Structure, Microstructure, and Mixed Conduction of [(ZrO2)0.92(Y2O3)0.08]0.9(TiO2)0.1

Abstract

[(ZrO2)0.92(Y2O3)0.08]0.9(TiO2)0.1 (titania-doped yttria stabilized circonia, 10TiYSZ) samples were prepared by solid state reaction from mixtures of 8 mol% yttria-doped ZrO2 (YSZ) and TiO2 and characterized in terms of structure, microstructure, and electrical properties. [(ZrO2)0.97(Y2O3)0.03]0.9(TiO2)0.1 (titania-doped tetragonal zirconia polycrystalline, 10TiTZP) was also prepared for comparison in some specific studies. Ionic transport properties were measured by impedance spectroscopy in air as a function of temperature. DC techniques including electromotive force (EMF) and Ion Blocking measurements (IB) were carried out in order to determine the electronic contribution to the total conductivity. The addition of titania to YSZ induces the tetragonal zirconia phase formation, thus [(ZrO2)0.92(Y2O3)0.08]0.9(TiO2)0.1 is a composite material and is constituted by two solid solutions, titania-doped yttria-stabilized zirconia (67.7 mole fraction) and titania-doped tetragonal zirconia (32.3 mole fraction). A decrease in bulk ionic conductivity, of one order of magnitude, when TiO2 is added to YSZ is observed in the whole temperature range. Furthermore, in the bulk conductivity vs the reciprocal of the temperature plot, a bending (from 550°C to higher temperatures) toward higher activation energies was detected. The bending could indicate the existence mainly of Ti4+–Vö associated pairs with an association energy of 0.43±0.02 eV. It could mean that Ti–O bonds become stronger and shorter and could produce the formation of microdomains of a ZrTiO4-like structure. The addition of titanium is effective in increasing the electronic conductivity under reducing conditions. Conductivity as a function of Po2 and IB results cannot be related to the formation of small polarons during the reduction process. Furthermore, according to the calculations based on the small polaron theory, inconsistent values for the radius of a small polaron (rp) are obtained in both 10TiYSZ and 10TiTZP. However, large polarons can explain the transport properties in these materials under reducing conditions in agreement with the experimental data.