There are a number of several types of sensors which you can use as essential parts in numerous designs for machine olfaction systems.
Electronic Nose (or eNose) sensors belong to 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 change in 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) will be examined, as they are well researched, documented and established as important element for various machine olfaction devices. The application, where proposed device is going to be trained to analyse, will greatly influence the choice of load cell sensor.
The response from the sensor is actually a two part process. The vapour pressure in the analyte usually dictates how many molecules exist within the gas phase and consequently how many of them is going to be in the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need in order to interact with the sensor(s) in order to produce a response.
Sensors types found in any machine olfaction device can be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. according to metal- oxide or conducting polymers. In some cases, arrays may contain both of the above 2 kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan in the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and therefore are easily available commercially.
MOS are created from a ceramic element heated by a heating wire and coated by a semiconducting film. They could sense gases by monitoring alterations in the conductance through the interaction of the chemically sensitive material with molecules that should be detected in the gas phase. Away from many MOS, the material which was experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Different types of MOS can 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 a longer period to stabilize, higher power consumption. This sort of MOS is a lot easier to produce and thus, cost less to get. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, more expensive to buy. On the other hand, it has greater sensitivity, and much lower power consumption compared to thick film MOS device.
Manufacturing process. Polycrystalline is easily the most common porous materials for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared in an aqueous solution, to which 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 blended with dopands (usually metal chlorides) then heated to recover the pure metal as a powder. For the purpose of screen printing, a paste is produced up from the powder. Finally, in a layer of few hundred microns, the paste will be left to cool (e.g. on a alumina tube or plain substrate).
Sensing Mechanism. Change of “conductance” in the MOS is the basic principle from the operation inside the button load cell itself. A change in conductance occurs when an interaction using a gas happens, the conductance varying depending on the power 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 cqjevg “oxidizing” vapours.
As the current applied involving the two electrodes, via “the metal oxide”, oxygen inside the air commence to interact with the surface and accumulate on the surface of the sensor, consequently “trapping free electrons on the surface from the conduction band” . In this manner, the electrical conductance decreases as resistance within these areas increase as a result of lack of carriers (i.e. increase effectiveness against current), as there will be a “potential barriers” involving the grains (particles) themselves.
When the sensor in contact with reducing gases (e.g. CO) then your resistance drop, as the gas usually interact with the oxygen and for that reason, an electron will be released. Consequently, the release of the electron boost the conductivity as it will reduce “the possible barriers” and let the electrons to start out to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the inline load cell, and consequently, due to this charge carriers is going to be produced.