A Review of Polyolefin-Insulation Materials in High Voltage Transmission; From Electronic Structures to Final Products.

This review focuses on the use of polyolefins in high-voltage direct-current (HVDC) cables and capacitors. A short description of the latest evolution and current use of HVDC cables and capacitors is first provided, followed by the basics of electric insulation and capacitor functions. Methods to determine dielectric properties are described, including charge transport, space charges, resistivity, dielectric loss, and breakdown strength. The semicrystalline structure of polyethylene and isotactic polypropylene is described, and the way it relates to the dielectric properties is discussed. A significant part of the review is devoted to describing the state of art of the modeling and prediction of electric or dielectric properties of polyolefins with consideration of both atomistic and continuum approaches. Furthermore, the effects of the purity of the materials and the presence of nanoparticles are presented, and the review ends with the sustainability aspects of these materials. In summary, the effective use of modeling in combination with experimental work is described as an important route toward understanding and designing the next generations of materials for electrical insulation in high-voltage transmission.

Coarse-Grained Model for Prediction of Hole Mobility in Polyethylene.

Electrical conductivity measurements of polyethylene indicate that the semicrystalline structure and morphology influence the conductivity. To include this effect in atomistic charge transport simulations, models that explicitly or implicitly take morphology into account are required. In the literature, charge transport simulations of amorphous polyethylene have been successfully performed using short oligomers to represent the polymer. However, a more realistic representation of the polymer structure is desired, requiring the development of fast and efficient charge transport algorithms that can handle large molecular systems through coarse-graining. Here, such a model for charge transport simulations in polyethylene is presented. Quantum chemistry calculations were used to define six segmentation rules on how to divide a polymer chain into shorter segments representing localized molecular orbitals. Applying the rules to amorphous systems yields distributions of segments with mode and median segment lengths relatively close to the persistence length of polyethylene. In an initial test, the segments of an amorphous polyethylene were used as hopping sites in kinetic Monte Carlo (KMC) simulations, which yielded simulated hole mobilities that were within the experimental range. The activation energy of the simulated system was lower compared to the experimental values reported in the literature. A conclusion may be that the experimental result can only be explained by a model containing chemical defects that generate deep traps.

Electronic conductivity of polymer electrolytes: electronic charge transport properties of LiTFSI-doped PEO.

The electronic structure of poly(ethyleneoxide) with and without a common electrolyte lithium bis(trifluoromethane)sulfonimide salt is calculated from first principles. Introducing the salt into the polymer electrolyte significantly reduces the band gap, down to 0.6 eV. Thus, this will have a significant impact on the leakage currents in polymer electrolytes used in all-solid-state batteries.

Communication: Band bending at the interface in polyethylene-MgO nanocomposite dielectric.

Polymer nanocomposite dielectrics are promising materials for electrical insulation in high voltage applications. However, the physics behind their performance is not yet fully understood. We use density functional theory to investigate the electronic properties of the interfacial area in magnesium oxide-polyethylene nanocomposite. Our results demonstrate polyethylene conduction band matching with conduction bands of different surfaces of magnesium oxide. Such band bending results in long range potential wells of up to 2.6 eV deep. Furthermore, the fundamental influence of silicon treatment on magnesium oxide surface properties is assessed. We report a reduction of the surface-induced states at the silicon-treated interface. The simulations provide information used to propose a new model for charge trapping in nanocomposite dielectrics.

Understanding the Ionic Conduction in Dielectric Polymers at High Electric Fields Using Molecular Dynamics Simulations.

Applying molecular dynamics, we have studied ionic transport in polyethylene at moderate and high electric fields. The ion mobility of a variety of species is calculated and compared with existing theories. It reveals that ion mobility starts to deviate from the Einstein relation and increase roughly linearly with field beyond 100 MV/m, which results in a superlinear increase of the ionic current at high field that is consistent with the experimental results. At high field, we argue that the accelerated ion molecule is able to facilitate the conformation change of the surrounding polymers through elastic scattering, which in turn accommodates its passage. The extent of the mobility enhancement depends on the mass and size of the ion molecule. The ion transport is a thermally activated process, but has smaller activation energy than diffusion due to the additional energy provided by the field.

Local Field Factors and Dielectric Properties of Liquid Benzene.

Local electric field factors are calculated for liquid benzene by combining molecular dynamic simulations with a subsequent force-field model based on a combined charge-transfer and point-dipole interaction model for the local field factor. The local field factor is obtained as a linear response of the local field to an external electric field, and the response is calculated at frequencies through the first absorption maximum. It is found that the largest static local field factor is around 2.4, while it is around 6.4 at the absorption frequency. The linear susceptibility, the dielectric constant, and the first absorption maximum of liquid benzene are also studied. The electronic contribution to the dielectric constant is around 2.3 at zero frequency, in good agreement with the experimental value around 2.2, while it increases to 6.3 at the absorption frequency. The π → π* excitation energy is around 6.0 eV, as compared to the gas-phase value of around 6.3 eV, while the experimental values are 6.5 and 6.9 eV for the liquid and gas phase, respectively, demonstrating that the gas-to-liquid shift is well-described.