Introduction and Objectives: Cordierite ceramics (Mg₂Al₄Si₅O₁₈) are recognized as advanced dielectric materials due to their low dielectric constant, very low dielectric loss, high thermal stability, and optimal performance in microwave and millimeter-wave frequency ranges. These materials are utilized in fifth- and sixth-generation (5G and 6G) communication systems. This study investigates the simultaneous substitution effects of Mn⁴⁺ and Ni²⁺ cations for Al³⁺ on the densification, crystalline structure, microstructure, mixing entropy (ΔS_mixing), and dielectric properties of cordierite ceramics.
Materials and Methods: Samples with different substitution levels (0, 0.25, and 0.50) were prepared by the solid-state synthesis method and subsequently sintered at various temperatures. Phase and microstructural analyses were carried out using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. A network analyzer was employed to measure the dielectric properties at microwave frequencies. In addition, the mixing entropy and lattice distortion were measured, analyzed, and discussed.
Results: It was observed that substituting Mn⁴⁺ and Ni²⁺ for Al³⁺ results in achieving the maximum density at a temperature about 100 °C lower than that of pure cordierite. X-ray diffraction and Raman spectroscopy analyses indicated that a complete solid solution is formed at x=0.25, whereas increasing x to 0.50 results in the appearance of a secondary phase, Mg_1.16Mn_0.84Si₂O₆, in addition to the cordierite phase. Microstructural observations also revealed that the average grain size increases with higher substitution levels. The optimal microwave dielectric properties were obtained for Mg₂Al_3.75(Mn⁴⁺Ni⁴⁺)_0.25Si₅O₁₈, with εᵣ= 4.58, Q×f =107333 GHz, and τ_f = –14.7 ppm/°C, while the value of ΔS_mixing was measured to be 2.30 J/mol · K.
Conclusion: Controlled substitution of Mn⁴⁺ and Ni²⁺ at an appropriate sintering temperature enables simultaneous optimization of dielectric properties and thermal stability.