OPTIMIZATION OF IONIC TRANSPORT THROUGH MIXED CONDUCTING OXIDE CERAMICS

Ceramics exhibiting both electronic and ionic conductivities can be obtained by doping highly oxygen conducting matrices with multivalent cations. These materials allow oxygen transport to occur under open circuit conditions driven by an oxygen chemical potential gradient only. In the present work, equations are developed which express this oxygen flux as a function of environmental and material variables. For small P(O2) gradients, the oxygen flux is maximized when t(O2-) = t(e-) = 0.5. To treat the case of significant P(O2) gradients, a more general analysis has been carried out using a model material system, ZrO2-Y2O3-MO2, where M is Ce or Ti. The extrinsic electron conductivity due to the multivalent dopant exhibits a maximum when plotted either as function of temperature or P(O2). As a result, an inflexion point is observed at a critical oxygen pressure, P(O2)c, in the curve describing oxygen flux through the material as a function of the environmental P(O2). Below P(O2)c, the oxygen flux rapidly approaches a saturation value, J(O2-)max. Both P(O2)c and J(O2-)max are shown to be functions of temperature and material composition. The variation of P(O2) through the material is also discussed.