What is a gas sensor?
Gas sensor is the core of the gas detection system and is usually installed in the detection head. In essence, gas sensors are converters converting a certain gas volume score into the corresponding electrical signal. The probe uses a gas sensor to adjust the gas samples. It usually includes filtering impurities and interfering with gas, drying or freezing treatment, sample pumps, and even chemical treatment of samples, so that chemical sensors can measure faster measurement.
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Features of gas sensor
Generally, the gas sensor has the following characteristics:
1. Stability
Stability refers to the stability of the gas sensor's basic response throughout the working time, depending on zero drift and interval drift. Zero drift refers to changes in the output response of the sensor without gas without gas. Interval drift refers to changes in the output response of the sensor placed in the gas, which is manifested as a reduction in the output signal of the sensor during working hours. Ideally, the annual zero drift of the sensor is less than 10%under continuous working conditions.
2. Sensitivity
Schipidity refers to the ratio of the output change of gas sensors to the input of the measured input, which mainly depends on the technology used in the sensor structure. Most gas sensors use biochemical, electrochemical, physical and optical design. The sensitivity technology of the gas sensor must have sufficient sensitivity to detect the threshold limit (TLV) or the lower limit of explosion (Lel).
3. Selectivity
Selectability is also called cross -sensitivity. It can be determined by measuring sensor response caused by a certain concentration of interference gas. The response is equivalent to a sensor response to a certain concentration of gas. This characteristic is very important in tracking the application of a variety of gases, because cross -sensitivity will reduce the repeatability and reliability of measurement, and the ideal gas sensor usually has high sensitivity and high selectivity.
4. Corrosion resistance
Corrosion resistance refers to the ability of sensors to be exposed to high volumes of gas. When a large amount of gas leaks, the probe should be able to withstand 10 to 20 times the volume score of gas. When the normal working conditions are resumed, the sensor drifting and zero correction should be as small as possible.
The four basic features of gas sensors are mainly determined by the choice of materials. Select the appropriate material and develop new materials to optimize the sensitivity of the gas sensor.
Types of gas sensors
There are many types of gas and different properties, so there are many types of gas sensors.
Let's take a look at several typical gas sensors:
1. Semiconductor gas sensor
Semiconductor gas sensors can be divided into resistance and non -resistance (knot type, MOSFET type, capacitive type). The principle of resistance gas sensor is that the resistance changes in sensitive materials caused by gas molecules; non -resistant gas sensors mainly include M (S) diode, knot diode, and field effect transistor (MOSFET). The principle, the principle structure is the same as the ISFET (ion -sensitive field effect transistor).
(1) Resistance semiconductor gas sensor
1) Working principle
It has been found that SNO2, ZNO, Fe2O3, CR2O3, MGO, NIO2 and other materials have a sensitive effect. The gas -sensitive film made of these metal oxides is an impedance device. Gas molecules and sensitive membranes occur with ion exchanges, reaction reactions occur, and the sensory membrane resistance changes. As a sensor, this reaction must be reversible, so oxidation reactions must occur to eliminate gas molecules. The heater in the sensor helps the oxidation reaction.
SNO2 thin -film gas -sensitive devices have the advantages of good stability, low working temperature, many types of gas types, and mature process. They are current mainstream products. In addition, FE2O3 is currently widely used and researching materials.
In addition to the traditional SNO, SNO2 and Fe2O3, a variety of new materials have also been developed, including single metal oxide materials, composite metal oxide materials and mixed metal oxide materials. The research and development of these new materials has widened the scope of application of gas sensors.
2) Structure
SNO2 resistance gas sensor usually uses sintering process. It uses a porous SNO2 ceramics as a substrate, and then adds several different substances to sinter through the ceramic process. The heating resistance wire and the measurement electrode are embedded during sintering.
In addition, thin film devices and multi -layer membrane devices made of steamed plating, sputtering and other processes have high sensitivity and good dynamic characteristics. There are also thick membrane devices and mixed -membrane devices made by screen printing. These devices have the advantages of high integration, easy assembly, convenient use, and mass production.
The figure below is a typical structure of the resistance gas sensor. It is mainly composed of SNO2 sensitive components, heater, electrodes, bases, and stainless steel mesh. The sensor structure is simple, convenient to use, and can detect restore gas, combustible gas, steam, etc.
Figure 2. Typical structure of resistance gas sensor
3) Main feature parameters
Inherent resistance R0 and working resistance RS
The inherent resistance RO is also called the normal resistance, which refers to the resistance value of the gas sensor under normal air conditions. The working resistance RS indicates the resistance value of the gas sensor in a certain concentration gas.
