Current research trends and development directions of new gas sensors (Part 1)

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Current research trends and development directions of new gas sensors (Part 1) [Copy link]

The gas sensor is the core of the gas detection system and is usually installed in the detection head. In essence, a gas sensor is a converter that converts the volume fraction of a certain gas into a corresponding electrical signal. The detection head conditions the gas sample through the gas sensor, which usually includes filtering out impurities and interfering gases, drying or refrigeration, sample aspiration, and even chemical treatment of the sample to allow the chemical sensor to make faster measurements.
　　The sampling method of the gas directly affects the response time of the sensor. At present, the main sampling methods of gas are simple diffusion or aspiration of the gas into the detector.
　　Simple diffusion uses the natural propagation characteristics of gas. The target gas passes through the sensor in the probe and produces a signal proportional to the volume fraction of the gas. Because the diffusion process gradually slows down, the diffusion method requires the probe to be located very close to the measurement point. One advantage of the diffusion method is that the gas sample is introduced directly into the sensor without physical and chemical transformation. Sample aspiration probes are usually used when the sampling location is close to the processing instrument or exhaust duct. This technology can provide a stable airflow with controllable speed for the sensor, so this method is more recommended when the airflow size and flow rate are often changing. The gas sample from the measuring point may pass through a distance to the measuring probe. The length of the distance depends mainly on the design of the sensor. However, a longer sampling line will increase the measurement lag time, which is a function of the sampling line length and the gas flow rate from the leak point to the sensor. For certain target gases and vapors, such as SiH4 and most biological solvents, the amount of gas and vapor samples may be reduced due to their adsorption or even condensation on the wall of the sampling tube.
　　Gas sensors are a major category of chemical sensors. From working principles, characteristic analysis to measurement technology, from materials used to manufacturing processes, from detection objects to application fields, they can all constitute independent classification standards, deriving a complex and complicated classification system. In particular, there is no unified classification standard at present, and it is difficult to conduct a strict systematic classification.
　　1 Main characteristics
　　1.1 Stability
　　Stability refers to the stability of the basic response of the sensor during the entire working time, which depends on zero drift and interval drift. Zero drift refers to the change in the sensor output response during the entire working time when there is no target gas. Interval drift refers to the change in the output response of the sensor when it is continuously placed in the target gas, which is manifested as a decrease in the sensor output signal during the working time. Ideally, a sensor has a zero drift of less than 10% per year under continuous working conditions.
　　1.2 Sensitivity
　　Sensitivity refers to the ratio of the change in sensor output to the change in the measured input, which mainly depends on the technology used in the sensor structure. Most gas sensors are designed based on biochemistry, electrochemistry, physics and optics. The first thing to consider is to choose a sensitive technology that has sufficient sensitivity to detect the percentage of the valve limit (TLV-thresh-old limit value) or the lower explosive limit (LEL-lower explosive limit) of the target gas.
　　1.3 Selectivity
　　Selectivity is also called cross sensitivity. It can be determined by measuring the sensor response generated by a certain concentration of interfering gas. This response is equivalent to the sensor response generated by a certain concentration of target gas. This feature is very important in applications that track multiple gases, because cross sensitivity will reduce the repeatability and reliability of the measurement. The ideal sensor should have high sensitivity and high selectivity.
　　1.4 Corrosion resistance
　　Corrosion resistance refers to the ability of the sensor to be exposed to a high volume fraction of the target gas. In the case of a large gas leak, the probe should be able to withstand 10 to 20 times the expected gas volume fraction. When returning to normal working conditions, the sensor drift and zero correction values should be as small as possible. The basic characteristics of gas sensors, namely sensitivity, selectivity and stability, are mainly determined by the selection of materials. Select appropriate materials and develop new materials to optimize the sensitive characteristics of gas sensors.
　　2 Main principles and classifications
　　Usually classified by gas-sensitive characteristics, they can be mainly divided into: semiconductor gas sensors, electrochemical gas sensors, solid electrolyte gas sensors, contact combustion gas sensors, photochemical gas sensors, polymer gas sensors, etc.
　　2.1 Semiconductor gas sensors
　　Semiconductor gas sensors are components made of metal oxides or metal semiconductor oxide materials. When interacting with gases, surface adsorption or reaction occurs, causing conductivity or volt-ampere characteristics or surface potential changes characterized by carrier movement. These are all determined by the semiconductor properties of the material.
　　Since the advent of semiconductor metal oxide ceramic gas sensors in 1962, semiconductor gas sensors have become the most commonly used and most practical type of gas sensors. According to their gas-sensitive mechanism, they can be divided into resistive and non-resistive types.
