Polypropylene (PP) contains promising application prospects in thermoplastic cables for high voltage direct current (HVDC) power transmission because of its outstanding thermal and dielectric properties. However, the problem of poor toughness and space charge has restricted the application of pure PP in HVDC cables. In this paper, polyolefin elastomer (POE) and nano-silica were blended thoroughly and added into a PP mixture by a melting method. Scanning electron microscopy (SEM) was employed to observe the dispersion of POE and nanoparticles. Thermal properties were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Mechanical properties were evaluated by tensile tests. The elastomeric properties of composites were improved as the dispersed POE could transfer and homogenize external mechanical forces. DC breakdown results showed that the fail strength of composite with 10 phr POE and 1 phr nano-silica was obviously enhanced. The pulsed electro-acoustic (PEA) results showed that the injection and accumulation of space charge was increased by the introduction of POE, while it was restrained by the collective effect caused by nano-silica filling. X-ray diffraction (XRD) spectrograms showed that secondary ordered structures existed in the composites of PP, POE, and nano-silica, and that the ordered structure around the nanoparticles contributed to the enhancement of breakdown strength. The mechanical and dielectric properties were modified synergistically, which made the modified PP a propitious insulation material for HVDC cables.

To investigate the inhomogeneous distribution of electric fields in insulating equipment and components, five nonlinear-conductance composite materials based on epoxy resin (EP) (nano-SiC/EP, nano-ZnO/EP, micro-ZnO/EP, nano-SiC/ZnO/EP, and nano-micro-SiC/ZnO/EP), were prepared using nano-SiC, nano-ZnO, and micro-ZnO particles as fillers. The mass fractions of the inorganic fillers were 1, 3, and 5 wt%, respectively. The direct current (DC) voltage characteristics of the composites showed that the electrical conductivities and nonlinear coefficients of the composites utilizing single-filler types increased with increasing inorganic filler content. Under the same conditions, the conductivity and nonlinear coefficient of SiC/EP were both larger than those of the nano-ZnO/EP and micro-ZnO/EP. However, the nonlinear coefficient of the composites was significantly affected by the simultaneous addition of the two inorganic fillers, micro-ZnO and nano-SiC. When the content ratio of micro-ZnO to nano-SiC was 2:3, the nonlinear coefficient of the composite reached a maximum value of 3.506, significantly higher than those of the other samples. Compared with the nano-SiC/EP, micro-ZnO/EP and nano-ZnO/EP composites with 5 wt% inorganic filler, the nonlinear coefficient of the two-filler composite was greater by a factor of 0.82, 2.48, and 5.01, respectively.

Low-density polyethylene (LDPE) is an important thermoplastic material which can be made into films, containers, wires, cables, etc. It is highly valued in the fields of packaging, medicine, and health, as well as cables. The method of improving the dielectric property of materials by blending LDPE with inorganic particles as filler has been paid much attention by researchers. In this paper, low-density polyethylene is used as the matrix, and montmorillonite (MMT) particles and silica (SiO₂) particles are selected as micro and nano fillers, respectively. In changing the order of adding two kinds of particles, a total of five composite materials were prepared. The crystallization behavior and crystallinity of five kinds of composites were observed, the ε_r and tanδ changes of each material were investigated with frequency and temperature, and the power frequency (50 Hz) AC breakdown performance of materials were measured. The differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results show that the crystallinity of the composites is higher than that of LDPE. Experimental data of dielectric frequency spectra show that the dielectric constants of micro–nano composites and composites with added MMT particles are lower than LDPE, the dielectric loss of composites can be improved by adding MMT particles. The experimental data of dielectric temperature spectra show that the permittivity of SiO₂-MMT/LDPE is still at a low level under the condition of 20~100 °C. In terms of breakdown field strength, the SiO₂/LDPE composite material increased by about 17% compared with the matrix LDPE, and the breakdown field strength of the materials SiO₂-MMT/LDPE and MMT-SiO₂/LDPE increased by about 6.8% and 4.6%, respectively.

Low density polyethylene (LDPE) doped with inorganic nano-MMT and nano-ZnO particles improved the dielectric properties of the cable insulation layer. In this article, nano-MMT/LDPE and nano-ZnO/LDPE composites were prepared by polymer intercalation and melt blending, respectively. The octadecyl quaternary ammonium salt and silane coupling agent were applied for surface modification in nano-MMT and nano-ZnO particles, and this then improved the compatibility of nanoparticles and polymeric matrix. These samples were characterized by FTIR, PLM, DSC and TSC, from which the effect of nanoparticles doping on polymer crystal habit and interface traps would be explored. In these experiments, the AC breakdown characteristics and space charge characteristic of different composites were studied. The experimental results showed that the interface bonding of nanoparticles and polymer was improved by coupling agents modifying. The dispersion of nanoparticles in matrix was better. When the mass fraction of nanoparticles doping was 3 wt.%, the crystallization rate and crystallinity of composites increased, and the crystalline structure was more complete. Besides, the amorphous regions in material decreased and the conducting channel was circuitous. At this time, the breakdown field strength of nano-MMT/LDPE and nano-ZnO/LDPE increased by 10.3% and 11.1%, compared to that of pure LDPE, respectively. Furthermore, the density and depth of interface traps in polymer increased with nanoparticles doping. Nano-MMT and nano-ZnO could both restrain the space charge accumulation, and the inhibiting effect of nano-ZnO was more visible.

In this article, low-density polyethylene (LDPE) was used as a matrix polymer, the Micro-ZnO and Nano-ZnO particles were used as the inorganic filler. With the melt blending method, the Nano-ZnO/LDPE(Nano-ZnO particles doping into LDPE), Micro-ZnO/LDPE(Micro-ZnO particles doping into LDPE) and Micro-Nano-ZnO/LDPE (Nano-ZnO and Micro-ZnO particles doping into LDPE in the same time) composites were prepared. Then, the inorganic filler and the composites were dealt with structural characterizations and analysis by Fourier transform infrared (FTIR), Polarization microscope (PLM), and Differential scanning calorimeter (DSC). Besides, these samples were dealt with (alternating current) AC breakdown performance test. The micro-experimental results showed that the Micro-ZnO and Nano-ZnO particles doping reduced the crystal size and increased the crystallization rate. With the change of cell structure, the crystallinity of composites increased. The crystallinity order of different samples was as follows: LDPE < Micro-ZnO/LDPE < Nano-ZnO/LDPE < Micro-Nano-ZnO/LDPE. From the breakdown of the experimental result, with the same mass fraction of the different inorganic doping of particles, the breakdown strength of these composites was different. The Nano-ZnO particle doping could improve the breakdown strength of composites effectively. Among them, the breakdown strength of Nano-ZnO/LDPE and Micro-Nano-ZnO/LDPE were 11% higher and 1.3% lower than that of pure LDPE, respectively. Meanwhile, the breakdown strength of Micro-composite was the lowest but its Weibull shape coefficient was the highest. Therefore, the Micro-ZnO doping was helpful for the Nano-ZnO dispersing in the matrix, which produced the Micro-Nano-synergy effects better.