Near-Zero TCF and High Quality Factor Achieved in Li ₂ Ti _{1– x} (Fe _1/2 Nb _1/2 ) _x O ₃ -Based Microwave Dielectric Ceramics via Phase Modulation Engineering

Modern wireless communication systems put higher requirements on microwave dielectric ceramics in terms of dielectric properties, temperature stability, and low-temperature sintering process. To address the challenges associated with high-frequency signal attenuation, thermal effects, and the high-density integration of multiple-input multiple-output systems, microwave dielectric materials based on the Li 2 Ti 1– x (Fe 1/2 Nb 1/2 ) x O 3 ceramic system were designed and prepared in this study. A monoclinic (cation-ordered)-cubic (cation-disordered) biphasic composite structure was engineered via (Fe 1/2 Nb 1/2 ) 4+ solid solution-induced phase transition, enabling effective tuning toward a near-zero temperature coefficient of resonance frequency (TCF). When x = 0.05, the composite ion synergistic effect significantly suppresses the oxygen vacancy defects and achieves an optimal Q × f value of 65,320 GHz. Furthermore, different proportions of LiF (in the range of 0–8 wt %) were investigated, and the introduction of 4 wt % LiF effectively reduces the sintering temperature of the ceramic to 925 °C, meeting the requirements of the low-temperature cofired ceramic process. The optimized ceramic exhibits excellent dielectric properties (ε r = 22.3, Q × f = 70,300 GHz (at 7.32 GHz), TCF = −6.1 ppm/°C) and strong compatibility with silver electrodes. A prototype microstrip patch antenna designed using this ceramic for the BeiDou B1C band (operating at 1.575 GHz) demonstrated high simulated radiation efficiency (96.1%) and gain (4.0 dBi). This study represents significant advancements in TCF precision tuning and low-temperature sintering, offering innovative material solutions for 5G/6G communication devices.