Low‐Loss Colossal‐Permittivity La‐Doped SrTiO ₃ via CuBi ₂ O ₄ Grain‐Boundary Modification

Base‐metal electrode (BME, Ni) co‐firing requires low oxygen partial pressure and sintering temperatures below 1350°C, conditions under which most colossal‐permittivity (CP) ceramics suffer from excessive loss or poor insulation resistance (IR). Here, a grain and grain‐boundary (GB) co‐design strategy is demonstrated for La‐doped SrTiO 3 (LST) using CuBi 2 O 4 (CB) as a transient‐liquid GB engineering aid. CB addition (0–3 wt%) enables the formation of dense ceramics at 1300°C, with pronounced grain refinement and improved re‐oxidation behavior. Structural analysis confirms a single‐phase cubic perovskite, while XPS reveals suppressed Ti 3 + and enhanced lattice/adsorbed oxygen, indicating strengthened GB re‐oxidation. Mott–Schottky and impedance analyses show that CB addition enhances back‐to‐back Schottky barriers at GBs, leading to increased barrier height, widened depletion layers, and elevated GB resistance, with the strongest effect observed at 3 wt% CB. As a result, CP‐level dielectric constants (∼10 4 ) are retained up to 1 MHz with suppressed dielectric loss and markedly improved IR; at 1 kHz, LST‐1 wt% CB achieves ε ′ ≈ 8.76 × 10 4 with tan δ ≈ 0.014. All compositions meet the X9P temperature‐stability requirements, and the optimized composition meets the X9D requirements. These results demonstrate a BME‐compatible pathway that combines low sintering temperature, high dielectric constant, low loss, and robust IR, offering practical potential to reduce MLCC layer count or footprint under Ni co‐firing conditions.