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

Conductivity sensors might be made up of metal oxide and polymer elements, both of which exhibit a modification of resistance when exposed to 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 applying, in which the proposed device is going to be trained to analyse, will greatly influence the option of multi axis load cell.

The response from the sensor is actually a two part process. The vapour pressure of the analyte usually dictates how many molecules exist within the gas phase and consequently how many of them will likely be at the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need in order to react with the sensor(s) in order to create a response.

Sensors types found 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. In some cases, arrays could have both of the above 2 kinds of sensors [4].

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

MOS are created from a ceramic element heated with a heating wire and coated with a semiconducting film. They can sense gases by monitoring changes in the conductance through the interaction of a chemically sensitive material with molecules that need to be detected in the gas phase. From many MOS, the material which has been experimented with the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types of MOS can include oxides of tin, zinc, titanium, tungsten, and iridium, doped having a noble metal catalyst such as 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 type of compression load cell is a lot easier to generate and for that reason, cost less to buy. Limitation of Thin Film MOS: unstable, hard to produce and thus, higher priced to purchase. On the other hand, it offers higher sensitivity, and far lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is regarded as the common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared within an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This can be later ground and combined with dopands (usually metal chlorides) and after that heated to recuperate the pure metal as a powder. Just for screen printing, a paste is produced up from your powder. Finally, in a layer of few hundred microns, the paste will be left to cool (e.g. on the alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” in the MOS is definitely the basic principle in the operation inside the sensor itself. A change in conductance occurs when an interaction having 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, whilst the p-type responds to “oxidizing” vapours.

Operation (n-type):

Because the current applied in 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 your conduction band” [2]. This way, the electrical conductance decreases as resistance during these areas increase due to lack of carriers (i.e. increase effectiveness against current), as you will have a “potential barriers” between the grains (particles) themselves.

If the weight sensor subjected to reducing gases (e.g. CO) then your resistance drop, since the gas usually react with the oxygen and thus, an electron is going to be released. Consequently, the discharge from the electron boost the conductivity because 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 from your surface of the sensor, and consequently, because of this charge carriers will likely be produced.