Impedance Spectroscopy of Nanocrystalline Y-Stabilized Tetragonal Zirconia
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P. MONDAL and H. HAHN Department of Materials Science, Thin Film Division, Darmstadt University of Technology, Petersenstr. 23, D-64287 Darmstadt, Germany ABSTRACT Impedance Spectroscopy, X-Ray Diffraction and High Resolution Scanning Electron Microscopy have been used to study the effect of extremely fine grain size on electrical properties such as dc-conductivity and activation energies in nanocrystalline Y-stabilized tetragonal zirconia. The samples were prepared from powders produced by the Inert Gas condensation method. X-Ray Diffraction was used to characterize phase and average grain size of the sample. In addition, grain size and microstructure of the sample was examined using High Resolution Scanning Electron Microscopy. With Impedance Spectroscopy relaxations of 0 2 -- ions in the lattice and in the grain boundaries could be resolved. The dc-conductivities of the lattice and the grain boundaries were deduced from the data. The activation energies for ac- and dc-conductivity of the lattice and grain boundary relaxation are reported. INTRODUCTION In conventional polycrystalline materials with grain sizes larger than 1 pm the fraction of atoms in the grain boundaries of the order of 10-4 or less is nearly negligible. Nanocrystalline structures exhibit extremely fine grain size and consequently large fractions of grain boundaries. Hence, the properties of these materials are expected to be dominated or at least modified by the properties of the grain boundaries [1]. It has been suggested that high angle grain boundaries in nanocrystalline materials reveal lower atomic density than in microcrystalline materials due to the different rigid body translations required to maximise the atomic density in the nanocrystalline material [1, 2], causing additional property changes. Because of its nearly pure ionic conductivity microcrystalline Y-stabilized zirconia (YSZ) is a well known material for oxygen sensors and solid oxide fuel cells for many years [3, 41. Conduction takes place by diffusion of 0 2 -- ions via 0 2 -- vacancies introduced by the substitution of Zr 4+ by Y3 + in the zirconia lattice. Impedance Spectroscopy is therefore a good method to investigate the proposed structural changes in the grain boundaries of the nanocrystalline material by the possibility of measuring the relaxation dynamics of the 0 2-- ions in lattice and grain boundaries in different temperature and frequency ranges [5]. EXPERIMENT The ZrO 2- and Y2 0 3 -powders used for the preparation of the nanocrystalline YSZ were produced by the Inert Gas Condensation Method [6]. As precursors ZrO and Y were used. The prepared powders were characterized by X-ray Diffraction (XRD) concerning the 273
Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
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20 Figure 1: Left: XRD-pattern of a nanocrystalline Y-stabilized zirconia sample with 87 % of the theoretical density after sintering at 1000 °C; right: HRSEM micrograph of a fracture surface of the same sample. phase and average
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