Phase composition, crystal structure, and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y3-xAl5NbxO12+x (0 <= x <= 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 <= x <= 0.1. Further increments in x prompted the precipitation of the YNbO4 secondary phase at the grain boundary of Y3Al5O12. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Qxf values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb5+ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of epsilon(r) similar to 10.99, Qxf similar to 280,387 GHz (13.5 GHz), and tau(f) similar to - 34.7 ppm/degrees C can be obtained in x = 0.1 (Y2.9Al5Nb0.1O12.1) sintered at 1700 degrees C for 6 h.