Study On Methane Gas Sensor Of Carbon Nanofibers Prepared By Electrospinning

In the field of gas sensors,methane(CH₄)is an important detection object,such as coal mine gas explosion,gas leakage,greenhouse gas emissions and other fields have a wide range of detection requirements.It is colorless,odorless,imperceptible,highly flammable and explosive,which can easily cause fire or explosion and eventually cause serious casualties,property losses and environmental damage,so it is very important to monitor the concentration of methane.However,most of the semiconductor methane gas sensors currently used generally have problems such as high operating temperature,high detection concentration,and serious cross-sensitivity.Therefore,the development of a new type of CH₄ gas sensor that can work at room temperature and be efficient and sensitive is of great significance to the safety of life and property and environment.Carbon nanofibers(CNFs)are considered as a kind of promising sensor material at room temperature because of their unique physical and chemical properties.This type of gas-sensitive material can be easily prepared by electrospinning technology,its fiber length-to-diameter ratio and morphology are easy to control,and can be modified with different gas-sensitive materials,then the carbon nanofibers composite film with the characteristics of large aspect ratio,high porosity and three-dimensional network framework structure can be obtained,which can promote the penetration and diffusion of gas molecules.Therefore,based on the electrospinning and annealing process,several different carbon nanofibers composite film gas sensitive materials were prepared in this thesis.By testing the gas sensitive response of CH₄ at room temperature,and combining with the characterization methods of material microstructure,crystal structure and element analysis,the gas sensitive performance of the sensor was studied,and its application in transformer fault detection was preliminarily explored.The main work and achievements of this thesis are as follows:(1)Soluble polyimide(PI)was used as the spinning precursor for electrospinning,the obtained PI nanofibers were pre-oxidized and then annealed in nitrogen atmosphere for carbonization,all the carbon nanofibers collapsed and melted.After that,the carbon nanofibers melting problem was solved by adding polyacrylonitrile(PAN)to the precursor for blending electrospinning.On this basis,the ratio of PI to PAN,the concentration of spinning solution,the parameters of electrospinning and annealing process were adjusted to study the influence on the morphology of carbon nanofibers.Finally,the optimal experimental parameters were determined as follows:the ratio of PI to PAN was 3:2,the concentration of spinning solution was 14%,the spinning voltage was 20kV,the injection rate was 5?L/min,the receiving distance was 18cm,the carbonation rate was 2?/min and carbonation temperature was 600?,and finally the finer diameter carbon nanofibers with good morphology were prepared.(2)Based on the above-mentioned optimal experimental parameters for preparing carbon nanofibers,added WCl₆,(NH₄)₂MoS₄ and SnCl₂·2H₂O into the spinning precursors respectively,WO₃/CNFs composite film,MoS₂/CNFs composite film and SnO₂/CNFs composite film were prepared by electrospinning and annealing.Then,the above three materials were tested using the characterization method of SEM,TEM,EDS,Raman and XPS.The analysis showed that the three composite fibers had good morphology,and proved the formation of WO₃,MoS₂ and SnO₂.Afterwards,the sensors of these three composite materials were tested and studied their gas sensitivity to CH₄ at room temperature.The test results exhibited that all three sensors can detect CH₄ with concentration as low as 10ppm effectively,the WO₃/CNFs sensor showed the highest response,and the response value reached 8.28%at 250ppm,which was 1.8 and 2.3 times that of MoS₂/CNFs and SnO₂/CNFs respectively,shorter response/recovery time(200s/40s)and better repeatability,and showed better selectivity to other fault characteristic gases of C₂H₄,H₂,CO and CO₂ in the transformer and transformer oil.The research results that can detect low-concentration CH₄ at room temperature lay a certain foundation for early warning of danger in practical applications.