● Sensitivity S
It is usually represented by S = RS/R0, and sometimes the ratio of the resistance in two different concentrations of gas C1 and C2 is also represented by sensors: s = RS (C2)/R0 (C1).
● Response time T1
It reflects the dynamic characteristics of the sensor, that is, the time required to reach the stable value from the moment when the sensor resistance is in contact with a certain concentration gas. It is usually indicated that the change rate of resistance value under this concentration reaches 63%.
● Recovery time T2
Also known as the extension time, it reflects the dynamic characteristics of the sensor. It refers to the time when the sensor leaves the value of the detection gas to the sensor resistance to the value of the sensor.
● Heating resistance RH and heating power pH
Rh provides the resistance value of the electric wire under the work temperature, and the pH is the heating power required to maintain the normal working temperature.
Resistance gas sensors have the advantages of low cost, simple manufacturing, high sensitivity, fast response speed, long life, low humidity sensitivity, simple circuit and other advantages. However, it is necessary to work at high temperature. The gas selectivity is poor, the component parameters are scattered, the stability is not ideal, and the power requirements are high. And it is easy to poison when detecting sulfides in the gas.
(2) Non -resistant semiconductor gas sensor
Non -resistant is also a relatively common semiconductor gas sensor. This type of device is easy to use and integrate, and does not need to set up working temperature. There are two main types: knot type and MOSFET type.
1) Elimination sensor
The knot gas sensor is also known as the gas -sensitive diode. These gas -sensitive devices use gas to change the rectification characteristics of diodes. The structure is shown in the figure below. The principle is: precious metal PD is selective for hydrogen, and contacts formed in contact with semiconductor.
Figure 3. The structure of the gas sensor
When the diode is biased forward, the electrons flow from semiconductor to metal will increase, so positive direction. When negative bias, the load is basically unchanged. This is the rectifier characteristics of the Schottky diode.
When we detect the atmosphere, due to the adsorption of hydrogen, the function of precious metals changes, the contact potential weakens, and the load increase. As a result, the positive current increases, and the rectifier characteristic curve of the diode will move to the left. The figure below is the characteristic curve of the PD-TIO2 gas-sensitive diode in different H2 concentration air. Therefore, we can detect the hydrogen concentration by measuring the positive current of the diode.
Figure 4 The characteristic curve of the PD-TIO2 qi-sensitive diode under different H2 concentration
2) MOSFET gas sensor
The left movement of the characteristic curve of the gas -sensitive diode can be regarded as changes in the diode -drive voltage. If this feature appears in the gate of the field effect tube, the threshold voltage UT of the field effect tube will change. Based on this principle, MOSFET gas sensors can be made.
Hydride MOSFET is one of the most typical gas -sensitive devices. It is made of a cricket pole with a metal (PD). In a hydrogen -containing atmosphere, due to the catalytic effect of 钯, the hydrogen molecule is decomposed into the interface of hydrogen atoms to the interface of 钯 and silica, which eventually causes the threshold voltage UT of MOSFET to change. Gallery-Discosit is often short-circuited when used to ensure that MOSFET works in the saturated area. At this time, the drain current ID = β (UGS-UT) 2. Using this circuit can measure the concentration of hydrogen.
The characteristics of hydrogen sensitivity MOSFET are:
● sensitivity
When the hydrogen concentration is low, the sensitivity of hydride MOSFET is high. When the hydrogen concentration changes 1ppm, the △ UT value can reach 10mV. When the hydrogen concentration is high, the sensitivity of the sensor will be reduced.
● Selectability to gas
The "gap" between the atoms is just allowed to pass the hydrogen atom. Therefore, the grille is only allowed to pass through the hydrogen, which is good.
● Response time
The response time of the device is affected by temperature and hydrogen concentration. The higher the temperature, the higher the concentration of the hydrogen, and the faster the response. The response time at room temperature is tens of seconds.
● Stability
In practical applications, UT will drift over time. Therefore, we can grow the SIO2 insulation layer in the HCL atmosphere to improve the UT drift.
2. Solid electrolyte gas sensor
Solid electrolyte is a solid substance with the same ion conductivity characteristics of the electrolyte solution, which is a battery when used as a gas sensor. It does not require gas to dissolve in the electrolytic solution through the air -ventilated membrane, which can avoid the evaporation of solution and electrode consumption. This sensor has been widely used in the fields of petrochemical, environmental protection, mining, food and other fields because of high -tech guidance, good sensitivity and selectivity.
