Stable Glass-Ceramic Nanocomposites as Compliant Seals for SOFCs

The lack of a functional sealing system to isolate various components of a solid oxide fuel cell (SOFC) remains today as one of the biggest factors inhibiting widespread use of SOFCs. Two criteria severely limit the use of inorganic, non-metallic sealing materials for SOFCs; 1) CTE mismatches of 2-3+ ppm/C within the SOFC structure, and 2) High operating temperatures (700C to 850C). From 2005 to 2007, Sem-Com Company collaborated as a subcontractor on a SBIR grant to Ceramatec, Inc. to develop a composite sealing glass for SOFC/Membrane applications. This work resulted in a composite sealing system consisting of a matrix glass that is semi-rigid at SOFC operating temperatures achieved by using a glass that has a transition point that is slightly lower than the SOFC operating temperature (CTE=8.8 ppm/C) combined with a secondary glass or filler (CTE=17 to 18 ppm/C) to achieve a composite CTE of 11 to 13 ppm/C. This high CTE secondary glass or filler was achieved by crystallizing a hexacelsian phase in a barium aluminosilicate glass. The composite sealing glass resulted in good seals that maintained hermeticity after temperature cycling from to 800C (5 cycles). This testing confirmed that the basic compliant feature of the seal however, high temperature aging tests revealed a new problem; a gradual downward drift in the composite CTE. From previous experience, it was concluded that the matrix glass was slowly dissolving the composite CTE adjuster; i.e. the hexacelsian glass. This well known phenomenon is usually not a factor in seals that are not subjected to high temperatures. In this new proposal, nanotechnology materials will be dispersed between the matrix glass and hexacelsian glass particles to prevent the matrix glass from dissolving the filler glass by one of the following mechanisms: Alumina nanoparticles as a sacrificial coating At high temperatures, these particles will be taken into solution by the matrix glass resulting in an alumina rich layer surrounding each filler glass particle. This very thin layer of boundary or transition glass will have substantially higher temperature characteristics than the pure matrix glass and be less aggressive towards the hexacelsian phase. Nanoparticles as a barrier coating In this case the material a refractory type material such as barium zirconate will act as a barrier between the matrix and hexacelsian glass systems. Nanotechnology materials are ideal for this application because the high surface area characteristics will minimize the amount needed to isolate the primary composite glasses. SCN-1, a Sem-Com glass that has been extensively studied for SOFC sealing with good results will also be included to determine if it can be further optimized; especially with respect to CTE and pore coalescence. While this proposed project focuses on the development of a sealing glass for SOFCs, there are actually three innovations involved: 1. An inorganic, non-metallic, composite sealing material that at elevated temperatures is compliant so as to provide hermetically seal materials with dissimilar (non-matching) CTE properties. There have been many attempts to achieve this goal but the proposers are not aware of any that have been successful in all important respects. 2. A glass-ceramic material with a CTE as high as 18 ppm/C and that can be predictably controlled and tailored by varying the heat treatment of the material. It is thought that these characteristics make this a truly unique material. 3. The use of nano-scale materials as barriers in inorganic composite systems to keep various components separate and distinct. These innovations, either separately or together, have the potential to provide technical solutions to a broad range of applications beyond sealing glasses for SOFCs. Over the years many applications have been abandoned because the mismatch between the sealing components was too great to be of practical consequence or because no high CTE glasses were available. A compliant seal technology will open up at least some of these applications to hermetic seal technology. The high CTE hexacelsian glass, especially with the addition of the nano-scale materials will allow a new family of high CTE sealing glasses to be developed that do not exist today. These high CTE glasses would be especially useful in glass-to-metal applications.