On the mixed ionic and electronic conductivity in polarized yttria stabilized zirconia

Electric transport in oxides with mixed conduction, being exposed to dc electric fields and being placed between blocking electrodes is discussed and exemplified for yttria stabilized zirconia (YSZ). This is relevant for cases of large current densities in solid oxide fuel cell (SOFC), solid oxide electrolysis cells (SOEC) and during flash sintering of oxide powders. In the present study equilibrium of defect reactions is not attained in the presence of blocking electrodes leading to a continuous generation of holes in p-regions and electrons in n-regions feeding the external current. In the remaining region between n- and p-region ion conduction is dominant. Thus a p-i-n junction is formed. The electronic species may be continuously generated by reaction with gaseous oxygen or by the creation and annihilation of single-charged vacancies. For both cases current voltage-relations are derived by assuming that the gradient of the chemical potential of double-charged vacancies is reduced below its value given by a zero electrochemical potential. The reduction is introduced as a consequence of the Le Chatelier Principle, which requires reactions to occur, which counteract the accumulation and depletion of double charged vacancies by the applied electric field and corresponding deviations from charge neutrality. Scenarios are discussed where electrons and holes recombine with a concomitant emission of light. This explains why the spectrum of the emitted light deviates from that of black body radiation for both examples of flash sintering and the Nernst glower.