Microwave dielectric properties of Li₂O–xMgO–ZnO–B₂O₃–SiO₂ glass-ceramics (x = 30–50 wt.%)

High crystallization, low-dielectric-loss Li2O–xMgO–ZnO–B2O3–SiO2 (x = 30, 35, 40, 45 and 50 wt.%) (LMZBS) glass–ceramics were prepared with different MgO contents using the solid-state method in this study. The effects of the MgO content on the microstructures, flexural strength and microwave dielectric properties of LMZBS were investigated. X-ray diffractometer results, for samples sintered at 920°C, for 30 wt.% ≤ x ≤ 35 wt.%, showed that the main phase was Mg2B2O5 and the secondary phase was Li2ZnSiO4. But a new Mg3B2O6 phase appeared when the MgO content exceeded 35 wt.%, becoming the main phase at x = 50 wt.%. Scanning electron microscopy showed that the average grain size decreased with increases in MgO and that a compact, homogeneous microstructure could be obtained at x = 40 wt.%. The Q×f value (product of qualify factor and resonant frequency) increased at first due to increases in the theoretical Q (quality factor) value and relative density, and then decreased due to a decrease in the average grain size and the appearance of large amounts of the liquid phase. Finally, the Li2O–xMgO–ZnO–B2O3–SiO2 glass-ceramics with 40 wt.% MgO exhibited excellent microwave properties and mechanical properties in terms of dielectric constants (εr = 6.66), Q×f = 54600 GHz, temperature coefficient of resonant frequency (τf = −35 ppm/°C) and flexural strength (σ = 160 MPa) after the samples were sintered at 920°C for 2 h in air, indicating their suitability as low-temperature co-fired ceramics.