Volume crystallization and microwave dielectric properties of indialite/cordierite glass by TiO2 addition
Indialite/cordierite (Mg₂2Al₄Si₅O₁₈) glass-ceramics with a low dielectric constant of 4.7 and a high Qf of >200 × 10³ GHz are predicted for use as micro/millimeter-wave materials in the fifth generation (5G) mobile communication systems. The glass-ceramics have a serious cracking problem caused by the anisotropic crystal growth during the surface crystallization. In this paper, the cracking was prevented by adding TiO₂ which acts as a seed. The glass-ceramics produced without cracking were composed of spherical crystals of approximately 10 μm diameter, formed by volume crystallization. Precipitated phases of the glass-ceramics crystallized at 1200–1350 °C/10 h and 20 h were indialite, cordierite, Al₂TiO₅ and rutile. The glass-ceramics crystallized for 10 h were analyzed by the Rietveld method. Indialite precipitated as an intermediate metastable compound at the lower temperature of 1200 °C and transformed to cordierite at the crystallization temperature. The reaction between cordierite and TiO₂ produced the new Al₂TiO₅ phase. The amounts of Al₂TiO₅ and rutile affected the microwave dielectric properties. In particular, the amount of rutile affected the TCf. In the cases of 10 wt % added TiO₂, and the crystallized at 1250 °C for 10/20 h the TCf values were improved to -2/-8 ppm/°C.