Ni–Co Complex Ionic Synergistically Modifying Octahedron Lattice of Cordierite Ceramics for the Application of 5G Microwave–Millimeter-wave Antennas

In this work, phase-pure Mg_1.8(Ni_1-xCo_x)_0.2Al₄Si₅O₁₈ (0 ≤ x ≤ 1) ceramics were synthesized by a high-temperature solid-state method. On the basis of Rietveld refinement data of X-ray powder diffraction and Phillips-Vechten-Levine theory, the atomic ionicity, lattice energy, and bond energy of the compound were calculated to explore their influence on the microwave dielectric properties of ceramics. The Mg_1.8Ni_0.1Co_0.1Al₄Si₅O₁₈ (x = 0.5) ceramic exhibited the best microwave dielectric properties: ε_r = 4.44, Qf = 73 539 GHz@13 GHz, and τ_f = -23.9 ppm/°C. (Ni_1-xCo_x)²⁺ complex ionic doping, compared with only Ni²⁺ or Co²⁺, is beneficial for improving the symmetry of [Si₄Al₂O₁₈] hexagonal rings and reducing distortion. Subsequently, 8 wt % TiO₂ was added to Mg_1.8Ni_0.1Co_0.1Al₄Si₅O₁₈, resulting in a near-zero τ_f and high Qf values for the composite ceramic, with ε_r = 5.22, Qf = 58 449 GHz@13 GHz, and τ_f = -2.06 ppm/°C. Finally, a 5G millimeter-wave antenna with a central operating frequency of 25.52 GHz was designed and fabricated using the Mg_1.8Ni_0.1Co_0.1Al₄Si₅O₁₈-8 wt % TiO₂ ceramics. Operating in the 24.7-26.0 GHz range, it demonstrated favorable radiation characteristics with a simulated efficiency of 85.2% and a gain of 4.58 dBi. The antenna's performance confirms the high potential of the cordierite composite for application in 5G communication systems.