Superior dielectric and varistor properties of ZnO or SnO2 diffused calcium copper titanate ceramics

Sub-micrometer-sized (0.33 mu m) polycrystalline powders of calcium copper titanate (CCTO) were synthesized via the solgel method. Compacted calcined powders were sintered at 900 degrees C/2h to obtain porous (< 80% density) pellets. Subsequently, ZnO or SnO2 pastes (similar to 10 mu m grain size) were smeared on either side of the pellets and sintered at 1100 degrees C/15 h, which yielded dense (> 95% density) ceramics. X-ray structural investigations revealed the presence of ZnO or SnO2 traces in the bulk of the CCTO ceramics. Scanning electron microscopy and energy dispersive spectroscopic studies confirmed the diffusion and the segregation of these oxides at the grain boundaries. The dielectric and varistor properties of the ZnO or SnO2 diffused samples were found to be superior to that of pristine CCTO ceramics. For instance, the dielectric constant (epsilon ' ) of ZnO diffused ceramics exhibited a value as high as 2.4 x 10(4) (1 kHz, at room temperature) and a dielectric loss (D) of 0.059. Similarly, SnO2 diffused ceramics exhibited a dielectric constant of 2.7 x 10(4) (1 kHz, at room temperature) associated with a dielectric loss of 0.047. The figure of merit of varistor performance, i.e., nonlinear coefficient (alpha = 10.6), of the SnO2 diffused sample is significantly higher than that of ZnO diffused (alpha = 7.4) and pristine CCTO (alpha = 4.5) ceramics. The dielectric data obtained for both the pristine and ZnO or SnO2 diffused ceramics were rationalized by invoking Cole-Cole analysis. The thermal activation energy was estimated from the temperature-dependent dielectric data besides current (I)-voltage (V) characteristics. Equivalent circuit modeling of the Nyquist plots demonstrated that the inclusion of ZnO and SnO2 layers in CCTO ceramics remarkably improved the grain boundary resistance (Rgb) by 5 fold and 20-fold, respectively, which resulted in making CCTO a better dielectric. This methodology of fabricating ceramics via interfacial engineering could pave the way for obtaining superior CCTO ceramics associated with exotic functional properties.