Influence of the grain boundaries on conductivity of yttrium stabilized zirconia
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Influence of the grain boundaries on conductivity of yttrium stabilized zirconia V. Petrovsky, P. Jasinski, H.U. Anderson and T. Petrovsky University Missouri-Rolla, EMARC, USA
ABSTRACT The influence of the grain boundaries on the ionic conductivity of yttrium stabilized zirconia (YSZ) was investigated. The initially nanocrystalline samples were prepared using a tape casting process. The samples were annealed at different temperatures in the range from 1000 to 1400oC to overlap the grain size from 100nm to ~10µm and investigated using impedance spectroscopy. Two distinct semicircles were found on all YSZ samples corresponding to the influence of the grain and grain boundary on the resistance. The activation energies for both resistances are very close (1.00 and 1.03eV correspondingly). The grain resistance does not change significantly during the annealing process, but the grain boundary resistance decreases after high temperature annealing which causes a decrease in the overall resistance of the material. The calculations show that the decrease in the grain boundary resistance is connected only with the increase in the grain size and the specific grain boundary resistance (per unit surface area of grain boundary) does not change with annealing. INTRODUCTION In order to make solid oxide fuel cells (SOFCs) more competitive with other electrical power generation source, there is a need to lower the processing temperatures of the cells which results in the use of nanocrystalline materials and the development of nanocrystalline structures [1…6]. For example: 1) Processing temperatures need to be lowered for cathode supported cells to prevent chemical reactions between electrolyte and cathode materials; 2) Nanocrystalline structure of the materials is of special importance for anode and cathode reaction interlayers to ensure high effective surface area and to decrease electrode overpotentials; and 3) Application of MEMS technology for micro-SOFCs suggests reduction in the thickness of the electrolyte and, consequently, the formation of nanocrystalline structured materials. It is well known that due to the high surface areas, the electrical properties of nanocrystalline materials are influenced by grain boundaries which can effect the final characteristics of the devices [7…15], so it is important to understand these parameters. The influence of the sintering temperature on the electrical properties of YSZ has a long history of study [16...22]. Impedance spectroscopy is the obvious approach for this investigation and was used by most of the authors. It has been shown that different microstructural defects such as pores, grain boundaries, cracks and insulating inclusions influence the impedance spectra, but the behavior is different for each defect. The porosity of the material impacts both the grain and grain boundary semicircles whereas two-dimensional defects (such as grain boundaries, cracks and insulating inclusions) mainly cause an appearance of the second (so called grain boundary) semicircle. The goal of t
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