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There are a number of various kinds of sensors which may beutilized as essential parts in different 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, which employing spectrometry-based sensing methods.

Conductivity sensors may be made up of metal oxide and polymer elements, each of which exhibit a change in resistance when in contact with Volatile Organic Compounds (VOCs). In this particular 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 essential element for various types of machine olfaction devices. The application, in which the proposed device will be trained onto analyse, will greatly influence the option of 3 axis force sensor.

The response in the sensor is really a two part process. The vapour pressure of the analyte usually dictates the amount of molecules are present inside the gas phase and consequently how many of them is going to be in the sensor(s). When the gas-phase molecules have reached the sensor(s), these molecules need so that you can interact with the sensor(s) so that you can create a response.

Sensors types utilized in any machine olfaction device can be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based on metal- oxide or conducting polymers. In some cases, arrays could have both of the aforementioned two types of sensors [4].

Metal-Oxide Semiconductors. These sensors were originally created 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 they are widely accessible commercially.

MOS are made from a ceramic element heated with a heating wire and coated by a semiconducting film. They could sense gases by monitoring alterations in the conductance throughout the interaction of the chemically sensitive material with molecules that need to be detected within the gas phase. Away from many MOS, the fabric which was experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped having 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 an extended period to stabilize, higher power consumption. This kind of miniature load cell is a lot easier to generate and therefore, cost less to get. Limitation of Thin Film MOS: unstable, challenging to produce and therefore, more expensive to get. On the contrary, it provides much higher sensitivity, and far lower power consumption than the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous materials used for thick film sensors. It is almost always 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 really is later ground and mixed with dopands (usually metal chlorides) then heated to recover the pure metal being a powder. For the purpose of screen printing, a paste is created up from the powder. Finally, in a layer of few hundred microns, the paste is going to 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 inside the sensor itself. A modification of conductance happens when an interaction with a gas happens, the conductance varying depending on the concentration of the gas itself.

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

Operation (n-type):

As the current applied between the two electrodes, via “the metal oxide”, oxygen within the air commence to interact with the top 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 during these areas increase because of lack of carriers (i.e. increase resistance to current), as you will have a “potential barriers” in between the grains (particles) themselves.

Once the load cell exposed to reducing gases (e.g. CO) then the resistance drop, since the gas usually interact with the oxygen and therefore, an electron is going to be released. Consequently, the release of the electron increase the conductivity since it will reduce “the possibility barriers” and allow the electrons to start to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the surface of the sensor, and consequently, due to this charge carriers will be produced.

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