Frequency response characteristics of gas sensors

Abstract

This paper presents the novel experimental observations and analysis of the frequency response of a sensed gas surface. The main objectives of this study were: (i) to characterise the gas sensors with additional A.C. parameters apart from reported D.C. conductance response, and (ii) to understand the A.C. conduction mechanism. The outcome of this study was that the gas sensors from Motorola, Capteur and Figaro behaved similarly with an A.C. frequency sweep. Essentially their behaviour can be characterised into three distinct frequency bands. For the Motorola micro gas sensor, the flat response from 10 Hz to around 100 kHz range was like a simple R P C P (a zero in admittance function) parallel circuit. From 100 kHz to around 1 MHz, the real part of admittance decreases indicating the presence of an inductive pole in admittance function. Beyond 1 MHz and up to 13 MHz, both real and imaginary parts of admittance increase significantly revealing the presence of higher order zeros. The frequency band differs for Figaro and Capteur sensors. The lumped circuit representation may be a form of representing the distributed reacting layers on the surface. Best fitting of the entire frequency range response using MATLAB indicated the presence of two poles and two zeroes or more. Cleaning by finite triangular or pulse waves restored the surface for sensing. R P, C P, f C, the frequency at which reactive impedance was maximum, the measured negative peak of reactive impedance ( Z i), the slope of estimated capacitance with frequency ( C P/ f) and pole/zero locations can be used as additional parameters to improve the sensitivity and selectivity of gas and other contaminating environments. The A.C. conduction mechanism indicates that the ohmic conduction (1/ R P) in the intergranular region follows the barrier theory. The majority charge carriers in the semiconductor increases with sensing temperature ( T). The barrier potential decreases at high absolute humidity and longer cyclic heating duration ( PW). The capacitance ( C P) may be due to the geometry of the semiconducting intergranular separation between grains. It increases with T due to the closeness at high T. The variation of C P with f may be due to the polarisation contribution of adsorbed gas molecules.