Phase evolution and microwave dielectric properties of MgO-B2O3-SiO2-based glass-ceramics

The densification and crystallization behaviors of MgO-B₂O₃-SiO₂ (MBS) glass with various amounts of TiO₂ additions (0-10 wt.%) were investigated by means of thermal analysis, X-ray powder diffraction and scanning electron microscopy. A microwave dielectric characterization was performed in order to evaluate the suitability of MBS glass-ceramics as a low-permittivity dielectric substrate. The densification of the MBS glass started below 700 °C. The main crystalline phases of Mg₂B₂O₅ and MgSiO₃ appeared at 800 and 950 °C, respectively. The Mg₃TiB₂O₈ and TiB_0.024O₂ phases additionally crystallized in TiO₂-added MBS glass-ceramics at 1000 °C. The permittivity increased from 6.1 in pure MBS glass to 6.9 in MBS glass with 10 wt.% of TiO₂. The addition of TiO₂ enhanced the crystallization and consequently increased the Qxf-values of the MBS glass (11 300 GHz) up to 16 500 GHz. The improvement of the Qxf-values became the most evident at 1050 °C. Dense MBS glass-ceramics sintered at 850 ≤ T ≤ 950 °C exhibited Qxf-values of 5000-8000 GHz (at ∼12 GHz), which are comparable with the values of CaO-B₂O₃-SiO₂-based glass-ceramics.