Polymers are the preferred materials for dielectrics in high-energy-density capacitors. The electrification of transport and growing demand for advanced electronics require polymer dielectrics capable of operating efficiently at high temperatures. In this review, we critically analyze the most recent development in the dielectric polymers for high-temperature capacitive energy storage applications. While general design considerations are discussed, emphasis is placed on the elucidation of the structural dependence of the high-field dielectric and electrical properties and the capacitive performance, including discharged energy density, charge-discharge efficiency and cyclability, of dielectric polymers at high temperatures. Advantages and limitations of current approaches to high-temperature dielectric polymers are summarized. Challenges along with future research opportunities are highlighted at the end of this article.

/pdf/dielectric-polymers-for-high-temperature-capacitive-energy-7gbn119lxl.pdf

Dielectric polymers for high-temperature capacitive energy storage.

The 2050 carbon-neutral vision spawns a novel energy structure revolution, and the construction of the future energy structure is based on equipment innovation. Insulating material, as the core of electrical power equipment and electrified transportation asset, faces unprecedented challenges and opportunities. The goal of carbon neutral and the urgent need for innovation in electric power equipment and electrification assets are first discussed. The engineering challenges constrained by the insulation system in future electric power equipment/devices and electrified transportation assets are investigated. Insulating materials, including intelligent insulating material, high thermal conductivity insulating material, high energy storage density insulating material, extreme environment resistant insulating material, and environmental-friendly insulating material, are categorised with their scientific issues, opportunities and challenges under the goal of carbon neutrality being discussed. In the context of carbon neutrality, not only improves the understanding of the insulation problems from a macro level, that is, electrical power equipment and electrified transportation asset, but also offers opportunities, remaining issues and challenges from the insulating material level. It is hoped that this paper envisions the challenges regarding design and reliability of insulations in electrical equipment and electric vehicles in the context of policies towards carbon neutrality rules. The authors also hope that this paper can be helpful in future development and research of novel insulating materials, which promote the realisation of the carbon-neutral vision. 

/pdf/insulating-materials-for-realising-carbon-neutrality-2ym29t1u.pdf

Insulating materials for realising carbon neutrality: Opportunities, remaining issues and challenges

Excellent energy storage performance of polyetherimide filled by oriented nanofibers with optimized diameters

Origins and effects of deep traps in functional group grafted polymeric dielectric materials

Metallised polypropylene film capacitors (MPPFCs) are widely used in power electronics and are generally degraded by elevated temperatures. This work aims to determine the relationships between the structural changes of MPPFC and the microstructural variations of the PP film during the thermal ageing of MPPFC at 100°C for 38 days. The capacitance of MPPFC has a slight decrease during thermal ageing. However, the breakdown voltage of the MPPFC decreases by 39.4% by the ageing. The partial discharge (PD) number of MPPFC increases linearly with ageing time. The tear-down analysis of the MPPFC reveals that the molecular structure of the PP film has not been altered but has led to molecular chain scission and the generation of some polar fragments/groups. Meanwhile, the relative permittivity of the PP films rises as the ageing time increases. Moreover, thermal ageing causes the conversion of aluminum to alumina in the metallised electrode, which is hydrophilic for polar groups and leads to an adhesion effect between the metallised electrodes and the PP film. Contact angle measurements prove that the surface hydrophilicity of the PP sample increased after thermal ageing. Therefore, the PD/breakdown voltage in the MPPFC increases/decreases due to the uneven adhesion of the metallised PP film. 

Revisiting the thermal ageing on the metallised polypropylene film capacitor: from device to dielectric film

Polypropylene (PP)-based nanocomposite is a promising insulation material for recyclable high-voltage direct current (HVDC) cables, where the coupling agent plays an important role. In this study, four silane coupling agents with different alkyl chain groups (methyl, propyl, octyl, and octadecyl) were used to surface-modify magnesium oxide (MgO) nanoparticles. The surface-modification effect on the electrical properties of PP/MgO nanocomposites was investigated. The results show that surface-modified nanoparticles introduce quantities of deep traps, whose quantity increases as the alkyl chain length increases. The similar tendency also occurs on DC volume resistivity. All these nanocomposites show remarkable space charge suppression ability and improved DC breakdown strength. Among them, the nanocomposites with octyl-modified MgO show the best electrical properties, which could be attributed to the introduction of a large quantity of deep traps. The work may give a reference for the selection of coupling agents in PP-based nanocomposite insulation material for HVDC cable.

Surface-modification effect of MgO nanoparticles on the electrical properties of polypropylene nanocomposite

Polypropylene (PP)-based composites including PP/thermoplastic polyolefin (TPO) elastomer blends and PP/TPO-based nanocomposite are rather promising insulation materials for HVDC cables. Aging is an inevitable issue in practical operation and needs to be investigated thoroughly. In this paper, PP/TPO blends and PP/TPO/ZnO nanocomposite were prepared and thermally aged at 130 °C for various periods (28 days at longest). The micro-morphology, thermal properties and high-temperature space charge behavior were investigated. The results show that PP and TPO still exhibited excellent compatibility without phase separation after thermally aging. The crystallinity was not influenced after thermally aging, but a new endothermic peak at about 140 °C appeared in the melting process of thermally aged composites. Space charges accumulated in the two thermally aged composites, but dissipated slowly in PP/TPO, while dissipated rapidly first and then reduced in PP/TPO/ZnO nanocomposite. The calculated apparent charge mobility and trap level distribution based on space charge dissipation show that deep traps increased in aged PP/TPO, but decreased in aged PP/TPO/ZnO nanocomposite, which may be correlated with the chemical variation of thermally aged polymer matrix. The work may give a reference on the aging research of PP-based composite material for recyclable HVDC cable insulation.

Thermal properties and space charge behavior of thermally aged polypropylene/elastomer blends nanocomposite

Improved High-Temperature Electrical Properties of Polymeric Material by Grafting Modification

Polypropylene (PP) is considered to be a rather potential material for recyclable HVDC cable insulation. Grafting modification has been proven to be effective to further enhance the dielectric properties of PP. To investigate the grafting effect on the performance of different PPs, homopolymerized PP (PPH), ethylene-propylene copolymer (PEC), and impact PP copolymer (IPC) are, respectively, grafting-modified by styrene, and the comprehensive properties are experimentally studied. The results indicate that all these grafting-modified PPs show enhanced dielectric properties, while the enhancement levels differ. Styrene-grafted PPH presents the best dielectric properties improvement and highest thermostability, but the elastic modulus is also the highest. PEC exhibits moderate dielectric properties enhancement after grafting modification. Grafting-modified IPC shows excellent mechanical properties, but the enhancements on dielectric properties are rather limited. Grafting styrene has no significant influence on the thermal and mechanical properties of PP. In conclusion, PPH is preferred to be the matrix for styrene-grafting to achieve enhanced dielectric properties. However, the mechanical properties still need further modification to meet the demands of cable insulation.

Shixun Hu

Papers

Surface-modification effect of MgO nanoparticles on the electrical properties of polypropylene nanocomposite

Origins and effects of deep traps in functional group grafted polymeric dielectric materials

Thermal properties and space charge behavior of thermally aged polypropylene/elastomer blends nanocomposite

Improved High-Temperature Electrical Properties of Polymeric Material by Grafting Modification

Comprehensive Comparisons of Grafting-Modified Different Polypropylene as HVDC Cable Insulation Material