Ultralow-Permittivity and Temperature-Stable Ba_1–xCa_xMg₂Al₆Si₉O₃₀ Dielectric Ceramics for C-Band Patch Antenna Applications

Ca-substituted Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 ceramics were prepared to explore the relationships among their crystal structural parameters, phase compositions, dielectric properties, and coefficients of thermal expansion and applications in C-band antenna. The maximum solubility of Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 was located at x = 0.25, and Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 ceramics (0 ≤ x ≤ 0.25) crystallized in the space group P 6/ mcc . In Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 single-phase ceramics, ε r was dominated by ionic polarizability and “rattling effects” of Ba 2+ and Al(2) 3+; Q × f was controlled by the roundness of [Si 4 Al 2 O 18 ] inner rings and total lattice energy; and τ f was affected by the bond valence of Si/Al(1)–O(1). Notably, the low average coefficients of thermal expansion (2.668 ppm/°C) at −150 °C ≤ T ≤ 850 °C and near-zero coefficients of thermal expansion (1.254 ppm/°C) at −150 °C ≤ T ≤ 260 °C were achieved for the Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 ( x = 0.1) ceramic. Optimum microwave and terahertz dielectric properties were obtained for the Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 ( x = 0.1) ceramic with ε r = 5.80, Q × f = 31,174 at 13.99 GHz, τ f = −7.10 ppm/°C, and ε r = 5.71–5.85 at 0.2 THz ≤ f ≤ 1.0 THz. Also, the Ba 1– x Ca x Mg 2 Al 6 Si 9 O 30 ( x = 0.1) ceramic substrate had been designed as a C-band patch antenna with a high simulated radiation efficiency (87.76%) and gain (6.30 dBi) at 7.70 GHz (|S 11 | = −38.41 dB).