Jiaming Yang

Space charge suppression induced by deep traps in polyethylene/zeolite nanocomposite

NaY zeolite nanoparticles doped in low-density polyethylene (LDPE) is investigated. The zeolite nanoparticles are uniformly distributed in LDPE. Space charge distribution from pulsed electro-acoustic method and trap level from thermally stimulated current test are obtained. The results indicate that zeolite doping enormously suppresses space charge accumulation and reduces the conduction current by importing abundant deep traps. It can be explained that the zeolite nanoparticles increase the interface regions and introduce small size cavity traps from the porous surface of zeolite. The deep traps greatly weaken impurity ionization and carrier mobility, and raise potential barrier for charge injection.

Influence of moisture absorption on the DC conduction and space charge property of MgO/LDPE nanocomposite

Experimental studies and theoretical explanations were carried out in order to understand the influence of moisture absorption on the dielectric properties of magnesium oxide (MgO)/low density polyethylene (LDPE) nano-composite. After exposing LDPE and our MgO/LDPE to 40% relative humidity (RH) for 9 days, the absorbed water content in LDPE and our MgO/LDPE nanocomposite was about 2.30/00 and about 3.60/00 respectively. Experimental results show that moisture absorption has a negative influence on the dielectric properties of our MgO/LDPE nanocomposite at high electric fields of about 40kV/mm. Damp nanocomposites can still maintain the ability to suppress space charge and keep the conducting current from increasing (which normally occurs when moisture is absorbed) at low electric fields below 20 kV/mm. Absorbed water molecules can be considered to be divided into three forms, free water molecules which are assumed into the polymer matrix, weakly bound water molecules which may gather around nano-particles and tightly bonded water molecules, bound by hydrogen bonds. Thermogravimetry (TG) test results indicate that tightly bonded water molecules cannot be removed from the nanocomposite after a drying process at 80 °C in vacuum conditions. For damp MgO/LDPE nanocomposite after the drying process, the j-E characteristics were completely recovered up to an electric field of 40 kV/mm, whereas the space charge suppression ability partially recovered. The reason for this partial recovery may be attributed to the influence of tightly bonded water molecules on the interfacial charge in nano-particles and matrix polymers.

Effects of interfacial charge on the DC dielectric properties of nanocomposites

The interfacial charge phenomenon of MgO/low-density polyethylene (LDPE) and SiO2/LDPE nanocomposites was measured by synchrotron radiation small-angle X-ray scattering. Based on the Porod theory, the Porod curve of SiO2/LDPE nanocomposite shows negative divergence but the LDPE and MgO/LDPE do not, which reveals that interfacial charge may exist in the SiO2/LDPE nanocomposite. The DC dielectric properties of the nanocomposites are closely related to the interfacial charge. Experimental results show that the SiO2/LDPE nanocomposite has lower DC conductivity, less space charge, and higher DC breakdown strength than the MgO/LDPE nanocomposite. It is thought that the interfacial charge has a positive effect on the DC dielectric performance of nanocomposites, and the mechanism could be attributed to the scattering effects of the interfacial charge on the carrier migration. There is no obvious interfacial charge in the MgO/LDPE nanocomposite, but it still has excellent DC dielectric properties compared with LDPE, which indicates that the interfacial charge is not the only factor affecting the dielectric properties; the dipole interface layer and the reduction of free volume can also inhibit the migration of carriers and decrease electrons free path, improving the dielectric performance.

Assessment of nano dielectrics interface charge by electrokinetic sonic amplitude and atom force microscopy

Research has predicted the existence of the interface charges between nano particles and matrix polymers in nano dielectrics, which may become a dominant factor as the electrical properties of composites continue to improve. But until now, no experimental evidence has been found to prove this prediction. This research try to prove that the charging abilities of the nano particles in liquid suspension are related to that of polymeric nanocomposites, even more to find the existence evidence of interface charge in polymeric nanocomposites. Zeta potential of surface modified nano particles SiO2, and MgO/ paraffinic wax suspension systems have been measured by electrokinetic sonic amplitude (ESA) method to assess the charging properties. The average of zeta potential value of -62.8 mV and 2.1 mV of SiO2, and MgO/ wax suspension systems were obtained. The interface charge domains were detected by means of atomic force microscopy (AFM) lift model phase scanning of atomic flat surface of the corresponding polymeric nano composites. The interface charge domains have been found in SiO2/ low density polyethylene (LDPE) nanocomposite but cannot be found in MgO/LDPE system, which agree with the tremendous deference of zeta potential of corresponding suspension. Three types of models are proposed to interpret the AFM phase scanning appearance of the domains. Measurement results of zeta potential and AFM phase scanning and j (current density)-E (electric field) characteristics of LDPE and two kinds of nano composites reveal that the interface charge zones would inhibit the conducting current densities of the composites, and the larger absolute value of zeta potential corresponds to the stronger inhibiting ability to the conducting current density in polymeric nanocomposites.

