Molecular Dynamics Study Of Nano-SiO₂ And Al₂O₃ Doped Crosslinked Polyethylene

Crosslinked polyethylene has become the main insulation material for power cables due to its extremely excellent electrical,mechanical and processing properties.As the voltage level increases,the requirements for the insulation performance of cross-linked polyethylene are becoming higher and higher.Nano-material doping modification has become an important means to improve insulation materials.With the rapid development of molecular simulation technology,it has been gradually applied to the field of high voltage insulation.Molecular simulation technology is used to study the thermal stability and mechanical properties of nano-SiO₂ and nano-Al₂O₃-doped cross-linked polyethylene cables.The nano-doping modification and the micro-mechanism of water tree have significant significance.The glass transition temperature,elastic modulus,and shear modulus of the above systems were studied.Finally,the water molecule diffusion behavior and the microscopic mechanism of nano-particles to inhibit water dendrites were analyzed in a temperature field,and the following conclusions were obtained:1)Comparing the glass transition temperature of pure cross-linked polyethylene to273K,nano-SiO₂ and nano-Al₂O₃ doped cross-linked polyethylene can increase the glass transition temperature of cross-linked polyethylene by 12K and 10K.The modulus of elasticity and shear modulus of cross-linked polyethylene at 300K were 2.061 Gpa and0.76 Gpa,respectively,and at 1.548 Gpa and 0.541 Gpa at 360 K.Nano-SiO₂ increased the elastic modulus and shear modulus of the cross-linked polyethylene by 51%and 57%at300K,respectively,and by 67%and 113%at 360K.Nano-Al₂O₃ increased the elastic modulus and shear modulus of cross-linked polyethylene by 45%and 53%at 300K,respectively,and by 59%and 111%at 360K.In terms of doping effect,nano-SiO₂ is better than nano-Al₂O₃.2)Performing 5%KH550 graft modification on the surface of nano-SiO₂ and nano-Al₂O₃ to further improve the glass transition temperature of the nanocomposite materials by 5K and 6K.The elastic modulus at 300K increased by 11%and 15%,shear modulus both increased by 18%.The 10%KH550 graft modification showed a decrease in glass transition temperature,elastic modulus,and shear modulus compared to 5%,which is not much different from the data of untreated nano-particle-doped composite XLPE materials.Graft modification of the surface of nano-particles can indeed further improve the glass transition temperature,elastic modulus and shear modulus of the nano-particles.However,the lifting effect is not completely positively correlated with the surface grafting content.When the grafting content is excessive,the effect of the surface grafting treatment will be reduced.3)With the increase of water content,the glass transition temperature,elastic modulus and shear modulus of XLPE,SiO₂/XLPE and Al₂O₃/XLPE all decreased.Under the same water content,the addition of two kinds of nano-particles can effectively increase the glass transition temperature,elastic modulus and shear modulus of XLPE.As the temperature increases and the water content increases,the water molecule diffusion coefficients in all three systems increase.Nano-particles can effectively form a hydrogen bonding effect with water molecules and inhibit the diffusion of water molecules in XLPE materials.In terms of the effect of nano-SiO₂ and nano-Al₂O₃ on water molecules,the interaction effect of nano-Al₂O₃ and water molecules is stronger.In the presence of water,doped nano-Al₂O₃ is better than nano-SiO₂.4)As the radius increases,the glass transition temperature of the SiO2/XLPE system is the highest at a radius of 6?,which is 290K.The Al2O3/XLPE system is the highest at a radius of 5?,which is 288K.The changes in the volume of the two systems are related to the spatial volume effect and surface effect of the nano-particles.At 300K and 360K,with the increase of the radius,the change trend of the mechanical modulus of the two systems is the same and both decrease.The change of mechanical modulus of the two systems is mainly affected by the unit surface binding energy of nano-particles and cross-linked polyethylene.