Crystallization kinetics and properties of nonstoichiometric cordierite-based thick-film dielectrics

Dielectric thick films based on a nonstoichiometric cordierite (2.4MgO·2Al₂O₃·5SiO₂ containing 3 wt% B₂O₃, 3 wt% P²O₅ and 3 wt% PbO) were investigated, in regard to their microstructure crystallization kinetics, and properties. A stable glass-ceramic thick-film microstructure that was formed on a 96% alumina substrate was observed after firing at a temperature of 915 °C for 30 min in a nitrogen atmosphere. No μ-cordierite was observed in the X-ray diffraction (XRD) patterns of the thick film. The crystallization kinetics were studied via quantitative XRD analysis using the Avrami equation and the rate constant increased as the temperature increased. The decreasing tendency of the Avrami parameter relative to temperature suggested a change in growth directionality during crystallization. The activation energy for crystallization of the thick film was determined to be approximately 83 kcal/mol (approximately 350 kJ/mol). The coefficient of thermal expansion (CTE) and the dielectric constant of the glass phase were evaluated using the bulk-sample data. For the case of a 3-wt%-PbO sample fired at 950 °C for 30 min in a nitrogen atmosphere, the remaining glass was estimated using the parallel mixing rule, to have a dielectric constant of 15.3 at 1 MHz. The dielectric constant of the remaining glass was dependent on the PbO content and the heat-treatment temperature. The estimated CTE of the remaining glass for the 3-wt%-PbO sample was 19×10^-6/°C.