Investigation of the humidity effects on SnO2-based sensors in CO detection
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0915-R07-05
Investigation of the humidity effects on SnO2-based sensors in CO detection Cesare Malagu', Michele Benetti, Maria Cristina Carotta, Alessio Giberti, Vincenzo Guidi, Luciano Milano, and Giuliano Martinelli Physics, University of Ferrara, via Saragat 1/c, Ferrara, 44100, Italy
ABSTRACT An algorithm for compensating water vapor pressure in CO detection is proposed here and tested on SnO2 thick-film gas sensors. For each sensor working at a fixed temperature, the conductance, G, is fitted by an analytical surface, whose expression can be inverted to determine the CO concentration once the water partial pressure is measured. As soon as the rate of watervapor pressure change is slower than about 300 Pa/min, G is a function of the temperature, water vapor and CO concentration. If quicker water vapor variations occur instead, the sensing film undergoes a non-negligible transitory phenomenon during which G assumes different values even at fixed water vapor pressure and temperature. This phenomenon prevents the compensation from working properly. An explanation of the behavior is offered by the interpretation of kinetics equations at surface. INTRODUCTION It is well known that humidity strongly interacts with SnO2 in the process of CO detection [1]. The main theoretical approach to investigate the humidity role deals with a competing effect between OH- groups and CO in the occupation of surface sites, through bond formation with chemisorbed oxygen species [2-3]. A different interpretation concerns the creation of surface dipoles that modify the surface energy by a change in the electron affinity of the material [4]. It must be pointed out that water partial pressure and not the relative humidity variations directly affect the sensors behavior. In fact, if the partial pressure of water vapor is kept constant while varying the temperature of the test chamber, large relative humidity variations are induced without a corresponding variation of the sensors signal, but for the temperature effect itself. On the other hand, the sensors conductance follows the water vapor pressure variations when the temperature is kept constant. From the Clapeyron equation one obtains the water vapor partial pressure, pH2O as a continuous function of RH and T [5], the relative humidity and the temperature measured in the test chamber. Aim of this work is to obtain sensors capable of detecting selectively CO even in the presence of a varying water vapor pressure, as it happens in real operating conditions. An algorithm to compensate the effects of water vapor on the sensors response is discussed, which may in principle be applied to other interfering gases. The limitations of this approach arise when large and rapid water vapor pressure variations are induced. In this case, a transitory phase originates, which prevents the correct application of the algorithm. We try here to give an explanation for this behavior, starting from the solution of kinetic equations involving the formation of those surface states responsible for the conductan
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