Sensor water-vapor-sensing material

Because in zeolites the silver clusters change their color when in contact with water, it has been suggested that these compounds may be used as water-vapor-sensing materials. Ozin et al. observed the vapor-pressure chromic, cathode-ray chromic, water chromic, photochromic, and thermochromic properties for silver-sodalite, and they proposed that these compounds hold promise for use as sensors.[30] In addition, the silver clusters in zeolites are also sensitive to other molecules, and therefore, they have great potential as chemical-sensing materials. 

Composite fibers of poly(o-anisidine]-PS was produced by electrospinning for chemical vapor sensing. Sensibility of the composite fibers were tested under water and ethanol vapor, the sensors elements responded better to the high polarity ofthe solvent. The CSA-doped POA/PS composition seems to be stable under the submitted ambient conditions to ethanol. The sensor could be reused several times without any change in sensing behavior and/ or damage to the sensing materials. 

In particular, Connolly et al. (2005) designed NH capacitive sensor with 500-nm-thick porous SiC film. The response in humidity was very low for RH<50 %, which was attributed to the porous dimensions. The exact sensing mechanism is still not clear, but NH levels as low as-0.5 ppm were detected. Porous alumina (AI2O3) has also been examined as a sensing material for capacitive gas sensors and in particular for humidity measurements (Nahar and Khanna 1982 Timar-Horvath et al. 2008). The Al Og-based humidity sensor was a volume-effect device based on physical adsorption. At low humidity, the walls of the pores are lined with one-molecular-thickness liquid layer. As the humidity increases, after saturating the walls, due to a capillary condensation effect, the water starts condensing in the pores (Boucher 1976 Neimark and Ravikovitch 2001). It was established that the water molecules, even at a partial pressure higher than the saturated vapor pressure tend to condense in capillary pores with a radius below the Kelvin radius r, which is defined as function (1) (Boucher 1976) ... 

A protonic conductor such as CaZro 9lno.i Os- , can be applied to sense hydrocarbon gases. In this case, two different electrodes were used. One is silver metal which is a material inert to hydrocarbons and the other is a perovskite-type oxide (Lao.6Bao.4Co03) which contains rare earth and accelerates the combustion of hydrocarbons. The chemical reaction occurring in the sensor is shown in fig. 69 (Hibino and Iwahara 1994). Hydrocarbons in the ambient atmosphere do not react on the Ag electrode. On the contrary, hydrocarbons react with oxygen and form water vapor and carbon dioxide on the oxide electrode surface. 

The outstanding merits of this type of hydrocarbon sensor are as follows. Firstly, the sensing characteristics are enhanced by choosing the appropriate combination of electrodes. Secondly, a standard electrode material is not necessary for detection because the water vapor which initially exists in the atmosphere is applied as the reference material. 

Commercial exposure facilities typically measure two main types of moisture relative humidity and wet time. Measurements of relative humidity by a shaded device, such as a wet bulb/dry bulb or solid-state sensor enclosed in a ventilated shelter, indicate the amount of water vapor in the air mass relative to its maximum capacity at the ambient temperature. Wet time is the amount of time liquid water is present on the surface of the material because of condensation and precipitation. It is measured either by increase in electrical conductivity of a cotton muslin wick when it is wet by rain or by the electrical potential developed in a moisture sensing galvanic cell described in ASTM G84 (13). 

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