Study On Structure And Dielectric Properties Of PE/OMMT Nanocomposite Dielectrics

Polyethylene(PE)has excellent electrical properties and physical mechanical properties.Cross-linking to form cross-linked polyethylene(XLPE)improves its heat resistance and environmental stress crack resistance.It is widely used in medium and high voltage power cable insulation.As the voltage level of the power system continues to increase,higher requirements are placed on the insulation performance of the cable.Cross-linking and nano-modification technology are important means to improve material properties.Studying the effect of cross-linking behavior on the structure and dielectric properties of nanocomposite dielectrics has important application value in the development of cable insulation.Polyethylene/montmorillonite(PE/OMMT)and cross-linked polyethylene/montmorillonite(XLPE/OMMT)nanocomposites with different mass fractions of organic montmorillonite(OMMT)were prepared by melt blending.The intercalation state of the organic OMMT in the matrix was analyzed,and the microstructure of the nanocomposite affected the mechanical properties and dielectric properties of the composites.Small-angle X-ray diffraction(XRD)was used to characterize the variation of interlayer spacing and intercalation dispersion of OMMT in the matrix.The results show that when the mass fraction of OMMT is 2%,the OMMT layer spacing is the largest,and OMMT achieves good exfoliation and dispersion in PE resin.status.Compared with before cross-linking,the interlayer spacing of OMMT in the same mass fraction composites decreases after cross-linking.Scanning electron microscopy(SEM)observation of the brittle fracture of composites was found in PE/OMMT composite systems,the OMMT is uniformly dispersed without obvious agglomeration,and the dispersion effect of 2% PE/OMMT was the best.After cross-linking,the dispersion of OMMT in the matrix is significantly worse and undistributed.The crystal morphology of the nanocomposite dielectric is observed by SEM.The observation results show that the addition of OMMT significantly improved the crystal morphology of the material and formed a "transcrystallization" along the OMMT sheet.The melting process and crystallization process of the nanocomposite dielectric were analyzed by differential scanning calorimetry(DSC)and successive self-nucleation annealing(SSA)analysis was carried out.The results show that the addition of nano-OMMT broadens the size distribution of the material,decreases the crystallinity and increases the thickness of the wafer.After cross-linking,the crystallinity and wafer thickness are further reduced.At the same time,doping of a small amount of OMMT canincrease the tensile strength and elongation at break of the materiall,but the doping amount is too high,OMMT dispersion Uneven,easy to form stress concentration points,thus affecting the mechanical properties of the composite.The results of dielectric constant,dielectric loss,volume resistivity and breakdown field strength of nanocomposites show that the uniformly dispersed lamellar OMMT is easy to form a strong interfacial force with the matrix during a small amount of doping.The movement of the molecular chain hinders the occurrence of steering polarization,decreases the polarizability,and decreases the dielectric constant.As the OMMT content continues to increase,the dielectric constant increases.As the doping amount increases,the dielectric loss value decreases first and then increases,reflecting the result of the interaction of the interface polarization and the dipole polarization of the polar group.The volume resistivity test results show that 2%PE/OMMT has the highest activation energy and the highest interface barrier.After cross-linking,the activation energy decreases.The breakdown field strength results show that the 2% PE/OMMT composite has the highest breakdown field strength and the smallest dispersion,which is related to the intercalation dispersion state of OMMT in PE.