Nucleation, grain growth and microstructure of CaO-B2O3-SiO2 glass-ceramics for low temperature co-fired ceramics

The nucleation and growth of A6M tape, a crystallisable CaO-B2O3-SiO2 glass for low temperature co-fired ceramics (LTCCs) produced by Ferro, was investigated. The crystalline phases wollastonite (CaSiO3) and calciborite (CaB2O4) formed from the glass at heat treatment temperatures of 830, 850 and 870 degrees C. The nucleation rate was found to be a function of temperature with a maximum at 850 degrees C; as the temperature increases or decreases from 850 degrees C, the nucleation rate reduces. Nucleation becomes very slow at a temperature lower than 750 degrees C or higher than 900 degrees C. Grain growth could be diffusion controlled according to the grain size versus time profile. A high nucleation rate is found to be crucial to generate a fine and uniform microstructure. The fine scale microstructure generated at 850 degrees C led to LTCCs with high fracture strengths experimentally, which is consistent with fracture analysis of ceramics. Since A6M tape has a glass transition temperature of about 635 degrees C, the tape could be sintered before heating up to the peak temperature at 850 degrees C in the firing temperature profile used for LTCC manufacture. The sinterability enables densification of the tape before crystallisation during LTCC manufacture.