There are a number of several types of sensors which can be used essential components in various 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 those employing spectrometry-based sensing methods.
Conductivity sensors might be made up of metal oxide and polymer elements, both of which exhibit a change in resistance when subjected to Volatile Organic Compounds (VOCs). In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will likely be examined, since they are well researched, documented and established as vital element for various types of 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 of the sensor is 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 percentage of them will likely be in the sensor(s). When the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) to be able to produce a response.
Sensors types found in any machine olfaction device can 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 above two types of sensors .
Metal-Oxide Semiconductors. These micro load cell were originally created in Japan inside the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and are easily available commercially.
MOS are made from a ceramic element heated with a heating wire and coated by way of a semiconducting film. They could sense gases by monitoring modifications in the conductance during the interaction of the chemically sensitive material with molecules that should be detected inside the gas phase. Away from many MOS, the fabric that has been experimented with all 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 using 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 period to stabilize, higher power consumption. This type of MOS is a lot easier to produce and therefore, are less expensive to purchase. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, more expensive to buy. On the contrary, it offers greater sensitivity, and far lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is easily the most 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, to which is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) and after that heated to recoup the pure metal as 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 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 of the operation inside 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 into 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, while the p-type responds to “oxidizing” vapours.
Since the current applied involving the two electrodes, via “the metal oxide”, oxygen within the air commence to interact with the top and accumulate on the surface of the sensor, consequently “trapping free electrons on the surface from the conduction band” . This way, the electrical conductance decreases as resistance during these areas increase due to absence of carriers (i.e. increase resistance to current), as you will have a “potential barriers” involving the grains (particles) themselves.
When the torque sensor exposed to reducing gases (e.g. CO) then the resistance drop, because the gas usually interact with the oxygen and therefore, an electron will be released. Consequently, the release in the electron raise the conductivity because it will reduce “the possible barriers” and let the electrons to begin to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the sensor, and consequently, due to this charge carriers will likely be produced.