Development and Testing of High Purity Alumina Ceramics for SOFC Stack Components

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.