Space charge and conductivity characteristics of CB/XLPE nanocomposites

In order to improve the DC dielectric properties of cross-linked polyethylene (XLPE) insulation materials, carbon black (CB)/ XLPE nanocomposites were prepared with the melt blending method. The pulsed electro-acoustic (PEA) method was used to measure the space charge distribution of XLPE and CB/XLPE nanocomposites. The relationship between the DC electrical conduction and the applied electric field strength of each material was measured under several constant temperatures. The research results show that the amount of space charges significantly reduces when a small quantity of CB was added into XLPE. The ability of composites to inhibit space charges is strong when the CB content is 1 phr. It was found that the electrically conductive properties of XLPE present nonlinear characteristics under lower electrical field intensity. The electrical conductivity significantly increases with temperature. The electrical conductivity of the nanocomposites with CB is almost a constant value under the electrical field below 20 kV/mm, and the effect of temperature on the DC electrical conductivity obviously decreases, when temperature goes up. The addition o CB into the XLPE can effectively inhibit the internal space charge accumulation and improve the DC conduction characteristics of the composites.

Effects of surface morphology on space charge formation in low density polyethylene

The effects of a modified surface morphology on space charge formation in low-density polyethylene (LDPE) were investigated with pulsed electro-acoustics. The change in morphology resulting from LDPE crystallization on atomically flat substrates was imaged by atomic force microscopy. An extensive lamellar morphology was observed, which contrasts with the crystal structure formed between PET films. The regular arrangement was confirmed with wide-angle X-ray diffraction. Space charge accumulation was characterized in samples with different surface morphologies. Charge injection occurs in both films, but is much larger in the LDPE-mica films than in the LDPE-PET films, and the difference increases over time. A suppression of current density in the LDPE-mica films was coincident with space charge formation.

DC conduction properties of SiO 2 /LDPE nanocomposite

The DC conduction characteristics of Silica (SiO 2 )/low density polyethylene(LDPE) nanocomposite was investigated at electric fields of 3 to 30kV/mm and at temperatures of 20 to 70°C. By the addition of nano-sized SiO 2 filler, the conductivity was decreased at least in an order of magnitude in the whole range of temperatures. SiO 2 /LDPE shows much lower thermal activation energy than LDPE. The current voltage (I–V) characteristic shows that SiO 2 /LDPE has a higher Space-charge-limited-current threshold than LDPE, which indicated the suppression of space charge injection. The interfacial electric charge region was suggested to be the reason for the improvement of electrical properties in Nanocomposite.

The suppression of space charge accumulation in CB/LDPE nanocomposites and its association with molecule relaxation

Abstract Space charge accumulation within insulating material poses a threat to the reliability in the operation of DC power cables. To investigate the influence of carbon black (CB) on the space charge accumulation of low density polyethylene (LDPE), both conductive carbon black (C-CB) and insulating carbon black (I-CB) were employed as filler particles. The space charge distributions of LDPE and CB/LDPE nanocomposites were obtained by the pulsed electro-acoustic (PEA) method. Additionally, dynamic mechanical analysis (DMA) and thermally stimulated current (TSC) spectroscopy were applied to explore the mechanism of improving space charge performance. Both the C-CB/LDPE and I-CB/LDPE nanocomposites can effectively suppress space charge accumulation. It was concluded that the improvement in space charge characteristics of CB/LDPE nanocomposites was attributable to the interaction between the CB particles and the LDPE, which reduces the number of defects formed from molecules participating in α relaxation and decreases the density of traps within the LDPE.

Space charge formation related to the structural relaxation of SiO2/LDPE nanocomposite

SiO2/LDPE nanocomposites can effectively decrease space charge accumulation at room temperature. However, space charge build up and suppress mechanism in nanocomposite are still not well understood. Experiments results showed that LDPE has a TSC peak around 60°C and the mechanical loss curve of LDPE which has a peak corresponding to the α-relaxation correspond well to the TSC peak. Space charge of LDPE at different temperatures were obtained by PEA then the sum of space charge amount showed that the space charge curve of LDPE at different temperatures still has a peak and it correspond well to the TSC and DMA peak. These results proved this point of view that space charge accumulation is associated with molecular motions. Compared with LDPE, the DMA loss peak of SiO2/LDPE was lower and shifted to higher temperature which implied that nanoparticles could restrict the movement of LDPE chain. The TSC peak appeared in LDPE is suppressed in SiO2/LDPE nanocomposite and the space charge peak correspond to TSC peak is also suppressed. These experiments results provided some evidences that the space charge suppress mechanisms of LDPE based nanocomposite has correlation with the bound effects on chain movement.