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Thin polypropylene film is having a strategic role at present because it is the dielectric of choice for high-energy density and high-power density dc-link capacitors. The fast evolution of green energy applications is demanding for higher energy and power density, with expected operating temperature and electric field up to 115°C and above 250V/µm respectively, at which the insulation resistance becomes a key performance factor. Studies on thin film polypropylene are rare in literature and the characterization of ultra-thin films poses several technical challenges. In this work, the use of relatively large capacitance wound film capacitors allowed to improve the sensitivity of conductivity measurements, and to perform multiple breakdown analyses. Two different commercial polypropylene films were tested. Films and capacitors were submitted to processes aimed at reducing the leakage current at high temperatures and high electric fields. The selected treatments made possible to improve the high temperature performance, with significant reduction of leakage current. One relevant observation highlighted in this work regards the constant decay of current during dc polarization experiments, combined with the absence of a steady state condition, as reported by few other authors before. The explanation of such observation required the formulation of a novel mechanism, deviating from a pure conductivity model. A theoretical model based on charge injection and electric field distortion was developed to explain the experimental leakage current results, with focus on the decay of leakage current with time. Numerical simulation was implemented and used to validate the model and qualitatively simulate experimental results. According to the proposed model, leakage current includes a dissipative contribute from conduction through the material and a reversible contribute due to injection of charge with formation of space charge and distortion of the internal electric field.

This article investigates the surface electrical properties of polyether-ether-ketone (PEEK) and cross-linked polyethylene (XLPE) before and after the fluorination modification. The samples are tested for surface conductivity, surface morphology, and surface antiaging characteristics. The samples are also tested for flashover voltage with ramping voltage under 100, 20, and 10 kPa. The results showed that the surfaces of PEEK and XLPE become slightly smoother with the increase in the fluorination time and the conductivity increased with the increase in the fluorination time. After fluorination, the surface of PEEK shows significant hydrophilicity, while the surface hydrophobicity of XLPE is slightly reduced, and there is a trend of recovery with the extension of the fluorination time. After fluorination, the surface antiaging performance is significantly improved. Compared with the untreated samples, the fluorinated PEEK has a higher flashover voltage at 100 kPa, and the flashover voltage is slightly increased at 10 and 20 kPa, while the flashover voltage of XLPE after fluorination shows a downward trend, especially under low pressures. This study provides a reference for fluorinated polymers as promising candidates for the insulation systems for all-electric aircraft.

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.

The work activities reported in this PhD thesis regard the functionalization of composite materials and the realization of energy harvesting devices by using nanostructured piezoelectric materials, which can be integrated in the composite without affecting its mechanical properties. The self-sensing composite materials were fabricated by interleaving between the plies of the laminate the piezoelectric elements. The problem of negatively impacting on the mechanical properties of the hosting structure was addressed by shaping the piezoelectric materials in appropriate ways. In the case of polymeric piezoelectric materials, the electrospinning technique allowed to produce highly-porous nanofibrous membranes which can be immerged in the hosting matrix without inducing delamination risk. The flexibility of the polymers was exploited also for the production of flexible tactile sensors. The sensing performances of the specimens were evaluated also in terms of lifetime with fatigue tests. In the case of ceramic piezo-materials, the production and the interleaving of nanometric piezoelectric powder limitedly affected the impact resistance of the laminate, which showed enhanced sensing properties. In addition to this, a model was proposed to predict the piezoelectric response of the self-sensing composite materials as function of the amount of the piezo-phase within the laminate and to adapt its sensing functionalities also for quasi-static loads. Indeed, one final application of the work was to integrate the piezoelectric nanofibers in the sole of a prosthetic foot in order to detect the walking cycle, which has a period in the order of 1 second. In the end, the energy harvesting capabilities of the piezoelectric materials were investigated, with the aim to design wearable devices able to collect energy from the environment and from the body movements. The research activities focused both on the power transfer capability to an external load and the charging of an energy stor