Sintering, crystallization, and properties of CaO doped cordierite-based glass–ceramics

The effects of replacement of MgO by CaO on the sintering, and crystallization behavior of MgO–Al2O3–SiO2 system glass–ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature (Tg) firstly increased, and then decreased. The melting temperature was lowered, and the crystallization temperature of the glass–ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt% CaO, the predominant crystalline phase in the glass–ceramics, fired at 900 °C, was found to be α-cordierite, and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt%, the predominant crystalline phase was found to be anorthite, and the secondary phase to be α-cordierite. Both the thermal expansion coefficient (TCE), and the dielectric constant of the samples increase by replacing MgO by CaO. The dielectric loss of the sample with 5 wt% CaO (C5), fired at 900 °C, has the lowest value of 0.08%. Only the sample C5 and the sample containing 10 wt% CaO, (C10), can be fully sintered before 900 °C. Therefore, a dense, and low dielectric loss glass–ceramic with predominant crystal phase of α-cordierite, and some amount of anorthite was achieved by using fine glass powders (D50 = 3 μm), fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases, and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends not only on its relative density, but also on its crystalline phases. The dense, and crystallized glass–ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10−3) at 1 MHz, a low TCE (4.0–4.25 × 10−6 K−1), very close to that of Si (∼3.5 × 10−6 K−1), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.