Influence of CO2 in dry and wet atmospheres on the response of Mg-doped SrTiO3 ceramic oxygen sensors

Mg-doped SrTiO3 thick film sensors fabricated by screen-printing proved to be very promising fbr the use as oxygen sensors. The resistance of such sen sors exhibits a Po-2 dependence according to Ro:Po-2(-1/4) The influence of CO2 in dry and wet atmospheres was evaluated in this work. The results obt ained show that the presence of CO2 has no influence on the oxygen-sensing properties of the sensor in dry conditions and the Mg-doped SrTiO3 sensor c an work even better than the ZrO2 oxygen sensor in the dry CO2-containing a tmospheres. In an atmosphere containing both CO2 and H2O, the Mg-doped SrTi O3 sensor can only operate properly as oxygen sensor at CO2 concentrations below 70%. The effects from CO2 and H2O become stronger at CO2 contents hig her than 80%. An interesting CO2-sensing characteristic instead of oxygen-s ensing of the Mg-doped SrTiO3 sensor is observed in a wet non-oxygen atmosp here. A model based on the defect chemistry, grain structures and conductio n mechanisms of this material is developed in order to explain the experime ntal results. It is proposed that CO2 may be absorbed at the surface of thi s oxide and an uncharged complex is formed in the dry conditions. In the pr esence of H2O, a partial proton conduction is introduced to the total condu ctivity due to the surface reactions between CO2 and H2O. This material may become a predominant proton conductor in a non-oxygen atmosphere containin g bath CO2 and H2O. (C) 1999 Elsevier Science Limited. All rights reserved.