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There are a number of various kinds of sensors which can be utilized as essential components 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, and those employing spectrometry-based sensing methods.

Conductivity sensors could be made from 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) will be examined, because they are well researched, documented and established as vital element for various types of machine olfaction devices. The applying, in which the proposed device will be trained on to analyse, will greatly influence the choice of 3 axis load cell.

The response in the sensor is really a two part process. The vapour pressure of the analyte usually dictates the amount of molecules exist inside the gas phase and consequently what percentage of them will likely be in 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 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 on metal- oxide or conducting polymers. In some instances, arrays might have both of the aforementioned two types of sensors [4].

Metal-Oxide Semiconductors. These sensors were originally created 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 available commercially.

MOS are made of a ceramic element heated by way of a heating wire and coated with a semiconducting film. They can sense gases by monitoring modifications in the conductance through the interaction of a chemically sensitive material with molecules that should be detected within the gas phase. From many MOS, the material 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 may include oxides of tin, zinc, titanium, tungsten, and iridium, doped with 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 an extended period to stabilize, higher power consumption. This sort of compression load cell is a lot easier to produce and for that reason, are less expensive to get. Limitation of Thin Film MOS: unstable, hard to produce and for that reason, more expensive to purchase. On the contrary, it has much higher sensitivity, and much lower power consumption compared to 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 within an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This is later ground and blended with dopands (usually metal chlorides) and then heated to recover the pure metal as a powder. Just for screen printing, a paste is made up through 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” inside the MOS is definitely the basic principle from the operation inside the sensor itself. A change in conductance takes place when an interaction with a gas happens, the conductance varying depending on the concentration 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 to “oxidizing” vapours.

Operation (n-type):

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

When the load cell sensor in contact with reducing gases (e.g. CO) then this resistance drop, as the gas usually interact with the oxygen and thus, an electron is going to be released. Consequently, the release in the electron boost the conductivity as 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 surface of the sensor, and consequently, because of this charge carriers will be produced.