There are a number of different types of sensors which you can use as essential components in different 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 composed of metal oxide and polymer elements, both of which exhibit a modification of resistance when exposed to Volatile Organic Compounds (VOCs). In 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 important element for various types of machine olfaction devices. The application form, in which the proposed device will likely be trained on to analyse, will greatly influence deciding on a weight sensor.
The response of the sensor is a two part process. The vapour pressure of the analyte usually dictates the number of molecules can be found in the gas phase and consequently what percentage of them will be on the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need in order to react with the sensor(s) to be able to create a response.
Sensors types utilized 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. Sometimes, arrays may contain both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan within the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and are easily available commercially.
MOS are created from a ceramic element heated by a heating wire and coated by way of a semiconducting film. They are able to sense gases by monitoring modifications in the conductance during the interaction of a chemically sensitive material with molecules that ought to be detected within the gas phase. Away from many MOS, the material which has been experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped using a noble metal catalyst such as 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 a longer time to stabilize, higher power consumption. This sort of MOS is easier to create and for that reason, cost less to purchase. Limitation of Thin Film MOS: unstable, difficult to produce and for that reason, higher priced to purchase. On the other hand, it offers higher sensitivity, and much 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 will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready inside an aqueous solution, that 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 is later ground and mixed with dopands (usually metal chlorides) then heated to recover the pure metal as a powder. Just for 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. over a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” inside the MOS is the basic principle in the operation inside the compression load cell itself. A change in conductance takes place when an interaction using a gas happens, the conductance 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 cqjevg “oxidizing” vapours.
Because the current applied in between the two electrodes, via “the metal oxide”, oxygen inside the air start to interact with the top 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 within these areas increase because of insufficient carriers (i.e. increase potential to deal with current), as you will have a “potential barriers” in between the grains (particles) themselves.
Once the sensor exposed to reducing gases (e.g. CO) then the resistance drop, as the gas usually interact with the oxygen and thus, an electron will be released. Consequently, the discharge in the electron boost 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 through the surface of the inline load cell, and consequently, due to this charge carriers is going to be produced.