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_O^2- = 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^c_O2 in the curve describing oxygen flux through the material as a function of the environmental P_O2. Below P^c_O2, the oxygen flux rapidly approaches a saturation value, J^max_O^2-. Both P^c_O2 and J^max_O^2- are shown to be functions of temperature and material composition. The variation of P_O2 through the material is also discussed.