Velocity-optimized diffusion for ultra-fast polymer-based resistive gas sensors

Abstract

Conducting polymers are an attractive candidate for use in thin film resistive gas and vapour sensors, but their use has been limited by their apparently slow response times ranging from tens of seconds to minutes. In studying this problem, the authors have discovered that the response time is highly dependent upon the velocity at which the analyte flows across the sensor surface. This type of sensor operates at ambient temperature, and so the authors have attributed this behaviour to the combination of a reduced time for diffusion of the analyte in the carrier gas (pulse broadening) and faster diffusion through a disturbed boundary layer on the polymer surface. A finite element model has been developed to explore this phenomenon. The actual responses of two types of carbon-black/polymer composite resistive sensors have been measured at velocities ranging from 50 to 1500 cm/s to pulses of ethanol and toluene vapour in air, using a purpose-built automated low-volume microchannel flow delivery system. The effect of velocity on the sensors response has been found to vary for different analytes, with a linear velocity coefficient of 1 to 15 s/cm for ethanol and toluene vapour in air, respectively in the diffusion coefficient. The magnitude and rise time of the sensor responses were computed for different velocities and agrees well with the theoretical finite element model. Under optimal conditions, the fastest sensor was found to exhibit a rise time of less than 100 ms, which would be expected to fall even further for thinner films. These results suggest that conducting polymer resistive sensors could be designed to work with a 1 s duty cycle making them attractive for applications where rapid monitoring is required, e.g. on unmanned airborne vehicles and land-based mobile robots.