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.