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 fall into five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and those employing spectrometry-based sensing methods.
Conductivity sensors could be made up of metal oxide and polymer elements, both of which exhibit a modification of resistance when subjected to Volatile Organic Compounds (VOCs). In this particular report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, because they are well researched, documented and established as important element for various machine olfaction devices. The application, in which the proposed device is going to be trained onto analyse, will greatly influence the choice of load sensor.
The response in the sensor is actually a two part process. The vapour pressure in the analyte usually dictates how many molecules can be found in the gas phase and consequently how many of them will be in the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need so that you can interact with the sensor(s) to be able to generate 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. Sometimes, arrays may contain both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These micro load cell were originally manufactured 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 widely accessible commercially.
MOS are made from a ceramic element heated by a heating wire and coated with a semiconducting film. They could sense gases by monitoring alterations in the conductance during the interaction of any chemically sensitive material with molecules that should be detected within the gas phase. Out of many MOS, the fabric which was experimented with all the most is tin dioxide (SnO2) – this is due to 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 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 an extended period to stabilize, higher power consumption. This sort of MOS is a lot easier to create and for that reason, cost less to get. Limitation of Thin Film MOS: unstable, challenging to produce and therefore, more expensive to buy. On the other hand, it provides higher sensitivity, and far lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is easily the most common porous material used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready within an aqueous solution, to which 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 mixed with dopands (usually metal chlorides) then heated to recuperate the pure metal being a powder. For the purpose of screen printing, a paste is made up through the powder. Finally, in a layer of few hundred microns, the paste will likely be left to cool (e.g. on the alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” within the MOS is definitely the basic principle from the operation within the sensor itself. A change in conductance happens when an interaction having a gas happens, the lexnkg varying depending on the power 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.
Since the current applied involving the two electrodes, via “the metal oxide”, oxygen in the air commence to interact with the surface and accumulate on the surface of the sensor, consequently “trapping free electrons on the surface through the conduction band” . This way, the electrical conductance decreases as resistance within these areas increase because of absence of carriers (i.e. increase effectiveness against current), as there will be a “potential barriers” involving the grains (particles) themselves.
When the torque sensor exposed to reducing gases (e.g. CO) then your resistance drop, since the gas usually interact with the oxygen and thus, an electron is going to be released. Consequently, the discharge from the electron boost the conductivity because it will reduce “the potential barriers” and allow the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the sensor, and consequently, as a result of this charge carriers will be produced.