There are a number of different types of sensors which can be used as 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 could be made up of 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 be examined, since they are well researched, documented and established as essential element for various types of machine olfaction devices. The application form, where proposed device is going to be trained onto analyse, will greatly influence the choice of load sensor.
The response in the sensor is a two part process. The vapour pressure from the analyte usually dictates how many molecules can be found within the gas phase and consequently what percentage of them is going to be at the sensor(s). When the gas-phase molecules are in the sensor(s), these molecules need in order to interact with the sensor(s) so that you can generate a response.
Sensors types used 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 may contain both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These miniature load cell were originally produced in Japan within the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and are widely accessible commercially.
MOS are created from a ceramic element heated by way of a heating wire and coated with a semiconducting film. They could sense gases by monitoring alterations in the conductance during the interaction of the chemically sensitive material with molecules that should be detected within the gas phase. Out of 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. Different types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped having 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 time to stabilize, higher power consumption. This type of MOS is easier to produce and for that reason, are less expensive to buy. Limitation of Thin Film MOS: unstable, difficult to produce and for that reason, more expensive to buy. On the contrary, it has much higher sensitivity, and a lot lower power consumption compared to the thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared inside an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This really is later ground and blended with dopands (usually metal chlorides) then heated to recover the pure metal as a powder. With regards to screen printing, a paste is made up from your powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” inside the MOS is the basic principle of the operation in the sensor itself. A change in 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, whilst the p-type responds to “oxidizing” vapours.
Since the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air start to react with the surface and accumulate on the surface of the sensor, consequently “trapping free electrons on the surface through the conduction band” . In this manner, the electrical conductance decreases as resistance within these areas increase because of lack of carriers (i.e. increase potential to deal with current), as you will have a “potential barriers” involving the grains (particles) themselves.
When the torque transducer in contact with reducing gases (e.g. CO) then this resistance drop, because the gas usually react with the oxygen and thus, an electron will likely be released. Consequently, the release of the electron increase the conductivity since it will reduce “the possibility barriers” and let the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the surface of the sensor, and consequently, because of this charge carriers will likely be produced.