　　Resistive semiconductor gas sensors mainly refer to semiconductor metal oxide ceramic gas sensors, which are impedance devices made of metal oxide films (such as SnO2, ZnO Fe2O3, TiO2, etc.), and their resistance varies with the gas content. Odor molecules undergo reduction reactions on the surface of the film to cause changes in the conductivity of the sensor. In order to eliminate the odor molecules, an oxidation reaction must also occur. The heater in the sensor helps the oxidation reaction process. It has the advantages of low cost, simple manufacturing, high sensitivity, fast response speed, long life, low humidity sensitivity and simple circuit. The disadvantages are that it must work at high temperatures, poor selectivity for odors or gases, dispersed component parameters, unsatisfactory stability, and high power requirements. When sulfides are mixed in the detected gas, it is easy to be poisoned. Now, in addition to the three traditional categories of SnO, SnO2 and Fe2O3, a number of new materials have been studied and developed, including single metal oxide materials, composite metal oxide materials and mixed metal oxide materials. The research and development of these new materials have greatly improved the characteristics and application range of gas sensors. In addition, by adding precious metals such as Pt, Pd, and Ir to the semiconductor, the sensitivity and response time of the element can be effectively improved. It can reduce the activation energy of chemical adsorption of the measured gas, thereby improving its sensitivity and accelerating the reaction rate. Different catalysts are beneficial to different adsorption samples, thus having selectivity. For example, various precious metals dope SnO2-based semiconductor gas-sensitive materials, Pt, Pd, and Au increase the sensitivity to CH4, and Ir reduces the sensitivity to CH4; Pt and Au increase the sensitivity to H2, while Pd reduces the sensitivity to H2. Metal oxide gas sensors manufactured using thin film technology and super particle thin film technology have the characteristics of high sensitivity (up to 10-9 level), good consistency, miniaturization, and easy integration. Non-resistance semiconductor gas sensors are MOS diode type, junction diode type, and field effect tube (MOSFET) semiconductor gas sensors. Its current or voltage changes with the gas content, and it mainly detects combustible gases such as hydrogen and silane gas. Among them, the working principle of MOSFET gas sensor is that volatile organic compounds (VOC) react with catalytic metals (such as buttons) in contact, and the reaction products diffuse to the gate of MOSFET, changing the performance of the device. VOC is identified by analyzing the changes in device performance. Sensitivity and selectivity can be optimized by changing the type and film thickness of catalytic metals, and the operating temperature can be changed. MOSFET gas sensors have high sensitivity, but the manufacturing process is relatively complex and the cost is high.
　　2.2 Electrochemical gas sensors
　　Electrochemical gas sensors can be divided into four types: galvanic cell type, controlled potential electrolysis type, coulometric type and ion electrode type. Galvanic cell type gas sensors detect the volume fraction of gas by detecting current. Almost all commercially available instruments for detecting hypoxia are equipped with this sensor. In recent years, galvanic cell type sensors for detecting acidic and toxic gases have been developed. Controllable potential electrolysis type sensors detect the volume fraction of gas by measuring the current flowing during electrolysis. Unlike galvanic cell type, they require a specific voltage to be applied from the outside. In addition to being able to detect gases such as CO, NO, N02, 02, and S02, they can also detect the volume fraction of oxygen in the blood. Coulometric gas sensors detect the volume fraction of gas by the current generated by the reaction between the measured gas and the electrolyte. Ion electrode type gas sensors appeared earlier. The main advantage of electrochemical gas sensors is that they have high sensitivity and good selectivity in detecting gas.
　　2.3 Solid electrolyte gas sensor
　　Solid electrolyte gas sensor is a chemical battery with an ion conductor as electrolyte. Since the 1970s, solid electrolyte gas sensors have developed rapidly due to their high conductivity, good sensitivity and selectivity. Now they are used in almost all fields such as environmental protection, energy conservation, mining, and automobile industry. They have large output and wide application, second only to metal oxide semiconductor gas sensors. Recently, some foreign scholars have divided solid electrolyte gas sensors into the following three categories:
　　1) Sensors in which the ions derived from the gas to be measured adsorbed in the material are the same as the mobile ions in the electrolyte, such as oxygen sensors.
　　2) Sensors in which the ions derived from the gas to be measured adsorbed in the material are different from the mobile ions in the electrolyte, such as gas sensors composed of solid electrolyte SrF2H and Pt electrodes for measuring oxygen.
　　3) Sensors in which the ions derived from the gas to be measured adsorbed in the material are different from the mobile ions in the electrolyte and the fixed ions in the material, such as the newly developed high-quality C02 solid electrolyte gas sensor composed of solid electrolyte NASICON (Na3Zr2Si2P012) and auxiliary electrode materials Na2CO3-BaC03 or Li2C03-CaC03, Li2C03- BaC03.
　Most of the newly developed high-quality solid electrolyte sensors belong to the third category. For example: sensors made of solid electrolyte NaSiCON and auxiliary electrode N02-Li2C03 for measuring NO2; sensors made of solid electrolyte YST-Au-W03 for measuring H2S; sensors made of solid electrolyte NH4-Ca203 for measuring NH3; sensors made of solid electrolyte Ag0.4Na7.6 and electrode Ag-Au for measuring NO2, etc.