The piezoelectric response and thermal charge stability of melt-blown polypropylene (PP) nonwovens have been investigated for as-received and orthophosphoric acid $\left(\mathrm{H}_{3} \text{PO}_{4}\right)$ -treated samples. Corona-charged nonwoven stacks with up to three intermediate uncharged layers have been studied via quasistatic d33 measurements, thermally stimulated charge decay (TSCD), and $\mathbf{d}_{\mathbf{3 3}}$ measurements as a function of temperature. The as-received PP nonwovens exhibit a room-temperature da3 of approximately $108-130 \text{pC} / \mathrm{N}$, exceeding PVDF by a factor of 4 to 6. The use of a 30 wt% blend of $\mathbf{H}_{\mathbf{3}} \mathbf{P O}_{\mathbf{4}}$-treated pellets (100°C, 24 h) with untreated pellets shifts the charge-decay onset by 20-30°C and improves the relative thermal stability but reduces $\mathbf{d}_{\mathbf{3 3}}$ by a factor of 3 to 4 due to morphological alterations of the fiber network. Optical microscopy confirms a 33% increase in mean fiber diameter for samples manufactured with treated pellets. The sensitivity-stability trade-off is quantified for the first time in the melt-blown nonwoven format, and the results indicate that post-formation treatment may be preferable to preserve the cavity microstructure while enhancing charge retention.

In a nuclear environment, the most significant aging factors of safety-related cables are elevated temperature and high-energy radiation. During installation, the bending radius of cables is often below the minimum limits, resulting in continuous mechanical strain on the insulation and jacket. Since mechanical strain is also an important aging factor of polymers, this study examines the effect of mechanical strain on conductive processes in cable insulation. For the investigation, samples were prepared from EPR-insulated and CSPE-jacketed Class 1E NPP instrumentation and control cable. A group of samples was irradiated at a dose rate of $\mathbf{0. 8 ~ k G y} / \mathrm{h}$ to a total dose of 1.2 MGy, while another group was bent to a 15 cm radius during irradiation. Polarization-depolarization current was measured in both sample groups. The results showed that the current is higher in bent samples, and the difference is even more significant as the absorbed dose increases. The increased conductivity can be explained by degradation by-products formed during degradation, such as carbonyls, hydroperoxides, and other polar groups. The results of the study revealed that mechanical strain is an additional degradation factor for cable insulation in a nuclear environment, resulting in a decline in electrical properties.

Polyimide (PI) films are widely employed as high-temperature insulation in aerospace and electrical systems, where long-term reliability strongly depends on their thermo-oxidative stability. To evaluate their aging behavior, PMDA-ODA PI films were thermally aged in air at 280 degrees C up to 60 days. Materials were tested by nondestructive techniques: Field Emission Scanning Electron Microscope (FE-SEM), Fourier Transform Infra-Red (FTIR) (using both ATR and transmission modes), dielectric spectroscopy, DC conductivity and thermally stimulated depolarization current (TSDC) measurements. The results reveal that the material remains stable during the first 10 days with limited oxidation. From 10 to 40 days, FTIR shows that oxidation gradually extends from the surface into the bulk, increasing the amount of carbonyl dipoles. This leads to stronger dipolar polarization, while microstructural rearrangements caused by chain scission enhance interfacial polarization and conductivity. In the later stage, both interfacial polarization and conductivity reach a stable plateau. DC conductivity and TSDC measurements jointly revealed a two-stage trap evolution: a pronounced reduction of activation energy and trap depth up to 30 days, followed by a slower stabilization phase. These findings deepen the understanding of polyimide aging and support the development of more reliable high-temperature insulation.

