CO₂ sensing properties of Zr-added porous CaFe₂O₄ powder

The gas sensing properties of Zr-added and pure CaFe₂O₄ powders for CO₂ in air were examined in the temperature range of 250-450 °C. The semiconductor-type gas sensor made from pure CaFe₂O₄ powder showed a fairly good response to CO₂. Furthermore, the addition of a small amount of Zr into CaFe₂O₄ powder was found to be effective for enhancing the CO₂ response of the present gas sensor. It was also found that the gas response, defined by the ratio of the resistance in air and that of the target gas reached maximum at the operating temperature of 300 °C. The gas response of the Zr-added CaFe₂O₄-based sensor at 300 °C was estimated to be 2.5 times higher than that of the sensor made from pure CaFe₂O₄ powder. However, the 90% response time of the Zr-added CaFe₂O₄-based sensor was much quicker at 350 °C than that at 300 °C. Thus, the optimal gas sensing performance of the Zr-added CaFe₂O₄-based sensor is expected to be obtained at the operating temperature of 350 °C, considering the still higher response to CO₂ gas at this temperature. It is noted that the present CaFe₂O₄-based sensor responded reversibly as well as continuously to CO₂ gas. Infrared analysis revealed that the sensing mechanism of the present CaFe₂O₄-based sensor is the change in the electric resistance of CaFe₂O₄ caused by reactive CO₂ adsorption with negatively charged oxide ions (O^-) resulting in the increase in the hole concentration in the base material of CaFe₂O₄.