Attempted optimisation of ionic conductivity in the yttria-stabilized zirconia system by co-doping

• Main Author: Darby, R. J.
• Published: University of Cambridge 2009
• Subjects: 541.37
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598277

Samples that spanned the full cubic-phase field of YSZ (containing 21-57 mol.% YO_1.5) were produced to investigate the binary system. The Raman spectra of these samples revealed short-range ordering similar to that found in the ordered delta-phase, Zr₃Y₄O₁₂, at yttria contents of 49 mol.% YO_1.5 and above. The conductivity measurements on these binary samples revealed an increasing activation enthalpy for conduction with increasing yttria content up to an yttria addition of 33 mol.% YO_1.5. By comparing this with enthalpy of formation data it was proposed that the cause was an increasing migration enthalpy contribution resulting from the increasing presence of relatively large yttrium ions. The potential co-doping technique investigated was termed solution-doping and involved infiltrating a porous ceramic with a dopant-containing nitrate solution, followed by drying and sintering. It was an efficient method of differently doping multiple samples and was successfully used to incorporate calcium ions into an YSZ solid solution. Comparison with samples produced by ball-milling showed an increased relative density with solution-doping but a decreased homogeneity. Co-doping of the YSZ system was investigated by adding a range of additional dopants to a YSZ composition containing 18 mol.% YO_1.5. The dopants added were between 1 and 4 mol.% of CaO, GaO_1.5, ZnO, GdO_1.5, YbO_1.5 or YO_1.5. Comparisons were made among samples of the same cation or anion vacancy content. The only co-dopant to consistently increase the conductivity of the samples was ytterbium. A greater ionic radius and valency mismatch between the dopant cation and the majority zirconium cation lead to a lower conductivity, increased activation enthalpy and increased segregation to the grain boundaries.