Gas Sensor

A device that converts the physical and chemical effects of specific gas components into electrical signals and measures them. Typical examples are small and easy-to-use devices such as semiconductor sensors for LPG found at construction sites, and are used in a wide range of fields. There are many types, and they can be used with a wide range of gases. The conversion of sensor output into an electrical signal can be active (where the sensor itself generates electricity, such as solid electrolyte sensors and electrochemical sensors) or passive (where the sensor observes changes in electrical signals from outside). [ I ] Semiconductor sensors: These utilize the change in electrical conductivity caused by the interaction between the semiconductor and the sample gas, and can be broadly divided into two types. (1) Surface-controlled type: These utilize the fact that when a gas is adsorbed onto the surface of an oxide-type semiconductor, electrons are exchanged, causing a change in electrical resistance. They are often used for combustible gases ( _H2 , _CH4 , CO, etc.), but can also be used for _NO2 , _O3 , and fluorocarbons. _SnO2 and ZnO are often used as sensor materials, but other metal oxides and organic semiconductors can also be used. (2) Bulk control type: A gas is introduced into the bulk portion of the semiconductor, and the change in electrical resistance accompanying changes in composition is examined. A semiconductor with lattice defects is used. [ II ] Surface potential sensor: Utilizes changes in surface potential and interfacial potential due to gas adsorption. (1) Diode type: Utilizes the phenomenon in which the rectification of a metal/semiconductor junction diode changes due to gas adsorption. For example, Pd/CdS _or Pt/TiO2 for _H2 , or Au/ _TiO2 for _SiH4 . (2) FET type: A MOSFET (metal oxide semiconductor field effect transistor) in which the metal at the gate is thinned and the change in threshold due to gas adsorption to the gate metal is utilized. For example, Pd/ _SiO2 /Si for _H2 , _NH3 , and _H2S . [ III ] Solid electrolyte sensor: An electrochemical cell with electrodes attached to both sides of a solid electrolyte cell, which measures electrical changes due to the reaction between the sample gas and the electrolyte of the cell. Depending on the method of detecting the response, sensors can be broadly divided into potential detection and electrolytic current detection types. Depending on the type of electrolyte, sensors are also available for measuring _H2 , _Cl2 , _NO2 , _CO2, etc. [ IV ] Catalytic combustion sensor: Alumina carrying fine particles of Pt or Pd as a catalyst is coated on the sensor element, the sample gas is burned, and the heat of combustion is measured. The change in heat is measured by the change in electrical resistance of a Pt wire embedded in the element. It is highly sensitive to combustible gases. It is used to detect CO in the exhaust of water heaters, for example. [ V ] Electrochemical sensor: A cell of aqueous electrolyte solution is fitted with both electrodes and a permeable membrane through which the detected gas can enter (for example, _Pt | _H2SO4 or NaOH|Pt). The sample gas is electrochemically oxidized or reduced in the cell and measured by the change in current. It is often used to measure CO, but can also be used for other gases, _SO2 , _O2 , _SO2 , fluorocarbons, phosphine, etc. [ VI ] Piezoelectric sensor: This sensor makes use of the fact that the resonance of a quartz (or ZnO, etc.) oscillator changes sensitively when a gas is adsorbed on the surface. An adsorbent is applied to the surface of an oscillator such as quartz or ZnO, and this sensor is useful for detecting trace amounts of moisture and other extremely trace components such as environmental and toxic gases, but is also susceptible to interference from other gases. Known examples of use for detecting _SO2 , _NH3 , _SO2 , etc. [ VII ] Optical sensor: An optical fiber equipped with a sensitive layer that adsorbs the sample gas. It measures light absorption or fluorescence emission or quenching. Examples of measuring _H2 , _O2 , CO, _NH3 , _NO2 , etc.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

特定の気体成分のもたらす物理・化学的効果を電気信号にかえて測定する装置．工事の現場などで見られるLPG用半導体センサーの例のように，小型で手軽な装置が代表例で，広い分野で実用されている．多くの種類があり，適用できる気体も多種類である．センサー出力の電気信号への変換は，能動型(センサー自体が発電するもの．たとえば，固体電解質センサー，電気化学センサーなど)と，受動型(外部からの電気信号の変化を見るものなど)がある．【Ⅰ】半導体センサー：半導体と検体ガスの相互作用で電気伝導率が変化することを利用したもので，次の２種類に大別しうる．(1)表面制御型；酸化物型半導体の表面にガスが吸着すると，電子の授受が起こって電気抵抗が変化することを利用したもの．可燃ガス(H₂，CH₄，COなど)に多く用いられるが，NO₂，O₃，フロンなどにも利用できる．センサー材にはSnO₂，ZnOなどが多用されるが，そのほかの金属酸化物や有機半導体も利用できる．(2)バルク制御型；半導体のバルク部分に気体を入れて，その組成の変化に伴う電気抵抗の変化を調べる．格子欠陥半導体を用いる．【Ⅱ】表面電位式センサー：ガス吸着による，表面電位や界面電位の変化を利用するもの．(1)ダイオード型；金属/半導体の接合ダイオードの整流作用がガス吸着によって変化する現象を利用する．たとえば，H₂ 用のPd/CdS，Pt/TiO₂やSiH₄用のAu/TiO₂など．(2)FET型；MOSFET(metal oxide semiconductor field effect transistor)で，ゲート部の金属を薄くして，ゲート金属への気体吸着によるしきい値の変化を利用する．たとえば，H₂，NH₃，H₂S用のPd/SiO₂/Siなど．【Ⅲ】固体電解質センサー：固体電解質のセルの両側に両電極を付けた電気化学電池で，検体の気体とセルの電解質との反応による電気的変化を測定する．応答の検知方式によって，電位検出式と電解電流検出式に大別される．電解質の種類により，H₂，Cl₂，NO₂，CO₂などの測定用のものもつくられている．【Ⅳ】接触燃焼式センサー：PtやPdの微粒子を触媒として担持させたアルミナなどを，センサー素子に被覆して，検体ガスを燃焼させ，その燃焼熱で測定する．熱量変化は，素子に埋め込んだPt線の電気抵抗の変化で調べる．可燃性気体に対して感度が大きい．給湯器の排気中のCO検知用などに利用される．【Ⅴ】電気化学式センサー：電解質水溶液のセルに両極と検知ガスが入る透過膜を付けたもので(たとえば，Pt|H₂SO₄またはNaOH|Ptなど)，セル内で検体ガスを電気化学的に酸化または還元してその電流変化で測定する．CO測定に多く利用されるが，そのほかの気体，SO₂，O₂，SO₂，フロン，ホスフィンなどにも利用されている．【Ⅵ】圧電体センサー：水晶(またはZnOなど)の振動子の共振は，表面にガスを吸着させると敏感に変化することを利用したもの．水晶またはZnOなどの振動子の表面に吸着媒を塗布したもので，微量の水分，そのほか環境ガス，毒性ガスなど極微量成分の検出に有用であるが，ほかの気体による妨害も受けやすい．SO₂，NH₃，SO₂などへの利用例も知られている．【Ⅶ】光学式センサー：光ファイバーに検体の気体を吸着させる感応層を付けたもの．光の吸収または蛍光の発光や消光を測定する．H₂，O₂，CO，NH₃，NO₂などの測定例がある．

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