Correlating grain-boundary electrical characteristics and mixed Cu valence states with dielectric and nonlinear electrical properties in Sr-substituted Ca1–1.5xSrxCu3Ti4.5O12 ceramics

Establishing the correlation between grain-boundary electrical characteristics, Cu valence states, and electrical performance is essential for understanding the transport mechanism in CaCu 3 Ti 4 O 12 (CCTO) ceramics. However, this relationship has not yet been clarified for nominal Ca 1-1.5x Sr x Cu 3 Ti 4.5 O 12 ceramics. In this work, Ca 1-1.5x Sr x Cu 3 Ti 4.5 O 12 ceramics (x = 0.03, 0.10, and 0.20) were synthesized via a polymer pyrolysis method and sintered at 1060 °C for 6 and 10 h. X-ray diffraction and Rietveld refinement identified the CCTO-related phase as the dominant crystalline phase together with a composition-dependent TiO 2 secondary phase. Sr substitution significantly modified the microstructure, grain-boundary resistance (R gb ), grain-boundary activation energy (E gb ), Schottky barrier height (Φ B ), and Cu oxidation-state distribution, resulting in distinct dielectric and nonlinear electrical responses. Among the investigated ceramics, Sr20-6h exhibited ε′ = 27,320, tan δ = 0.083, Δε′ ≤ ±15% over -40 to 110 °C, R gb = 520.0 kΩ.cm, E gb = 0.760 ± 0.001 eV, α = 5.61, and E b = 1859.40 V·cm⁻¹ . High-resolution XPS revealed the coexistence of Cu + , Cu 2+ , and Cu 3+ , confirming mixed-valence Cu states associated with semiconducting-grain behavior. Correlating the impedance, Schottky, and XPS analyses demonstrates that the electrical performance is closely associated with the synergistic regulation of grain-boundary electrical characteristics and mixed Cu valence states induced by Sr substitution. These findings provide new insights into the transport mechanism and grain-boundary engineering of CCTO ceramics for dielectric and nonlinear electrical applications.