Gas Sensing Properties Studies On Metal Doped Carbon Nanotubes Detecting SF₆ Decomposed Products

There will be inevitable defects occured in SF₆ insulated equipment during preparation and long time running processes,which may induce partial discharge,spark discharge and arc discharge inner SF₆ insulated equipment and affect the insulation of equipment.Besides,due to the strong latency of discharge faults at early stage,it is hard to find the discharge faults by regular maintenance and off-line detection methods.In order to ensure the running reliability of SF₆ insulated equipment,it is urgent to find an effective method to analyze the discharge patterns and severity in SF₆ insulated equipment on-line.When discharge faults occur in SF₆-insulated equipment,SF₆ will decompose to various typical decomposition products: SO₂,H₂ S,SOF₂ and SO₂F₂ by reacting with traces of H₂O and O₂.It is found that the composition and concentration of generated decomposition products have corresponding relations with discharge patterns and severity.Hence,it is effective to realize the on-line monitoring for SF₆-insulated equipment by detecting the generation properties of typical SF₆ decomposition productsDue to the outstanding properties of carbon nanotubes?CNTs?,it has been widely studied in gas detection field.However,intrinsic CNTs exist limit in gas detection.Generally,it is common to enhance its gas sensing properties by doping method.And single metal atom and metal cluster doping are the effective ways to improve its gas sensing properties.In this paper,Ni,Pd and Al doped CNTs materials are taken as the research object to analyze its gas sensing properties for SO₂,H₂S,SOF₂ and SO₂F₂ typical decomposition products by theoretical and experimental study.Following are the main study contents:?1?Based on DFT of first principle theory,this work systematically analyzes the adsorption properties of single and double metal atoms doped CNTs,and metal?111?plane to SO₂,H₂ S,SOF₂ and SO₂F₂,and calculates the adsorption structures,adsorption energy,charge distribution,density of states?DOS?and molecular orbitals in detail.And we have studied the modification mechanism of Ni,Pd and Al doping,and its gas sensing mechanism to SO₂,H₂ S,SOF₂ and SO₂F₂.Theoretical calculation reveals the influence of single and double metal atoms doped CNTs,and metal?111?plane to SO₂,H₂ S,SOF₂ and SO₂F₂.After single and double metal atoms doping,the metal atoms protrudes out of CNTs surface,acting as active adsorption sites,decrease the adsorption barrier to gas molecules,making gas molecules easy to adsorb on the surface of metal doped CNTs by physisorption or chemisorption.The adsorption capacity of metal atom doped CNTs to gas molecules increase with the quantity of metal atoms.The order of adsorption energy of Ni,Pd and Al doped CNTs to SF₆ decomposition products sorts in following order: Al>Ni>Pd.Gas adsorption induces the charge re-distribution,SO₂,SOF₂ and SO₂F₂ receive electrons when they adsorb on the surface of single and double metal atoms doped CNTs,Ni?111?and Al?111?plane,and lose electrons when they interact with Pd?111?.H₂ S molecules lose electrons at all adsorption models.The charge re-distribution leads to the change of DOS and molecular orbitals,resulting in the change of conductivity of adsorption system after SO₂,H₂ S,SOF₂,SO₂F₂ adsorption.?2?By controlling the gas flow rate and deposition temperature of CVD?chemical vapor deposition?,we prepared intrinsic CNTs with uniform morphology and less impurity,and its radius and width are about 10²0 nm and 30 ?m.In addition,Ni,Pd and Al doped CNTs are prepared by chemical reduction method.XRD analysis shows that there are few impurity elements in prepared material.And SEM analysis shows that Ni and Pd nanoparticles are uniformly distributed on the surface of CNTs,while there is partial aggregation of Al nanoparticles on the surface of CNTs.According to the characteration results of XRD and SEM,it confirms that the methods adopted for metal doped CNTs preparation in this study are effective.In the end,we prepared Ni,Pd and Al doped CNTs gas sensor by uniformly spraying the solution of metal doped CNTs on interdigitated electrodes through spray method.?3?The gas sensing properties of Ni,Pd and Al doped CNTs gas sensors to SO₂,H₂ S,SOF₂ and SO₂F₂ were tested under ambient temperature and pressure,including resistance change,concentration,response-recovery,selectivity and stability characters.It is found that the gas sensing speed of sensors are slow,it takes about 5 min for the resistance of sensors to reach stable.The value of sensing response increases with the gas concentration,while the sensor sensitivity decreases with gas concentration.After multiple response-recovery tests,the value of sensing response shows little reduction,signifying its good reuse property.Ni,Pd and Al doped CNTs sensors show different gas sensing properties to SO₂,H₂ S,SOF₂ and SO₂F₂.The detection limit of Ni-CNTs sensor to SF₆ decomposition products reaches 1 ?L/L,and its sensing response value to 100 ?L/L SO₂,H₂ S,SOF₂ and SO₂F₂ are-1.62%,-9.5%,-5.91% and-0.7%.The sensing response value of Pd-CNTs sensor to 100 ?L/L SO₂,H₂ S,SOF₂ and SO₂F₂ are-11%,-13.5%,-9% and 3%.The sensing response value of Al-CNTs sensor to SO₂,H₂ S,SOF₂ and SO₂F₂ are-5.3%,-7.6%,2.3% and-4.4%.According to the different sensitivity and selectivity of Ni,Pd and Al doped CNTs sensors to SF₆ typical decomposition products,it is feasible to on-line monitor and diagnose the insulation faults in SF₆-insulated equipment by preparing sensor array that detects the composition and concentration of SF₆ typical decomposition products.