Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3+xTi4O12 ceramics via a Cu-nonstoichiometric approach

Abstract

In this work, the effects of Cu composition on the thermal stability of the dielectric and nonlinear properties of CaCu3+xTi4O12 (−0.2 ≤ x ≤ 0.2) ceramics obtained via a polymer-pyrolysis chemical process were studied. The mean grain sizes of Cu-stoichiometric (x = 0), Cu-deficient (x < 0) and Cu-excess (x > 0) CaCu3+xTi4O12 ceramics were found to be ~3.2, ~3.4 and ~3.7 μm, respectively. Interestingly, very good dielectric properties (0.020 ≤ tanδ ≤ 0.038 and 4000 ≤ ε′ ≤ 7065) were attained in CaCu3+xTi4O12 (−0.2 ≤ x ≤ 0.1, excluding x = 0.2) ceramics. Moreover, the variation of dielectric constant (ε′) within a limit of ±15% (Δε′± 15%) over a wide temperature range (TR) of −70 – 220 °C with low tanδ < 0.05 (tanδ<0.05) over a TR of −70 to 80 °C were achieved in a CaCu2.8Ti4O12 ceramic. These results suggest that this ceramic could be applicable for X9R capacitors and energy storage devices that require high thermal stability. Additionally, the nonlinear properties of Cu-nonstoichiometric ceramics could be improved when compared with those of the Cu-stoichiometric material. The incremental changes of dielectric and nonlinear properties of CaCu3+xTi4O12 (−0.2 ≤ x ≤ 0.2) ceramics revealed the significant role of Cu composition on grain boundary resistance (Rgb), which was confirmed by impedance spectroscopy analysis. In addition, XANES results revealed the proper ratios of Cu+:Cu2+ and Ti3+:Ti4+ found in these ceramics, indicating the semiconducting behavior of these grains.