Enhancement of densification and microwave dielectric properties in LiF ceramics via a cold sintering and post-annealing process

LiF is a low-firing fluoride with excellent microwave dielectric properties, however densification of LiF ceramics is challenging owing to their low surface free energy. In this study, a cold sintering process (150 °C, 250 MPa, 60 min) was employed to pre-densify LiF ceramics to 78 % relative density. Post-annealing treatments between 650 °C and 800 °C led to significant grain growth which was accompanied by an increase in relative density to 92 %. The microwave quality factor (Qf) increased with increasing annealing temperature to a maximum of 110,800 GHz at 800 °C, 1.5 times higher than the value obtained via conventional sintering (78,800 GHz), with relative permittivity ε_r = 8.2 and temperature coefficient of resonant frequency, τ_f =–135 ppm/°C. Such high values of Qf and its compatibility with Ag electrode suggest that cold sintered LiF has great potential as a component or additive in low temperature co-fired ceramic formulations.