Dielectric And Decomposition Characteristics Of EP/SrTiO₃ Nanocomposites With High Thermal Conductivity

Epoxy resin,as a popular insulation material,has been widely used in preparation of various electrical insulation parts.But with the modernization of modernized electrical equipment toward high power,high efficiency,high voltage and miniaturization,great efforts and higher standard were put in the improvement in the electrical and thermal properties of epoxy resin to solve the heat dissipation problem and enhance dielectric strength,which also represents a new direction for the development of insulation materials.Nanocomposites tends to be well known for its simple and flexible preparation method to sovle both electrical and thermal problems of insulation materials.Specifically speaking,nanocomposites can be prepared according to the practical insulation level in the electrical equipment via changing various physical parameters of polymer matrix and filler in order to meet the demand of engineering situation but not all the nanocoposties can possess strong dielectric strength and high thermal conductivity.In our article SrTiO₃/epoxy nanocomposites were prepared to improve electrical and thermal properties of epoxy resin.Through a variety of testing methods the structure and morphology characteristics were illustrated and some significant electrical and thermal parameters were measured,and mechanism of improvement in both electrical and thermal properties was discussed in the classical mathematic model.At the same time,influence of thermal aging in air and SF₆ atmosphere on electrical characteristics of EP/SrTiO₃ nanocomposite was investigated.In attempting to illustrate the micro mechanism of composite materials,the rapidly developed computing technology offers a feasible and effective path so that by using molecular dynamic simulation method thermal and mechanical properties of EP/SrTiO₃ were further studied.Compared with experimental nanocomposites samples some physical parameters of nanocomposite cell model including mechanics and thermology were analyzed.Interface interaction enery was also calculated via interface model to better reveal the improvement mechanism.Decomposition of epoxy resin at high temperature and its chemical interation with SF₆ can drastically change concentration of decomposed gases of SF₆.To better understand the generation principle of decomposed gases,synchronous thermal analyzer and gas chromatography were combinedly used to measure decomposition components concentration after nanocomposites were heated at different temperatures.Finally based on the results of tests,a preliminary assessment formula was put forward to judge whether in the SF₆-infused electrical equipment there is existing an overheating disfunction involving epoxy resin decomposition.Also after flashover discharge happening on the surface of epoxy resin,decomposition component concentration of SF₆ was tested,the law of gases generation was summarized,and the basic method of judging whether there was exsiting flashover discharge fault on epoxy resin surface was put forwarded.First of all,SrTiO₃ was selected as filler owing to its high thermal conductivity,thermal stability and proper dielectric constant value before being modified by silane coupling agent.Then,epoxy/SrTiO₃ nanocomposites were fabricated via the facile solution method.Structure and morphology characteristics of modified nanoparticles and nanocomposites were analyzed via a series of test methods such as SEM,IR,TEM,XRD and so on.The results demonstrated that silane coupling agent had been coated on the surface of nanoparticles and homogeneous distribution of nanoparticles in the nanocomposites without holes,gaps and cracks.Thermal conductivity tested by the heat flow method increased with the weight fracitons of nanoparticles that was further verified by infiltration theory to demonstrate that theoretical value kept well consistant with the experimental value.Thermal statbility also reinforced with weight frations and tended to get to a saturation level in the nanocomposite with a higher weight fraction.The dielectric constant also increased with weight fractions and temperature while dielectric loss kept in a relatively low range,breakdown strength of nanocomposites also had a small rise with weight fractions.The Lichtenecker mixed logarithmic equation was adopted to explain the reason of difference between theoretical value and measured value of dielectric constants.Combined with infrared thermal imager,the discharge characteristics along surface of nanocomposites was further studied.The results showed that nanocomposites had a higher breakdown voltage and a faster temperature dissipation rate compared with pure epoxy resin.The mechanism of thermal breakdown under pluse power was adopted to explain the improvement in surface breakdown characteristics.Compared with EP/BaTiO₃ nanocomposites,it was found that thermal conductivity,dielectric constant,and flashover voltage of EP/SrTiO₃nanocomposite were superior to EP/BaTiO₃ nanocomposites under the same experiment condition.After aging for 500 h at 90°C,EP/SrTiO₃ nanocomposite still maintains high breakdown strength and flashover voltage,demonstrating after filling SrTiO₃ nanoparticles,nanocomposite can tolerate long-term work under 90°C.Secondly,in order to better explain the enhancement of nanocomposites in thermal dielelctric characteristics,molecular dynamics methods were adopted to illustrate the effects of SrTiO₃weight fractions on thermodynamic parameters,during which WLF equation emphasing on cooling rate influence was also adopted to prove the differences between theoretical value and experimental vaule of glass transition temperature.EP/SrTiO₃ cell model was also established to analyze the weight fraction influence on the mechanical and thermal properties of nanocomposites via mean-square deviation?MSD?and dipole autocorrelation functions?DACF?.The results showed that incorporation of SrTiO₃ nanoparticles can significantly weaken the movement of molecular chain segment below glass transition temperature.EP/SrTiO₃ interface model was also constructured to calculate the interface interaction energy.The results showed that the interface interaction energy is higher.Dielelctric constant,steric parameter and volume resistivity of pure epoxy resin were also simply predicted via an infinite loop molecular chain to show that the theoretical value and measured value match well.Finally,based on its high thermal conductivity and excellent dielelctric properties decomposition characteristics of nanocomposites in SF₆ atmosphere was explored by combining synchronized thermal analyzer with gas chromatograph.Compared with pure SF₆decomposition process,epoxy resin decomposition at high temperature can exacerbate the SF₆decomposition so that the gas decomposition component concentrations were measured and the generation principle was studied in order to estabilish a fundermental assessement method to judge whether in electrical equipment there is existing an overheat fault involving decomposition of epoxy resin under high temperature.When the CO₂ concentration reaches five times concentration of SO₂ and H₂S,and H₂S concentration is higher than 0.5 ppm,overheating failure involving thermal decomposition of epoxy resin tends to occur in electrical equipment.Thermal analysis found that there were more exothermic peaks during decomposition of expoy resin in SF₆.XPS and EDS analysis of residues demonstrate the existence of fluorine element and sulfur element.Besides,the relationship between the content of the decomposition components of SF₆ and the number of flashover discharge on epoxy resin surface was analyzed.The gas generation rate was arranged as follows:SOF₂>CF₄>SO₂>CO₂>CS₂>H₂S>SO₂F₂.It was verified again that the presence of epoxy resin would have a great impact on the concentration of SF₆ decomposition components.Also the basic method to judge whether in the electrical equipment there happens flashover discharge was put forward.When SOF₂ concentrations is twice bigger than concentration of CF₄,and sum of concentration of CF₄ and SOF₂ is far greater than concentration of H₂S,flashover discharge failure on epoxy resin surface may happen.Compared with the experiment result of heating epoxy resin,the reasons for the differences in the concentration of SF₆ gases byproduct were analyzed.