Confirming phase transitions and achieving excellent microwave dielectric properties in barium silicate ceramics and their LTCC materials for application in dielectric antennas

The phase transition and the evolution of microwave dielectric properties in barium silicate ceramics were determined for x Ba₅Si₈O₂₁- (1-x) BaSi₂O₅ (0.13 ≤ x ≤ 0.45) system. It was found that a high sintering temperature (≥ 1285 °C) and a near-equimolar ratio could promote the transformation of Ba₅Si₈O₂₁ and BaSi₂O₅ phases into the Ba₃Si₅O₁₃ phase. Notably, x Ba₅Si₈O₂₁- (1-x) BaSi₂O₅ (x = 0.35) ceramic sintered at 1270 °C (comprising 71 wt% of Ba₅Si₈O₂₁ and 29 wt% of BaSi₂O₅) exhibited a two-phase composition with high relative density (> 97%) and excellent microwave dielectric properties: ε_r = 7.72, Q×f = 25410 GHz and τ_f = 1.45 ppm/°C. Furthermore, the sintering temperature of the x Ba₅Si₈O₂₁- (1-x) BaSi₂O₅ (x = 0.3₅) ceramic was reduced to 850 °C by adding 3 wt% LiF as a sintering aid. The resulting ceramic demonstrated moderate microwave dielectric properties and good chemical compatibility with Ag electrodes. The LiF sintering aid had a significant effect on the phase compositions of x Ba₅Si₈O₂₁- (1-x) BaSi₂O₅ system, indicating that stable Ba₂Si₃O₈ phase could be induced by adding LiF sintering aids. A thermally stable 0.35 Ba₅Si₈O₂₁-0.6₅ BaSi₂O₅-3 wt% LiF low-temperature co-fired ceramic (LTCC) was successfully synthesized. This material was further engineered into a dielectric patch antenna and a cylindrical resonator antenna. The dielectric resonator antenna, operating at a center frequency of 7.79 GHz, exhibited outstanding performance with a simulated radiation efficiency of 97.5% and a gain of 6.34 dBi. These results highlight the potential of barium silicate ceramics for X-band communication applications, owing to its low dielectric loss and excellent thermal stability.