LOW-FREQUENCY A.C. RESPONSE OF POLYPYRROLE GAS SENSORS

Electrically conducting organic polymers change their conductivity and relative permittivity when exposed to volatile chemicals [1,2]. The c hanges depend on the frequency, the concentration and the type of the chemical. In particular the frequency dependence could be used for mak ing more selective sensors. A polypyrrole (PPY) film eas sensor was de signed and constructed in order to probe it. The low frequency (20 Hz- 10 KHz) a.c. response was studied when it was exposed to 200 ppm of fo ur different vapours: methanol, acetone, ethyl acetate and ethanol. Re sponse patterns to each vapour were obtained by varying the measuremen t frequencies. The pattern for each vapour investigated was very diffe rent from the others, and so it has been possible to discriminate betw een them. The results demonstrate that it is possible to increase the sensor selectivity using the low frequency a.c. response rather than d .c. resistance change. Ageing and temperature dependence of resistance and capacitance are also reported. (C) 1997 Elsevier Science S.A.