There are a number of various kinds of sensors which can be used 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, 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 subjected to Volatile Organic Compounds (VOCs). In this particular report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) is going to be examined, as they are well researched, documented and established as essential element for various types of machine olfaction devices. The application, where proposed device will be trained on to analyse, will greatly influence the option of weight sensor.
The response of the sensor is actually a two part process. The vapour pressure of the analyte usually dictates how many molecules can be found inside the gas phase and consequently what number of them is going to be at the sensor(s). Once the gas-phase molecules have reached the sensor(s), these molecules need so that you can react with the sensor(s) to be able to produce 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. In some instances, arrays may contain both of the above two kinds of sensors .
Metal-Oxide Semiconductors. These miniature load cell were originally created in Japan within the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and they are easily available commercially.
MOS are made of a ceramic element heated by way of a heating wire and coated with a semiconducting film. They can sense gases by monitoring alterations in the conductance through the interaction of the chemically sensitive material with molecules that should be detected within the gas phase. From many MOS, the content that has been experimented with all the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Various kinds of MOS may include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst including 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 get. Limitation of Thin Film MOS: unstable, challenging to produce and therefore, higher priced to buy. On the other hand, it provides higher sensitivity, and a lot lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is the most common porous material used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready within an aqueous solution, 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 mixed with dopands (usually metal chlorides) and after that heated to recover the pure metal being a powder. For the purpose of screen printing, a paste is made up from 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” in the MOS will be the basic principle from the operation inside the sensor itself. A modification of conductance happens when an interaction having a gas happens, the lexnkg varying depending on the power of the gas itself.
Metal oxide sensors fall under 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 start to react with the surface and accumulate on the top 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 as a result of absence of carriers (i.e. increase potential to deal with current), as there will be a “potential barriers” between the grains (particles) themselves.
If the rotary torque sensor exposed to reducing gases (e.g. CO) then the resistance drop, as the gas usually interact with the oxygen and for that reason, an electron will be released. Consequently, the release of the electron raise the conductivity because it will reduce “the possible barriers” and allow the electrons to start out to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the top of the sensor, and consequently, because of this charge carriers will be produced.