There are a number of various kinds of sensors which can be used as essential parts in numerous 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, and those employing spectrometry-based sensing methods.
Conductivity sensors could 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 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 machine olfaction devices. The application form, where proposed device will likely be trained on to analyse, will greatly influence deciding on a weight sensor.
The response in the sensor is actually a two part process. The vapour pressure from the analyte usually dictates the amount of molecules can be found within the gas phase and consequently how many of them will be at the sensor(s). If the gas-phase molecules have reached the sensor(s), these molecules need in order to interact with the sensor(s) to be able to create a response.
Sensors types utilized 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 aforementioned two kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan in the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and they are widely available commercially.
MOS are created from a ceramic element heated by a heating wire and coated by way of a semiconducting film. They could sense gases by monitoring alterations in the conductance throughout the interaction of the chemically sensitive material with molecules that should be detected inside the gas phase. From many MOS, the fabric which has been experimented with the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst such as 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 sort of MOS is easier to generate and thus, cost less to purchase. Limitation of Thin Film MOS: unstable, challenging to produce and thus, more expensive to get. On the contrary, it provides greater sensitivity, and far lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is easily 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 in an aqueous solution, to which is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This is later ground and mixed with dopands (usually metal chlorides) and then heated to recover the pure metal as a powder. With regards to screen printing, a paste is produced 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” within the MOS will be the basic principle of the operation inside the button load cell itself. A modification of conductance takes place when an interaction using a gas happens, the conductance varying depending on the concentration of the gas itself.
Metal oxide sensors belong to 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, while the p-type responds cqjevg “oxidizing” vapours.
Since the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air start to interact with the outer lining 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 during these areas increase as a result of insufficient carriers (i.e. increase effectiveness against current), as there will be a “potential barriers” in between the grains (particles) themselves.
When the sensor in contact with reducing gases (e.g. CO) then your resistance drop, as the gas usually interact with the oxygen and therefore, an electron will be released. Consequently, the release of the electron raise the conductivity as it will reduce “the potential barriers” and let the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the inline load cell, and consequently, because of this charge carriers is going to be produced.