There are a number of various kinds of sensors which can be used as 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 those employing spectrometry-based sensing methods.
Conductivity sensors may be made from metal oxide and polymer elements, each of which exhibit a change in resistance when subjected to Volatile Organic Compounds (VOCs). Within 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 important element for various machine olfaction devices. The application, where the proposed device will be trained to analyse, will greatly influence the choice of weight sensor.
The response from the sensor is a two part process. The vapour pressure in the analyte usually dictates the amount of molecules exist inside the gas phase and consequently what number of them will be at the sensor(s). When the gas-phase molecules have reached the sensor(s), these molecules need in order to react 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. based upon metal- oxide or conducting polymers. Sometimes, arrays might have both of the above two kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan in the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and are easily available commercially.
MOS are made from a ceramic element heated by a heating wire and coated by way of a semiconducting film. They can sense gases by monitoring modifications in the conductance throughout the interaction of any chemically sensitive material with molecules that need to be detected within 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. Different types of MOS may 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 simpler to produce and therefore, cost less to buy. Limitation of Thin Film MOS: unstable, hard to produce and thus, more expensive to get. On the contrary, it offers much higher sensitivity, and a lot lower power consumption compared to thick film MOS device.
Manufacturing process. Polycrystalline is easily the most common porous materials for thick film sensors. It will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready in an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This really is later ground and combined with dopands (usually metal chlorides) and then heated to recoup the pure metal being a powder. For the purpose of screen printing, a paste is produced 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” inside the MOS is definitely the basic principle of the operation in the miniature load cell itself. A change in conductance occurs when an interaction with a gas happens, the conductance varying depending on the power of the gas itself.
Metal oxide sensors fall under 2 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 cqjevg “oxidizing” vapours.
Since the current applied in between the two electrodes, via “the metal oxide”, oxygen within the air commence to react with the top and accumulate on the surface 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 lack of carriers (i.e. increase resistance to current), as there will be a “potential barriers” in between the grains (particles) themselves.
When the sensor exposed to reducing gases (e.g. CO) then your resistance drop, because the gas usually interact with the oxygen and therefore, an electron will be released. Consequently, the discharge from the electron increase the conductivity since it will reduce “the possible barriers” and let the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the inline load cell, and consequently, due to this charge carriers will likely be produced.