Synergistic Grain Refinement and Insulating Phase Formation via Zr4+ Doping for Low-Loss Colossal Permittivity in (In,Nb) Codoped TiO2 Ceramics

Abstract Colossal permittivity (In,Nb) codoped TiO2 ceramics are promising for microelectronic applications but suffer from high low-frequency dielectric loss primarily due to leakage currents across grain boundaries. This work demonstrates that Zr4+ doping at Ti-sites in (In0.5Nb0.5)0.05Ti0.95O2 effectively mitigates this issue. A series of (In0.5Nb0.5)0.05Ti0.95-xZrxO2 (x = 0–0.20) ceramics were synthesized via solid-state reaction. Zr4+ incorporation refines grain size from ∼56.1 μm (undoped) to ∼2.5 μm (x = 0.20) and promotes the precipitation of an insulating ZrTiO4 secondary phase at grain boundaries for compositions with x ≥ 0.10. These microstructural modifications synergistically enhance the grain boundary resistance, which peaks at 10.50 × 106 Ω·cm for the x = 0.10 composition, nearly an order of magnitude higher than the undoped ceramic. Consequently, the low-frequency dielectric loss is significantly reduced, with the x = 0.10 composition sintered at 1450 °C exhibiting a minimal loss tangent. Complex impedance analysis reveals the introduction of a new interfacial polarization associated with the ZrTiO4/INTO phase boundaries. XPS analysis confirms that Zr doping suppresses oxygen vacancy concentration without altering the Ti3+/Ti4+ ratio crucial for electron-pinned defect dipoles. The optimized composition (x = 0.10) also shows improved nonlinear coefficient (α = 2.8) and breakdown field (Eb = 530 V/cm), alongside stable low loss (tanδ < 0.05) over a broad temperature range at 1 kHz. This study presents a viable defect-engineering strategy utilizing Zr4+ doping to achieve low-loss colossal permittivity in (In,Nb) codoped TiO2-based ceramics for practical applications.