Abstract
A zirconia (ZrO 2) darkening phenomenon in oxygen sensors has been discovered and a 9 mol% Y 2O 3-stabilized ZrO 2 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 ZrO 2 ball is measured with an oxygen-pumping approach; electron conduction is observed at high temperatures. The ZrO 2 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 ZrO 2 surface and body in contact with the carburization atmosphere. After use for four to five months, it is proposed that the darkened ZrO 2 be reoxidized to prolong the lifetime of the oxygen sensor.
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