Effect Of Morphology On Breakdown Characteristics Of BiFeO₃/LDPE Composites Treated With Stable Magnetic Field

Low density polyethylene（LDPE）is widely used in the power material industry because of its good thermal workability and excellent insulation properties.It has been shown that adding appropriate nanoparticles or micron particles to LDPE can not only preserve the stable physicochemical properties of LDPE matrix,but also significantly improve the dielectric properties of the composite material.As a kind of soft material,polymer responds to the stimulation of external fields（such as temperature field,electric field,magnetic field,etc.）in space.Therefore,it has become an important research direction in the field of composite materials to prepare new composite materials with excellent properties.In this thesis,nano composites and micron composites were prepared by adding0.2%,0.5%,1%and 2%bismuth nano-Bi Fe O₃ and bismuth micron ferrate（micro-Bi Fe O₃）with LDPE as the polymer matrix.The structure of micro and nano bismuth ferrite powder particles and their micro and nano composites were characterized and analyzed by atomic force microscopy（AFM）,Fourier infrared spectroscopy（FT-IR）,differential scanning calorimetry（DSC）and other microscopic characterization methods.The results show that the crystallinity of pure LDPE and composite materials is improved and the melting peak temperature moves to the high temperature region after the magnetization treatment.The magnetic field changes the intermolecular and intramolecular interactions of LDPE,and the macromolecular orientation is enhanced,which promotes the formation of spherulites of LDPE,resulting in the increase of crystallinity of the samples after magnetization.In this thesis,the dielectric properties of micro and nano BiFeO₃/LDPE composites were prepared and their electrical properties were tested.The effects of the mass fractions of dopants with different particle sizes and the steady magnetic field treatment on the breakdown field strength,conductance current（E-I characteristic curve）and dielectric properties of the composites were analyzed.The results show that there is a negative correlation between the dc breakdown strength and the doping mass ratio when the dc electric field is applied.The breakdown field strength of the bismuth ferrite nanometer composite material showed the change characteristics of first rising and then falling with the increase of the mass ratio of bismuth ferrite nanometer composite material,but the breakdown field strength of all the doping contents was better than that of pure LDPE.When an ac electric field is applied,the addition of bismuth ferrite micron and nano micron can effectively improve the ac breakdown field strength of the material,and when the mass fraction of bismuth ferrite micron or nano micron in the material increases,the breakdown field strength first increases and then decreases.The breakdown strength of the sample magnetized at room temperature is higher than that of the unmagnetized sample,and the breakdown strength of the sample magnetized at high temperature is lower than that of the unmagnetized sample.The conductance current of the composite material will be reduced by magnetization treatment,and the conductance current of the sample under room temperature treatment is lower than that of the high-temperature magnetized sample.The dielectric constant and loss factor of the composite were increased when bismuth ferrite particles were added.The dielectric constant and loss factor can be reduced by magnetization.Combined with the crystallization test results of the composite material,it can be seen that the crystallinity of LDPE and the composite material after the magnetization treatment is greater than that of the unmagnetized sample.With the increase of crystallinity,the proportion of internal amorphous area is reduced,and traps are prone to appear.This provides support for the formation of discharge channels,thus changing the dielectric resistance performance of the composite material.