Microwave dielectric properties of (1−x)MgO–x(0.8LiF–0.2CaF2) ceramics for low-temperature Co-fired ceramics

The effects of eutectic addition of 0.8LiF–0.2CaF2 on microwave dielectric properties and structure of MgO ceramics were investigated. Addition of LiF–CaF2 decreased the sintering temperature of MgO ceramics to 900 °C but did not affect the dielectric properties of the ceramics. The dielectric constant (εr) and temperature coefficient of the resonant frequency (τf) of MgO ceramics were not influenced by the LiF–CaF2 eutectic content within the range of 0.02–0.08. However, the quality factor (Q×f) was dependent on the amount of LiF–CaF2 added. Moreover, open or closed sintering condition influenced the microstructure and microwave dielectric properties of LiF–CaF2-doped MgO ceramics. The X-ray diffraction and impedance spectroscopy analyses indicated that Li and Ca substitution for Mg and F substitution for O occurred in MgO ceramics, and a high amount of F was incorporated into the MgO lattice under closed sintering condition. Overall, 0.96MgO–0.04(0.8LiF–0.2CaF2) ceramics sintered at 950 °C in an open crucible exhibited excellent microwave dielectric properties of εr=9.18, Q×f=130198 GHz and τf=−65 ppm/°C. This study shows that eutectic addition of LiF–CaF2 effectively decreases the sintering temperature of MgO without deteriorating the ceramic performance.