Reactions between Stabilized Zirconia for Oxygen Sensor and Iron Oxides

Abstract

Three kinds of stabilized zirconia solid electrolytes with 15 mol%MgO, 15 mol%CaO, or 8 mol%Y₂O₃, were allowed to react at 1573 K with Fe₂O₃ in air and with Fe_xO in a CO-CO₂ mixture (CO/CO₂=1). The reaction zone formed at each reaction couple was investigated by metallographic observation and by electron probe microanalyzer survey. The reactivity of reaction couples was different from one another. Results obtained were as follows:
(1) All stabilized zirconia electrolytes examined were attacked with Fe_xO more severely than with Fe₂O₃. In particular, ZrO₂ (8 mol%Y₂O₃) was drastically attacked with Fe_xO.
(2) In the ZrO₂ (15 mol%CaO)/Fe₂O₃ (or Fe_xO) couples, pores were concentrated and enlarged at the grain boundary of ZrO₂ (15 mol%CaO) phase near the reaction interface.
(3) In the ZrO₂ (15 mol%CaO)/Fe_xO couple, a trace of an original interface remained in Fe_xO phase, showing that the interface moved with proceeding of reaction.
(4) In the ZrO₂ (15 mol%MgO)/Fe₂O₃ couple, an intermediate phase, as identified to be magnesioferrite (MgFe₂O₄), was formed in Fe₂O₃ phase slightly apart from the interface. No intermediate phase was detected in the other reaction couples.
(5) The reactivity of stabilized zirconia with iron oxides can be estimated to a considerable extent on the basis of phase diagrams concerned.