Effect of Ge4+-substituted on the structure characteristics and microwave/terahertz dielectric properties of ultra-low εr, high Q·f cordierite ceramics

In this study, Ge⁴⁺-substituted cordierite, Mg₂Al₄(Si_1–xGe_x)₅O₁₈ ceramics, were successfully prepared by the traditional solid-state method to broaden its application potential. Notably, the excellent dielectric properties with ε_r = 4.90, Q·f = 128,200 GHz, and τ_f = −21.01 ppm °C^–1 were achieved. The increase in ε_r value is mainly due to the heightened content of Ge⁴⁺ with high polarizability. The Q·f value improved by 2.21 times compared to the cordierite matrix, which can be primarily attributed to enhanced lattice energy, bond covalency, and hexagonal ring symmetry. The alteration in τ_f value arises from the variation of bond energy, bond strength, and distortion in the [MgO₆] octahedra. These conclusions provide valuable insights for the design of silicate ceramics with higher Q·f values. In addition, the dielectric properties in the microwave and terahertz bands were compared. The higher Q·f and lower ε_r values in the terahertz band mainly result from the withdrawal of partial polarization mechanisms and differences in measurement methods. Mg₂Al₄(Si_0.92Ge_0.08)₅O₁₈ ceramics, demonstrating an ultra-low ε_r value of 4.54 and an ultra-high Q·f value of 286,533 GHz in the terahertz band, emerge as formidable contenders for future terahertz communications materials. Finally, a microstrip patch antenna was fabricated, achieving a bandwidth of 150 MHz at 4.78 GHz, which confirms the application in the n79 band for wireless communication.