From very high to negative thermal expansion in alkaline earth zinc silicatesWednesday (28.09.2016) 11:15 - 11:30 Part of:
The thermal expansion behavior of glasses and glass ceramics is one of the main properties, which has to be adjusted in order to achieve sealing materials for applications such as solid oxide fuel cells or high-temperature reactors in general. Therefore, glass-ceramics with very high but tunable coefficients of thermal expansion are needed. Recently we found a new crystalline phase, which can be easily crystallized in high concentrations from silicate glasses. This phase has unexpected low and partially negative thermal expansion properties, which strongly depend on the crystallographic direction as well as on the surrounding matrix. The concentration of this phase can be controlled by slight changes in the glass composition. Very high concentrations of this phase also lead to zero or even negative thermal expansion of the bulk materials, which can also be controlled by the microstructure of the glass-ceramics.
This paper describes glass-ceramics based on BaZn2Si2O7, which forms solid solution crystals in wide concentration ranges. These solid solutions can show totally different thermal expansion properties depending on the composition. In this phase, different ions such as Mg2+, Co2+, Mn2+, Cu2+, Ni2+, Sr2+, and Ge4+ can be incorporated. The thermal expansion properties are studied with dilatometry and high-temperature X-ray diffraction up to 1000 °C as a function of the composition. Furthermore, sealing experiments were performed in which the glass-ceramics were joint to metallic alloys as they are used in solid oxide fuel cells. The interfacial reactions between the sealing components were characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy.