Normalized dielectric constants for as-deposited and annealed samples under different frequencies. Frequencies: 100Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. The grain size of the annealed sample (9.55 nm) is larger than the as-deposited sample (8.83 nm), of which the grain size values are extracted from the XRD data (Figure 2). It is clear that dielectric relaxation for the as-deposited sample (triangle symbol) is much worse than that of the annealed one (square symbol). The Cole-Davidson fitting data are represented by solid lines. Normalized dielectric constants for the CaCu3TiO12 (CCTO) samples  under different frequencies (100Hz, 1 kHz, 10 kHz, and 100 kHz) are given in the inset as supporting evidence. Similar to CeO2, dielectric relaxation for the medium-grain-size CCTO sample is superior to the small sample within the entire frequency range. Moreover, the large-grain-size sample is better than the medium one in terms of dielectric relaxation. Therefore, grain size makes a significant impact on dielectric relaxation.
Zhao et al. Nanoscale Research Letters 2013 8:172 doi:10.1186/1556-276X-8-172