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 composed of metal oxide and polymer elements, each 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, because they are well researched, documented and established as vital element for various machine olfaction devices. The application, where the proposed device is going to be trained on to analyse, will greatly influence deciding on a weight sensor.
The response in the sensor is a two part process. The vapour pressure in the analyte usually dictates the amount of molecules exist in the gas phase and consequently what number of them is going to be on the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need in order to interact with the sensor(s) to be able to generate a response.
Sensors types utilized in any machine olfaction device may be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based on metal- oxide or conducting polymers. In some cases, arrays might have both of the aforementioned two kinds of sensors .
Metal-Oxide Semiconductors. These micro load cell were originally manufactured in Japan in the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and are widely available commercially.
MOS are created from a ceramic element heated by a heating wire and coated by a semiconducting film. They could sense gases by monitoring modifications in the conductance during the interaction of the chemically sensitive material with molecules that need to be detected in the gas phase. Out of many MOS, the content which was experimented using the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Different types of MOS can 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 an extended period to stabilize, higher power consumption. This kind of MOS is easier to create and therefore, are less expensive to get. Limitation of Thin Film MOS: unstable, hard to produce and for that reason, more costly to buy. On the contrary, it offers greater sensitivity, and much lower power consumption compared to 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 that is dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This is later ground and blended with dopands (usually metal chlorides) and after that heated to recuperate the pure metal being a powder. With regards to screen printing, a paste is created 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” in the MOS is definitely the basic principle from the operation in 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.
As the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air begin to react with the top and accumulate on the top of the sensor, consequently “trapping free electrons on the surface through the conduction band” . In this way, the electrical conductance decreases as resistance within these areas increase as a result of absence of carriers (i.e. increase resistance to current), as you will see a “potential barriers” between the grains (particles) themselves.
If the torque sensor in contact with reducing gases (e.g. CO) then the resistance drop, as the gas usually interact with the oxygen and for that reason, an electron is going to be released. Consequently, the production in the electron boost the conductivity because it will reduce “the possibility barriers” and enable the electrons to start out to circulate . 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.