Exceptional dielectric and varistor properties of Sr, Zn and Sn co-doped calcium copper titanate ceramics

Calcium copper titanate (CCTO) powders associated with the chemical formula Ca_1- Sr Cu_3- Zn Ti_4- Sn O₁₂ (where x, y, z varying from 0 to 0.1) were synthesized via a solid-state reaction route. Dense ceramics (>96% of theoretical density) were obtained by sintering these powders comprising micrometer-sized grains at appropriate temperatures. X-ray powder diffraction studies confirmed the formation of monophasic CCTO cubic phase, with no traceable secondary phases present. The lattice parameter 'a' was found to increase on increasing the dopant concentration. The microstructural studies performed on these ceramics confirmed a decrease in mean grain size (18 μm to 5 μm) with the increase in Sr, Zn and Sn doping concentrations as compared to that of undoped CCTO ceramics though they were sintered at the same temperature and duration (1100 °C/15 h). The dielectric studies (dielectric constant (ε') and the dielectric loss (D)) conducted in a wide frequency range (10²-10⁷ Hz) demonstrated an increase in ε' and a decrease in D on increasing the doping concentration. Impedance analysis (Nyquist plots) performed on these ceramics revealed a significant increase in grain boundary resistance. The highest value of grain boundary resistance (6.05 × 10⁸ Ω) (in fact this value was 100 times higher than that of pure CCTO) was obtained for the composition corresponding to x = y = z = 0.075 and intriguingly the ceramic pertaining to this composition exhibited enhanced ε' (1.7 × 10⁴) and lower D (0.024) at 1 kHz. Further, these co-doped CCTO ceramics exhibited substantial improvement in breakdown voltages and nonlinear coefficients (α). The temperature independent (30 -210 °C) dielectric response of these samples qualifies them to be suitable dielectric materials for the fabrication of multilayer ceramic chip capacitors.