DC Dielectric Properties Improvement Mechanisms Of CB/LDPE Nanocomposites

Polyethylene is a kind of material which is used extensively. However, the application of polyethylene in high voltage DC cable has been severely limited.One important reason is that the polyethylene can accumulate space charge under DC electrical field. Space charge accumulation increases the local electrical field and thus greatly threatens the safe operation of the cable system. The nanofillers are homogeneously dispersed into polyethylene matrix, which can modify the space charge property of polyethylene. A lot of researches have also confirmed that the addition of nanoparticles into polyethylene can effectively improve the properties of polyethylene. These nanoparticles are mostly oxide nanoparticles with nonconductive characteristic. The Dow chemical company produced polyethylene insulating material modified by conductive nanoparticle, which has excellent space charge inhibition property. However, the related modification mechanism is rarely reported. So it is necessary to carry out experimental and theoretical analysis of polyethylene modified by the conductive nanoparticles.In this work, two different carbon black (CB), marked as CB1 and CB2,were employed as nanofiller, respectively. The blending method was used to prepare low density polyethylene(LDPE) based nanocomposites. The microstructure and surface chemical characteristics of the two kinds of CB particles were characterized by the transmission electron microscope (TEM),Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) methods, respectively. The results demonstrated that the CB1 particle has more rough surface and higher surface activity compared with CB2 particle,which can be more effectively interacted with LDPE matrix. The CB particles can restrain the movement of molecules around them through physical adsorption or chemical bonding. The CB particle play anchoring effect, which decreases the intensity of a relaxation of nanocomposites. The CB1 particle more effectively retrains a relaxation compared with CB2 because the interaction between CB1 and LDPE is more remarkable.The space charge distribution of LDPE and its nanocomposites were obtained by the pulsed electro-acoustic (PEA) method. Both CB1/LDPE and CB2/LDPE nanocomposites can effectively suppress the space charge accumulation, and the ability of the former to suppress space charge accumulation is stronger. Analyzing the process of space charge thermal release and a relaxation characteristic of LDPE, we found that the detrapping process of space charge is related with a relaxation motion. It indicates that the space charge accumulated in LDPE is captured by the traps formed from molecules that participate in a relaxation and mostly locates in the amorphous region between the lamellae. The trap characteristic and deep trap formation mechanism in nanocomposites were researched by the thermally stimulated current (TSC)method and molecular dynamics simulation method. The results confirm that the addition of CB particles introduces a large number of deep traps within LDPE,and the generating mechanism of these deep traps is attributed to the CB particles having strong ability of capturing electrons. The injected electrons captured by the deep traps accumulate in the surface layer of composite and form negatively charged layer, which can reduce the electric field near electrode and inhibit the electron emission. In addition, CB particles interact with molecules between lamellae, which reduces the number of defects formed from molecules that participate in a relaxation and decreases the density of traps in the LDPE,Therefore, the original space charge capture mechanism of LDPE is suppressed.It is concluded that both mechanisms work together to improve effectively the ability of inhibiting space charge accumulation.The MgO/LDPE and SiO2/LDPE nanocomposites also conform to the mechanism of deep trap inhibition space charge by the comparison analysis with CB/LDPE nanocomposite. The generating mechanism of different composite systems may exist differences, therefore, the generating mechanism of different composite systems were explored based on the atomic force microscope (AFM)measurement technology. The deep traps of SiO2/LDPE nanocomposites could be due to the interface charge, however, deep-trap-generating mechanism of MgO/LDPE nanocomposites would probably suite the electric field induced deep trap model theory. AFM test further confirmed that CB particle itself carries the deep traps, and the deep trap within CB/LDPE nanocomposites is due to the CB particles itself having the ability of capturing electrons.Experimental study of the conductivity characteristics of LDPE and CB1/LDPE nanocomposite show that CB1/LDPE nanocomposite has smaller DC conductivity and weaker conductivity dependence on the temperature compared with LDPE. Moreover, simulation calculation for the electrical field distribution of cable insulation indicates that CB1/LDPE nanocomposite can suppress the electrical field outside the insulation layer remarkable increase under high temperature gradient, which due to its reasonable conductivity. So CB1/LDPE nanocomposite enhances the operation reliability of cable. The breakdown field strength of both CB1/LDPE and CB2/LDPE nanocomposites is lower than that of LDPE at low temperature(30?). However, the breakdown field strength of both the nanocomposites is slight lower or higher than that of LDPE at high temperature(70?). The breakdown field strength of cable is dominated by the lowest breakdown field strength during the working temperature range. Therefore,decrease in breakdown field strength of the nanocomposites at low temperature would not influence their application as potential cable insulation materials.