This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.
The successful attainment of many of the next generation Siemens SOFC Advanced Module features is dependent on development of key components required to provide fuel and process air to a stack of Delta cells. The overall objectives of this development effort included design and analysis of key stack components, fabrication of low cost net shape castings, characterization of high purity alumina ceramic material, and validation through full scale testing in Single and Multi-Cell Test Articles. The manufacturing process chosen for fabrication of stack components is a unique injection molding process referred to as the Blasch process. The Blasch process is a relatively low cost manufacturing process which allows for the fabrication of complex, close tolerance, near net shapes in a range of high alumina ceramic compositions without the need for expensive secondary machining. The Blasch process allows engineers to design virtually without restrictions related to other forming processes such as slip casting, extrusion, or pressing. The process utilizes nanotechnology to strongly bind together ceramic slurries containing one of a series of proprietary binders that can be activated by utilization of specific time/temperature processing. After casting into engineered molds, the binders in these slurries are caused to precipitate irreversibly and, upon firing, form a particularly thermal shock resistant ceramic bond containing no free silica. Ceramic shapes formed in this process shrink minimally and predictably, during firing, and therefore this is one of the few processes that can be claimed as true net shape manufacturing. Considerable effort went also in the development of a new class of failure tolerant alumina ceramics for SOFC stack components for service in reducing atmosphere at temperatures up to 1000°C. Pressureless infiltration of freeze cast alumina parts with chromium oxide was conducted to improve material’s strength. Strengthening of the porous alumina matrix is postulated to be a combination of both fracture toughness increase and crack size decrease, as a result of the infiltration process. Final results suggest that mechanical properties of infiltrated ceramics are superior to conventional porous freeze cast alumina material. This paper addresses the approach to ceramic castings design for SOFC stack components, the fabrication challenges with respect to shape complexity and the experimental tests performed to validate the material choice.
One of the enabling technologies required for commercialization of high efficiency solid oxide fuel cell (SOFC) stacks is the development of low cost ceramic refractories capable of withstanding the harsh environment during start-up and steady state operation. Although low density, high purity fibrous alumina materials have been used for more than two decades in manufacturing of SOFC stack components, their low mechanical strength and high cost have precluded their use in the next generation pre-commercial generator modules. A current trend in SOFC stack design is to use high strength, low purity mullite bonded, cast ceramics which can be produced in large volume at a relatively low cost. Sufficient strength is required to provide structural support of the stack and its upper internals in addition to withstanding the severe thermal gradients in both steady state and transient conditions. To reduce costs while achieving suitable mechanical strength, thermal shock, and creep resistance, certain levels of silica and other impurities are present in the refractory ceramic. Silica, however, has been established to poison SOFC anodes thus degrading cell performance and stack life. Therefore, silica transport within the stack has become a dominant issue in SOFC generator design. As a result, an important design requirement for the stack ceramic materials is to develop a fundamental understanding of the silicon species transport process based on refractory composition and gas atmosphere in effort to minimize silicon species volatilization through the porous material. The vaporization behavior of the Al-Si-O system has been investigated in numerous studies and verified experimentally. It is well known that when aluminum silicate components are exposed to a reducing atmosphere, the partial pressure of oxygen is low, therefore this causes formation of volatile SiO(g). This SiO(g) gaseous phase is transported by the fuel stream to the anode/electrolyte interface and electrochemically oxidizes back into SiO2 over the triple phase boundaries (TPB) by the oxygen transported via the fuel cell. This re-deposition process of SiO2, known also as Si poisoning, blocks the reaction of fuel oxidation as it takes over the reactive sites, leading to noticeable degradation in cell performance. In this paper, the status of research on formation of volatile silicon species in aluminosilicate SOFC insulation materials is examined. The formation of volatile SiO(g), SiO(OH)(g), and SiO(OH)2(g) are indicated to facilitate silicon transport in anode fuel streams. Silica deposition is shown to degrade fuel cell anode performance utilizing a novel SOFC silicon poisoning test setup, and silica deposition is only observed on YSZ in the electrochemically active regions of the cell.
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