Dissolution of Alumina, Sintering, and Crystallization in Glass Ceramic Composites for LTCC

Sintering and microstructure evolution of alkali-free calcium-alumo-borosilicate glass/alpha-Al2O3 composites (mean particle size ca. 2 mu m) for low-temperature cofired ceramics were studied during heating at 5 K/min by heating microscopy, thermal analysis (DTA), X-ray diffraction (XRD), and electron microscopy (SEM). Composites fully densify at approximate to 830 degrees C, not essentially influenced by the dissolution of alumina and glass crystallization. Thus wollastonite, as first crystalline phase, was detectable at 840 degrees C. Above 900 degrees C, a pronounced crystallization of anorthite is evident, reaching 60 wt% at 1050 degrees C. Rietveld analyses of XRD data revealed that anorthite precipitates at the expenses of alumina, which declines from approximate to 33 to < 10 wt%, and wollastonite, which fully declines from its maximum of approximate to 19 wt%. Based on XRD, we discuss the evolution of crystal mass fractions, the residual glass composition, the glass viscosity, and the effective shear viscosity of the composites under study during heating.