate dependence with a SCG exponent of -33, indicating susceptibility of the material to slow crack growth failures. Thermal cycling was found to increase the Weibull modulus of both the dense MMA manifold materials and porous MMA tube materials. Fractography showed that regardless of loading rate or environment, large surface pores resulting from processing were the origins of failure in the weakest samples of both the manifold and tube MMA materials. The results from this investigation should make a significant contribution to the literature concerning the high temperature behavior of structural ceramics
