Yongjun Li

The organic electric‐field regulator enables the polymer to have nonlinear conductivity and to self‐regulate electric field. However, it may migrate out from the polymer matrix due to its small molecular weight, causing nonreversible damage to the electrical performance of the composite. In this work, to improve the long‐term stability, the allyloxy polyethylene glycol (APEG) with electron‐donating groups was grafted onto the low‐density polyethylene (LDPE) chains by chemical grafting method. The composites were mainly characterized by weight loss rate, breakdown strength, conductivity, space charge distribution, and surface potential decay. In addition, the quantum chemical calculations were carried out on APEG and grafting APEG/LDPE composites. The results show that grafting APEG on LDPE macromolecules not only improves the long‐term stability but also enhances the nonlinear conductivity, which is attributed to the increase in the electrostatic potential of electron‐donating groups of APEG. The nonlinear conductivity helps homogenize the electric field distribution in polymer composites and helps dissipate the surface charges. This work lays a foundation for the improvement of all‐organic nonlinear dielectrics.

Moisture penetration causes the degradation of cross-linked polyethylene (XLPE). To explore the effects of morphology on moisture and water treeing resistance, in this work, the XLPE samples were prepared under an assisted electric field, and their morphology, moisture resistance, and antiwater treeing performance were investigated. The scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) measurement results indicated that the assisted electric field reduced the spherulite size and enhanced the crystallinity of XLPE. Molecular dynamics simulations further demonstrated that the assisted electric field reduced the free volume, which consequently lowered the water diffusion coefficient. These morphological modifications effectively impeded water molecule migration, led to a reduction in moisture absorption, and further enhanced the moisture and water tree resistance. This work provides a novel strategy for the enhancement of the moisture and water treeing resistance of polymers, which helps improve the long-term reliability of terrestrial and submarine cables.

Electrical insulating materials with high breakdown strength and low space charge accumulation are very important for the development of high-voltage direct current (HVDC) transmission. In this work, low-density polyethylene (LDPE)-based composites were prepared by adding 0, 0.1, 0.3, and 0.7 wt % fluorinated graphene. The morphology, breakdown strength, space charge distribution, and surface potential decay were characterized. The experimental results illustrate that when the mass fraction of fluorinated graphene increases from 0 to 0.7 wt %, the breakdown strength increases at first and then decreases, whereas the space charge density reduces initially and grows later. The 0.3 wt % composite has the highest breakdown strength, increasing by 54.2%, and the lowest space charge density, reducing by 26.4%, compared with pure LDPE. According to the quantum chemistry calculations and the trap distribution, the effects of fluorinated graphene on charge transport and breakdown strength are revealed. This work is very helpful for improving the electrical properties of polymeric materials.

Blending low‐density polyethylene (LDPE) with high‐density polyethylene (HDPE) is an effective strategy to improve electrical properties for the high voltage direct current (HVDC) power cable insulation. However, the effects of HDPE on the space charge accumulation of the HDPE/LDPE blends are rarely reported. In this paper, the HDPE/LDPE blends are prepared with different HDPE contents, and their space charge distribution, conductivity, breakdown strength, surface potential decay, and morphology are measured, respectively. The results show that the optimal content range for the space charge inhibition is from 3 to 10 wt%. For the 10 wt% blend, it is 29.5% smaller in average space charge density than pure LDPE. Moreover, the 10 wt% blend is 85.5% lower in conductivity and is 33.7% higher in breakdown strength compared with pure LDPE. Thick lamellae structure is introduced into HDPE/LDPE by HDPE, and it leads to the formation of deep traps, contributing to the improvement of the electrical properties of the blends. This study provides a valuable foundation for the design of the new HVDC power cable insulating material.

An organic molecule of poly(propylene glycol) bis(2-aminopropyl ether) (ATPE) with electron-donating groups is blended with low-density polyethylene (LDPE) to prepare all-organic nonlinear resistive field grading material. Quantum chemical results show that ATPE has an electron-donating ability. Experimental results demonstrate that, compared with traditional inorganic SiC particles, ATPE reduces the LDPE composite's percolation threshold and improves the nonlinear conductivity, achieving much lower filler loading, which is attributed to the electron-donating groups of ATPE. In addition, ATPE also provides higher breakdown strength and surface flashover voltage as well as better toughness for LDPE composites than SiC particles. It provides the basis for high-performance all-organic nonlinear resistive field grading material.

The insulating polymer with high electrical properties is demanded for the high voltage direct current (HVDC) transmission. Regulating the aggregation structure of the insulating polymer is an effective strategy to improve its electrical properties. In this paper, the effects of the DC assisted electric field on the aggregation structure and electrical properties of low-density polyethylene (LDPE), a typical insulating polymer, were investigated. The LDPE treated with the assisted electric field of 0, 0.1, 0.3, 0.5, 1.0 and 2.0 kV mm⁻¹ were prepared, respectively. Aggregation structure, conductivity, breakdown strength and surface potential decay were characterized. The experimental results show that the assisted electric field is helpful for the increase in spherulite amount and for the reduction in spherulite size, which leads to more deep traps to improve the electrical properties. With the increase in the assisted electric field, the conductivity decreases at first and then increases, whereas the breakdown strength is just the opposite. It is worth noting that the LDPE treated with the assisted electric field of 0.5 kV mm⁻¹ has the lowest conductivity and the highest breakdown strength. This work provides a guideline for the design of electrical insulating polymers.

The direct current (DC) electrical properties of insulating polymers (e.g., low-density polyethylene, LDPE) are very important for the development of high-voltage direct current (HVDC) cables. The electric field assist can improve the DC electrical properties of the electrical insulating polymers. However, there are rarely reports about the effect of the alternating current (AC) electric field on the electrical properties of insulating polymers. In this article, the effects of the AC-assisted electric field on the DC electrical properties and the morphology of LDPE are studied. LDPE is prepared under the AC-assisted electric field of 0, 0.3, 0.5, 1.0, and 2.0 kV/mm, respectively. The DC conductivity, space charge distribution, breakdown strength, surface potential decay, and morphology of LDPE are characterized. Experimental results show that the AC-assisted electric field not only reduces the DC conductivity and space charge accumulation but also increases the breakdown strength of LDPE. The AC-assisted electric field increases the nucleation rate of LDPE and contributes to forming more spherulites, which introduces more deep traps to improve the DC electrical properties of LDPE. Notably, compared with the untreated LDPE, the LDPE prepared with the AC-assisted electric field of 1.0 kV/mm decreases by 96% in conductivity and reduces by 52.6% in average space charge density, whereas its DC breakdown strength is enhanced by 35%. This work provides a foundation for the improvement of the electrical properties of the insulating polymers.