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There are a number of different types of sensors which can be used as essential components in various designs for machine olfaction systems.

Electronic Nose (or eNose) sensors belong to five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and those employing spectrometry-based sensing methods.

Conductivity sensors could be composed of metal oxide and polymer elements, both of which exhibit a change in 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, because they are well researched, documented and established as essential element for various types of machine olfaction devices. The application form, where proposed device will likely be trained onto analyse, will greatly influence the choice of weight sensor.

The response of the sensor is actually a two part process. The vapour pressure of the analyte usually dictates the amount of molecules are present inside the gas phase and consequently what number of them will likely be on the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) so that you can produce a response.

Sensors types found in any machine olfaction device could be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. according to metal- oxide or conducting polymers. In some cases, arrays might have both of the aforementioned two kinds of sensors [4].

Metal-Oxide Semiconductors. These compression load cell were originally produced in Japan within the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and are easily available commercially.

MOS are made of a ceramic element heated by way of a heating wire and coated by a semiconducting film. They could sense gases by monitoring modifications in the conductance during the interaction of the chemically sensitive material with molecules that ought to be detected in the gas phase. From many MOS, the fabric which has been experimented with the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Different types of MOS can include oxides of tin, zinc, titanium, tungsten, and iridium, doped having 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 time to stabilize, higher power consumption. This sort of MOS is simpler to generate and for that reason, cost less to purchase. Limitation of Thin Film MOS: unstable, challenging to produce and for that reason, more costly to purchase. On the other hand, it provides greater sensitivity, and a lot lower power consumption than the thick film MOS device.

Manufacturing process. Polycrystalline is easily the most common porous materials for thick film sensors. It will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared inside 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 really is later ground and blended with dopands (usually metal chlorides) and then heated to recover the pure metal being a powder. Just for screen printing, a paste is produced up from your 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” within the MOS is the basic principle in the operation within the sensor itself. A change in conductance takes place when an interaction with a gas happens, the lexnkg varying depending on the concentration 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.

Operation (n-type):

As the current applied between the two electrodes, via “the metal oxide”, oxygen within the air commence to interact with the outer lining and accumulate on the top of the sensor, consequently “trapping free electrons on the surface through the conduction band” [2]. This way, the electrical conductance decreases as resistance during these areas increase because of insufficient carriers (i.e. increase effectiveness against current), as you will see a “potential barriers” between the grains (particles) themselves.

Once the rotary torque sensor exposed to reducing gases (e.g. CO) then your resistance drop, as the gas usually interact with the oxygen and thus, an electron will be released. Consequently, the release from the electron boost the conductivity because it will reduce “the possibility barriers” and allow the electrons to start to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your surface of the sensor, and consequently, as a result of this charge carriers will likely be produced.