Low dielectric polymer materials play a critical role in modern microelectronics, telecommunications, power systems, and aerospace industries, and the development of their preparation technologies continuously drives the progress of related industries. The research background of low dielectric polymer material fabrication techniques was elaborated in this paper. Beginning with material design principles, the relationship between molecular structures, functional group characteristics, and dielectric properties was deeply analyzed. The research progress of low-dielectric polymers, including polyolefins, fluorinated polymers, silicon-containing polymers, and aromatic polymers, was systematically classified and discussed, aiming to provide a reference for further research and application of low-dielectric polymer materials.

Aiming to address the challenges in composite insulation, low dielectric constant and loss in high-voltage, high-frequency transformers, this paper aims to investigate the dielectric and breakdown characteristics of a novel composite insulation material composed of fluorocarbon (FC-40) and insulation paper. Firstly, the basic physico-chemical properties of the fluorocarbon were studied, and a Nomex paper-fluorocarbon composite insulation sample was prepared by fully vacuum impregnating Nomex paper with the fluorocarbon. The dielectric and breakdown properties of the composite insulation were investigated using a broadband dielectric spectroscopy system under high-temperature and high-pressure and an AC/DC breakdown testing platform, combined with the Havriliak-Negami (H-N) relaxation distribution model. The results indicate that the Nomex paper-fluorocarbon composite insulation exhibits weak interfacial polarization, leading to a negligible increase in the imaginary part of the dielectric constant in the low-frequency range. The high-frequency relaxation peak shifts to higher frequencies as the temperature increases. Both the permittivity and dielectric loss of the Nomex paper-fluorocarbon composite insulation are significantly lower than those of conventional oil-paper insulation. The AC and DC breakdown strengths of the Nomex pa-per-fluorocarbon composite insulation are substantially higher than those of pure fluorocarbon, and are comparable to those of conventional oil-paper composite insulation. Overall, the Nomex paper-fluorocarbon composite insulation exhibits low permittivity, low dielectric loss, excellent breakdown performance, and high stability, making it a promising candidate for applications in the insulation system of high-frequency power electronic devices.

To address the problems of low time-frequency resolution and cross-term interference in the identification and localization of subway cable defects using time-frequency domain reflectometry (TFDR), this paper proposed a parameterized resampling time-frequency transform (PRTF) method as a time-frequency analysis approach for TFDR test signals. The PRTF method employed a Gaussian-envelope linear frequency modulation signal as the test signal, combined it with a dynamic resampling operator and an adaptive time-frequency atomic decomposition technique, and further introduced a normalized time-frequency cross-correlation function. By extracting time-delay information from local peaks, accurate defect localization was achieved. Simulation experiments were conducted on a flexible control cable in flame-retardant class B1 for subways, and a defect was preset at a distance of 3 900 m from the head end to verify the effectiveness of the proposed method. The results show that compared with the traditional Wigner-Ville distribution (WVD), short-time Fourier transform (STFT), and continuous wavelet transform (CWT) methods, the proposed method achieves a localization error of only 5.5 m, and its time-frequency cross-correlation function curve is free of interference terms. In contrast, the WVD method suffers from significant interference terms, while the localization errors of the STFT and CWT methods both exceed 16 m. The proposed method significantly improves the detection sensitivity and localization reliability of weak local defects in subway cables.

To improve the electrical performance of transformer oil under high electric field strength and extreme operating conditions, in this paper, modified transformer oils containing different concentrations of SiO₂ and ZnO nanoparticles were prepared, the terahertz time-domain spectra of different oil samples were measured experimentally, and the differences in their terahertz time and frequency domain spectral characteristics were analyzed. The results show that the absorption coefficients of different oil samples exhibit similar variation trends in the lower frequency range of 0.2-1.0 THz, while absorption is enhanced in the higher frequency range of 1.0-2.0 THz, where the differences in absorption characteristics among the oil samples are significant. Compared with pure transformer oil, the refractive index and permittivity of the nano-modified transformer oils in the terahertz frequency range increase significantly. In the frequency range of 0.2-2.0 THz, as the nanoparticle concentration increases, the average dielectric constant of both types of nano-modified transformer oils first increases and then decreases slightly. Meanwhile, the average dielectric loss factor of the ZnO nano-oil exhibits a fluctuating downward trend, whereas that of the SiO₂ nano-oil first decreases and then increases.

