The influence of the TEOS (Si(OC 2 H 5 ) 4 ) addition, which decomposes into SiO 2 at high temperature, on the sintering process, morphology, and microwave dielectric properties in Y 3 Al 5 O 12 (YAG) ceramics were investigated in this work. The Y 3 Al 5 O 12 + x wt.% TEOS (0≤ x ≤ 1) ceramics were synthesized through solid-state reaction. The XRD results indicated that all YAG samples possessed a pure cubic garnet structure. The dense, pore-free crystalline microstructure of the specimen was measured by SEM. The Rietveld method was employed to refine the lattice parameters of YAG samples. Results showed that the samples' cellular parameters decreased with increasing amounts of TEOS. The addition of TEOS lowered the sintering temperature and improved the Q × f values of Y 3 Al 5 O 12 ceramics. The YAG system was added by 0.5 wt% TEOS can be exhibited excellent microwave dielectric properties with ε r =10.82, Q × f = 229,423 GHz, τ f = -29.1 ppm/°C sintering at 1670 °C for 12 h.
High-performance dielectric ceramics are vital for next-generation microwave devices, requiring advanced doping and composite strategies to enhance material properties. The (1- x )MgAl 2 O 4 - x Y 3 Al 5 O 12 composite ceramics were found in the MgO-Al 2 O 3 -Y 2 O 3 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 Mg 2+ /Y 3+ cation transition zone, promotes the formation of a partial solid solution, thereby alleviating interfacial stress at the grain boundaries between MgAl 2 O 4 and Y 3 Al 5 O 12 . Notably, the 0.3MgAl 2 O 4 -0.7Y 3 Al 5 O 12 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 Y 3+ by Mg 2+ with a smaller radius induced the “rattling” effect and abnormal large permittivity in the [YO 8 ] site. The 27 Al 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 )MgAl 2 O 4 - x Y 3 Al 5 O 12 ceramics have promising applications in the field of HTCC millimeter wave communications.
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