Glass-ceramic sealant reinforcement for high-temperature applications

In the development of solid oxide fuel cells, the components most in need of improvement are still the sealants. Over the last decade, several types of sealants have been investigated for use under high temperatures, such as compressive, compliant, and rigidly bonded seals. Of these three types, rigidly bonded glass-ceramic seals are the most promising. Their properties can be tailored to match the requirements of SOFC sealants. These include the coefficient of thermal expansion, joining temperature, crystallization behavior, electrical insulation, and gas-tightness. Nevertheless, in the past, the developed sealant compositions failed to demonstrate sufficient mechanical strength. This property is extremely important to avoid catastrophic failure of the rigid seals during SOFC operation. Additionally, there is a lack of standardized methods to characterize the mechanical strength of joined components in the research community. This makes it difficult to rely on the results of the state of art measurements, to reproduce them, and indeed to compare them. In order to improve the mechanical strength of glass-ceramic sealants, this work proposes reinforcing the glass-ceramic sealant with different metallic and ceramic particles. A new concept of laminate sealant, known as a multilayer design, was developed in an attempt to combine the properties of two types of composites in one joint. In addition, three possible methods for mechanical strength characterization were developed. The reinforcement concept is mainly based on adding fillers to the glass matrix named “87”, which is a composition from the system BaO-CaO-SiO2. The chosen fillers were metallic particles including nickel (Ni), nickel-chromium (NiCr) (80-20), copper (Cu), and silver (Ag), as well as ceramic fillers such as gadolinium-doped ceria (CGO) particles and yttrium-stabilized zirconia (YSZ) particles or fibers. These materials were tested in different weight concentrations in the glass matrix to form the composites. This approach showed that adding filler materials (metallic or ceramic) improved the mechanical strength values. The multilayer design was also proven to be effective in combining the properties of two different composite layers in one joint. Electrically insulating samples with sufficient mechanical strength were produced with single layers of reinforced sealant as well as with the multilayer approach.