The XLPE/EPDM interface is one of the weak points in the insulation system of HVDC cable joints, where interface charge is easily accumulated, causing electric field distortion, thus leading to discharges and breakdown. The effects of interface roughness, the presence or absence of silicone oil at the interface, and the ratio of different XLPE/EPDM thicknesses on the interfacial charge are still not clear. XLPE/EPDM with different interfacial roughness (P600, P400, P180, P80) and thickness ratios (5:3,1:1,5:8) were prepared, and silicone oil was coated at the interface of the samples with interfacial roughness of P80, and interfacial charge distribution was investigated by the pulsed electroacoustic method (PEA). The interfacial roughness significantly affects the interfacial charge. In particular, the interfacial charge accumulation increases slowly with the increase of roughness at first, then increases sharply. This is related to the interfacial barrier, which is controlled by the contact area at the interface. Silicone oil applied at the interface decreases the interfacial charge, which could be associated with a better contact between the two layers at the interface. Different thickness ratios slightly affect the magnitude of the interfacial charge. A slight increase in interfacial charge build-up with increasing XLPE thickness (EPDM thickness remaining unchanged), likely resulting from the enhanced blocking effect of the interfacial barrier.

This paper investigates the aging behavior of PVC using dielectric spectroscopy, DC conductivity measurements, and Fourier transform infrared spectroscopy (FTIR) under accelerated radiation (860 Gy/h, room temperature) and radiation-thermal conditions (820 Gy/h at 70° C). The real part of permittivity in the low frequency and conductivity show pronounced changes during the initial aging phase, while further aging causes the reaching of a plateau of the investigated properties under irradiation alone, whereas combined aging with temperature leads to stronger degradation at long aging times. FTIR results show a continuous increase in the -OH absorption band throughout aging, indicating a strong oxidation in the materials.

Challenges such as complex natural environments and high altitudes impose stringent requirements on the insulation performance of electrical equipment. Being oil-paper, the primary insulating material used for high-voltage electrical equipment, its space charge characteristics play a crucial role in influencing electric field distortion, thereby affecting its performance, which is dependent on physical treatment of the materials themselves. This work investigates the effect of dielectric barrier discharge plasma treatment on the space charge transportation of oil-impregnated paper. Under applied DC voltage, the charge amount in untreated samples increases gradually, while in plasma-treated specimens, the charge amount reaches a maximum value within minutes and then decreases. This characteristic is linked to reduced trap depth and increased carrier mobility due to plasma treatment, which enhances charge recombination probability. The effect of plasma treatment is closely related to the energy dose. Treatments providing energy at or below 7.36 J·cm-2 are ineffective, and a threshold exists between 14.73 J·cm-2 and 22.09 J·cm-2, where excess energy is redundant and may cause aging or mechanical damage due to excessive temperature rise. For treatments with energy doses ranging from 7.36 J·cm-2 to 22.09 J·cm-2, beneficial post-treatment effects persist, with higher delivered power (for comparable energy dose) recommended to maintain longer efficacy.

ABSTRACT This study investigates the effects of radiation‐induced ageing on the physical, chemical, mechanical and electrical properties of Poly[4,4′‐oxydiphenylene‐ pyromellitimide] (PMDA‐ODA) polyimide, a high‐performance insulation material widely used in aerospace applications. Using a combination of Fourier‐transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and dielectric spectroscopy, the degradation mechanisms were systematically analysed under varying radiation dose rates and thermal conditions. Results reveal a dual‐path degradation process: at lower dose rates (< 860 Gy/h), microstructural modifications, such as chain scission predominate, under more severe conditions (∼2 kGy/h and 70°C), oxidation becomes the dominant mechanism. Despite these changes, TGA confirms exceptional thermal stability with almost invariant residual mass at 800°C (∼60%) and mechanical testing shows increased stiffness with ageing, attributed to chain crosslinking. Dielectric analysis indicates an increase in permittivity, dissipation factor and DC conductivity with ageing severity, whereas AC breakdown voltage remains largely unaffected. These findings highlight the complex interplay between molecular degradation and macroscopic performance, providing a basis for the nondestructive evaluation and reliable deployment of PMDA‐ODA in extreme environments.

