AC Conductivity Based Microstructural Characterization of Yttria Stabilized Zirconia
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AC Conductivity Based Microstructural Characterization of Yttria Stabilized Zirconia G. Sauti and D.S. McLachlan School of Physics and Material Physics Research Institute, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa ABSTRACT AC conductivity experiments, or Impedance Spectroscopy, at temperatures between 100 and 400°C, made on 8 mole % Yttria Stabilized Zirconia (8YSZ) with different grain sizes, are presented. The results are analyzed using Effective Media Theories, the Brick Layer Model and percolation theory. The results obtained are satisfactory, only if the measured frequency dependant complex conductivity parameters of the grains are inserted in the equations. INTRODUCTION Impedance spectroscopy is routinely used to measure the electrical transport properties of composite electro-ceramics, and to characterize their microstructures. The usual way of analyzing the results is to use an equivalent circuit analysis but this does not directly give the complex conductivity, (dielectric constant) of the grains ( σ h ), the grain boundaries ( σ l ) and the volume fraction of grains (φ). Effective Media Theories (EMT) and the Bricklayer Model (BLM) enable one to arrive at these parameters, but a detailed comparative study using different grain sizes and impurity concentrations have not yet been made. This is partially rectified in the current paper and the possibility of using percolation theory is also examined. In order to understand the implications of fitting the results to the Maxwell-Wagner (HashinShtrikman Boundaries) Theory (MW) also known as the Maxwell-Garnet Theory [1,2], the BLM [3] and percolation theory [2] and understand the resulting parameters the underlying microstructures, which are given in figure 1, must be understood. As applied to Yttria Stabilized Zirconia (YSZ) and many other ceramics the MW microstructure (figure 1a) consists of an infinite array of space filling YSZ spheres (the grains, white), coated with a uniform boundary layer of more electrically resistive inter-granular material (black). The BLM microstructure (figure1b) consists of cubic bricks (YSZ grains, white) interspersed with a uniform layer of cement (inter-granular) material (gray). The percolation microstructure (figure1c), for sintered YSZ, can be visualized as YSZ grains covered by strings of dislocations (black dots), which form the grain boundaries. As the volume fraction (1-φ) of grain boundaries is reduced the intergrain boundaries, lying between the grains, will vanish first leaving grain boundary material along the triple grain edges and the corners. When sufficient inter granular boundaries have vanished, an inter-granular percolation path, which passes through no insulating grain boundary barriers, should span the sample at the critical volume fraction (φc). The Maxwell-Wagner equation, where σ m is the complex medium conductivity is: (σ m − σ l ) (σ m + 2σ l ) = φ (σ h − σ l ) (σ h + 2 σ l ) R8.6.1
(1)
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b
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Figures 1a-c illustrate the microstructures described by the Ma
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