Structural behavior of zirconia thin films with different level of yttrium content
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Structural behavior of zirconia thin films with different level of yttrium content V. Petrovsky, H.U. Anderson, T. Petrovsky, and E. Bohannan Electronic Materials Applied Research Center, University Missouri-Rolla, Rolla, MO, USA ABSTRACT The preparation of dense, high conductive electrolyte layers is important for the development of intermediate temperature solid oxide fuel cells and other devices based on oxygen-ion conductivity. Thus a number of techniques have been used to produce these structures. This study makes use of one of these methods to produce dense nanocrystalline 0.1 to 1 micron layers of zirconia. Polymeric precursors were used to prepare zirconia films with different level of yttrium substitution. The films were annealed at a series of temperatures in the range of 400 to 1000oC and were characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that initially (after 400oC annealing) the films had cubic structure and grain size of ~5 nm regardless of Y content. The situation changed when the annealing temperature was increased. Y (16mol %) stabilized zirconia (YSZ) did remain cubic over the entire temperature region investigated (up to 1000oC), but for compositions with lower Y content changes in crystal structure occurred. The samples with 4 and 8mol% Y transformed to the tetragonal phase at about 700oC, and undoped zirconia became monoclinic at the same temperature. The results were compared with sintered zirconia and it was shown that the behavior of thin films is quite similar to that of the sintered material, if the annealing temperature was high enough (> 700oC). The main differences between polymeric prepared films and sintered material are the existence of the cubic structure at low temperatures (< 600oC) and lower transition temperatures to the high temperature phase, which can be explained by small initial grain size in polymer-derived zirconia.
INTRODUCTION Yttrium substituted zirconia is a well-known ionic conductor and has been extensively investigated. In particular, the phase transformations have been studied and are well understood [1-8]. Most of the experimental results have been obtained using zirconia (micron grain size) samples prepared by high temperature sintering. The use of high processing temperatures has some limitations in the fact that reactions with the electrode materials can result and grain growth takes place during the sintering procedure, which limits the minimum thickness of the electrolyte layer. As a result alternative techniques for the preparation of zirconia based electrolytes are important. Polymeric precursor processing is one of the promising techniques which allow for lower processing temperatures.. Previous investigations did show the potential of decreasing the thickness of the electrolyte layer and lowering the processing temperature by use of this type of processing [9-11]. As a result, a number of different nanocrystalline metal oxide coatings have been prepared and investigated including Y stabilized zir
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