The cross-linked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) interface is one of the weakest elements in the cable insulation system, where failure is frequently due to space charge accumulation. Even if the interface charge characteristics have been studied in the past, the space charge characteristics at coated interfaces under temperature gradient (TG) and polarity reversal (PR) remain unclear, and the underlying mechanisms require further understanding. This study investigates the interfacial charge characteristics of EPDM/XLPE systems coated with non-polar (PDMS) and polar (PMTFS) silicone oils using the pulsed electro-acoustic (PEA) method. By integrating carrier mobility, trap distributions, and interfacial potential barriers, the underlying mechanisms were elucidated. The results show that in the EPDM/PDMS/XLPE interface structure, where the interfacial barrier is relatively low, deep traps in the EPDM play a significant role in the hysteresis of interfacial charge polarity during PR. The difference between electron and hole traps in the EPDM leads to different interfacial charge accumulation before and after PR. In the EPDM/PMTFS/XLPE interface structure, due to the high hole potential barrier at the EPDM/PMTFS interface, the interfacial charge polarity does not follow the applied voltage polarity on the XLPE side under negative voltages and low TG. Under high TG and negative voltages, negative interface charges decrease. In addition, the XLPE/PMTFS and EPDM/PMTFS electronic potential barriers cause electrons to readily be trapped within the PMTFS layer, which accelerates the transition of interfacial charge polarity toward negative polarity and promotes the accumulation of negative charges. This may explain why, at low TG, more negative charges accumulate at the interface than positive charges when the applied voltage switches from positive to negative.

EVA is a PE copolymer containing vinyl acetate side groups which confer significant polarity to the system. This polymer exhibits high mechanical properties, the ability to incorporate high filler loadings without embrittlement and a thermoplastic behaviour. The EVA properties depend on the VA content, which directly affects the crystalline fraction, the polarity and the thermal stability of the polymeric system. In this work three different VA content have been selected, namely 7%, 19% and 28%, together with a LDPE sample as a 0% VA content reference. The space charge build-up dynamics and the charge mobility have been investigated by means of the PEA technique and conductivity measurements, respectively. Tests have been performed at 20 kV/mm and 25 °C. Although the PEA results of EVA are comparable with LDPE results, over a 7% VA content a significant heterocharge formation close to the cathode is detected. Conductivity increases as the VA content increases, confirming the relationship between the extension of the amorphous phase and the charge mobility. The findings of this study lead to considering the EVA with low VA content suitable as outer insulation for MV and HV transmission system accessories, such as joints and terminations, where components are subjected to high mechanical stress and a reduced temperature gradient.

The y-irradiation initiates several degradation mechanisms in insulating polymers, including cross-linking, chain scission, and oxidation, thereby degrading both the mechanical and electrical properties of the materials. Traditionally, the elongation properties are used to qualify the functionality of polymers in radiation environments; however, there is a growing need for non-destructive material testing. One of the substantial advantages of testing electrical properties is that they can be non-destructively measured. Therefore, the conductive properties of y-irradiated EPR insulation were investigated in this study. The subject of the research, EPRinsulated cable samples, was investigated using current and extended voltage response (EVR) measurements. The samples were irradiated at a dose rate of 0.8 kGy/h. The total absorbed dose was 1.2 MGy. The results showed that the conductive current and the slope of decay voltage increased steadily with absorbed dose. The activation energies of conductivity increased from the initial 0.8 eV to 2 eV after the total dose. From the trap distribution data, the de-trapping rates were calculated for shallow and deep traps. The de-trapping rate showed a strong correlation with conductivity; however, the de-trapping rate was greater for shallow traps, indicating the dominant role of shallow traps in conduction.

Structural Health Monitoring (SHM) of composite laminates often relies on embedded sensors, which can detrimentally compromise the material's mechanical integrity. In contrast, micro and nanomaterials have been shown to exert minimal impact on the mechanical performance of laminates while providing effective sensing capabilities for compressive, tensile, and tactile loads. Nevertheless, the detection of elastic wave propagation continues to depend on ceramic piezoelectric lead zirconate titanate (PZT) sensors. This study presents an innovative approach by integrating PZT nanofibers directly into the composite laminate structure, effectively combining the superior piezoelectric properties of PZT with the advantages of a nanostructured, minimally intrusive material. Specifically, PZT nanofibers are strategically interleaved and positioned at the corners of a rectangular glass fiber reinforced plastic (GFRP) panel (30 x 20 cm). Upon impact on the laminate surface, the high sensitivity of the PZT nanofibers enables the detection of elastic waves propagation, generating a piezoelectric signal that allows for precise impact localization through a triangulation algorithm. Compared with a traditional panel with commercial PZT disks interleaved, the nanostructured panel exhibits a higher impact localization error (2.50 +/- 2.46 cm versus 3.74 +/- 2.84 cm), while exerting less influence on its mechanical properties. Indeed, low-velocity impact tests confirm the superior impact resistance of the nano-modified laminate compared to its counterparts with traditional PZT sensors.

