THE INFLUENCE OF WATER-VAPOR ON CARBON-MONOXIDE SENSITIVITY OF ALPHA-FE2O3 MICROPOROUS CERAMIC SENSORS

The influence of humidity on the carbon monoxide (CO) sensitivity of a lpha-Fe2O3 porous ceramic sensors was investigated by precision volt-a mperometric and impedance spectroscopy techniques in the 0-300 ppm CO range. The humidity varied from 0 to 95% r.h. The sensors were prepare d by sintering a 130 m2/g specific area alpha-hematite powder at 800, 850 and 900-degrees-C, respectively. The microstructural properties we re characterized by X-ray diffraction, N2 adsorption and mercury intru sion porosimetric techniques. The variation of the resistance and cond uctivity activation energy were evaluated in high vacuum, dry and wet air conditions, as a function of the temperature. The resistance versu s temperature relationship was explained, according to literature data , as thermally activated adsorption/desorption mechanisms of the surfa ce oxygen-related species like O-, O2- and OH-. Alpha-Fe2O3 reacts as an n-type semiconductor when exposed to CO and water vapour atmosphere s. The maximum CO and water vapour sensitivity, expressed as DELTAR/R variations, were found at 300-degrees-C for both gases. Cross-sensitiv ity tests carried out at 300 and 400-degrees-C activation temperatures showed that water vapour interferes with CO increasing the DELTAR/R v ariation. A catalytic action of the water vapour at 300-degrees-C is p roposed.