Gas sensors are mainly of semiconductor type, contact combustion type, chemical reaction type, light interference type, thermal conduction type, infrared absorption and dispersion type, and the like. Among them, semiconductor gas sensors are more widely used.
The semiconductor gas sensor is composed of a gas sensing part, a heating wire, and an explosion-proof net. It is a sensor that adds sensitizers such as Pt and Pd to metal oxides such as SnO2, Fe2O2, and ZnO2 in the gas sensing part. The selectivity of the sensor is controlled by the amount of sensitizer added. For example, for ZnO2 series sensors, if Pt is added, the sensor has higher sensitivity to propane and isobutane; if Pd is added, it is more sensitive to CO and H2 .
Structural characteristics of semiconductor gas sensors
How semiconductor gas sensors work
The gas sensor uses a ceramic tube as a frame and is covered with a layer of sensitive film material, and the gold-plated pins at both ends of the film are used for measurement. The most commonly used materials for sensitive membranes are metal oxides, polymer materials, and colloidal sensitive membranes. Its two key parts are the heating resistor and the gas-sensitive membrane. The gold electrode connects the two ends of the gas-sensitive material, making it equivalent to a resistance whose resistance value changes with the concentration of the external gas to be measured. Because metal oxides have high thermal stability, and this sensor only produces a reversible redox reaction on the surface layer of the semiconductor, the internal chemical structure of the semiconductor remains unchanged, so high stability can be obtained for long-term use. The principle is as follows:
Once the metal oxide is heated, the oxygen in the air will take away the electrons from the donor level of the metal oxide semiconductor crystal particles, and the negative electrons will be adsorbed on the crystal surface, which will increase the surface potential and thus hinder the movement of conductive electrons. The gas sensor has a constant resistance value in the air. At this time, the reducing gas reacts with the oxygen adsorbed on the surface of the semiconductor, and the surface potential of the gas molecule changes due to the desorption of gas molecules. Therefore, the resistance value of the sensor changes. For reducing gases, the resistance value decreases; for oxidizing gases, the resistance value increases. In this way, the concentration of the gas can be detected based on the change in the resistance value.