Impedance spectroscopy of grain boundaries in nanophase ZnO

Sintered compacts of nanophase ZnO (∼60 nm average grain size, presintered at 600 °C) were made from powders (∼13 nm) prepared by the gas-condensation technique. Impedance spectra were taken as a function of temperature over the range 450–600 °C and as a function of oxygen partial pressure over the range 10⁻³−1 atm (550 and 600 °C only). The activation energy was determined to be 55 kJ/mole (0.57 eV) and was independent of oxygen partial pressure. The oxygen partial pressure exponent was −1/6. Impedance spectra exhibited nonlinearI-Vbehavior, with a threshold of approximately 6 V. These results indicate that grain boundaries are governing the electrical properties of the compact. Ramifications for oxygen sensing and for grain boundary defect characterization are discussed.