There are a number of several types of sensors that you can use as essential components in numerous 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 might be made up of metal oxide and polymer elements, both of which exhibit a change in resistance when exposed to Volatile Organic Compounds (VOCs). In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, because they are well researched, documented and established as important element for various types of machine olfaction devices. The application form, in which the proposed device will be trained to analyse, will greatly influence deciding on a weight sensor.
The response from the sensor is actually 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 likely be in the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need to be able to react with the sensor(s) to be able to create a response.
Sensors types used in any machine olfaction device can be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based upon metal- oxide or conducting polymers. In some cases, arrays may contain both of the above two kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan within 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 made of a ceramic element heated with a heating wire and coated by way of a semiconducting film. They could sense gases by monitoring changes in the conductance throughout the interaction of a chemically sensitive material with molecules that should be detected in the gas phase. Out of many MOS, the fabric that 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 may 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 type of MOS is a lot easier to generate and for that reason, cost less to buy. Limitation of Thin Film MOS: unstable, challenging to produce and thus, more costly to buy. On the contrary, it provides much higher sensitivity, and a lot lower power consumption than the 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 in an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide that 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) then heated to recover the pure metal as being a powder. For the purpose of screen printing, a paste is created up from your 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” within the MOS will be the basic principle of the operation in the miniature load cell itself. A modification of conductance occurs 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, as the p-type responds cqjevg “oxidizing” vapours.
As the current applied involving the two electrodes, via “the metal oxide”, oxygen inside the air begin to react with the outer lining and accumulate on the top of the sensor, consequently “trapping free electrons on the surface through the conduction band” . 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 sensor exposed to reducing gases (e.g. CO) then the resistance drop, because the gas usually react with the oxygen and thus, an electron will be released. Consequently, the release in the electron raise the conductivity since it will reduce “the potential barriers” and let the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your top of the tension load cell, and consequently, as a result of this charge carriers is going to be produced.