Cold sintering optimized SrF2 microwave dielectric ceramics for the development of dielectric resonator antenna at 5G millimeter-wave band

Abstract

SrF2 is a promising low-εr fluoride with outstanding microwave dielectric properties, while densification of SrF2 ceramics is challenging via traditional thermal sintering (TTS). In this work, dense SrF2 ceramics with 93.4%–97.2% relative density have been obtained via cold sintering (300 MPa–900 MPa, 150 °C, 1 h) and subsequent post-annealing at 950 °C/3 h. The pretreated cold sintering process is beneficial for microstructure optimization, where the maximum Qf value (62,037 GHz) is obtained at 750 MPa, nearly three times higher than the TTS sample (21,080 GHz). An ultra-low dielectric constant (εr) of 5.94 is simultaneously obtained, together with a temperature coefficient of resonant frequency τf = −78.26 ppm/°C. Good chemical compatibility between SrF2 ceramics and silver is verified, indicating great promise for their use in LTCC technology. Moreover, the low-εr and high Qf values of the cold sintering optimized SrF2 ceramics suggest great potential in the 5G millimeter-wave antenna systems. A SrF2-based dielectric resonator antenna is designed and fabricated, which resonates at the desired 24.5 GHz and exhibits an outstanding S11 of −43.95 dB and a broad bandwidth of 4.51 GHz.