There are a number of several types of sensors which can be used as essential components in numerous designs for machine olfaction systems.
Electronic Nose (or eNose) sensors belong to five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and these employing spectrometry-based sensing methods.
Conductivity sensors may be made from metal oxide and polymer elements, each of which exhibit a modification of resistance when subjected to Volatile Organic Compounds (VOCs). Within this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) is going to be examined, as they are well researched, documented and established as essential element for various types of machine olfaction devices. The application, where the proposed device will be trained onto analyse, will greatly influence the choice of load sensor.
The response of the sensor is actually a two part process. The vapour pressure in the analyte usually dictates the amount of molecules exist in the gas phase and consequently what percentage of them will be at the sensor(s). If the gas-phase molecules have reached the sensor(s), these molecules need to be able to react with the sensor(s) so that you can produce a response.
Sensors types utilized 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 could have both of the above 2 kinds of sensors .
Metal-Oxide Semiconductors. These compression load cell were originally produced in Japan within the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and they are easily available commercially.
MOS are created from a ceramic element heated with a heating wire and coated by a semiconducting film. They can sense gases by monitoring modifications in the conductance through the interaction of any chemically sensitive material with molecules that need to be detected in the gas phase. Out of many MOS, the material which has been experimented with the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Various kinds of MOS can 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 an extended period to stabilize, higher power consumption. This type of MOS is easier to produce and thus, cost less to purchase. Limitation of Thin Film MOS: unstable, hard to produce and therefore, 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 the most common porous material used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared in an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) then heated to recover the pure metal being a powder. For the purpose of screen printing, a paste is made up from the powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on the alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” in the MOS is the basic principle from the operation within the sensor itself. A modification of conductance happens when an interaction with a gas happens, the lexnkg varying depending on the power of the gas itself.
Metal oxide sensors fall under 2 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 to “oxidizing” vapours.
Because the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air commence to interact with the surface and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from the conduction band” . This way, the electrical conductance decreases as resistance in these areas increase due to insufficient carriers (i.e. increase effectiveness against current), as you will see a “potential barriers” between the grains (particles) themselves.
When the torque sensor subjected to reducing gases (e.g. CO) then the resistance drop, because the gas usually interact with the oxygen and for that reason, an electron will likely be released. Consequently, the production in the electron increase the conductivity since it will reduce “the potential barriers” and let the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the top of the sensor, and consequently, as a result of this charge carriers will likely be produced.