There are a number of different types of sensors which can be used essential components in various 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 these employing spectrometry-based sensing methods.
Conductivity sensors might be made up of metal oxide and polymer elements, each of which exhibit a change in resistance when in contact with Volatile Organic Compounds (VOCs). In this particular 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 important element for various machine olfaction devices. The application, where proposed device will 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 of the analyte usually dictates the amount of molecules can be found inside the gas phase and consequently how many of them will be at the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need in order to react with the sensor(s) so that you can generate a response.
Sensors types utilized in any machine olfaction device can be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based on metal- oxide or conducting polymers. In some cases, arrays might 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 been used more extensively in electronic nose instruments and they are easily available commercially.
MOS are made of a ceramic element heated with a heating wire and coated by way of a semiconducting film. They are able to sense gases by monitoring changes in the conductance during the interaction of any chemically sensitive material with molecules that ought to be detected inside the gas phase. Away from many MOS, the material which was experimented using the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Various kinds of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst like platinum or palladium.
MOS are subdivided into two 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 kind of MOS is a lot easier to produce and thus, are less expensive to buy. Limitation of Thin Film MOS: unstable, hard to produce and for that reason, more costly to purchase. On the other hand, it offers higher sensitivity, and a lot lower power consumption compared to the thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous materials used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready in an aqueous solution, to which is added ammonia (NH3). This precipitates tin tetra hydroxide which can be dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This can be later ground and mixed with dopands (usually metal chlorides) and after that heated to recoup the pure metal being a powder. Just for screen printing, a paste is made up through the powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. over a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” inside the MOS is definitely the basic principle from the operation in the sensor itself. A modification of conductance takes place when an interaction using a gas happens, the lexnkg varying depending on the concentration of the gas itself.
Metal oxide sensors fall under 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 to “oxidizing” vapours.
As the current applied in between the two electrodes, via “the metal oxide”, oxygen within the air begin to react with the top and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from your conduction band” . In this way, the electrical conductance decreases as resistance within these areas increase because of insufficient carriers (i.e. increase potential to deal with current), as there will be a “potential barriers” in between the grains (particles) themselves.
Once the rotary torque sensor subjected to reducing gases (e.g. CO) then your resistance drop, as the gas usually react with the oxygen and for that reason, an electron will likely be released. Consequently, the discharge in the electron raise the conductivity since it will reduce “the possibility barriers” and enable the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your surface of the sensor, and consequently, because of this charge carriers will be produced.