Abstract
CaCu3Ti4-x((A0.05Nb0.05))xO12 ceramics (A: Al and Bi; x = 0, 0.3) were synthesized by high-energy mechanical ball milling and reactive sintering at 1050 °C in air. Rietveld refinement of XRD data revealed the pure and (Al3+, Nb5+) cosubstituted ceramics contained a minor CuO secondary phase with a mole fraction of about 3.2% and 6.9%, respectively, along with a CaCu3Ti4O12 (CCTO)-like cubic structure. In addition, (Bi3+, Nb5+) cosubstituted ceramics had a pyrochlore (Ca2(Ti, Nb)2O7) secondary phase of about 18%. While the (Al3+, Nb5+) cosubstituted CCTO showed the highest relative permittivity (ε’ = 3.9 × 104), pure CCTO showed the lowest dielectric loss (tanδ = 0.023) at 1 kHz and 300 K. Impedance-spectroscopy (IS) measurements showed an electrically heterogeneous structure for the studied ceramics, where a semiconducting grain was surrounded by highly resistive grain boundary. The giant relative permittivity of the ceramics was attributed to the Maxwell–Wagner polarization effect at the blocking grain boundaries and domain boundaries. The higher tanδ of the cosubstituted samples was correlated with their lower grain boundary’s resistivity, as confirmed by IS analysis. Modulus-spectrum analysis revealed two relaxation processes for the pure and (Bi3+, Nb5+) cosubstituted CCTO samples. Dissimilar behavior was observed for the (Al3+, Nb5+) cosubstituted CCTO, where three relaxation mechanisms were observed and attributed to the grain, domain-boundary, and grain-boundary responses.
Highlights
Materials with colossal relative permittivity (ε’ > 103 ) are important for numerous energy-storage-related applications
Considering this polycrystalline structure for CCTO, the model of the internal barrier layer capacitor (IBLC) was proposed by Sinclair et al [3] and was successfully used in the literature to interpret the dielectric behavior of CCTO
We investigated the structural and dielectric properties of cosubstituted CCTO
Summary
Materials with colossal relative permittivity (ε’ > 103 ) are important for numerous energy-storage-related applications. Several studies evidenced an electrically inhomogeneous structure for CCTO and its related materials, i.e., semiconductor grains surrounded by insulating grain boundaries [2] Considering this polycrystalline structure for CCTO, the model of the internal barrier layer capacitor (IBLC) was proposed by Sinclair et al [3] and was successfully used in the literature to interpret the dielectric behavior of CCTO. According to the IBLC model, the colossal permittivity of the ceramics is a result of the internal capacitances that form due to the accumulation of charge carriers at the internal resistive boundaries of the tested sample [3] Considering this structure, the static relative permittivity ε0s of IBLC ceramics depends on the thickness of the grain boundary The dielectric properties of the prepared ceramics were studied in a wide range of frequencies (1 Hz–10 MHz) and temperatures (120–400 K)
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