Thermal Oxidative Aging Characteristics Of Micro Nano BBiFeO₃/LDPE Composites Under Stable Magnetic Field

Low density polyethylene（LDPE）has been widely used for its excellent insulation and mechanical properties.Inorganic fillers can improve the insulation properties of LDPE.During the long-term operation of LDPE,it will continue to be subject to electrical,thermal and mechanical effects,leading to material aging and insulation performance degradation.In order to improve the service life of insulating materials,it is explored whether the composite materials still maintain good insulating properties after thermal aging.Therefore,this paper studied the physical and chemical properties,dielectric properties and space charge characteristics of bismuth ferrite/low density polyethylene（BiFeO₃/LDPE）composites with nano,micro spherical and micro lamellar structures with different doping concentrations after thermal aging after magnetic field treatment at different temperatures.The main work is as follows:（1）BiFeO₃ particles were prepared and composites were synthesized to characterize the materials.Nanometer BiFeO₃ particles were prepared by sol gel method,micro BiFeO₃ particles were prepared by hydrothermal synthesis method,and were melted and co mixed with LDPE to form 0.2wt%,0.5wt%,1wt%,2wt%composites.The composite materials were magnetized at different temperatures,and then thermally aged.Scanning electron microscopy（SEM）results show that thermal aging has a significant effect on the flatness and density of LDPE and composites,and the structural effect is weakened after magnetization treatment.After aging,the cross section of the composite is smooth,and the lamellar composite has a certain orientation under the action of magnetic field.Fourier transform infrared spectroscopy（FTIR）results showed that with the increase of aging time,the oxidation reaction continued to increase,and the chemical bond breakage led to the increase of carbonyl content,the carbonyl content of nanocomposites increased less,and the carbonyl content of micrometer composites increased slowly.Differential scanning calorimetry（DSC）results show that the addition of BiFeO₃ plays a role as heterogeneous nucleating agent,and the orientation of magnetic field changes the arrangement of molecular chains and improves the crystallinity of the composite.（2）The dielectric and breakdown properties of the material are tested The aged composite material has lower dielectric constant and loss factor than LDPE.0.2wt%nano and micron BiFeO₃/LDPE composites have significant anti-aging ability,and the dielectric constant and loss factor of the composites can be further reduced by magnetic field treatment.After aging,the AC breakdown field strength of the composite was higher than that of LDPE,and after aging,the breakdown field strength was further increased after magnetization treatment.After 28 days of aging,the AC breakdown field strength of LDPE decreased by 19.4%,that of nanocomposites decreased by 13%,that of micron spherical composites decreased by8.47%,and that of micron laminated composites decreased by 11.3%,all of which were smaller than that of LDPE.After 28 days of aging,the AC breakdown field strength of each composite is higher than that of LDPE,with an average increase of19.3%.The dielectric properties and breakdown test results show that the doping of BiFeO₃ and magnetic field treatment can change the micro structure of the composite,increase the density of the material,and make the composite have better dielectric properties and higher breakdown field strength.（3）The space charge property of the material is tested BiFeO₃/LDPE composites have the ability to inhibit space charge before and after thermal aging,among which 1wt%nano and 0.2wt%micro composites have the best inhibition effect,and the space charge accumulation decreases after magnetization.The addition of BiFeO₃ particles introduces into the interface area,increases the crystallinity,improves the trap energy level,trap density,charge injection barrier,reduces the carrier mobility,reduces the accumulation of heteropolar charges,and reduces the accumulation of space charges.