Investigation On The Trap Characteristics And Electrical Conduction Properties Of LDPE Matrix GNPs Nanodielectrics

Polymer nanodielectrics have superior electrical properties.It can be regarded as the third generation of insulating materials,which have been the hot topic in the field of electrical insulation.Traps have prominent effects on the electrical conduction,space charge and breakdown of LDPE.The investigation on the trap characteristics of LDPE matrix composite and the effect of trap characteristics on electrical properties are helpful to accelerate the industrialization process of LDPE matrix nanocomposite.In this thesis,the doping effect of graphene nanoplatelets(GNPs)on the trap characteristics and dielectric properties of LDPE is studied by combining experimental and theoretical methods,and the mechanisms corresponding to the electrical properties of LDPE and GNPs/LDPE composite are studied from macroscopic to molecular scale.The GNPs/LDPE composites with different flake sizes and filling amounts are prepared using a mixing machine.The result of SEM test shows that the thickness of GNPs prepared in this study are all about 40-50 nm and the diameters of flake plane of the GNPs are about 1500 nm,850nm and 650 nm,respectively.The XRD spectrum shows that the LDPE filled with GNPs has higher crystallinity than the pure LDPE.The thermally stimulated depolarization current(TSDC)spectrum indicates that GNPs enlarge the trap level densities of the trap existing in the LDPE originally and introduce novel traps into LDPE,which makes the distribution of the trap in a lager energy interval and the average volume trap density of GNPs/LDPE much higher comparing with the pure one.Another verification experiment,photo stimulated discharging(PSD),shows that the discharging peak of GNPs/LDPE exits in the original wavelength interval of pure LDPE,and a new discharging peak exits in the position of smallerwavelength.The PSD testing result is a strong evidence of the validity of TSC testing result.The conduction test indicates that GNPs can reduce the conductivity of LDPE markedly,and this result is related to the enhancement of trap level density.For further proving the carrier trapping effect of GNPs,the band structure,the projected density of state and the transmission spectrum are calculated in computer software by combining density function theory(DFT)and NonEquilibrium Green Formula(NEGF)。The calculation result certifies that GNPs can be evaluated as carriers trap in LDPE from two varied perspectives,real space and Hilbert space.The trap depth is relevant to the size of graphene segments,especially,the smaller graphene segments has much more obvious effect on capturing carriers.The theoretical analysis correlates well with the results from the experiment.Therefore,doping GNPs with small size is beneficial to trapping charge carriers and enhancing insulation ability,which is suggested as an effective strategy to explore novel insulation materials used in direct current cables.