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There are a number of different types of sensors which can be utilized as essential components in numerous designs for machine olfaction systems.

Electronic Nose (or eNose) sensors belong to five categories [1]: 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, each of which exhibit a modification of resistance when in contact with 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 important element for various machine olfaction devices. The application, where proposed device will likely be trained on to analyse, will greatly influence the choice of multi axis load cell.

The response of the sensor is actually a two part process. The vapour pressure from the analyte usually dictates the amount of molecules can be found in the gas phase and consequently what percentage of them will likely be at the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need so that you can react with the sensor(s) to be able to generate a response.

Sensors types utilized in any machine olfaction device can be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. according to metal- oxide or conducting polymers. In some cases, arrays may contain both of the above two types of sensors [4].

Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan in the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and therefore are easily 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 could sense gases by monitoring modifications in the conductance throughout the interaction of the chemically sensitive material with molecules that ought to be detected inside the gas phase. Away from many MOS, the fabric which has been experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types 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 sort of compression load cell is simpler to generate and therefore, are less expensive to buy. Limitation of Thin Film MOS: unstable, hard to produce and thus, more expensive to purchase. On the contrary, it offers greater sensitivity, and far lower power consumption than the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous materials used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared 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 can be later ground and blended with dopands (usually metal chlorides) and then heated to recoup the pure metal as a powder. Just for screen printing, a paste is produced up through the powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on the alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” within the MOS will be the basic principle from the operation in the sensor itself. A change in conductance happens when an interaction with a gas happens, the conductance varying depending on the concentration 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, while the p-type responds to “oxidizing” vapours.

Operation (n-type):

As the current applied involving the two electrodes, via “the metal oxide”, oxygen in the air commence to interact with the surface and accumulate on the surface of the sensor, consequently “trapping free electrons on rocdlr surface from the conduction band” [2]. This way, the electrical conductance decreases as resistance during these areas increase due to insufficient carriers (i.e. increase potential to deal with current), as there will be a “potential barriers” involving the grains (particles) themselves.

Once the load cell sensor in contact with reducing gases (e.g. CO) then your resistance drop, as the gas usually react with the oxygen and therefore, an electron will likely be released. Consequently, the discharge of the electron increase the conductivity because it will reduce “the possibility barriers” and let the electrons to start out to circulate . 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 is going to be produced.