Only the same electrolyte has obvious conductivity at high temperature. Oxidation (ZRO2) is a typical gas sensor material. Pure oxidation has a single oblique crystal structure at room temperature. When the temperature rises to about 1000 ° C, the same alien transformation will occur, the single -diagonal structure is transformed into a polycrystalline structure, accompanied by volume contraction and heat absorption reaction. Therefore, it is an unstable structure.
Therefore, ZRO2 is mixed with stabilizers such as alkaline earth CAO or rare earth Y2O3, making it a stable fluorite cubic crystal. The degree of stability is related to the concentration of the stabilizer. After ZRO2 adds a stabilizer, it sinters in an atmosphere of 1,800 ° C. Some ions are generated by calcium ions (ZRO · CAO). Since CA2+is a binary positive ion, ZR4+is a quadrior valence positive ion. To keep the electrical neutral, oxygen ion O2-acupoints will be generated in the crystal. This is the reason why (Zro · CAO) transferred oxygen ions at high temperature. Therefore, (Zro · CAO) becomes a conductor of oxygen ion at 300-800 ° C.
However, in order to truly pass the oxygen ion, the two sides of the solid electrolyte should have different oxygen pressure division (poor oxygen power) to form a thick battery. The structure is shown in the figure. The two sides are porous precious metal electrodes, and the middle of the dense Zro · CAO material is in the middle.
Figure 5. The structure of solid electrolyte gas sensor
The oxygen voltage voltage on both sides of the electrode is PO2 (1) and PO2 (2). The two electrodes occur the following reactions:
(+) Pole: PO2 (2), 2O2- → O2+4E
(-) pole: PO1 (1), O2+4E → 2O2-
The electric potential of the above reaction is expressed in the Energy equation:
It can be seen that at a certain temperature, PO2 (1) is fixed, and the oxygen concentration can be detected by the upper formula.
Fixed PO2 (1) is actually a fixed electrode on the (-) electrode, that is, reference electrodes. There are gas reference electrodes and coexist reference electrodes. Gas reference electrodes can be air or other hybrid gases, such as H2-H2O and CO-CO2 can also form a fixed PO2 (1). Coexistence electrodes of coexistence refer to the mixed powder (solid phase) of metal-metal oxides and low-priced metal oxides-high-priced metal oxides. These mixtures are mixed with oxygen (gas phase) to generate oxidation reactions and form the same oxygen pressure.
In addition to the measurement of oxygen, solid electrolyte sensors such as β-Al2O3, carbonate, NASICON gas sensor, etc. can also be used to measure gas and other gases such as CO, SO2, NH4. In recent years, gas sensors that can be used at low temperatures can also be used to detect positive ions.
3. Infrared gas sensor
(1) Working principle
The molecules composed of different atoms have a unique vibration and rotation frequency. When they are irradiated with infrared rays of the same frequency, infrared absorption will occur, causing changes in infrared light. The gas concentration can be measured by measuring changes in infrared strength.
It should be noted that vibration and rotation are two different forms of movement, which will correspond to different infrared absorption peaks. In addition, vibration and rotation itself have diversity. Therefore, in general, a gas molecule has multiple infrared absorption peaks. According to the position of a single infrared absorption peak, we can only determine which groups in the gas molecules. To accurately determine the type of gas depends on the absorption peak position of the gas in the infrared area, that is, the infrared absorption of the fingerprint map of the gas.
However, when environmental conditions are known, the type of gas can be roughly judged according to the position of a single infrared absorption peak. Because all substances above zero will produce infrared radiation related to temperature, like catalytic elements, in order to eliminate infrared radiation changes caused by changes in ambient temperature, infrared gas sensors will consist of a pair of infrared detectors.
A complete infrared gas sensor consists of infrared light sources, optical cavity, infrared detector and signal conditioning circuit.
Figure 7. The basic structure of infrared absorption
(2) Advantages and D isadvantages
1) Advantages:
● Except for gas composed of the same atom, all gas can be measured.
● Complete variety.
● The induction process itself does not interfere with the induction.
2) Disadvantages:
● Excessive. The infrared gas sensor is essentially a temperature sensor. It changes the temperature of the detector through infrared radiation, thereby changing the electrical characteristics. The sensing process is complicated. The system requires the following characteristics:
The light source must have stable infrared radiation;
Stable optical performance of light cavity; stable;
Filtering filters and infrared detectors are stable.
These problems can be solved by reasonable process and technology, but the manufacturing cost is high, resulting in high price.
● In the common design of broadband infrared light sources and filters and detectors, the filter itself cannot achieve ideal selective filtering, so interference, especially water interference, have always existed. The deeper reason for selective problems is that many different gas molecules will have the same chemical bonds, so similar or even overlap infrared absorption will occur.
● Dust, background radiation, strong adsorption, easy transformation