Superconducting magnets in accelerators and fusion machines are cooled in liquid helium, but transitions from liquid to gas phase or leaks into vacuum insulation can locally degrade the dielectric strength of the insulation system. The aim of this work is to characterize DC breakdown in gaseous helium at roomtemperature, comparing the results under uniform and nonuniform electric fields. The study integrates Finite Element Method (FEM) electric field simulations to correlate breakdown voltage with field distribution, alongside a preliminary analysis of the discharge regime through a dielectric barrier. Results highlight deviations from classical Paschen behavior, introduced by the non-uniformity of the field, and a polarity dependence. Notably, in low-pressure regimes, non-uniform geometries exhibited higher breakdown voltage compared to uniform fields. These findings offer a baseline for helium-cooled system design and future cryogenic campaigns.

This study investigates the thermal aging behavior of bamboo paper and wood paper and their epoxy resin impregnated composites. The results reveal that although both materials follow a common “degradation-reorganization” aging path, their mechanisms differ fundamentally due to inherent structures. Bamboo paper exhibits higher chemical reactivity arising from its greater hemicellulose and lignin content, leading to more extensive oxidative degradation, as evidenced by a 66% increase in carbonyl index versus 53% for wood paper, a faster decline in thermal decomposition temperature, and a higher char residue rate. Crystallinity analysis shows a non-monotonic evolution in both papers, with wood paper maintaining a consistently higher crystallinity index throughout aging. Dielectric measurements demonstrate that the epoxy resin impregnated bamboo paper composites develop a more reactive interface, producing broader interfacial polarization, higher relaxation strength and conductivity, and a more dynamic three-stage dielectric evolution, with parameters 30% to 50% higher than those of the epoxy resin impregnated wood paper composites. Havriliak-Negami model analysis reveals that the epoxy resin impregnated bamboo paper composites possess higher charge mobility and a wider trap energy distribution. Separate analysis of the shape parameters further identifies fundamentally different interfacial polarization evolution modes, with the bamboo composites exhibiting α1-dominated broadening of the relaxation time distribution and stable β1, whereas the wood composites display a β1-dominated decline. These findings demonstrate that wood paper undergoes structure-modulated aging with relatively stable progression, while bamboo paper follows chemically driven aging characterized by more complex degradation and interfacial restructuring.

This study investigates the effects of DC corona discharge on the surface and electrical properties of three epoxybased insulating materials: the neat resin and two nanocomposite formulations containing $\mathbf{1 0 \% w t}$ and $\mathbf{2 0 \% w t \mathbf {T i O} _ {\mathbf {2}} \text{. Prolonged co-}}$ rona exposure induces surface degradation primarily through oxidative reactions triggered by ion acceleration, leading to the formation of oxygen-containing functional groups and progressive material erosion. The extent of these transformations strongly depends on the filler content. Among the tested materials, the 10% $\mathbf{T i O}_{\mathbf{2}}$ nanocomposite exhibits the highest stability under corona ageing, showing the lowest surface erosion identified as the dominant degradation mechanism in this ageing mode and superior electrical robustness, with reduced surface conductivity. In contrast, the neat resin and the $\mathbf{2 0 \%} \mathbf{T i O}_{\mathbf{2}}$ formulation undergo more pronounced morphological and electrical changes. These findings clarify how nanoparticle loading may enhance the corona resistance of epoxy-based materials and help guide the optimization of their properties for high-voltage insulation applications.

This study examines the impact of DC corona discharge on the electrical and chemical properties of polyethylene (PE), epoxy resin (EP), and polyimide (PI). The discharge process induces surface modifications, primarily through the formation of carbonyl and hydroxyl functional groups, resulting from the dissociation of environmental oxygen and moisture due to ion acceleration effects. The extent of these chemical transformations is material-dependent, with PE exhibiting the most pronounced variations due to its lower binding energy. In contrast, $E P$ and $P I$ demonstrate greater structural stability owing to the presence of aromatic and imide functionalities, respectively. Changes in surface conductivity suggest nonuniform charge accumulation dynamics, likely influenced by surface reorganization mechanisms. Surface erosion is observed across all samples. These findings provide essential insights into the degradation mechanisms associated with dielectric materials under sustained corona exposure, contributing to the optimization of material selection and engineering strategies in high-voltage applications.

