Ionic conductivity of directionally solidified zirconia–mullite eutectics

The properties of directionally solidified eutectic (DSE) zirconia–mullite composites are presented. These materials combine two oxide ion conductors in eutectic microstructures stable over broad temperature (570–1415 °C) and pO2 (10− 20 to 105 Pa) ranges, which are pertinent for their application as high temperature (> 1200 °C) solid electrolytes, e.g. in Nernstian sensors. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy results reveal a composite structure comprising eutectic crystals of mullite and zirconia, and an intergranular amorphous phase rich in Al–Y–Si–O. The amorphous phase is crystallised upon annealing at 1400 °C, and the resulting composites have a composition close to the nominal eutectic consisting of 79 vol.% mullite and 21 vol.% zirconia. The electrical conductivity of these materials is rationalized in terms of percolation, fraction and properties of each phase, attaining values in excess of 0.01 S/cm at 1370 °C and displaying Arrhenius behaviour with activation energy of 70 kJ/mol. The broad electrolytic domain of these solid electrolytes is demonstrated by conductivity measurements carried out from 600 °C to 1370 °C in O2, air, Ar and 10%H2 + 90%N2 atmospheres.