Influence of CO 2 in dry and wet atmospheres on the response of Mg-doped SrTiO 3 ceramic oxygen sensors

Abstract

Mg-doped SrTiO 3 thick film sensors fabricated by screen-printing proved to be very promising for the use as oxygen sensors. The resistance of such sensors exhibits a P O 2 dependence according to R∝P O 2 −1/4. The influence of CO 2 in dry and wet atmospheres was evaluated in this work. The results obtained show that the presence of CO 2 has no influence on the oxygen-sensing properties of the sensor in dry conditions and the Mg-doped SrTiO 3 sensor can work even better than the ZrO 2 oxygen sensor in the dry CO 2-containing atmospheres. In an atmosphere containing both CO 2 and H 2O, the Mg-doped SrTiO 3 sensor can only operate properly as oxygen sensor at CO 2 concentrations below 70%. The effects from CO 2 and H 2O become stronger at CO 2 contents higher than 80%. An interesting CO 2-sensing characteristic instead of oxygen-sensing of the Mg-doped SrTiO 3 sensor is observed in a wet non-oxygen atmosphere. A model based on the defect chemistry, grain structures and conduction mechanisms of this material is developed in order to explain the experimental results. It is proposed that CO 2 may be absorbed at the surface of this oxide and an uncharged complex is formed in the dry conditions. In the presence of H 2O, a partial proton conduction is introduced to the total conductivity due to the surface reactions between CO 2 and H 2O. This material may become a predominant proton conductor in a non-oxygen atmosphere containing both CO 2 and H 2O.