There are a number of various kinds of sensors which may beused as important components in different designs for machine olfaction systems.
Electronic Nose (or eNose) sensors fall into five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and these employing spectrometry-based sensing methods.
Conductivity sensors could be composed of metal oxide and polymer elements, both of which exhibit a modification of resistance when subjected 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, because they are well researched, documented and established as essential element for various machine olfaction devices. The application, in which the proposed device will be trained onto analyse, will greatly influence deciding on a multi axis load cell.
The response from the sensor is really a two part process. The vapour pressure in the analyte usually dictates the number of molecules can be found within the gas phase and consequently what percentage of them is going to be on the sensor(s). When the gas-phase molecules are in the sensor(s), these molecules need in order to react with the sensor(s) so that you can generate a response.
Sensors types used in any machine olfaction device may be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based upon metal- oxide or conducting polymers. In some instances, arrays may contain both of the above two types of sensors .
Metal-Oxide Semiconductors. These sensors were originally created in Japan within the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and they are widely accessible commercially.
MOS are created from a ceramic element heated by a heating wire and coated by a semiconducting film. They can sense gases by monitoring modifications in the conductance through the interaction of any chemically sensitive material with molecules that need to be detected within the gas phase. Out of many MOS, the fabric 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 using a noble metal catalyst like 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 miniature load cell is easier to generate and for that reason, cost less to buy. Limitation of Thin Film MOS: unstable, difficult to produce and for that reason, more expensive to get. On the contrary, it provides much higher sensitivity, and much lower power consumption compared to thick film MOS device.
Manufacturing process. Polycrystalline is easily the most common porous material used for thick film sensors. It is almost always 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 which is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This is later ground and combined with dopands (usually metal chlorides) and after that heated to recuperate the pure metal as a powder. For the purpose of screen printing, a paste is made up from your powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” within the MOS is the basic principle from the operation inside the sensor itself. A change in conductance occurs when an interaction with a gas happens, the conductance varying depending on the concentration of the gas itself.
Metal oxide sensors belong to 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.
Since the current applied in between the two electrodes, via “the metal oxide”, oxygen in the air commence to react with the outer lining and accumulate on the surface of the sensor, consequently “trapping free electrons on rocdlr surface from the conduction band” . In this way, the electrical conductance decreases as resistance within these areas increase due to insufficient carriers (i.e. increase resistance to current), as you will have a “potential barriers” involving the grains (particles) themselves.
Once the load cell sensor in contact with reducing gases (e.g. CO) then the resistance drop, since the gas usually react with the oxygen and for that reason, an electron will be released. Consequently, the discharge from the electron boost the conductivity since it will reduce “the possible barriers” and enable the electrons to begin to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the sensor, and consequently, as a result of this charge carriers will be produced.