Physical and structural characteristics of sol–gel derived CaO–B2O3–SiO2 glass-ceramics and their dielectric properties in the 5G millimeter-wave bands

In this study, sol–gel derived CaO–B2O3–SiO2 glass-ceramics with a set B2O3 content of 22.2 mol% and CaO/SiO2 ratios ranging between 0.15 and 0.27 were used for low-temperature cofired ceramic applications in the 5G millimeter-wave bands. X-ray diffraction analysis, scanning electron microscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy data indicated that, unlike the typical CaO–B2O3–SiO2 glass-ceramics prepared via melting resulted in the presence of calcium silicates, the CaO–B2O3–SiO2 glass-ceramics in this study comprised only an amorphous phase containing different amounts of CaB2O4 crystallites depending on the CaO/SiO2 ratio. Among the formulations evaluated, the 14.5CaO‒22.2B2O3‒63.3SiO2 glass-ceramic sintered at 950 °C exhibited a dielectric constant of 4.33 and a dielectric loss of 0.0012 at 60 GHz, which conferred its low signal propagation delay and low signal attenuation in applications. In addition, the electrical resistivity, breakdown strength, thermal conductivity, and coefficient of thermal expansion of the 14.5CaO‒22.2B2O3‒63.3SiO2 glass-ceramic were 1.72 × 1012 Ω cm, 15.49 kV/mm, 1.70 W/mK, and 4.1 ppm/°C, respectively. The 14.5CaO‒22.2B2O3‒63.3SiO2 glass-ceramic exhibited excellent insulating properties, facilitating its use as substrate material; moreover, its thermal properties matched those of Si and GaAs.