Part Ι: Effects of two-stage heat treatment on densification, microstructural features and dielectric properties of CaO–MgO–SiO2 glass-ceramics with ZrO2 nucleating agents

Abstract

Densification, microstructures and dielectric properties of CaO–MgO–SiO2 glass-ceramics were investigated with ZrO2 nucleating agent using two-stage heat treatment in the low temperature co-fired ceramic (LTCC) method. The amorphous cullet (abbreviated as CMSZ glass) was synthesized by mixing CaO, MgO, and SiO2 powders with ZrO2 nucleating agent, which were melted at 1500°C before the quenching process. The results show that CMSZ glasses annealed below the critical nucleation temperature (Tcn) and sintered at 950°C exhibit obvious bloating phenomena. On the other hand, CMSZ glasses annealed above Tcn demonstrate that zirconia nucleation occurs at the first-stage heat treatment (annealing). The second-stage heat treatment (sintering) is characterized by the rapid crystal growth of CaMgSi2O6 phase around the ZrO2 crystallites, indicating growth of nuclei over the critical size above Tcn. Moreover, specimens fabricated using two-stage heat treatment and annealed above Tcn exhibit appropriate microwave dielectric properties and high mechanical strength.