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: εr = 10.1 ± 0.24, Q × f = 195,000 ± 12,000 GHz, and τf = -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 τf 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.