Enhanced microwave dielectric properties of spinel-garnet ceramics based on MgO-Al2O3-Y2O3 ternary systems: A study on solid solutions and composite synergistic strategies

High-performance dielectric ceramics are vital for next-generation microwave devices, requiring advanced doping and composite strategies to enhance material properties. The (1-x)MgAl₂O₄-xY₃Al₅O₁₂ composite ceramics were found in the MgO-Al₂O₃-Y₂O₃ pseudoternary phase diagram and synthesized via solid-phase reaction. The two phases coexist to improve grain size distribution and achieve a low eutectic temperature, allowing x = 0.1–0.7 samples to gain a dense microstructure (ρ > 96 %) when sintered at 1650–1670 °C. The semi-coherent interface, coupled with the Mg²⁺/Y³⁺ cation transition zone, promotes the formation of a partial solid solution, thereby alleviating interfacial stress at the grain boundaries between MgAl₂O₄ and Y₃Al₅O₁₂. Notably, the 0.3MgAl₂O₄-0.7Y₃Al₅O₁₂ ceramic exhibits satisfying microwave dielectric properties: ε_r = 10.1 ± 0.24, Q × f = 195,000 ± 12,000 GHz, and τ_f = −11.9 ppm/°C. In addition to phase composition and microstructure, the enhanced performance can be attributed to the optimized crystal structure. The partial substitution of Y³⁺ by Mg²⁺ with a smaller radius induced the “rattling” effect and abnormal large permittivity in the [YO₈] site. The ²⁷Al NMR spectra revealed an increase in the Mg-Al inversion degree in the Mg-deficient spinel structure, resulting in the elevated total covalency of the Al-O bond. The structural changes corresponded to a decrease in intrinsic dielectric loss in Mg-Al spinel and an increase in τ_f in the Y-Al garnet phase. These findings indicate the (1-x)MgAl₂O₄-xY₃Al₅O₁₂ ceramics have promising applications in the field of HTCC millimeter wave communications.