Evaluating the effect of A- and B-site cobalt doping on the structural, morphological, dielectric, and non-ohmic properties of CaCu3Ti4O12 ceramics prepared by the hydrothermal method

Abstract

CaCu₃Ti₄O₁₂ (CCTO), CaCu_3-xCo_xTi₄O₁₂ (CCC_xTO) and CaCu₃Ti_4-xCo_xO₁₂ (CCTC_xO) ceramics with x = 0.1 were synthesized by the hydrothermal process at 200 °C for 24 h. The influence of cobalt substitution on the copper and titanium sites in CaCu₃Ti₄O₁₂ on the structural, morphological, and physical properties was investigated. It was shown through the analysis of X-ray diffractograms of CCTO, CCC_xTO, and CCTC_xO compounds that they crystallized in a pure perovskite structure without the presence of secondary phases. The refinement of the spectra using the Rietveld method showed an efficient formation of the crystalline phase of the cubic structure (Im$\overline{3}$), which remains unchanged, with an increase in the unit cell due to the substitution of Co²⁺/Co³⁺ in the Cu²⁺ and Ti⁴⁺ sites of the CCTO ceramic. Raman spectroscopy was used as a complementary characterization method to XRD in order to detect vibrational bands and highlight any changes in the crystal lattice. SEM results showed that cobalt insertion increased the average grain size. The dielectric properties were studied by complex impedance spectroscopy in a frequency range from 1 kHz to 1 MHz at different temperatures, where the insertion of cobalt in the Cu²⁺ and Ti⁴⁺ sites has a significant effect on the permittivity value (ε_r) and dielectric losses (tanδ). The non-ohmic characteristics showed that the change in grain size due to cobalt incorporation is beneficial to improving the breakdown strength (E_b) and nonlinear coefficient (α), which can be attributed to the grain boundary properties of the Internal Barrier Layer Capacitor (IBLC) model and the behavior of the Schottky barrier.