Grain boundary analysis and ionic conductivity of superplastic cubic zirconia for solid oxide fuel cell electrolytes

Yttrium stabilized zirconia (YSZ) is the material most commonly used for solid oxide fuel cell (SOFC) electrolytes because it has high oxygen ion conductivity at elevated temperatures. Manufacturing and sealing of the SOFC YSZ electrolyte is relatively expensive and cost could be reduced if the ceramic could be net shape formed. Methods to net shape form YSZ by superplastic deformation have been developed by introducing SiO₂ as a second phase, but the impact of this approach on ionic conductivity was not known. This dissertation focuses on understanding how SiO2 affects the ionic conductivity of YSZ.; The present work necessitated the design and fabrication of an appropriate impedance spectroscopy test capability and the preparation and evaluation of a matrix of samples with various silica amounts and grain sizes. Impedance spectroscopy is the figure of merit used to measure and evaluate ionic conductivity. Impedance spectroscopy at temperatures from 350 to 700°C and analytical electron microscopy were used to characterize grain boundary conductivity and grain boundary segregation of in 8 mol% yttrium cubic stabilized zirconia (8Y-CSZ). 1 to 10 wt% of silica was added as an intergranular phase. Grain growth experiments were conducted at temperatures of 1350°C to 1600°C for times from 0.1 to 100 hours. Grain boundary widths were determined from impedance spectroscopy data using a brick layer model. Average grain boundary widths were also determined from analytical electron microscopy conducted at Oak Ridge National Laboratory and the amount of yttrium and silicon segregation at grain boundaries was determined from chemical composition line scans.; Results indicate that the addition of intergranular SiO₂ to 8Y-CSZ leads to smaller grain size (due to grain boundary pinning) therefore increased grain boundary area and reduced total ionic conductivity. For a constant grain size, the specific grain boundary and the total ionic conductivity is not significantly affected at SiO₂ concentrations less than 5 wt%. There is a strong correlation between the grain boundary widths determined by impedance spectroscopy and analytical electron microscopy. Yttrium segregation at the grain boundaries of greater than 10 mol% compared to 8 mol% in the grain interiors along with yttrium depletion layers as low as 6 mol% on each side of the grain boundary and the presence of covalent SiO₂ (2 to 3 wt%) at the grain boundary in all samples may in part explain why the specific grain boundary conductivity is generally two to three orders of magnitude lower than the grain interior conductivity. Small amounts of intergranular SiO₂ may be used to enhance superplastic forming and will not lower the overall ionic conductivity if post process annealing can restore the grain size to the equivalent used in SOFC electrolytes today.