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There are a number of various kinds of sensors which can be used as essential components in various 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 made from metal oxide and polymer elements, both of which exhibit a change in resistance when in contact with Volatile Organic Compounds (VOCs). In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, as they are well researched, documented and established as essential element for various machine olfaction devices. The application, in which the proposed device will likely be trained onto analyse, will greatly influence the choice of load cell.

The response in the sensor is a two part process. The vapour pressure from the analyte usually dictates the number of molecules are present in the gas phase and consequently how many of them will be in the sensor(s). When the gas-phase molecules have reached the sensor(s), these molecules need to be able to 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. according to metal- oxide or conducting polymers. Sometimes, arrays may contain both of the above 2 kinds of sensors [4].

Metal-Oxide Semiconductors. These compression load cell were originally produced in Japan in the 1960s and utilized 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 created from a ceramic element heated by a heating wire and coated by a semiconducting film. They could sense gases by monitoring changes in the conductance throughout the interaction of a chemically sensitive material with molecules that need to be detected within the gas phase. From many MOS, the fabric which has been experimented using the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Different types of MOS may include oxides of tin, zinc, titanium, tungsten, and iridium, doped using 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 kind of MOS is simpler to create and thus, cost less to get. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, higher priced to get. On the other hand, it provides greater sensitivity, and much lower power consumption compared to the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous materials for thick film sensors. It will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared within an aqueous solution, to 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 mixed with dopands (usually metal chlorides) then heated to recover the pure metal as being a powder. Just for 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. over a alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” in the MOS is the basic principle of the operation inside the sensor itself. A change in conductance takes place when an interaction using a gas happens, the lexnkg 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, whilst the p-type responds to “oxidizing” vapours.

Operation (n-type):

Because the current applied between the two electrodes, via “the metal oxide”, oxygen in the air begin to interact with the top and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from the conduction band” [2]. In this manner, the electrical conductance decreases as resistance during these areas increase as a result of insufficient carriers (i.e. increase resistance to current), as you will have a “potential barriers” involving the grains (particles) themselves.

When the torque transducer in contact with reducing gases (e.g. CO) then this resistance drop, as the gas usually interact with the oxygen and therefore, an electron is going to be released. Consequently, the discharge of the electron raise the conductivity as it will reduce “the possible barriers” and enable the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the surface of the sensor, and consequently, because of this charge carriers will likely be produced.

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