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There are a number of various kinds of sensors which may beutilized as important 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, and those employing spectrometry-based sensing methods.

Conductivity sensors might be composed of metal oxide and polymer elements, each of which exhibit a modification of resistance when exposed to Volatile Organic Compounds (VOCs). In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will likely be examined, as they are well researched, documented and established as essential element for various types of machine olfaction devices. The application form, where proposed device will be trained on to analyse, will greatly influence the option of 3 axis load cell.

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

Sensors types utilized 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. Sometimes, arrays could have both of the above two types of sensors [4].

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

MOS are made from a ceramic element heated with a heating wire and coated with a semiconducting film. They could sense gases by monitoring changes in the conductance during the interaction of a chemically sensitive material with molecules that need to be detected inside the gas phase. Away from many MOS, the material which has been experimented with all the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Different types of MOS can 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 a longer period to stabilize, higher power consumption. This sort of compression load cell is simpler to create and therefore, are less expensive to buy. Limitation of Thin Film MOS: unstable, hard to produce and thus, more costly to get. On the contrary, it has much higher sensitivity, and a lot lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is easily the most common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready in an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide which can be dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This can be later ground and combined with dopands (usually metal chlorides) and after that heated to recover the pure metal being a powder. For the purpose of screen printing, a paste is made up from your powder. Finally, in a layer of few hundred microns, the paste will 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 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 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 in the air commence to interact with the outer lining and accumulate on the surface of the sensor, consequently “trapping free electrons on rocdlr surface through the conduction band” [2]. In this manner, the electrical conductance decreases as resistance during these areas increase because of lack of carriers (i.e. increase resistance to current), as you will have a “potential barriers” involving the grains (particles) themselves.

Once the load cell exposed to reducing gases (e.g. CO) then the resistance drop, because the gas usually react with the oxygen and therefore, an electron will likely be released. Consequently, the release from the electron boost the conductivity as it will reduce “the potential barriers” and enable the electrons to start to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your surface of the sensor, and consequently, as a result of this charge carriers is going to be produced.