As one of the critical insulating materials with low dielectric constant and low dielectric loss for high-voltage and high-power semiconductor devices, silicone gel is highly prone to electrical treeing degradation due to severe operating conditions of high frequency, high voltage, and elevated temperatures. However, existing researches have no enough comprehensive investigations into the growing models and suppression methods of electrical tree in silicone gel, impeding further advancement in high-voltage and high-power devices. This paper established a growing model for electrical tree in silicone gel based on the theory of bond-breaking and self-healing, and validated the accuracy of the model through electrical tree growth and inhibition experiments under high-frequency sinusoidal and bipolar square wave voltage. The results demonstrate that the bond-breaking rate (R_b) and self-healing rate (R_s) collectively regulate the development of electrical tree in silicone gel as the voltage application time increases: when R_b>>R_s, electrical tree grows rapidly; when R_b is slightly greater than R_s, electrical tree exhibits slow growth; when R_b<R_s, electrical tree propagation stagnates. It is indicated that electrical tree in silicone gel under high-frequency sinusoidal or square wave voltages exhibit a stepped propagation characteristic, which aligns with the proposed model. Furthermore, combined with existing data in literature, it is further illustrated that the proposed growth model of electrical trees in silicone gel can be universally applicable to the scenarios driven by different driving factors such as voltage waveforms and temperatures. A method of doping ultraviolet absorber into silicone gel is proposed to suppress electrical tree development. The effectiveness of this method is confirmed through discharge luminescence spectroscopy and laser confocal fluorescence microscopy characterization. This study provides theoretical foundations and experimental references for the further development of high-voltage and high-power encapsulation insulation.

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 insulation failure of epoxy resin (EP) used in stator windings of large generator caused by long-term coupling degradation effect of electrical and thermal stress, it is urgent to develop epoxy resin composite dielectrics with both excellent thermal conductivity and mechanical strength. In this study, SiCn@SiCp/EP composite dielectrics with varying SiCn and SiCp contents were prepared by using epoxy resin as the matrix, silicon carbide particles (SiCp) as the nonlinear conductivity base elements, and high aspect-ratio silicon carbide nanowires (SiCn) as the network skeleton. The results show that the multi-scale three-dimensional functional network formed by the mutual matching bridging of SiCn and SiCp provides transmission channels for external stress and internal phonons. The tensile strength, toughness, and thermal conductivity of composite dielectrics reach 64.8 MPa, 1.86 MJ/m³, and 0.326 W/(m·K), respectively. Meanwhile,affected by the percolation network, the composite dielectrics exhibit superior nonlinear conductive characteristics at relatively low SiC content, with a nonlinearity coefficient as high as 7.79 and a threshold field strength of only 2.12 kV/mm. The mutual matching of different morphologies of the same material avoids the complex issues arising from differences in interfacial interactions among multiple heterogeneous fillers in previous studies.

Four types of organic soluble polyimide (PI) resins with semi-cyclic structures were prepared by one-step high-temperature solution polycondensation using alicyclic dianhydride 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (ccHPMDA) and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride (ctHPMDA) with two aromatic diamines 9,9-bis[(4-amino-3-methyl)benzene]fluorene (MFDA) and 2,2′-dimethylbenzidine (DMBZ), respectively. Four types of PI films were prepared by dissolving the above four resins in N,N-dimethylacetamide and baking at high temperature. The thermal, optical, and mechanical properties of the PI films were tested. The results show that functional groups including six-membered cyclohexane ring, bulky fluorene substituents, ortho methyl substituent, and rigid biphenyl units endow these PI films with excellent thermal stability. The glass transition temperature of the four films are all higher than 400℃ and the 5% thermal weight loss temperatures are higher than 500℃ in nitrogen. Meanwhile, the four PI films show good optical transparency, with transmittance at 450 nm all surpassing 84.8%, yellow-blue index ranging from 1.09 to 2.15, and haze value ranging from 0.16% to 1.21%. In addition, the DMBZ based PI films exhibit superior mechanical properties, with tensile strength and tensile modulus exceeding 118 MPa and 3.8 GPa, respectively.

