Impedance spectroscopy of grain boundaries in nanophase ZnO
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J. -H. Hwang, J. J. Mashek, and T. O. Mason Department of Materials Science, Northwestern University, Evanston, Illinois 60208
A. E. Miller Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556
R. W. Siegel Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 6 September 1994; accepted 17 April 1995)
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 -l 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 nonlinear I-V behavior, 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.
I. INTRODUCTION Nanophase materials have received considerable attention since the pioneering work of Gleiter and coworkers.1"3 Much of the work since then has focused on nanophase metals; however, Siegel et al.4~6 and others7 have successfully prepared nanophase ceramics by the gas-condensation method.8^10 Like their metallic counterparts, nanophase ceramic powders possess distinct advantages including (i) enhanced sinterability at lower temperatures resulting from high driving forces and short diffusion distances,4-7 (ii) high surface (and grain boundary) purity due to the relative cleanliness of the gas-condensation technique, and (iii) when consolidated, potentially superior (or at least different) properties from corresponding microphase materials, owing to the fact that a large fraction of ions reside in interfaces.11 In particular, superplastic deformation of nanophase ceramics has been demonstrated at elevated temperatures,12 albeit with significant concurrent grain growth. There have been few reports of the electrical properties of nanophase materials. Huang et al.13 measured higher resistivities and a larger temperature dependence thereof in nanophase copper compacts, as compared to conventional bulk copper. These results were explained on the basis of increased electron scattering at grain boundaries. It has been demonstrated that the local structure, chemistry, and properties of surfaces and grain boundaries in ceramics can be significantly J. Mater. Res., Vol. 10, No. 9, Sep 1995
different from those of the bulk. (See Ref. 14 and the references therein.) It is therefore of interest to examine the electrical behavior of nanophase ceramics, where a large fraction of ions reside at or within a few atomic layers of the surfaces or grain boundaries. Zinc oxide, the material selected for the present study, is technologically important as the main constituent of vo
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