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THMG149 – Metal Oxide Sensors (Deep)

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In this episode we go a little deeper than normal into the Metal Oxide Sensors than we typically do, but at the end we bring it back home to a superb finish (as usual)

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Metal Oxide Semiconductor (MOS) Sensors

Metal Oxide Semiconductors, or MOS, are used for detecting toxic gases (commonly carbon monoxide and combustible gases) and work via a gas sensitive film that is composed of tin or tungsten oxides.

The sensitive film reacts with gases, triggering the device when toxic levels are present. Generally, metal oxide sensors are considered efficient due their ability to operate in low-humidity ranges. In addition, they can detect a range of gases, including combustibles. 

 

The chemistry basics of Metal Oxide Semiconductor (MOS) sensors include Reduction reactions, Oxidation reactions and understanding semiconductor functions.  

    1. Reduction is chemical reaction that involves the gaining of electrons by one of the atoms involved in the reaction.  The term refers to the element that accepts electrons, as the oxidation state of the element that gains electrons is lowered.
      1. An example of a reduction is when iron reacts with oxygen, forming a chemical called rust. In that example, the iron is oxidized, and the oxygen is reduced.
    2. Oxidation is any chemical reaction that involves the moving of electrons. Specifically, it means the substance that gives away electrons is oxidized.
      1. When iron reacts with oxygen it forms a chemical called rust because it has been oxidized (The iron has lost some electrons.) and the oxygen has been reduced (The oxygen has gained some electrons.).
      2. Oxidation is the opposite of reduction.
        1. A reduction-reaction always comes together with an oxidation-reaction.
        2. Oxidation and reduction together are called redox (reduction and oxidation).
        3. Oxygen does not have to be present in a reaction, for it to be a redox-reaction.
        4. Oxidation is the loss of electrons.
    3. Oxidizing agent can have two meanings.
      1. It could be a chemical that releases oxygen atoms. For example, potassium chlorate has a chemical formula of KClO3. When it oxidizes a reducing agent, such as powdered aluminum metal, it loses its oxygen to the aluminum and becomes potassium chloride, KCl.
      2. Another definition is a chemical that accepts electrons from a reducing agent. For example, potassium permanganate has an oxidation state of +7. In acid solution, it gains 5 electrons (e), becoming a manganese compound with an oxidation state of +2. Most oxidizing agents of the second (electron-accepting) definition have oxygen, but not all. For example, fluorine (F2), the most powerful oxidizing agent, does not have any oxygen in it. When it acts as an oxidizing agent, it gains an electron to transfer from an oxidation state of 0 to an oxidation state of -1.
      3. Oxidation is gain of electrons and we can remember it from LEO-GER: Loss of Electron Oxidation and Gain of Electron Reduction.
    4. Reducing agent is a chemical that gives away electrons to another chemical compound, the oxidizing agent.
      1. For example, zinc is a reducing agent. When it reacts with an oxidizing agent, it gives up two electrons, changing its oxidation state from 0 to +2.
      2. All chemical elements have an oxidation state of 0.
    5. Semiconductor
      1. semiconductor material has an electrical conductivity value falling between that of a conductor – such as copper, gold etc. – and an insulator, such as glass.
      2. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal.
      3. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities (“doping“) into the crystal structure.
        1. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electronsions and electron holes at these junctions is the basis of diodestransistors and all modern electronics.
      4. Semiconductor sensors detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor.
        1. Tin dioxide is the most common material used in semiconductor sensors, and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas.
        2. The resistance of the tin dioxide is typically around 50 kΩ in air but can drop to around 3.5 kΩ in the presence of 1% methane.
        3. This change in resistance is used to calculate the gas concentration.
        4. Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gases such as carbon monoxide.
          1. One of the most common uses for semiconductor sensors is in carbon monoxide sensors. They are also used in breathalyzers.
          2. Because the sensor must come in contact with the gas to detect it, semiconductor sensors work over a smaller distance than infrared point or ultrasonic detectors.

 

  • Operating Principle

 

    1. When semiconductor particles (typically tin dioxide) are heated in air at high temperature, oxygen is adsorbed on the particle surface by capturing free electrons.
    2. In the most extreme case where oxygen concentration is 0%, when metal oxide sensor material (typically tin dioxide [SnO2-x]) is heated at high temperature such as 400˚C, free electrons flow through the conjoined parts (grain boundary) of tin dioxide crystals.
    3. In clean air (approx. 21% O2), oxygen is adsorbed on the metal oxide surface.
      1. With its high electron affinity, adsorbed oxygen attracts free electrons inside the metal oxide, forming a potential barrier (eVs in air) at the grain boundaries.
      2. This potential barrier prevents electron flow, causing high sensor resistance in clean air. When the sensor is exposed to combustible gas or reducing gas (such as carbon monoxide), the oxidation reaction of such gas with adsorbed oxygen occurs at the surface of tin dioxide.
      3. As a result, the density of adsorbed oxygen on the tin dioxide surface decreases, and the height of the potential barrier is reduced.
      4. Electrons easily flow through the potential barrier of reduced height, and the sensor resistance decreases. Gas concentration in air can be detected by measuring the resistance change of MOS-type gas sensors. The chemical reaction of gases and adsorbed oxygen on the tin dioxide surface varies depending on the reactivity of sensing materials and working temperature of the sensor.

 


 As described above, MOS type gas sensors change resistance (R) as a result of a change in adsorbed oxygen concentration.

      1. If this is used adequately, one can detect reducing gases like carbon monoxide.
      2. The adsorbed oxygen formed in clean air will be consumed on contact with carbon monoxide, the resulting decrease of R being used to estimate the concentration of carbon monoxide.
      3. The sensor recovers the original level of resistance when carbon monoxide is off.

Such a detection mechanism is operative in tin dioxide-based gas sensors

The Hazmat Guys

Author: The Hazmat Guys

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