Formation of monoclinic zirconia at the anodic face of tetragonal zirconia polycrystalline solid electrolytes

Abstract

The stability of yttria tetragonal zirconia polycrystalline (Y-TZP) materials with and without the addition of alumina has been investigated during charge flow in solid electrolyte cells. A considerable amount of monoclinic zirconia is formed (up to 50–60%) on the anodic side of the solid electrolyte discs during current flow. The thickness of the surface layer in which maximum transformation takes place was determined to be 3–4.5 μm. On the cathodic side, the amount of monoclinic zirconia detected was relatively small (< 5%). The amount of monoclinic formed on the anodic side varied with the microstructure of the ceramic and was considerably less in materials free of pores and with uniform grain size distribution. Relaxation experiments indicate that the tetragonal to monoclinic zirconia phase transformation is related to the oxygen evolution reaction and is not due to oxygenion transport within the solid electrolyte. The observed behaviour has been explained in terms of the creation of space charge layers at the electrode/electrolyte interface leading to the saturation of vacancies by oxygen ions and instability of the tetragonal phase in the surface region on the anodic side of the solid electrolyte.