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

Electronic Nose (or eNose) sensors fall into 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, both of which exhibit a modification of 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) is going to be examined, since they are well researched, documented and established as important element for various machine olfaction devices. The application, in which the proposed device is going to be trained onto analyse, will greatly influence the choice of load cell sensor.

The response of the sensor is really a two part process. The vapour pressure of the analyte usually dictates the amount of molecules can be found in the gas phase and consequently what percentage of them is going to be on the sensor(s). When the gas-phase molecules are in the sensor(s), these molecules need to be able to react with the sensor(s) to be able to produce a response.

Sensors types found in any machine olfaction device could be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based upon metal- oxide or conducting polymers. Sometimes, arrays might have both of the above 2 kinds of sensors [4].

Metal-Oxide Semiconductors. These sensors were originally produced in Japan in the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and therefore are widely accessible commercially.

MOS are created from a ceramic element heated by a heating wire and coated with a semiconducting film. They are able to sense gases by monitoring modifications in the conductance through the interaction of the chemically sensitive material with molecules that ought to be detected inside the gas phase. Out of many MOS, the fabric which was experimented using 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 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 a longer period to stabilize, higher power consumption. This sort of MOS is easier to create and therefore, cost less to get. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, more expensive to buy. On the contrary, it provides greater sensitivity, and much lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous materials 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 can be dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This can be later ground and combined with dopands (usually metal chlorides) and after that heated to recuperate the pure metal as being a powder. Just for screen printing, a paste is produced up from your 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” in the MOS will be the basic principle of the operation in the compression load cell itself. A change in conductance takes place when an interaction having a gas happens, the conductance varying depending on the power of the gas itself.

Metal oxide sensors belong to 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, as the p-type responds cqjevg “oxidizing” vapours.

Operation (n-type):

Since the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air commence to react with the outer lining and accumulate on the top of the sensor, consequently “trapping free electrons on the surface through the conduction band” [2]. This way, the electrical conductance decreases as resistance in these areas increase because of lack of carriers (i.e. increase effectiveness against current), as you will have a “potential barriers” between the grains (particles) themselves.

Once the sensor subjected to reducing gases (e.g. CO) then this resistance drop, as the gas usually interact with the oxygen and therefore, an electron will be released. Consequently, the discharge of the electron increase the conductivity since it will reduce “the possible barriers” and enable the electrons to start out to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the inline load cell, and consequently, as a result of this charge carriers will be produced.

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