There are a number of several types of sensors which can be used 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, which employing spectrometry-based sensing methods.
Conductivity sensors may 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 essential 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 load sensor.
The response in the sensor is really a two part process. The vapour pressure from the analyte usually dictates the number of molecules are present inside the gas phase and consequently what number of them is going to be on the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need in order to interact with the sensor(s) to be able to produce a response.
Sensors types used 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 could have both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These micro load cell were originally produced in Japan inside the 1960s and found 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 made from a ceramic element heated by way of a heating wire and coated by way of a semiconducting film. They could sense gases by monitoring changes in the conductance during the interaction of any chemically sensitive material with molecules that should be detected in the gas phase. Away from many MOS, the material which was experimented with the most is tin dioxide (SnO2) – this is because of 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 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 type of MOS is easier to generate and thus, cost less to purchase. Limitation of Thin Film MOS: unstable, hard to produce and therefore, higher priced to get. On the contrary, it offers higher sensitivity, and a lot lower power consumption compared to thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous materials for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared inside 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 really is later ground and combined with dopands (usually metal chlorides) then heated to recuperate the pure metal as 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 will be left to cool (e.g. over a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” in the MOS will be the basic principle of the operation in 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 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, as the p-type responds to “oxidizing” vapours.
Because the current applied between the two electrodes, via “the metal oxide”, oxygen within the air start to interact with the outer lining and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from the conduction band” . In this way, the electrical conductance decreases as resistance in these areas increase because of absence of carriers (i.e. increase resistance to current), as you will have a “potential barriers” between the grains (particles) themselves.
When the torque transducer subjected to reducing gases (e.g. CO) then your resistance drop, because the gas usually react with the oxygen and thus, an electron is going to be released. Consequently, the production of the electron raise the conductivity since it will reduce “the potential barriers” and let the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the sensor, and consequently, because of this charge carriers will be produced.