Influences of CaO on Crystallization, Microstructures, and Properties of BaO-Al2O3-B2O3-SiO2 Glass-Ceramics

We have developed BaO-CaO-Al2O3-B2O3-SiO2 glass-ceramics with high thermal coefficient of expansion (TCE) to overcome thermal mismatch at board level. The crystalline phases include quartz (major), cristobalite (minor), and bazirite BaZrSi3O9 (minor). Calculations from whole-pattern fitting show that the crystallinity varies slightly within the range of 33.48% to 34.89%, while the mass fraction of the phases changes remarkably with the CaO content. This indicates that CaO cannot promote crystallization of Ba-Al-B-Si glass, but effectively suppresses the phase transformation from quartz to cristobalite, making the thermal expansion curves linear. An empirical equation for the TCE versus the temperature and the amount of CaO is established. Furthermore, the densification mechanism of Ca modifiers is revealed. Due to its higher field strength than Ba, substitution of Ca increases the glass viscosity and inhibits ion diffusion. Excessive CaO is thus harmful to the density, bending strength, and electrical properties. The sample with 10 wt.% CaO sintered at 950A degrees C exhibited high bending strength (154.1 MPa) and high TCE (12.38 ppm/A degrees C) as well as good electrical properties (epsilon = 6.2, tan delta = 5 x 10(-4), rho = 3.8 x 10(12) Omega cm).