Study On Gas Sensing Characteristics Of Zinc Oxide N-N/N-P Heterojunctions Based Gas Sensors

Dissolved gas analysis(DGA)is an effective method to diagnose potential faults of oil-immersed power transformers.High performance gas sensor technology has always been the direction of on-line monitoring of dissolved gases in transformer oil.As a typical n-type semiconductor material,ZnO plays an important role in various MOS materials due to its high carrier mobility of conduction electrons,good chemical and thermal stability.Traditional ZnO gas sensors generally operate at a temperature of200-500?.The grains growth of ZnO is easily induced by high temperature operation,which reduces the stability of the sensor and shortens its service life.Based on the morphological control of pure ZnO,metal additive doping modification and ultraviolet assisted irradiation can improve the long-term stability and sensitivity of the sensor,which lays a foundation for the development of high-performance ZnO based gas sensor.In this paper,H₂,CO and C₂H₂ are selected as the target gases.Based on the first principle,six common N/P type metal ion doped-ZnO models are established to systematically study the doping modification mechanism,providing theoretical guidance for the selection of doping elements of sensing materials.By optimizing the process parameters,the three-dimensional hierarchical porous structures with higher sensing capabilities were synthesized by hydrothermal method.Based on the simulation results,the optimal combination mode was optimized to synthesize n-n/n-p heterojunctions with different mole doping amounts,and the crystal structure information changes before and after doping were analyzed.The gas adsorption models at different adsorption sites on the material surface were constructed.The simulation results,microscopic characterization and macroscopic gas sensitivity were verified.The influence of ultraviolet light in gas sensing process on the service life of the sensor was studied.The main research work of this paper is as follows:(1)The doping mechanism of metal ion displacement of N/P metal oxides was systematically studied.Pure and common N/P metal ion doping models:tin(Sn),titanium(Ti),tungsten(W),nickel(Ni),cobalt(Co),chromium(Cr)were established,and comprehensive analysis was carried out from the perspectives of doping formation energy,charge population and electron state density.The simulation results show that the n-type metal doping system W/ZnO and the p-type metal doping system Ni/ZnO are the most likely to be doped.The formed system structure is the most stable with the largest amount of charge transfer,which is the best doping combination.(2)Hierarchical porous pure ZnO and ZnO-based heterojunction materials with different doping amounts were prepared and their microstructures were characterized.The hierarchical porous pure ZnO structures with large specific surface area,high porosity and oxygen adsorption were prepared by hydrothermal method.The material with the best microscopic characterization results,networked pure ZnO,was selected as the source of ZnO in the heterojunctions.WO₃/Ni O-ZnO heterojunctions with different mole doping amounts were prepared,and the microstructures were characterized to analyze the crystal structure information before and after doping.(3)The response characteristics of the sensors made of pure and heterogeneous materials to three important gases dissolved in oil(H₂,CO and C₂H₂)were experimentally studied.Based on the gas adsorption models,the gas mechanisms of heterojunctions-based sensors were analyzed.The temperature,gas concentration characteristics and dynamic response-recovery characteristics of the prepared materials to three gases were tested,and the adsorption models of the gases at different sites on the material surface were established.From four aspects of the charge density,charge distribution,electron density difference maps and total states density are analyzed,and compared with parameters before adsorption.The improvement mechanism of metal oxide doping on the performance of sensor is revealed.In addition,the materials with the best gas performances were selected to study their performance decay and microstructure change during the 60 days accelerated aging test,and the reason of sensor aging is analyzed.(4)The anti-aging capability of the sensor was studied by introducing ultraviolet light induced gas sensing test.The UV absorption spectrum of the optimal 5.0 at%WO₃/Ni O-ZnO based sensors were tested,and the excitation wavelength was selected to be 365 nm.The optimal operating temperature,gas concentration characteristics and response-recovery characteristics of the sensor under ultraviolet irradiation were tested.The results showed that the optimal operating temperature after illumination decreased to as low as 60?,and the response-recovery speed was faster.Under ultraviolet excitation,the anti-aging ability is also significantly improved.The property attenuation is obviously slowed down,and the damage degree of microstructure is reduced.