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 fall into five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, which employing spectrometry-based sensing methods.
Conductivity sensors might be made from metal oxide and polymer elements, both of which exhibit a modification of 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 be examined, since they are well researched, documented and established as vital element for various machine olfaction devices. The application form, where proposed device will be trained on to analyse, will greatly influence the option of weight sensor.
The response in the sensor is actually a two part process. The vapour pressure of the analyte usually dictates the amount of molecules exist in the gas phase and consequently what number of them is going to be in the sensor(s). When the gas-phase molecules are in 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. based upon metal- oxide or conducting polymers. In some cases, arrays may contain both of the above two types of sensors .
Metal-Oxide Semiconductors. These sensors were originally produced in Japan in the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and they are widely available commercially.
MOS are created from a ceramic element heated by a heating wire and coated by way of a semiconducting film. They could sense gases by monitoring alterations in the conductance through the interaction of the chemically sensitive material with molecules that ought to be detected in the gas phase. Out of many MOS, the material that has been experimented using 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 including 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 kind of MOS is a lot easier to create and thus, cost less to buy. Limitation of Thin Film MOS: unstable, challenging to produce and therefore, more costly to get. On the other hand, it offers greater sensitivity, and much lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous material used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready in an aqueous solution, 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 is later ground and mixed with dopands (usually metal chlorides) and then heated to recoup the pure metal being a powder. Just for screen printing, a paste is made up through the powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on the alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” within the MOS is the basic principle of the operation inside the miniature load cell itself. A change in conductance takes place when an interaction having a gas happens, the conductance varying depending on the concentration 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 inside 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” . This way, the electrical conductance decreases as resistance within these areas increase due to lack of carriers (i.e. increase effectiveness against current), as there will be a “potential barriers” involving the grains (particles) themselves.
If the sensor exposed to reducing gases (e.g. CO) then your resistance drop, as the gas usually interact with the oxygen and therefore, an electron is going to be released. Consequently, the release in the electron boost the conductivity as it will reduce “the possibility barriers” and allow the electrons to start out to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the tension compression load cell, and consequently, because of this charge carriers is going to be produced.