Transduction principles and related novel devices

The change of the electrical properties, related to the surrounding atmosphere, can be transduced as a change of resistance, impedance or work function. The easiest measurable parameter is the sensor resistance in DC conditions. It may be measured by a voltamperometric technique at constant bias but, in commercial chemical sensors, the sensing film is usually inserted inside a voltage divider. 

A typical kinetic response of conductance as a function of an introduction of a concentration step is shown in Fig. 8.2. After the reducing species is introduced at time ti, the sensor conductance G, increases to Gf, in the time needed to reach the new thermodynamic equilibrium of the surface reactions. If the metal oxide is not stable, or if there is an irreversible chemisorption, a steady state may not be reached. 

Response time is the time necessary for the conductance to reach a threshold value (usually 90%) of the difference between G( and Gj. Recovery time is the time necessary for the conductance to recover to a band expressed as a percentage fraction (usually 90%) of G(-Gi. Concerning the response of chemical sensors, the linearity hypothesis is not verified, and the response when working with gas mixtures cannot be deduced by the superimposition principle, with a simple sum of the individual response. 

The sensor response towards a reducing species and an n-type metal oxide may be defined as the relative change in conductance  

2 Conductance variation of the sensor produced by the introduction of a step concentration of a reducing gas. 

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