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

Dielectric thick films based on a nonstoichiometric cordierite (2.4MgO . 2A l(2)O(3). 5SiO(2), containing 3 wt% B2O3, 3 wt% P2O5, and 3 wt% PbO) were i nvestigated, in regard to their microstructure, crystallization kinetics, a nd properties. A stable glass-ceramic thick-film microstructure that was fo rmed on a 96% alumina substrate was observed after firing at a temperature of 915 degrees C for 30 min in a nitrogen atmosphere. No p-cordierite was o bserved in the X-ray diffraction (XRD) patterns of the thick film. The crys tallization kinetics were studied via quantitative XRD analysis using the A vrami equation, and the rate constant increased as the temperature increase d. The decreasing tendency of the Avrami parameter, relative to temperature , suggested a change in growth directionality during crystallization. The a ctivation energy for crystallization of the thick film was determined to be similar to 83 kcal/mol (similar to 350 kJ/mol), The coefficient of thermal expansion (CTE) and the dielectric constant of the glass phase were evalua ted using the bulk-sample data. For the case of a 3-wt%-PbO sample fired at 950 degrees C for 30 min in a nitrogen atmosphere, the remaining glass was estimated, using the parallel mixing rule, to have a dielectric constant o f 15.3 at 1 MHz, The dielectric constant of the remaining glass was depende nt on the PbO content and the heat-treatment temperature. The estimated CTE of the remaining glass for the 3-mt%-PbO sample was 19 x 10(-6)/degrees C.