EVALUATION OF COMMERCIAL ZIRCONIA POWDERS FOR SOLID OXIDE FUEL-CELLS

Yttria-zirconia electrolytes prepared from powders obtained from sever al suppliers have been evaluated for use in solid oxide fuel cells. Tw o compositions with Y2O3 content of 3 and 8 mol% have been studied. Io nic conductivities were measured with a four-probe dc technique over t he 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 operati ng temperature of 1000-degrees-C. The microstructure (grain size, dist ribution, 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 E lektron which had somewhat lower density. At 1000-degrees-C the conduc tivity 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 r esistivity was a function of the SiO2 content in the starting powders. As a consequence of annealing of specimens at 1000-degrees-C, an incr ease in both the grain boundary and the intragrain resistivity (measur ed at low temperatures) was observed but the effect was much higher on the grain boundary impedance especially for specimens with Y2O3 conte nt in the vicinity of 8 mol%. At the fuel cell operating temperature o f 1000-degrees-C, the difference in the conductivity of specimens prep ared from powders supplied by different manufacturers was insignifican t apart from the role of actual dopant content.