Isothermal Crystallization of a Solid Oxide Fuel Cell Sealing Glass by Differential Thermal Analysis

The crystallization kinetics of a solid oxide fuel cell sealing glass were studied using a new isothermal differential thermal analysis (DTA) method. The weight fraction of glass crystallized after an isothermal heat treatment was determined from the DTA crystallization peak area and the crystallization kinetic parameters were determined using the classical Johnson-Mehl-Avrami equation. The glass, an alkaline earth-zinc-silicate composition, crystallized in the temperature range between 740° and 950°C. The activation energy for crystallization varied with glass particle size and decreased from 570±25 to 457±30 kJ/mol as the average particle size decreased from 425-500 to ∼10 μm. The activation energy for crystallization, E, increased from 520±20 to ∼600±20 kJ/mol when glass particles (45-53 μm) were mechanically mixed with 10 vol% of micrometer-sized Ni or YSZ powders. This increase in E reflects the effect of a second phase in composite seal systems, but is independent of the chemical nature of the additives. The measured values of the Avrami exponent (n) indicate that surface crystallization is the dominant crystallization mechanism for this glass, particularly for small particle sizes, e.g. n=0.9±0.1 for ∼10 μm