Defect-mediated Q×f degradation and near-zero temperature coefficient of resonant frequency in CeO2-SrTiO3 microwave dielectric ceramics

(1- x )CeO 2 -xSrTiO 3 ( x = 0.04-0.10) ceramics were prepared by the conventional solid-state reaction method, and their phase constitution, microstructure, defect behavior, and microwave dielectric properties were systematically investigated. XRD combined with Rietveld refinement confirmed a two-phase composite of cubic CeO 2 and SrTiO 3 , and EDS mapping provided direct evidence of Sr-Ce cation interdiffusion across the phase boundaries. The relative density and ε r increased with both SrTiO 3 content and sintering temperature. For the x = 0.04 composition sintered at 1550 °C, a near-zero τ f (+6.5 ppm/°C), ε r ≈ 25.21, and a maximum Q×f (∼20,540 GHz) were obtained. However, increasing the sintering temperature to 1600 °C increased the relative density to ∼95.7% and the mean grain size from 2.98 to 4.15 μm but reduced Q×f to ∼13,588 GHz, indicating that the variation of Q×f does not follow the density trend. EPR, XPS and ac conductivity analyses showed that oxygen-vacancy-related defects and grain-boundary barrier-layer conduction contribute significantly to the Q×f degradation, whereas the increasing SrTiO 3 phase fraction, its intrinsically low Q×f , residual porosity and grain size also play a role. This work identifies defect migration and interfacial electrical heterogeneity as important contributors to the Q×f variation in CeO 2 -SrTiO 3 composite ceramics.