Crystallization behaviour and properties of BaO-CaO-B2O3-SiO2 glasses and glass-ceramics for LTCC applications

The influence of BaO content (up to 15 mol%) on the crystallization behaviour, structure, thermal properties and microwave dielectric properties of the BaO-CaO-B2O3-SiO2 glasses and glass-ceramics system was investigated. The glasses were produced by melting at 1400 degrees C and quenching into water, and the glass-ceramics were produced via heat treatment at temperatures between 750 and 800 degrees C. The results of X-ray diffraction analysis showed that increasing the BaO content raised the resistance of the glass against crystallization and favoured the transformation of beta-CaSiO3 and alpha-CaSiO3 phases, which crystallized in the Ba-free and in low BaO content compositions, into SiO2 and Ba4Si6O16, which crystallized in compositions with higher concentrations of BaO. The BaO content had little influence on the glass transition temperature (T-g) and the linear coefficient of thermal expansion (CTE), but strongly reduced the softening point (T-s). Even the addition of BaO as minor additives resulted in a dramatic reduction of the T-s; for example, the T-s decreased from 902 degrees C for the Ba-free composition to 682 degrees C for the BaO-containing one (5%). Low values of the dielectric constant (5.9 <= epsilon(r) <= 6.63) and dielectric loss (1.12 x 10(-3) <= tan delta <= 3.15 x 10(-3)) were measured.