To address the problem of poor interfacial compatibility between polyimide (PI) and poly(vinylidene fluoride) (PVDF) during blending, a fluorinated copolyimide (FPI) was synthesized by introducing -CF₃ groups into the PI main chain via copolymerization, and then FPI/PVDF all-organic composite films were prepared by blending FPI with PVDF. The chemical structure, micromorphology, thermal stability, and mechanical properties of the composite films were characterized, with a particular focus on their dielectric and energy storage performance at high temperature. The results show that after introducing -CF₃ groups into the PI main chain, the interfacial gap between the FPI phase and PVDF phase is reduced to below 100 nm, and the conduction loss of the composite films is significantly suppressed at high temperature. The improved compatibility between PI and PVDF remarkably reduces the dielectric loss factor of the composite films at high temperature and enhances the frequency stability and temperature stability of the dielectric constant. When the mass fraction of PVDF is 5% and the molar fraction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in FPI is 30%, the FPI/PVDF composite films achieve an electric strength of 515.30 kV/mm and a maximum energy storage density of 2.96 J/cm³ at 150℃, which is 85.0% higher than that of the PI/PVDF composite film with the same PVDF content but without fluorinated interfacial modification. Furthermore, the introduction of 6FDA and PVDF contributes to an increase in both the trap density and the deep trap energy level of the composite, thereby enhancing the high-temperature energy storage performance of the composite films.

Thermoplastic polypropylene (PP) cables have been widely used due to their recyclability, simple and efficient manufacturing process, low energy consumption, and good electrical and mechanical properties. However, research on evaluation methods for their insulation ageing state remains relatively scarce. In this paper, three types of PP were subjected to accelerated thermo-oxidative ageing in the laboratory. The changes in carbonyl group content of the aged PP were characterized using infrared spectroscopy (IR) and ultraviolet spectroscopy (UV). Based on the observed change law, the correlation between the carbonyl content and the elongation at break was investigated to evaluate the ageing degree of each PP. The accuracy of the proposed evaluation method was verified using samples aged for 200 days. The results show that the carbonyl index of the three PPs measured by IR exhibits considerable variation with increasing ageing time, and it does not correlate with the elongation at break data, making it difficult to effectively characterize the ageing degree of PP. In contrast, UV spectroscopy reveals that the carbonyl content increases linearly with ageing time during the PP ageing process, based on which a correlation between carbonyl content and elongation at break can be established. Validation using samples aged for 200 days shows that the differences between the actual elongation at break of the three PP samples and the predicted values calculated from the carbonyl content are small. The errors for copolypropylene PPB1 and PPB2 are 5.02% and 6.55%, respectively, while the error for blended polypropylene PPH is slightly larger at 16.54%. It is demonstrated that the proposed method for evaluating the ageing degree of PP using UV spectroscopy possesses high accuracy.

To improve the oil-paper insulation performance in oil-immersed transformers, this paper utilized hexamethyldisilazane coupled with dielectric barrier discharge (DBD) plasma to modify cellulose insulation paper. The physical morphology and chemical composition of the surface and cross-section of the modified insulation paper were characterized, and the influence of the medium flow rate on the insulation paper properties and oil-paper insulation performance was investigated. The results show that at a medium flow rate of 15 mL/min, the oil-paper insulation performance is significantly enhanced. The dielectric constant of the modified insulation paper decreases to 2.5, the flashover voltage and electric strength increase by 33.8% and 25.0%, respectively, the water contact angle increases by 194.3%, and the water absorption decreases by 44.9%. Silicon-containing thin films are deposited on the surface of the insulation paper and penetrate into the interior of the fibers, which can introduce deep traps to restrict charge migration and fill the fiber pores with hydrophobic substances. The introduction of the low-polarity film reduces the dielectric constant of the insulation paper, mitigates the electric field distortion at the oil-paper interface, and improves the interfacial bonding ability of the oil-paper interface, thereby enhancing the oil-paper insulation performance.

