Dielectric properties of CaO-B2O3-SiO2 glass-ceramic systems in the millimeter-wave frequency range of 20-60 GHz

The dielectric and structural properties of the as-quenched melts of three CaO-B2O3-SiO2 compositions (denoted CBS-1, CBS-2, and CBS-3) were investigated to determine their suitability for use in millimeter-wave applications. The CBS-1 glass-ceramic exhibited the lowest coefficient of thermal expansion (CTE = 3.2 ppm/degrees C), lowest dielectric constant (epsilon(r) = 4.04) at 60 GHz, and highest dielectric loss (tan delta = 0.0029) at 60 GHz, which were attributed to the presence of quartz (SiO2) as the major phase. In contrast, as the major phase constituent of the CBS-2 and CBS-3 glass-ceramics was beta-CaSiO3, they presented relatively high CTEs (6.6 and 5.9 ppm/degrees C, respectively), relatively high dielectric constants at 60 GHz (6.29 and 7.61, respectively), and relatively low dielectric losses at 60 GHz (0.0020 and 0.0012, respectively). The CBS-1 glass-ceramic exhibited the highest dielectric loss because of the presence of SiO2 as the major phase constituent as well as lattice scattering induced by the high glassy phase content. The thermal conductivities (kappa) of the CBS-1, CBS-2, and CBS-3 glass-ceramics were determined to be 2.43, 1.06, and 0.82 W/mK, respectively. Structural analysis using Raman and Fourier transform infrared spectroscopy revealed an absence of nonbridging oxygen in the CBS-1 glass-ceramic, while the high CaO content (>40 mol%) of the CBS-2 and CBS-3 glass-ceramics triggered the formation of nonbridging oxygen in the tetrahedral silicate units. The increase in CaO content of the glass-ceramics increased the number of nonbridging oxygen atoms, thereby resulting in the relaxation of the structure. Consequently, the CBS-2 and CBS-3 glass-ceramics exhibited low thermal conductivity. All the prepared glass-ceramics presented high electrical resistivities of greater than 5 x 10(11) Omega cm. The CBS-1 glass-ceramic displayed the highest breakdown strength of 15.20 kV/mm. Overall, the excellent microwave dielectric properties and thermal properties of the CBS glass-ceramics will facilitate the utilization of these materials in millimeter-wave applications.