Darkening of zirconia: A problem arising from oxygen sensors in practice

A zirconia (ZrO2) darkening phenomenon in oxygen sensors has been discovered and a 9 mol% Y2O3-stabilized ZrO2 ball of an oxygen sensor that has been used in a carburization furnace for four months is studied in the present work. No apparent phase change is detected by X-ray diffraction analysis. However, an oxygen deficiency is recorded in a thermogravimetric experiment. No valence-state changes of Zr, Y and O are discovered by X-ray photoelectron spectroscopy. The ionic transport number of the ZrO2 ball is measured with an oxygen-pumping approach; electron conduction is observed at high temperatures. The ZrO2 darkening is thus considered to be mainly due To the formation of F-centres resulting from the electrochemical reaction occurring at the cathode and the ZrO2 surface and body in contact with the carburization atmosphere. After use for four to five months, it is proposed that the darkened ZrO2 be reoxidized to prolong the lifetime of the oxygen sensor.