There are a number of different types of sensors which can be used as essential parts in various designs for machine olfaction systems.
Electronic Nose (or eNose) sensors belong to 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 exposed to Volatile Organic Compounds (VOCs). In this particular 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 essential element for various types of machine olfaction devices. The application, in which the proposed device will be trained on to analyse, will greatly influence deciding on a 3 axis load cell.
The response in the sensor is really a two part process. The vapour pressure in the analyte usually dictates how many molecules are present within the gas phase and consequently how many of them is going to be on the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) to be able to generate a response.
Sensors types found in any machine olfaction device may be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. according to metal- oxide or conducting polymers. Sometimes, arrays could have both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally produced in Japan within the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and therefore are widely available commercially.
MOS are made from a ceramic element heated by way of a heating wire and coated by way of 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. Away from many MOS, the content which was experimented with the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Several types of MOS can 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 a longer time to stabilize, higher power consumption. This type of compression load cell is easier to create and thus, are less expensive to buy. Limitation of Thin Film MOS: unstable, hard to produce and therefore, higher priced to get. On the other hand, it has higher sensitivity, and much lower power consumption compared to the thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous materials for thick film sensors. It will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready inside an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which can be dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) and after that heated to recuperate the pure metal as a powder. With regards to screen printing, a paste is produced up from the powder. Finally, in a layer of few hundred microns, the paste will likely be left to cool (e.g. over a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” within the MOS is definitely the basic principle in the operation in the sensor itself. A modification of conductance occurs when an interaction using a gas happens, the conductance varying depending on the power of the gas itself.
Metal oxide sensors fall into 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 start to react with the surface and accumulate on the top of the sensor, consequently “trapping free electrons on rocdlr surface through the conduction band” . In this way, the electrical conductance decreases as resistance during these areas increase as a result of absence of carriers (i.e. increase resistance to current), as there will be a “potential barriers” in between the grains (particles) themselves.
Once the load cell exposed to reducing gases (e.g. CO) then this resistance drop, as the gas usually interact with the oxygen and for that reason, an electron will likely be released. Consequently, the production from the electron raise the conductivity because it will reduce “the potential barriers” and allow the electrons to start out to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the sensor, and consequently, due to this charge carriers will be produced.