Binary, ternary and quaternary silicates of CaO, BaO and ZnO in high thermal expansion seals for solid oxide fuel cells studied by high-temperature X-ray diffraction (HT-XRD)

Gas-tight seals based on glasses suitable for joining of materials with high thermal expansion coefficients are for example required for solid-oxide fuel cells. If these seals are to be used at high temperatures, they can only be fabricated from glasses which enable the crystallization of phases with high thermal expansion coefficients. This paper reports on some components from systems suitable for high thermal expansion seals: binary calcium silicates, CaSiO3, Ca3Si2O7 and Ca2SiO4 zinc silicates, Zn2SiO4, ternary silicates of BaO, CaO and ZnO, BaCa2Si3O9, Ca2ZnSi2O7, and one quaternary silicate, Ba2CaZn2Si6O17, studied by high-temperature X-ray diffraction. Only CaSiO3, Ca3Si2O7 and BaCa2Si3O9 exhibit thermal expansion coefficients in the range suitable for high thermal expansion seals of 11.2–11.8 × 10−6 K−1 (100–800 °C). The thermal expansions strongly depend on the respective crystallographic axis. The coefficient of thermal expansion of a sealing glass is not only affected by the thermal expansions of the crystalline phases, but also by that of the residual glassy phase as well as by the elastic properties. The phase formation should carefully be controlled also with respect to aging.