Conducting polymer humidity sensors

An electronic conduction-type humidity sensor was developed by Ishida et al. [56J. The sensor structure is very similar to the one illustrated in Figure 20-41. A cross-linked hydrophilic polymer in which carbon particles are dispersed is used as the humidity-sensitive film. The swelling of the polymer disturbs the ohmic contacts between dispersed carbon particles and thus the electronic resistance of the element increases sharply as the relative humidity approaches 100< o, as shown in Figure 20-41. This element was put into practical use as a dew detector for the VTR cylinder in 1978. 

Since protection of individuals and the environment has become more important, improved sensors to measure chemical and physical changes such as chemical spedes, temperature, pressure, or humidity are required. Much research has been carried out to apply conducting polymers as sensor materials. 

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To improve classification ability, the effect of environmental parameters in different data preprocessing algorithms was evaluated. The algorithm that allowed the least humidity, temperature, and day correlation was selected. PCA was used to determine discriminating ability. The results indicate that MOS sensors were much better at discriminating the different odors when compared with the conducting polymer sensors. It was also shown that the location of samples for MOS sensors depended on the quality of water. 

Conducting polymers such as polyfuran and polythiophene may have valuable uses in humidity sensors and radiation detectors as the conductivity of these doped polymers varies considerably when exposed to humidity or radiation. 

Among the various interesting and useful properties of the new class of polymers, their switchable electrical conductivity has proven the most attractive to the community of chemists and physicists, and so it is understandable that these polymers are called conducting polymers. Much effort has been spent on measurements of their electrical conductivity and on determinations of the factors that affect its value [44,113,119,124,151,213,314,327,328,344,345,393 17]. The use of the conventional ex situ dc four-point method [44,393,398,399] or the ac impedance technique in a metal [polymer metal sandwich arrangement [119,124,410] for measurements of the conductivity of dry polymer samples is straightforward. However, the conductivities of dry polymers are affected by humidity and any gas present. Indeed, this is the property that is utilized in gas sensors. Conductivity can also be measured in situ, i.e., under controlled electrochemical and chemical conditions [151,394,395,397,400,402,406,408,417]. 

Gas sensors based on conducting polymers have high serrsitivities but (usitally) low selectivities. They respond to different gases (NH3, CO2, CO, HCl, H2S) and vapors, e g., alcohols, acetone or nitroaromatic explosives [161] water (humidity) [136,140] also irtfluences their properties. However, the composition of gas mixture can be calcirlated by using an array of several imits containing different... 

We have found that a composite film consisting of a conducting polymer and an insulating polymer is useful for a humidity sensor (II), The PANA as... 

Conducting polymer sensors Ambient temperature operation, sensitive to many VOCs, short response time, diverse sensor coatings, inexpensive, resistance to sensor poisoning Sensitive to humidity and temperature, sensors can be overloaded by certain analytes, sensor life is limited... 

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On the other hand, most sensing polymers are either hydrophilic or hydrophobic. In the case of humidity sensors, the absorption of water will result in the modification of conductive or dielectric properties with humidity. 

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