EVALUATION OF COMMERCIAL ZIRCONIA POWDERS FOR SOLID OXIDE FUEL-CELLS

Yttria-zirconia electrolytes prepared from powders obtained from several suppliers have been evaluated for use in solid oxide fuel cells. Two compositions with Y2O3 content of 3 and 8 mol% have been studied. Ionic conductivities were measured with a four-probe dc technique over the temperature range of 400-1000-degrees-C and impedance spectroscopy over the range 300-450-degrees-C. In addition, the effect of annealing on the conductivity has been studied at the current fuel cell operating temperature of 1000-degrees-C. The microstructure (grain size, distribution, shape, pore size and its distribution) has been investigated with scanning electron microscopy. Most specimens could be densified to near theoretical density except for powders supplied by Magnesium Elektron which had somewhat lower density. At 1000-degrees-C the conductivity of specimens with 8 MOI% Y2O3 content was higher by a factor of about three compared with 3 MOI% Y2O3-ZrO2 specimens. However, below 400-degrees-C the conductivities were comparable. The grain boundary resistivity was a function of the SiO2 content in the starting powders. As a consequence of annealing of specimens at 1000-degrees-C, an increase in both the grain boundary and the intragrain resistivity (measured at low temperatures) was observed but the effect was much higher on the grain boundary impedance especially for specimens with Y2O3 content in the vicinity of 8 mol%. At the fuel cell operating temperature of 1000-degrees-C, the difference in the conductivity of specimens prepared from powders supplied by different manufacturers was insignificant apart from the role of actual dopant content.