Colossal permittivity and low losses in Ba_1-xSr_xTiO_3-δ reduced nanoceramics

The oxalate route offers a controlled approach to synthesize pure Ba_1-xSr_xTiO₃ (BST) (0≤x≤1) nanoparticles (φ≈150nm in diameter). Reduced BST dense nanoceramics were obtained by spark plasma sintering (SPS) and then annealed for a short time to reach colossal permittivity (ε'r=10⁵) with low dielectric losses (tan δ=0.03) at 1kHz and 300K. The effects of Ba-Sr substitution on structural, microstructural and electrical properties were analyzed. Comprehensive analysis of the electrical properties indicates that polaron hopping, mediated by Ti³⁺ ions and oxygen vacancies is the main contributing mechanism to colossal permittivity in Ba-rich BST compounds. Substitution of Ba by Sr reduced the contribution of polaron hopping and led to a decrease of real and imaginary parts of permittivity, while preserving interfacial polarization and yielding better temperature stability. The lowest temperature coefficient of capacitance, or TCC (variation of capacitance between 310K and 450K) value, i.e., 44ppmK^-1, is obtained for SrTiO₃.