Gas Sensors

Background

Gas sensors

Introduction

Several experiments and research studies investigated the role of sensors for the detection of gas. The implementation of gas sensing technologies has proved to be very useful in many applications of specific gas detection. These application areas include: detection of specific gases especially methane emerging from industrial wastes, in automobile industry for detecting the polluting gases, in medical areas such as the use of electric nose for simulating and in environmental monitoring like detection of analyte in air and water. Development of gas sensors mainly depends on the material used for developing sensitive film, specific sensing principles and fabrication methods. However the fabrication methods implemented is generally very expensive. Therefore, developing a cost effective way of fabricating sensors is the main future aim in the field of gas sensors.

The gas sensors are generally characterised by different performance parameters such as (i) Sensitivity: Which is the change in relative response of the sensor per different range of gas concentrations; (ii) Selectivity: It is the sensors ability to respond selectively to a particular gas or mixture of gases ; (iii) Stability: it is generally determined on the basis of stable response and steady baseline achieved by the sensor; (iv) Limit of detection: is the lowest concentration of a gas detected by the sensor at operating conditions; (v) Operating temperature: The temperature at which the sensing results are performed; (vi) Dynamic range: range between the lowest and highest concentration detected; (vii) Reversibility: whether the sensor is able to reach to its baseline and produce accurate results; (ix) Response time: the time interval in which the sensor is exposed to an analyte and (x) Recovery time: the time taken by the sensor to reach to its baseline. On the basis of these parameters, various sensing results obtained for a specific analyte are studied and discussed. The different gas sensors that are currently used commercially for detecting gases like methane (CH4), ammonia (NH3), hydrogen (H2) and Nitrogen (N2) includes metal oxides sensors, conducting polymers sensors, optical sensors, calorimetric methods and sensors based on nanotubes of carbon. In these chapter different methods used for developing the above gas sensors with their properties are evaluated and compared with other gas sensing techniques.

Metal Oxides Sensors

Sensors made up of metal oxides semiconductors are the most commonly used sensors for detection of gases. They are generally known for their high selectivity and sensitivity. Normally metal oxide sensors which are used for detection of gases are based on n-type semiconductors. The basic principle used for gas sensing by these sensors is driven by redox reaction which occurs between the target analyte and sensitive layer of oxide. Redox reaction leads to a two way phenomenon: a) gas to be detected reacts with the O- ions distributed on the oxide layer resulting into the rise of electrical potential; b) this change in potential is transduced as a resistance vs time plot in the form of sensing output.