Study On Gas Sensing Mechanism And Characteristics Of Molybdenum Disulfide-Based Sensor For Transformer Fault Characteristic Gas

Dissolved gas analysis can effectively identify and predict the type and degree of internal faults in transformers by detecting the composition and concentration of characteristic fault gas.DGA is an important technology for status evaluation and failure prevention of power transformers.With the rapid development of modern sensing technology,semiconductor gas sensor detection method has become a pivotal measures in the field of dissolved gas analysis.Molybdenum sulfide（MoS₂）has been widely applied attribute to its excellent physicochemical property and effective detection performances for a variety of gases.However,pure MoS₂ gas sensitive device exhibits poor performances for dissolved gas,so it is necessary to enhance its properties through metal doping treatment.The microscopic doping modification mechanism and gas sensing mechanism of sensitive materials are still incomplete.Therefore,the theoretical adsorption characteristics and practical detection performances of MoS₂ based sensor for characteristic fault gas should be explored simultaneously.Depending on the National Natural Science Foundation of China,the fault characteristic gases CO,C₂H₄ and C₂H₂ in transformer oil were selected as the analysis objects in this paper.In order to comprehensively explore the modification mechanism of noble metal（Au,Ag）doped MoS₂ and the gas sensing mechanism of modified materials,the structure,energy and electronic characteristics of noble metal doped MoS₂ and gas adsorption MoS₂ systems were discussed in detail based on first principles.The intrinsic and doped MoS₂ samples were prepared by hydrothermal synthesis method,and the chemical element composition and microstructure morphology of the samples were investigated by characterization.The sensor structure with good thermal performances was optimized according to the finite element simulation design and the corresponding MoS₂ based gas sensors were developed.The gas-sensing performances of the MoS₂based sensors to fault characteristic gases CO,C₂H₄ and C₂H₂ were conducted.The main work and conclusion of this paper are as follows:（1）Based on first principles,the modification doping mechanism of noble metal for MoS₂ and the gas sensing mechanism of modified materials were explored from the microscopic point of view,and the detection performances of sensitive materials to characteristic fault gas were predicted.The intrinsic and noble metals Au,Ag doped MoS₂crystal model and their CO,C₂H₄ and C₂H₂ molecule adsorption models were established.By calculating the energy change,charge transfer,structure parameters,the band structure,electron density of states of each models,it can be found that after Au and Ag doping,the dopant atoms transferred 0.043 e and 0.138 e charge to MoS₂ respectively,meanwhile,the band gap and systems obviously narrowed.The small adsorption energy of CO,C₂H₄and C₂H₂ molecules absorbed by MoS₂ implied that the adsorption effect is very weak.After Au or Ag doping,the adsorption energy is still negative,which indicates that the adsorption process was spontaneous.Besides,the absolute value of the adsorption energy in doped systems increased to more than 0.6 e V,suggesting the reaction transformed from weak physical adsorption to chemical adsorption.For CO and C₂H₄ molecule,the maximum amount of charge transfer is 0.191 e and 0.301 e when adsorbed on Au-MoS₂system.As for the adsorption of C₂H₂,the maximum charge transfer is 0.234 e when adsorbed on the Ag-MoS₂ monolayer.（2）The design and fabrication of the MoS₂-based planar sensor with excellent thermal properties were completed.According to the hydrothermal process,the sensitive material samples（MoS₂,Au-MoS₂ and Ag-MoS₂）were successfully prepared,and it was found that the samples were uniform microspheres formed by nanosheets.The samples were proved to be hexagon-crystal system MoS₂.The simulation design of the planar sensing device was conducted based on Comsol sofware,and the thermal properties of the suspended planar sensor based on different insulation layers（SiO₂,Si₃N₄）and heating electrodes（Pd,Cr,Pt）were analyzed.The results showed that the maximum temperature and thermal uniformity of heating area and the temperature decrease degree of non-heating zone were excellent when SiO₂ and Pt were selected as insulation layer and heating electrode.Based on the prepared sensitive materials and the optimized sensor structure,MoS₂,Au-MoS₂and Ag-MoS₂based planar sensors were successfully fabricated.（3）The gas-sensitive detection performances of MoS₂,Au-MoS₂and Ag-MoS₂based planar sensors for fault characteristic gas CO,C₂H₄ and C₂H₂were explored macroscopically based on AES-4SD platform.It was found that the doping of Au and Ag can reduce the optimal operating temperature of MoS₂ sensor,improve the detection response,enhance the effective detection limit and accelerate the response recovery.Under the optimal operating temperature,for 30μL/L CO and C₂H₄ detection,the best performance was Au-MoS₂ sensor with the response of 26.52 and 32.85 respectively,while the highest response（31.82）of 30μL/L C₂H₂ appeared in its detection based on Ag-MoS₂ sensor.（4）It is found that the larger band gap decrease value calculated by theory corresponds to the lower optimal operating temperature in the experimental test.In the detection of the same gas by the different MoS₂-based sensors,the higher the charge transfer amount in the adsorption system,the higher the response value in the gas sensing experiment.Therefore,the correlations between the change value of band gap and the optimum operating temperature,and the charge transfer amount and the response value were proposed by the comprehensive analysis of the simulation results and the detection performances.The first-principles simulation calculation was confirmed to be valuable for gas sensing detection by this study,and the research results completed the gas sensing mechanism of noble metal Au,Ag doped MoS₂ based sensor to dissolved fault characteristic gases CO,C₂H₄ and C₂H₂.