Zirconia-based electroceramic materials for SOFC applications

Abstract

The phase relations, electrical properties and structural characteristics of doped cubic stabilised Zirconia based electroceramic materials have been investigated using a number of characterisation techniques. The phase relations of the ternary systems ZrO2 -Y2O3 -TiO2 and ZrO2 -Gd2 O3 - TiO2 at 1500°C have been investigated. Electrical characterisation in air and in low oxygen partial pressures has been carried out using 2-probe A.C. Impedance Spectroscopy and 4-probe D.C. resistivity measurements to ascertain whether compositions within these systems could be utilised as the anode materials in Solid Oxide Fuel Cells. The effect of porosity on the ionic and electronic conducting properties of the ZrO2 -Y2 O3 -TiO2 system has been investigated to provide a clearer understanding of the effect of the porosity within candidate anode materials. The effect of Al2O3 additions on the electrical properties and stability of the Solid Oxide Fuel Cell material of choice, 8 mol% Yttria stabilised Zirconia, has been investigated. Al2 O3 has been found to remain primarily as a second phase within the 8YSZ, however a small quantity of Al3+ does dissolve into the fluorite matrix. Al2 O3 has been found to have a negligible effect on the high temperature ionic conductivity of 8YSZ and improves the resistance of 8YSZ to hydrothermal degradation by stabilising the cubic structure. High temperature Time of Flight Neutron Diffraction has been used to link the change in activation energy observed in 8YSZ to a break down in local ordering of oxygen ions. Extended X-ray absorption Fine Structure Spectroscopy has been used to characterise the local structure of the cations in 8 mol% Yttria-stabilised Zirconia. Analysis of the high temperature data reveals that the local structure is quite different from the average crystallographic structure. The oxygen vacancies were determined to be associated with Zirconium ions and found to disorder at high temperatures.