What is a Gas Sensor?
Gas Sensor is the core of the gas detection system and is usually installed in the detection head. Essentially, a gas sensor is a converter that converts a certain gas volume fraction into a corresponding electrical signal. The probe uses the gas sensor to adjust the gas sample, which usually includes filtering out impurities and interfering gases, drying or refrigeration treatment, sample pump, and even chemical treatment of the sample, so that the chemical sensor can make faster measurements. In the following, we’ll mainly discuss the properties, typical types, and application fields of gas sensors.
Types of Gas Sensors
There are many types of gases with different properties, so there are many types of gas sensors.
Let’s see several typical types of gas sensors:
1, Semiconductor Gas Sensor
Semiconductor gas sensors can be divided into resistive and non-resistive (junction type, MOSFET type, capacitive type). The principle of the resistive gas sensor is that gas molecules cause changes in the resistance of sensitive materials; non-resistive gas sensors mainly include M()s diodes, junction diodes, and field-effect transistors (MOSFET), which uses the principle that the sensitive gases will change MOSFET turn-on voltage, and the principle structure is the same as ISFET (Ion-Sensitive Field Effect Transistor).
(1) Resistive Semiconductor Gas Sensor
1) Working PrincipleIt has been found that SnO2, ZnO, Fe2O3, Cr2O3, MgO, NiO2, and other materials have gas sensing effects. The gas-sensitive thin film made of these metal oxides is an impedance device. The gas molecules and the sensitive film can exchange ions, thus a reduction reaction occurs, and the resistance of the sensitive film changes. As a sensor, this reaction must be reversible, so an oxidation reaction must take place to eliminate gas molecules. The heater in the sensor contributes to the oxidation reaction progress.
SnO2 thin-film gas sensing devices are currently the mainstream products due to their advantages such as good stability, ability to work at lower temperatures and detect many types of gases, and mature processes. In addition, Fe2O3 is currently a widely used and studied material.
In addition to the traditional SnO, SnO2, and Fe2O3, a number of new materials have been 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 broadened the application range of gas sensors.
2) Structure
SnO2 resistive gas sensor usually adopts the sintering process. It adopts porous SnO2 ceramic as the base material, and then several different substances are added and sintered through the ceramic process, and the heating resistance wire and the measuring electrode are embedded during sintering.
In addition, there are thin-film devices and multilayer film devices made by processes such as evaporation and sputtering, which have high sensitivity and good dynamic characteristics. There are also thick-film devices and mixed-film devices made by silk-screen printing. These devices have the advantages of high integration, easy assembly, convenient use, and mass production.
(2) Non-resistive Semiconductor Gas Sensor
The non-resistive type is also a relatively common type of semiconductor gas sensor. This type of device is easy to use and integrate and does not need to set the operating temperature. There are mainly two kinds: types of junction and MOSFET.
2) MOSFET Gas Sensor
The left shift of the characteristic curve of the gas-sensitive diode can be regarded as a change in the diode turn-on voltage. If this characteristic occurs at the gate of the field-effect transistor, the threshold voltage UT of the field-effect transistor will change. Based on this principle, MOSFET gas sensors can be made.
Hydrogen-sensitive MOSFET is one of the most typical gas-sensitive devices. It is made of a palladium gate with metal palladium (Pd). In an atmosphere containing hydrogen, due to the catalytic effect of palladium, hydrogen molecules decompose into hydrogen atoms that diffuse to the interface between palladium and silicon dioxide, and eventually cause the threshold voltage UT of the MOSFET to change. The gate-drain is often shorted when used to ensure that the MOSFET works in the saturation region. At this time, the drain current ID=β(UGS-UT)2. Using this circuit, the concentration of hydrogen can be measured.
2, Solid Electrolyte Gas Sensor
A solid electrolyte is a solid substance having the same ion conductivity characteristics as an aqueous electrolyte solution, and when used as a gas sensor, it is a battery. It does not require the gas to dissolve in the electrolyte through the gas-permeable membrane, which can avoid problems such as solution evaporation and electrode consumption. Because of its high conductivity, good sensitivity, and selectivity, this sensor has been widely used in petrochemical, environmental protection, mining, food, and other fields.
3, Infrared Gas Sensor
(1) Working Principle
Molecules composed of different atoms have unique vibration and rotation frequencies. When they are irradiated with infrared rays of the same frequency, infrared absorption will occur, which will cause changes in infrared light intensity. The gas concentration can be measured by measuring the changes in infrared intensity.
4, Catalytic Combustion Gas Sensor
Generally, a high-purity platinum coil with a diameter of 15um, 20um, or 30um is wrapped with a supported catalyst in the form of a sphere. At a certain temperature, when the flammable gas contacts with the sphere, a drastic flameless combustion reaction is produced by the reaction between the gas and the adsorbed oxygen on its surface. The heat released by the reaction causes the temperature of the platinum coil to change, which leads to resistance change. The resistance can be measured by measuring the gas concentration.
Therefore, rather than saying that the catalytic element is a gas sensor, it is better to say that it is a temperature sensor. In order to overcome the interference caused by the change in ambient temperature, the catalytic element will form a complete element in pairs. One pair reacts to the gas, the other only responds to the ambient temperature, so that the two components collide with each other to eliminate the interference caused by the change in ambient temperature.
Unlike the semiconductor element, the sensing process of the catalytic element is more complicated. The former is the chemical reaction that occurs after the gas contacts the sensor, which directly causes the sensor resistance or electrical signal to change. The latter is the chemical reaction that occurs on the catalytic element, which results in a temperature change on the surface of the sensor carrier and inside the carrier. The temperature change of the carrier eventually leads to a resistance change of the platinum coil through heat transfer, completing the entire sensing process.