Gas Sensors Made of Polymers and Gels

A rather simple sensor design is based on a sensitive layer placed between two electrodes of metallic conductance (Fig. 5.7). The question whether the device is a resistive or a capacitive sensor cannot be answered a priori. First, the geometry must be considered. Resistive sensors have a larger ratio of electrode surface to receptor layer volume than capacitive sensors. Capacitive receptor layers may eventually have an infinite value of electric resistance. A measurable change in capacity is achieved by interaction of the dielectric with sample components. If the dielectric coefficient changes its value, then the capacity also changes. 

Some types of chemoresistors and chemocapacitors for gaseous components are composed of electrically conducting polymers. For example, a polypheny-lacetylene layer may interact with CO, CO2, N2 and CH4. The result is a distinct variation of the AC resistance. 

If solids are wetted, then their lattice components become hydrated, and mobile ions can form. As a result, their conductance increases. This behaviour is the basis for resistive humidity sensors. Commonly, a thin layer of the sensitive solid is placed between two electrodes. A useful material e.g. is phoshorous pentoxide. 

Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric. Absorption of polar water molecules has a strong effect on the dielectric constant of the material. The magnitude of this effect increases with a large inner surface which can accept large amounts of water. An example of this type of dielectric is porous j8-alumina. Colloidal ferric oxide, certain semiconductors, perowskites and certain polymers have also been used. /1-alumina is characterized by ionic conductance. Materials of this type can be characterized by a complex resistance composed of real (ohmic) as well as capacitive terms. The behaviour of such solids can be symbolized by a model and an associated equivalent circuit as given in Fig. 5.8. 

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