There are a number of different types of sensors which may beutilized as essential parts 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 could be made up of metal oxide and polymer elements, both of which exhibit a change in resistance when exposed to Volatile Organic Compounds (VOCs). Within 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 important element for various types of machine olfaction devices. The application, where proposed device will be trained onto analyse, will greatly influence deciding on a 3 axis force sensor.

The response from the sensor is a two part process. The vapour pressure in the analyte usually dictates the amount of molecules are present within the gas phase and consequently how many of them will be on the sensor(s). When the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) so that you can produce a response.

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

Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan inside the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and they are easily available commercially.

MOS are made of a ceramic element heated with a heating wire and coated with a semiconducting film. They could sense gases by monitoring modifications in the conductance throughout the interaction of the chemically sensitive material with molecules that should be detected in the gas phase. Away from many MOS, the content 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 may include oxides of tin, zinc, titanium, tungsten, and iridium, doped using a noble metal catalyst such as 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 sort of compression load cell is easier to produce and for that reason, are less expensive to get. Limitation of Thin Film MOS: unstable, difficult to produce and for that reason, higher priced to purchase. On the other hand, it provides higher sensitivity, and far lower power consumption compared to the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready within an aqueous solution, to which is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This can be later ground and combined with dopands (usually metal chlorides) then heated to recoup 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 likely be left to cool (e.g. over a alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” inside the MOS is the basic principle from the operation within the sensor itself. A change in conductance takes place when an interaction having a gas happens, the conductance varying depending on the concentration of the gas itself.

Metal oxide sensors fall under 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):

As the current applied in between the two electrodes, via “the metal oxide”, oxygen inside the air begin to interact with the top and accumulate on the top of the sensor, consequently “trapping free electrons on rocdlr surface through the conduction band” [2]. In this way, 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 weight sensor in contact with reducing gases (e.g. CO) then the resistance drop, since the gas usually interact with the oxygen and thus, an electron will be released. Consequently, the release of the electron boost 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 top of the sensor, and consequently, because of this charge carriers is going to be produced.