PURPOSE: To activate a sensor element without bringing about the structural change to zirconia by alternately impressing between electrodes a processing voltage whereby the partial pressure of oxygen at an interface between an electrode and a solid electrolyte of the sensor element becomes not smaller than a limit value causing blackening. ;CONSTITUTION: When sensor cells 3, 5 are driven for a long time, oxygen is adsorbed to the electrodes 11, 14, which raises the internal resistance. Therefore, each sensor cell 3, 5 is activated during control or before and after control of the air fuel ratio. At this time, if an alternating voltage to be applied is too large, zirconia constituting the sensor cells 3, 5 blackens, with deteriorating the solid electrolyte. The deterioration can be prevented by controlling a processing voltage so that the partial pressure of oxygen at a three-phase interface of the electrodes 11, 14 which are cathodes when the voltage is applied is not smaller than a safe limit thereof, i.e., 10-34atm not causing blackening at 500-800°C.;COPYRIGHT: (C)1994,JPO&Japio
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