Abstract
Na0.5Bi0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were synthesized by mechanochemical milling followed by spark plasma sintering (SPS) at 800–900°C. X-ray diffraction confirmed the cubic perovskite structure, with minor CuO traces at lower sintering temperatures and phase-pure NBCTO at 900°C. Scanning electron microscope and energy-dispersive X-ray spectroscopy revealed dense, fine-grained microstructures with average grain sizes below 400 nm, significantly smaller than those observed in conventionally sintered samples. All SPS-NBCTO ceramics exhibited colossal dielectric permittivity (ε′ > 10⁴) across wide frequency and temperature ranges, with values of 2.8–4.0 × 10⁴ at 1 kHz and room temperature. Impedance analysis revealed an internal barrier layer capacitance mechanism, with semiconducting grains and more resistive grain boundaries. Grain conductivity (∼3 × 10⁻² S/cm) and activation energies (0.089–0.096 eV) were consistent with CaCu₃Ti₄O₁₂-based systems, while grain boundary conductivity was unexpectedly high (2.5–3.5 × 10⁻³ S/cm) with relatively low activation energies (0.192–0.211 eV). These properties, linked to oxygen-vacancy-related conduction, led to unusually large dielectric losses. Relaxation analysis using the electric modulus confirmed thermally activated grain and grain boundary processes with activation energies matching conductivity values. The results demonstrate that SPS enables fine-grained, dense NBCTO ceramics with colossal dielectric constants, though excessive grain boundary conduction remains a challenge.