There are a number of different types of sensors which can be used essential components in numerous 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 from metal oxide and polymer elements, both of which exhibit a change in resistance when exposed 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, because they are well researched, documented and established as essential element for various machine olfaction devices. The application, where proposed device will be trained onto analyse, will greatly influence the choice of weight sensor.
The response in the sensor is a two part process. The vapour pressure of the analyte usually dictates the number of molecules are present within the gas phase and consequently what number of them is going to be in the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need so that you can interact with the sensor(s) to be able to produce a response.
Sensors types utilized 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. Sometimes, arrays could have both of the aforementioned two kinds of sensors .
Metal-Oxide Semiconductors. These miniature load cell were originally manufactured in Japan in the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and therefore are easily available commercially.
MOS are made of a ceramic element heated by way of a heating wire and coated by a semiconducting film. They can sense gases by monitoring changes in the conductance throughout the interaction of the chemically sensitive material with molecules that need to be detected in the gas phase. From many MOS, the content that has been experimented using the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Various kinds of MOS may include oxides of tin, zinc, titanium, tungsten, and iridium, doped having 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 create and therefore, are less expensive to get. Limitation of Thin Film MOS: unstable, difficult to produce and thus, more expensive to buy. On the contrary, it offers higher sensitivity, and far lower power consumption compared to 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 in an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which can be dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This is later ground and mixed with dopands (usually metal chlorides) and then heated to recover the pure metal as being a powder. Just for screen printing, a paste is produced up from your powder. Finally, in a layer of few hundred microns, the paste will likely be left to cool (e.g. on a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” in the MOS will be the basic principle in the operation in the sensor itself. A change in conductance occurs when an interaction with a gas happens, the lexnkg 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, as the p-type responds to “oxidizing” vapours.
Because the current applied involving the two electrodes, via “the metal oxide”, oxygen in the air begin to interact with the surface and accumulate on the top 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 absence of carriers (i.e. increase resistance to current), as you will see a “potential barriers” involving the grains (particles) themselves.
If the torque transducer exposed to reducing gases (e.g. CO) then the resistance drop, since the gas usually react with the oxygen and for that reason, an electron will likely be released. Consequently, the release from the electron increase the conductivity as it will reduce “the possibility barriers” and let the electrons to begin to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the sensor, and consequently, because of this charge carriers will likely be produced.