There are a number of different types of sensors which you can use as essential components in numerous designs for machine olfaction systems.
Electronic Nose (or eNose) sensors fall under five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and those employing spectrometry-based sensing methods.
Conductivity sensors might be made up of metal oxide and polymer elements, each of which exhibit a modification of resistance when exposed to Volatile Organic Compounds (VOCs). In this particular report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will likely be examined, as they are well researched, documented and established as vital element for various types of machine olfaction devices. The applying, where proposed device will likely be trained onto analyse, will greatly influence the choice of weight sensor.
The response in the sensor is actually a two part process. The vapour pressure from the analyte usually dictates how many molecules are present within the gas phase and consequently how many of them will be at the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need so that you can react with the sensor(s) in order to produce a response.
Sensors types used in any machine olfaction device may be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based upon metal- oxide or conducting polymers. In some instances, arrays might have both of the above two types of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan within the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and they are widely accessible commercially.
MOS are made from a ceramic element heated by a heating wire and coated with a semiconducting film. They can sense gases by monitoring changes in the conductance through the interaction of the chemically sensitive material with molecules that need to be detected in the gas phase. Away from many MOS, the fabric which has been experimented with all the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Different types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped using a noble metal catalyst like platinum or palladium.
MOS are subdivided into 2 types: Thick Film and Thin Film. Limitation of Thick Film MOS: Less sensitive (poor selectivity), it require a longer period to stabilize, higher power consumption. This type of MOS is a lot easier to create and for that reason, are less expensive to get. Limitation of Thin Film MOS: unstable, challenging to produce and for that reason, higher priced to buy. On the contrary, it offers greater sensitivity, and much lower power consumption compared to the thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous material used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready within an aqueous solution, to which is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) then heated to recoup the pure metal as being a powder. For the purpose of screen printing, a paste is created up from your powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” in the MOS will be the basic principle in the operation inside the button load cell itself. A modification of conductance takes place when an interaction using a gas happens, the conductance varying depending on the concentration of the gas itself.
Metal oxide sensors belong to two types:
n-type (zinc oxide (ZnO), tin dioxide (SnO2), titanium dioxide (TiO2) iron (III) oxide (Fe2O3). p-type nickel oxide (Ni2O3), cobalt oxide (CoO). The n type usually responds to “reducing” gases, whilst the p-type responds cqjevg “oxidizing” vapours.
As the current applied in between the two electrodes, via “the metal oxide”, oxygen within the air start to react with the surface and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from the conduction band” . In this manner, the electrical conductance decreases as resistance in these areas increase due to insufficient carriers (i.e. increase resistance to current), as you will have a “potential barriers” between the grains (particles) themselves.
Once the sensor subjected to reducing gases (e.g. CO) then this resistance drop, since the gas usually react with the oxygen and thus, an electron will likely be released. Consequently, the release in the electron boost the conductivity as it will reduce “the possibility barriers” and allow the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the inline load cell, and consequently, due to this charge carriers will likely be produced.