Study On Gas Sensing Properties And Mechanism Of ZnO Based Acetylene Gas Sensors Doped With Various Metals

Dissolved gas analysis is one of the most effective methods for detecting faults in oil-immersed power transformers at early stage. The core of the technique is gas sensing, which directly affects the accuracy, stability and service life of the online analysis system. The current application of gas sensor due to low sensitivity, aging or poisoning and other characteristics, has restricted the application of gas on-line monitoring technology. For further study of gas sensing technology and develop new gas detection sensor to enhance the level of dissolved gas in transformer oil online monitoring has the positive significance. One commonly used semiconductor gas sensor is zinc oxide(Zn O) gas sensor. On one hand, Zn O gas sensor is inexpensive, easy to maintain and sensitive to all types of fault characteristic gases; on the other hand, it has several drawbacks, such as, high operating temperature, selective and short service life. Moreover, Zn O gas sensor cannot achieve standard accuracy when used for C2H2 detection. This research is funded by the National Natural Science Foundation of China(No. 51277185). The main focus is C2H2 detection for transformer discharge fault diagnosis. We have built Zn O gas sensors with four different types of nanostructures, as well as Ni-doped and Ag-doped sensors. The measurements include analysis of the temperature and density characteristics, the minimum detectable concentration, response recovery characteristics, stability and selectivity of the sensors. The models(pure zinc oxide, Ni-doped zinc oxide, Ag-doped zinc oxide, oxygen adsorption and C2H2 gas adsorption) are built based on first principles theory. We simulated the doping characteristics and gas adsorption properties of Zn O. The macroscopic and microscopic simulation analysis produced the following results.We researched pure Zn O nanomaterial on gas-sensing properties of C2H2; Samples with four different morphology, particle, rod, hierarchy flower, hierarchy urchin shaped, are prepared by hydrothermal process. Experiments were done using trace gas test platform and the results show that urchin shaped Zn O is the most sensitive to C2H2 sensing. Additionally, it has the lowest operating temperature and fastest response-recovery time. A wurtzite Zn O gas sensing mechanism was built by applying surface adsorption model. Test results indicate urchin shaped Zn O has the best performance due to a large surface area and a significant amount of porosity. The structure of urchin shaped Zn O allows better gas diffusion, and therefore, increases the amount and the speed of surface charge transfer.We researched Ni-doped Zn O nanomaterial on gas-sensing properties of C2H2; The results of gas sensing measurements show that all Ni doped Zn O has a better performance and is more selective than other sensors, especially with 6 mole percent. We created doping models of Zn O(0001), oxygen adsorption models and C2H2 adsorption models by employing first principles density functional theory. By calculating the state density around doped Zn O(0001) surface, we notice that Ni doping narrows Band gap of the Zn O(0001) surface, and simplifies the movements of charge from valence band to conduction band. Oxygen adsorbed at doping position has more charge movements and the gas sensing properties are improved.We researched Ag-doped Zn O nanomaterial on gas-sensing properties of C2H2; The morphology and particle size of Zn O have no changes after doping Ag. Experiment results indicate that 10 mol% doped nanorod produce lower operating temperature, higher sensitivity and less sensing limitation. In addition, it could pick hydrogen from gas with carbon monoxide and hydrogen. The sensing mechanism of C2H2 of Ag doped Zn O sensors can be explained by barrier model. Doped Ag introduces impurities level near the top of valence band, and the conduction band of Zn O moved towards the low energy end. Consequently, the band gap became even narrower than the Ni doped ones. The interaction between Zn O surface and adsorbed gas is more frequent after gas is adsorbed at Ag site. As a result, a significant increase of charge transfers is noticed compared to pure Zn O and Ni doped Zn O(0001) surface. We observe that the sensing capacities are improved. The experimental results prove that our models can be applied to explain acetylene gas sensing mechanism of Ag doped Zn O.In this paper, we studies the pure Zn O, Ni, Ag doped Zn O C2H2 gas detection feature, based on the first principle of density functional theory establish the pure Zn O, Ni, Ag doped Zn O surface oxygen adsorption model and C2H2 gas adsorption model, simulates the C2H2 with Zn O material surface reaction for the entire process; establish wurtzite Zn O C2H2 gas gas sensor simulation mechanism, reveals the metal doped with different metal doped Zn O acetylene gas sensor mechanism, and improve the performance of gas sensor for development of high performance semiconductor Zn O acetylene gas sensor to provide a theoretical guidance.