Aiming at the thermal ageing problem of composite insulator silicone rubber sheds, relevant research using nonlinear ultrasonic detection technology was conducted in this paper. First, accelerated thermal ageing tests were conducted on the silicone rubber sheds of composite insulators, and the material parameters of samples with different thermal ageing time were measured. Then, a simulation model for nonlinear ultrasonic detection of composite insulator silicone rubber was established, and a nonlinear ultrasonic detection platform was built to carry out ultrasonic detection experiments. Finally, the relationship between the thermal ageing time of the composite insulator silicone rubber material and the ultrasonic nonlinear coefficient obtained from both simulations and experiments was verified. The results show that as thermal ageing time increases, both the fundamental and second harmonic amplitudes of the thermally aged samples exhibit a decreasing trend, while the relative nonlinear coefficient increases significantly. The linear fitting degree between the relative nonlinear coefficient measured in the experiment and the thermal ageing time is high. Thermal ageing leads to changes in the macroscopic mechanical properties of the material, manifested as decreased density and increased elastic modulus, which causes attenuation of acoustic wave energy, while the change in internal structure of material enhances the nonlinear effect. The second-order relative nonlinear coefficient can effectively characterize the thermal ageing degree of composite insulator silicone rubber materials, providing a new method for the non-destructive testing of the thermal ageing state of composite insulators.

Composite insulators in long-term outdoor operation undergo ageing due to the coupled effects of environmental factors. Accurately assessing the performance and ageing status of insulators that have been in service for many years is of great significance for the operation and maintenance of transmission lines. In this paper, a composite insulator that has been in service for 16 years on a 220 kV transmission line in Linyi area was investigated. By conducting multi-scale characterization analyses of the silicone rubber materials of different positions of the composite insulator umbrella sheds, including surface morphology, thermal properties, mechanical properties, electrical properties, and chemical structure, the spatial distribution law of the ageing degree of the silicone rubber material was investigated. The results show that after long-term service, the ageing degree of the composite insulator material exhibits spatial distribution characteristics. In the silicone rubber umbrella sheds of the composite insulator, the ageing degree of the large umbrella sheds is higher than that of the small umbrella sheds, the ageing degree at the high-voltage end is higher than that at the low-voltage end, and the ageing degree on the outer side is higher than that on the inner side. In addition, the differences in heat flow and thermogravimetric loss characteristics of the silicone rubber material at different umbrella shed positions are relatively small, while key parameters such as microstructure, hydrophobicity, chemical groups, electric strength, and creep compliance can effectively characterize the ageing degree of the umbrella sheds and significantly reflect the spatial ageing differences.

To enhance the energy storage performance of PVDF-based polymer dielectric films, layered structured polymer nanocomposites were prepared by solution casting method. The middle layer utilized poly(methyl methacrylate) (PMMA) with a volume fraction of 30% to suppress energy loss, while the outer layers employed poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) to provide high dielectric constant. Inorganic BaTiO₃ (BT) nano-ceramic particles were uniformly dispersed as fillers in the outer layers. By adjusting the filler content, its effects on the dielectric constant, electric strength, and energy density of the composites were investigated, along with the durability and stability of the films’ energy storage performance under different cycle numbers and in different regions. The results show that the addition of a low content of BT increases the dielectric constant, suppresses the dielectric loss, and reduces the leakage current density of the films. The composite with 5% mass fraction of nano-fillers achieves a discharge energy density as high as 15.5 J/cm³ at an electric field of 425 kV/mm, which is higher than the energy storage densities of most reported PVDF and its copolymer-based composites. The variations in discharge energy density after 10 000 cycles and across eight different regions do not exceed 19% and 7%, respectively.