ABSTRACT The advancement of high‐energy‐density capacitors necessitates dielectric films with superior breakdown strength. Although enhancing crystallinity is a common strategy to improve dielectric breakdown performance, it is not the sole determinant of breakdown strength. This work systematically investigates the effect of annealing (60°C–140°C) on the microstructure and electrical breakdown behaviour of biaxially oriented polypropylene (BOPP) films. Annealing above 110°C is found to not only increase crystallinity and lamellar thickness but also promotes a rearrangement of the amorphous phase. A combination of in situ fourier transform infrared spectroscopy (FTIR), low‐field solid‐state nuclear magnetic resonance (LF‐SSNMR), and Positron annihilation lifetime spectroscopy (PALS) reveals that annealing reduces free volume and significantly suppresses molecular chain mobility in the amorphous phase, as the annealing temperature raise to 140°C, the fractional free volume decreased from 0.15% to 0.09%, and the segment mobility was significantly suppressed. Consequently, the energy storage density has increased from 1.27 J/cm 3 to 2.7 J/cm 3 , and the charge‐discharge efficiency of the 140°C‐annealed film still reached as high as 94.4%, meanwhile, the breakdown filed strength of the metallised 140°C‐annealed film has also increased by 12.6% (≈67 MV/m). These findings demonstrate that amorphous phase mobility including mobility of functional groups, in addition to crystallinity, is a dominant factor governing breakdown behaviour. The constrained chain mobility impedes charge carrier transport and limits energy accumulation, thereby suppressing the initiation of electron avalanche breakdown. This study establishes a new structure–property relationship centred on the mobility of amorphous chains, identifying it as a key descriptor for design of high‐performance dielectric polymers via annealing.
ABSTRACT High‐voltage power semiconductor devices are subjected to repetitive frequency square‐wave pulse voltages during actual operation, making the online assessment of their packaging insulation material degradation challenging. Based on the principle of charge–atom coupling, this paper proposes a novel assessment method utilising molecular vibration acoustic waves induced by forced charge motion at square‐wave pulse edges. By directly employing the rising and falling edges of high‐voltage square‐wave pulses inherent in device operation as excitation sources, this method stimulates molecular chain vibrations within the insulation material and achieves in situ degradation assessment through synchronous detection of acoustic signals. Results indicate that pulse repetition frequency has no significant effect on vibration amplitude; vibration amplitude is proportional to the square of the pulse voltage amplitude and temperature exhibits different effects on the two materials—silicone gel shows a monotonic increase in vibration amplitude with rising temperature, whereas silicone rubber initially increases and subsequently decreases. Under fast edge excitation, low cross‐linking density insulation materials exhibit a double‐peak waveform, whereas high cross‐linking density materials consistently exhibit a single‐peak waveform. This phenomenon reveals intrinsic differences in material chain segment dynamics, providing a new technical pathway for the online monitoring of power device insulation systems without requiring external Gaussian pulse generators.
ABSTRACT Quantifying nanoparticle dispersion remains a central challenge in the development of polymer nanodielectrics, where electrical and dielectric properties are strongly governed by the three‐dimensional (3D) dispersion state of inorganic fillers. This study is aiming to establish a new quantitative framework that enables direct inference of the 3D dispersion state—expressed as the cluster size—from experimentally accessible two‐dimensional (2D) transmission electron microscopy (TEM) images. The framework integrates TEM image analysis, 3D numerical modelling, and machine‐learning regression, thereby linking 2D morphological descriptors to a physically interpretable 3D dispersion parameter. TEM images of three kinds of epoxy nanocomposites with different filler sizes and dispersion conditions were processed to extract particle contours, Feret maximum diameters, and inter‐particle distance distributions, from which a normalised full width at half maximum (FWHM) was obtained as a 2D descriptor of spatial heterogeneity. To clarify the physical origin of this descriptor, a 3D Thomas‐cluster model was constructed to generate particle configurations with controlled dispersion states and filler contents. Cross‐sectional slices extracted from the simulated structures reproduced the experimentally observed relationship between the normalised distance‐distribution width and the underlying 3D dispersion state, represented by the cluster size parameter. Using these simulation‐derived datasets, a LightGBM regression model was trained to predict the 3D dispersion state directly from two TEM‐extracted features: the normalised FWHM and the filler content. The model successfully inferred the relative dispersion states of the experimental samples, demonstrating that these 2D descriptors contain sufficient information to estimate the underlying 3D organization of nanoparticles. The proposed framework provides a physically grounded, data‐driven methodology for quantitative dispersion assessment and establishes cluster size as a robust, transferable metric for describing nanoparticle dispersion in polymer nanodielectrics.
ABSTRACT High‐crystallinity polypropylene (PP) has broad applications due to its excellent mechanical properties, thermal stability and processing properties. However, its capacitive energy storage performance deteriorates severely under high electric fields. In this paper, two surface modification methods, ultraviolet irradiation and surface fluorination, were adopted to modify the commercially biaxially stretched polypropylene (BOPP) film, and the effects of these methods on the surface structure, chemical composition, thermal properties, mechanical properties, dielectric properties and energy storage properties of the material were systematically studied. The results show that the introduction of oxygen‐containing polar groups by ultraviolet irradiation and the formation of C‐F bonds through fluorination treatment can effectively augment the electron injection barrier, suppress charge injection and migration and significantly improve breakdown strength, energy density and charge–discharge efficiency. The breakdown strength was significantly improved from 750 MV/m of pristine BOPP film to 790 MV/m of UV irradiated film and 831 MV/m of fluorinated sample, respectively. As a result, the energy density of the ultraviolet irradiated sample is 5.62 J/cm 3 and the efficiency is 92%. The energy density of the fluorinated sample can reach 5.9 J/cm 3 and the efficiency is 93%.
Bismuth ferrite (BiFeO3, BFO) is a prominent lead-free multiferroic material, whereas strontium titanate (SrTiO3, STO) is a high-permittivity perovskite oxide widely used to tailor dielectric performance. In this work, the solid-solution formation of (1 - x)BFO-xSTO (0.1 <= x <= 0.4) nano-ceramics was synthesised via a low-temperature molten salt method using NaCl as a flux. Rietveld refinement of X-ray diffraction patterns confirmed the formation of a single-phase perovskite structure without secondary phases. The FESEM micrographs indicated the uniform granular-shaped grain morphology of BFO. The XPS revealed the presence of mixed-valence Fe ions, indicating partial reduction within the BFO lattice. The incorporation of STO suppressed oxygen vacancies, enhancing the dielectric constant (similar to 151 at 100 kHz for x = 0.4) and reducing the dielectric loss with increasing STO content. M-H loops exhibited weak ferromagnetic behaviour, attributed to enhanced spin canting and lattice distortion. These results highlight the effective tuning of dielectric and magnetic properties through STO substitution, demonstrating the potential of BFO-STO ceramics for multifunctional electronic device applications such as spintronics and dielectric storage components.
