Volume crystallization and microwave dielectric properties of indialite/cordierite glass by TiO2 addition

Indialite/cordierite (Mg2Al4Si5O18) 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 mm 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.