This study investigates the physical-chemical and electrochemical performances of innovative separators for Li-ion batteries based on poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) nanofibrous membranes. The nanofibrous mat is produced through the electrospinning process, ensuring high surface to volume (S/V) ratio and allows large-scale production under suitable conditions. The materials investigated in this work aim at overcoming some limitations specific to the commercial separators, for example, mechanical shrinkage and electrolyte uptake. This is achieved by adding nanoparticles of different types, for example, ZrO2, SnO2, SiO2 at different concentrations. Results claim that nanofibrous separators improve the mechanical and thermal stability of the mat without significantly impacting its electrochemical performances. In particular, the addition of 7 wt% of ZrO2 nanoparticles to the nanofibrous separator showed to outperform commercially available solutions (i.e., Celgard) in terms of mechanical and thermal stability exhibiting, also, electrochemical performances.

This study investigates the gamma radiation aging behavior of EPDM insulation material using dielectric spectroscopy and Fourier transform infrared (FTIR) analysis. Specimens were aged under dose rates of $50.3,860$, and $1930 ~\text{Gy} / \mathrm{h}$ at room temperature, as well as under a combined radiationthermal condition ($820 ~\text{Gy} / \mathrm{h}$ at 70° C), with periodic sampling. Dielectric measurements show that, under all conditions, tan $\delta$ at 100 kHz monotonically increases with accumulated dose, indicating its potential as a general aging indicator. FTIR results found that oxidative degradation occurs primarily through the formation of metal carboxylates, evidenced by the raised absorbance at $\mathbf{1 6 5 1} \mathbf{~ c m}^{\mathbf{- 1}}$. Low dose rates tend to promote more uniform oxidation during prolonged radiation aging. Notably, the trends in dielectric and chemical properties exhibit a strong positive correlation, proving that $\tan \delta$ at 100 kHz serves as a reliable and non-destructive technique for evaluating radiationinduced aging in polymeric insulation.

This paper has the goal to investigate a new type of ionomer as a possible substitute for cross-linking polyethylene (XLPE) in HVDC insulating systems. XLPE is a thermosetting polymer non melt-processable and affected by cross-linking byproducts, cause of electric field distortion. Ionomers under test contain a small amounts of ion-pair comonomers comprising amine terminated methacrylates and methacrylic acid. They are able to provide a thermo-mechanical behavior similar to XLPE. PEA and conductivity test were performed on ionomers different for ion content; results revealed that ionic clusters may have a remarkable impact on DC performance. Overall, ionomers showed electrical properties comparable to XLPE, confirming the hypothesis as a possible substitute.

This article investigates the dielectric properties of novel polymeric materials for high-voltage direct current (HVDC) cable insulation, focusing on their potential for reprocessability and recyclability. Specifically, a reprocessable vitrimer, VIT2.6 characterized by the presence of thermally reversible cross-links, and its thermoplastic precursor, PE-HEMA, were comprehensively analyzed. Their performance was directly compared against cross-linked polyethylene (XLPE), the current benchmark material for HVDC applications. The study primarily focused on assessing space charge accumulation, utilizing the Pulsed Electroacoustic (PEA) method, and measuring electrical conductivity. This work aims to provide crucial insights into the dielectric behavior of these promising alternatives, particularly highlighting that the vitrimer exhibited less space charge accumulation and lower electrical conductivity, thus demonstrating significantly improved performance compared to PE-HEMA in meeting the stringent requirements for sustainable HVDC insulation.

This work aims to investigate the density and microstructure impact on DC performance of pure polyethylene. Plaques of PE-based materials, characterized by different densities namely LLDPE, LDPE and HDPE were investigated through DC electrical tests. These latter include space charge and conductivity tests performed at high electric fields (40 and $20 \text{kV} / \text{mm}$, respectively). Results show that electrical properties decrease as material density increases, defining a dependance between the microstructural arrangement within the polymer matrix and the conduction properties of the insulation system.