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There are a number of various kinds of sensors which may beused as important parts in various designs for machine olfaction systems.

Electronic Nose (or eNose) sensors fall under 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, each of which exhibit a modification of 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 likely be examined, since they are well researched, documented and established as vital element for various machine olfaction devices. The application, where the proposed device will likely be trained on to analyse, will greatly influence the option of 3 axis load cell.

The response in the sensor is a two part process. The vapour pressure of the analyte usually dictates how many molecules can be found within the gas phase and consequently how many of them will likely be at the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need in order to interact with the sensor(s) to be able to produce a response.

Sensors types used in any machine olfaction device can 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 aforementioned two types of sensors [4].

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

MOS are made of a ceramic element heated by way of a heating wire and coated by a semiconducting film. They can sense gases by monitoring alterations in the conductance during the interaction of a chemically sensitive material with molecules that should be detected inside the gas phase. Out of many MOS, the fabric which was experimented with all the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Various kinds of MOS may include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst like 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 time to stabilize, higher power consumption. This kind of compression load cell is easier to generate and therefore, cost less to buy. Limitation of Thin Film MOS: unstable, challenging to produce and thus, higher priced to buy. On the other hand, it provides higher sensitivity, and a lot lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is easily the most common porous materials for thick film sensors. It will always be 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 that is dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) then heated to recoup the pure metal as a powder. For the purpose of screen printing, a paste is created 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” within the MOS will be the basic principle of the operation inside the sensor itself. A modification of conductance occurs when an interaction having a gas happens, the conductance varying depending on the concentration of the gas itself.

Metal oxide sensors fall into 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, whilst the p-type responds to “oxidizing” vapours.

Operation (n-type):

Since the current applied between the two electrodes, via “the metal oxide”, oxygen in the air begin to react with the surface 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 as a result of insufficient carriers (i.e. increase resistance to current), as you will see a “potential barriers” in between the grains (particles) themselves.

Once the load cell sensor exposed to reducing gases (e.g. CO) then the resistance drop, as the gas usually interact with the oxygen and for that reason, an electron will be released. Consequently, the discharge of the electron raise the conductivity since it will reduce “the possible barriers” and let the electrons to start to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the top of the sensor, and consequently, due to this charge carriers will be produced.