Preparation And Nonlinear Conductivity Of Low-doped Graphene/Silicone Rubber Composites

The insulation structure of HVDC cable accessories is complex,and its distribution of electric field is extremely uneven.It has always been the weakest and most important link in the cable insulation structure.In this regard,the industry professionals propose to use nanocomposites with nonlinear conductivity or dielectric to homogenize the electric field.Due to the large specific surface area of graphene,the graphene/polymer nanocomposites can reach the leakage threshold at a low content,which can overcome the shortcomings of the existing nonlinear fillers(SiC or ZnO varistor ceramics,etc.)that lead to the decline of the mechanical properties of the composites due to the excessive content.It is expected to be widely used in the insulation of high-voltage electrical equipment.Therefore,in this paper,the preparation technology of graphene/silicone rubber nanocomposites was firsrtly studied by solution method and electric field induced orientation method,and the effect was characterized by SEM,FTIR and UV-Vis.The results show that graphene sheet is uniformly dispersed in the silicone rubber matrix and oriented along the direction of electric field.Secondly,the nonlinear conductivity of the composites in the range of 0?15kV/mm and 30?70? was tested,and the effects of diameter,content and distribution of filler on the threshold electric field Eb and nonlinear coefficient ? were studied.The results show that the electric field induced orientation of fillers can not only cause conductivity nonlinearity but also enhance the original nonlinearity of composites.With the increase of ultrasonic time,? decreases first and then increases.With the increase of filler content,Eb of composite sample decreases gradually;? of sample without induction increases with the increase of filler content,and the effect of filler content on ? is related to the diameter of sheet.Thirdly,the results of the experiments of conductivity and space charge show that the nonlinear conductivity mechanism of composite materials is that the inner layer and interface overlap to form a conductive channel.When the matrix gap between the layers decreases to a certain value,the charge transfer mechanism changes from Schottky effect to tunneling effect.Changing the distribution mode,diameter and mixing amount of filler can directly affect the gap between the layers in the conducting path of the sample,and thus change the nonlinear conductivity.Finally,by comparing ? and Eb,two kinds of nonlinear materials and their preparation processes are selected,which are:1)the sample with ultrasonic 1h,5.0wt%and induction by electric field,its Eb is 1.68kv/mm and ? is 8.87;2)the sample with ultrasonic 3h,5.0wt%and induction by electric field,its Eb is 0.68kv/mm and ? is 4.57.The feasibility of two kinds of composite materials used in 200kV DC cable joint is analyzed by thermal-electric calculation.The results show that compared with pure silicone rubber and fixed high-conductivity materials,these two kinds of nonlinear materials have significant homogenization effect on the local strong field in the DC cable joint,and less heating,which has a broad prospect in the field of high-voltage electrical equipment insulation.