Glass ceramics for high-temperature sealing applications: synthesis and physicochemical properties of modified CaO-MgO-Al2O3-SiO2 materials, with a view to recycling of industrial waste

Solid Oxide Fuel cells (SOFC) and dense gas separation membranes based on mixed ionic and electronic conductors have gained increased interest the resent years due the search for new technologies for clean energy generation. These technologies can be utilized to produce electricity from fossil fuel with low CO2 emission compared to conventional gas or coal based energy plants. One crucial challenge with SOFCs is the sealing of the active membranes/electrolytes to prevent leakage of air to fuel side or vice versa. Due to the high operating temperatures of typical 600-1000°C the selection of reliable sealing materials is limited. The seals have to remain gas tight during the life time of the reactor/SOFC, they need to be chemical compatible with the sealed materials and stable in reducing and oxidizing atmospheres containing water vapor and CO2, and finally they should be cheap, readily available and easy to process. The main purpose of the present work was to evaluate rigid bonded glass ceramic seals for dense oxygen ion and proton conducting membranes and electrolytes for SOFCs. First, a review of sealing technologies has been carried out with emphasis on SOFC and ceramic membranes technologies applicable for zero emission power plants. Regarding sealing, the best and cheapest materials at the present time are based on silicate glass and glass ceramics. The aim is to provide a systematic study of the properties of glasses and glass-ceramics as a function of the glass composition in the CaO-MgO-Al2O3-SiO2 system, which represent the most advanced sealing technology for high-temperature Solid Oxide Fuel Cells (SOFC). Two glass systems have been evaluated, aluminosilicate and boroaluminosilicate and their variants obtained adding five different oxides acting as nucleating agents, like TiO2, MnO2, ZnO and SnO2. Fabrication and characterization of the glasses are reported with special focus on the thermal and thermochanical properties, glass forming ability, kinetic crystallization and phase evaluation. The influence of different additives in glass and glass ceramic properties has been analyze in depth. In the last section, the research has been extended to the recycling of aluminosilicate basted waste to characterize a new type of glass ceramic derived from industrial waste, like Kaolin clay waste.