Behavior of Aging, Micro-Void, and Self-Healing of Glass/Ceramic Materials and Its Effect on Mechanical Properties

Abstract

Among the high-efficiency energy conversion devices that are environmentally friendly with little or no toxic emissions, the solid oxide fuel cell (SOFC) continues to show great promise as a future power source. It has potential applications in stationary power generation and as auxiliary power units. Among various SOFC designs under development, anode-supported planar cells have shown great potential in delivering high performance at reasonable costs (Teagan et al., 2000; Chung et al. 2005). Planar SOFCs offer a significant advantage of a compact design along with higher power densities. In the meantime, they require that hermetic gas seals be incorporated for efficient and effective channeling of fuel and oxygen. Seals are the most critical components in commercializing the planar SOFC technology (Nielsen et al., 2007; Choi & Bansal, 2005; Smeacetto et al., 2008). They must adequately prevent air and fuel from leaking, effectively isolate the fuel from the oxidant, and insulate the cell from short circuit. Essentially, there are two standard methods for sealing: compressive sealing and rigid bonding (Singh, 2007; Fergus, 2005; Chou et al., 2003). In compressive sealing, a compliant, high-temperature material is captured between the two sealing surfaces and compressed with a load frame external to the stack to deliver hermetic sealing. The sealing surfaces can slide with respect to one another without disrupting the hermeticity of the seal. This technology, however, remains incomplete due to the lack of a reliable, high-temperature, sealing material (Simner & Stevenson, 2001) and the difficulty of designing the appropriate load frame under high-operating temperatures. Rigid seals rely on effective bonding of the seal material to the sealing surfaces. They offer significant advantages over compressive seals that suffer from problems of oxide scaling and chemical stability under highly reactive environments in addition to the disadvantages of incorporating an externally applied load (Singh, 2007). As a rigid seal, glass joining provides a cost-effective and relatively simple method for bonding ceramic and metal parts. However, the softening point of the glass component typically limits the maximum operating temperature to which the joint may be exposed. As discussed by Weil et al. (2004), there are a number of other key materials and processing variables that can influence the performance of glass seals, including the composition of the metal substrate against which the seal is made, the operating parameters, such as the