techniques based on the detection of absorptions at multiple wavelengths to more accurately identify gases. Such technologies are diffi cult to miniaturise and make cost effective, however it is possible to manufacture sensors that can detect multiple types of contaminant.
The metal-oxide semiconductor or MiCS sensor employs a different approach to NDIR. This sensor type takes advantage of the chemical reactivity of atmospheric oxygen and the physical changes that result from transformations that take place on the surface of a metal-oxide material. Key advantages of the MiCS-based approaches lie in their ability to sense a wide range of contaminants and to be miniaturised to the chip-package level thanks to the use of MEMS production techniques.
A common construction uses tin oxide deposited onto the surface of an insulating substrate, which is then heated to improve reactivity. In clean air, donor electrons from the tin oxide are attracted to oxygen that adsorbs onto the surface. This reduces the current fl ow through the tin-oxide layer. Reducing gases, such as VOCs or CO, can react with the adsorbed oxygen molecules, which then move away from the sensor’s surface. As a result, the current increases and electrical resistance falls. Oxidising gases on the other hand tend to increase the resistance of the tin-oxide layer, because they add to the concentration of adsorbed species that attract donor electrons from the tin-oxide layer.
The resistance changes of a MiCS sensor can easily be read using an analogue-to-digital converter. Often the MiCS sensor is used with a constant supply of heat and power. However, more advanced pulsing techniques under software control make it possible to home in on signals from specifi c types of gas. For example, CO reacts at a lower temperature than most VOCs because the molecules do not need to dissociate before forming new bonds with oxygen. Removing heat and applying pulses periodically lowers the average temperature so that the sensor’s operation can home in on CO concentrations.
Ion conductor (solid or liquid)
Working electrode (Pt)
Technology review
One of the main advantages of the MiCS-type sensor is that it requires less power to run than other types of gas sensors, as the heat requirement is relatively low – a factor helped by the ability to make highly miniaturised devices. The change in resistance is typically not a linear response to the change in gas concentration. It is often a more complex polynomial relationship. The key advantage of MiCS sensors is that they react quickly to changes in gas concentration, making them highly suitable for detecting when contaminants rise above a threshold level. As a result, the sensors may be used in combination with other detectors, such as those based on NDIR, to provide an early indication of change with the NDIR systems used to provide a more accurate assessment of overall concentration of a target gas, if required. Specialist suppliers such as SGX Sensortech provide both forms of gas sensor.
Thanks to advances in miniaturisation and the development of MEMS processes, the benefi ts of air quality measurement can now be introduced to a wide range of applications in buildings and moving vehicles. With those sensors in place, air conditioning systems can ensure the air we breathe is as good as possible and delivered without increasing energy consumption.
CO2
CO H+
Ion conductor where H+ can move
A Counter electrode (Pt) In clean air An example of a metal-oxide semiconductor sensor in clean air vs. in carbon monoxide. 29 H2 H+ H+ O O2 H+ e_ In carbon monoxide
H2
O H+ H+ A
e_
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