Realistic dielectric response of high temperature sintered ZnO ceramic: a microscopic and spectroscopic approach

High temperature sintering (1200-1400 °C) has been performed on ZnO ceramics. An X-ray Absorption Fine Structure (XAFS) study shows that high sintering temperature introduces a constant amount of V_O and V_Zn defects without any significant effect on the crystal or electronic structure of Wurtzite ZnO. The combined effects of grain boundaries and voids are considered responsible for the apparent colossal dielectric constant (ε') > 10⁴ at low frequency (∼10² Hz) for all the sintered ZnO ceramics. The superior contact among grains of the ZnO-1200 sample enhances both the interfacial and orientational polarization of the Zn²⁺-V_O dipoles, which results in the increase of low and high frequency dielectric constants (ε') and the corresponding dielectric loss (tan δ) also increases. On the other hand, high temperature sintering of ZnO at 1300 °C and 1400 °C introduces voids at the expense of reduced grain and grain boundary contact areas, thus affecting both the interfacial and orientational polarization with corresponding reduction of dielectric constant (ε') and dielectric loss. Orientational polarizations due to Zn²⁺-V_O dipoles are suggested to remain fixed and it is the microstructure which controls the dielectric properties of high temperature sintered ZnO ceramics.