Study On Preparation Of BiFeO₃/LDPE Nanocomposites And Its Dielectric Properties Induced By Magnetic Field

The third generation of insulation polymer based inorganic nanocomposite occupys very important position in research and development of the engineering dielectric field,providing the effective way in order to improve the dielectric properties of electrical equipment insulating material.Under the action of external magnetic field,the state of aggregation of polymer has made response to the effect of external magnetic field.The crystalline region and amorphous region and molecular orientation will be changed,then the dielectric properties of insulating material are affected.Using the influence of the polarization of the dielectric materials and dielectric properties under external magnetic field,functional inorganic bismuth ferrite(BiFeO3)nanopowder with higher dielectric constant and multiferroic properties is filled,to study the correlation of microstructure changes and macro dielectric properties and indicate correlation theoretical mechanism.In this thesis,the dielectric properties of multiferroic materials BiFeO3 nanoparticles modified low density polyethylene(LDPE)nanocomposites are studied under external magnetic field.The main achievements of this thesis are listed as follows:The multiferroic materials BiFeO3 nanoparticles of dispersion uniformity were synthesized by the citric acid sol-gel method.The results indicated that the preparation and purity of BiFeO3 nanoparticles have a profound influence on the precursor solution pH value of sol process and the calcined temperature of the xerogel.The optimum reaction conditions were the precursor solution pH =7-8 and calcined temperature for 600.It is found that BiFeO3 nanoparticles with 50-200nm in size and good dispersion were synthesized under the optimum reaction conditions.Under the optimum reaction conditions the dielectric constant and dielectric loss are 9.38 and 0.04 at 105Hz.Under 30kOe magnetic field the saturation magnetization(MS)and the remanent magnetization(MR)of BiFeO3 nanoparticles are 1.08emu/g and 0.13emu/g,respectively.BiFeO3/LDPE nanocomposites were prepared by two different preparation methods of solvent-melt blending method and direct blend method.BiFeO3/LDPE nanocomposites were applied external magnetic field under different temperatures.Coupling agent modification can enhance BiFeO3 and LDPE intermolecular association by direct blend method with good dispersibility.The state of aggregation of the nanocomposite has a great influence on applying external magnetic and BiFeO3 nanoparticles will enrichment to the plane in parallel of the magnetic field direction,changing the matrix distribution of BiFeO3 nanoparticles in the LDPE and the macromolecular orientation,reducing the crystallinity of the composite material.LDPE has diamagnetism characteristics of non-polar materials.When filling BiFeO3 nanopowder,Magnetization intensity M of BiFeO3/LDPE nanocomposites showed the trend of increased and then decreased with the increase of magnetic field intensity H.When H reaches a certain value,the magnetism of BiFeO3/LDPE nanocomposites will be transition from antiferromagnetism to diamagnetism,and magnetic response can be strengthen with the increase of particle concentration.After external magnetic field remanent magnetization MR of BiFeO3/LDPE nanocomposites were increased and nanocomposites of coupling agent modification were more significant.BiFeO3/LDPE nanocomposites breakdown characteristics show that AC breakdown field strength of nanocomposite is increased by 14 percent above LDPE base and nanocomposite of external magnetic field is increased by 22 percent.The reasons are possible the interfacial effect of crystalline region and amorphous region,nanoparticles and base and free volum.Ageing breakdown time is more than 10 times of LDPE and the nanocomposites of the different percentage from 0.2%to 5%of coupling agent modification is higher than that of the LDPE.The reasons are possible the characteristics and higher electrical conductivity of BiFeO3 nanoparticles playing homogenizing electric field and morphological structure changes and effect of interface traps.The dielectric constants of LDPE are increased after filling BiFeO3 nanoparticles,and the tendency of the dielectric loss is different because of difference of polarization mechanism.The dielectric constants and dielectric losses of nanocomposite of external magnetic field are slightly rise.The current of BiFeO3/LDPE nanocomposites shows a trend of monotonic decay with time,and the current mainly includes the absorption current caused by polarization and space charge and steady conduction current of carriers.The volume resistivity of nanocomposites is about 2 to 12 times that of LDPE because of scattering and steric effect.The sample of continuous magnetized test is about 20 times that of LDPE with the most significant influence.The conduction current of nanocomposites is about 1/12 to 1/4 that of LDPE at least at 35kV/mm,and magnetization at room and high temperature has little effect but the sample of continuous magnetized test has great effect and the least current is about 1/36 that of LDPE.The reasons are state of aggregation change of BiFeO3/LDPE nanocomposites and distribution change of nanoparticles under external magnetic field,and these factors have a great influence on the carrier mobility and carrier density.The depth of trap and number of capture charge are changed after BiFeO3 nanoparticles filled,and trap deepens and number of capture charge is more.Because of the trap effect of structure and interfacial effect between nanoparticles and LDPE base,the carriers to take off the traps are more difficult.