Cordierite - mullite composites are used to produce glass ceramic substrates for microelectronic packaging. The composite material is obtained here by thermal transformation of Mg-exchanged zeolite P. The crystallization process is described for two precursor zeolites with different Mg exchange rates by quantitative Rietveld analyses and differential thermal analyses. X-ray powder diffraction experiments are performed in situ between room temperature and 1450 degrees C as well as on the quenched samples. At about 300 degrees C, zeolite Mg-P is rendered amorphous and crystallizes to Mg-P-quartz and mullite above 950 degrees C and furtheron to a pyroxene phase above 1050 degrees C, where the amorphous component is completely exhausted. At higher temperatures, Mg-beta-quartz and pyroxene fractions decrease, accompanied by the formation of sapphirine and cristobalite. At 1250 degrees C, 6.6 formula units Mg-beta-quartz, 9.4 mullite, 1.1 enstatite, 0.8 sapphirine, and 8.2 cristobalite coexist with the first appearence of 0.1 cordierite. Above 1250 degrees C, sapphirine and cristobalite react to yield increased amounts of cordierite, giving 3.1 mullite, 1.2 cristobalite, and 7.6 cordierite at the final firing step at 1400 degrees C. All reactions represent dynamic, non-equilibrium conditions, similar to phase formation in the ceramic sinter process. The high-temperature form of cristobalite is partly stabilized as a Mg-stuffed derivative.
