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

Ca-substituted Ba_1-xCa_xMg₂Al₆Si₉O₃₀ 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-xCa_xMg₂Al₆Si₉O₃₀ was located at x = 0.25, and Ba_1-xCa_xMg₂Al₆Si₉O₃₀ ceramics (0 ≤ x ≤ 0.25) crystallized in the space group P6/mcc. In Ba_1-xCa_xMg₂Al₆Si₉O₃₀ single-phase ceramics, ε_r was dominated by ionic polarizability and "rattling effects" of Ba²⁺ and Al(2)³⁺; Q × f was controlled by the roundness of [Si₄Al₂O₁₈] 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-xCa_xMg₂Al₆Si₉O₃₀ (x = 0.1) ceramic. Optimum microwave and terahertz dielectric properties were obtained for the Ba_1-xCa_xMg₂Al₆Si₉O₃₀ (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-xCa_xMg₂Al₆Si₉O₃₀ (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 (|S₁₁| = -38.41 dB).