To address the problems of poor solution processability, high dielectric constant, and low adhesion of thermoplastic polyimide (TPI) used for adhesive-free flexible copper clad laminates (FCCL), a soluble easter-containing TPI (PI-10) was synthesized via a two-step method involving solution copolycondensation and chemical imidization. Then PI-10 was chemically modified with dicyclopentadiene epoxy resin (HP7200) and fabricated into FCCL. Firstly, the chemical structure of PI-10 was confirmed by FTIR and ¹H-NMR, and then its organic solubility and molecular weight were tested. Subsequently, the curing kinetics behavior and process of HP7200 modified PI-10 (PI-10/HP7200) were investigated by non-isothermal DSC analysis. Finally, the properties of PI-10, PI-10/HP7200, and the prepared FCCL were studied. The results show that PI-10 has good organic solubility and high molecular weight. Compared with PI-10, the tensile strength of PI-10/HP7200 film increases from (97.37±1.68) MPa to (102.25±4.77) MPa, and the dielectric constant (D_k) at 10 GHz decreases from 3.10 to 2.83. However, its glass transition temperature (T_g) decreases from 213.5℃ to 204.2℃, the 5% weight loss temperature (T_d5) decreases from 496.9℃ to 451.8℃, and the coefficient of thermal expansion (CTE) within 50-150℃ increases from 66.20×10^-6℃^-1 to 73.15×10^-6℃^-1, water absorption increases from 2.59% to 3.57%, and the dielectric loss factor (D_f) at 10 GHz increases from 0.009 5 to 0.015 8. In addition, PI-10/HP7200 significantly enhances the 90° peel strength of the prepared FCCL, achieving a maximum increase of about 77% compared to PI-10 without sacrificing other properties, reaching 2.12 N/mm.

In this paper, binary copolymerized polyimide precursors polyamic acid PAA-A and PAA-B were first synthesized using pyromellitic dianhydride (PMDA) with 4,4′-diaminodiphenyl ether (ODA) and p-phenylenediamine (PPDA), respectively. The corresponding polyimides PI-A and PI-B were then obtained via thermal amide cyclization. Subsequently, block copolymerized polyamide acid PAA-AB was obtained through the reaction of PAA-A and PAA-B, and block copolymerized polyimide PI-AB was also obtained through thermal amide cyclization. Finally, the chemical structure, aggregation structure, and cross-sectional morphology of the three PI were characterized, and their mechanical properties, dielectric properties, electric strength, and electronic structure were tested and analyzed. The results show that the introduction of the third monomer PPDA and ODA does not induce the formation of defects in PI-AB. Furthermore, due to the alteration in chemical structure, the molecular chain spacing of PI-AB slightly increases compared to that of PI-A, which results in a decrease in its glass transition temperature. The electric strength of PI-AB is 6.18% higher than that of PI-A. The dielectric loss factor of PI-AB remains below 0.07 in the frequency range. Moreover, the band gap of PI-AB is wider than that of PI-A, making electron excitation more difficult, which demonstrates that PI-AB has superior insulating properties.

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.

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.

In this paper, power frequency breakdown tests were conducted on mixed insulating oil containing mineral oil and ethyl laurate. The insulation breakdown characteristics of mineral oil, lauric acid modified ester, and the mixed insulating oil were compared and analyzed. An insulating oil model containing H₂O and H₃O⁺ was established, and the microscopic influence mechanism of ethyl laurate on the power frequency breakdown characteristics of the mixed insulating oil was elucidated through molecular dynamics simulations. The results show that under a 2 mm oil gap, the breakdown voltages of ethyl laurate and the mixed insulating oil are 29.8% and 20.8% higher than that of mineral oil, respectively. The failure field strength distributions of mineral oil, ethyl laurate, and the mixed insulating oil range from 5.62 kV/mm to 12.9 kV/mm, with both ethyl laurate and the mixed insulating oil exhibiting higher failure field strengths than mineral oil. Compared with mineral oil molecules, ethyl laurate forms more hydrogen bonds with H₂O and H₃O⁺, exhibits stronger interaction energy, and has a smaller free volume fraction. In summary, the diffusion motion of impurities such as H₂O and H₃O⁺ in ethyl laurate and the mixed insulating oil is inhibited, making it difficult to form conductive "small bridges",and thus the insulation performance of ethyl laurate and the mixed insulating oil is superior to that of mineral oil.

To investigate the influence of SiC inverter characteristics on the voltage distribution in traction motor windings, this paper first proposed an equivalent circuit model for the high-frequency distributed parameters of motor windings that accounted for the proximity capacitance effect of the conductor. Subsequently, the voltage stress distribution characteristics of the winding under single-phase and three-phase pulse output from the SiC inverter were analyzed, and the correctness of the model analysis results was verified using an experimental test platform. Finally, the effects of the switching frequency, dv/dt, and grounding method of the SiC inverter on the winding voltage stress distribution were analyzed. The results show that under single-phase pulse operation, the maximum voltage stress to ground of the windings occurs on the winding near the inverter of the other two phases. Under three-phase pulse operation, the maximum voltage stress to ground appears on the middle-position winding of the respective phase. The switching frequency does not affect the maximum values of either the voltage stress to ground or the interturn voltage stress of the winding. The dv/dt significantly affects the interturn voltage stress distribution, when dv/dt increases from 2.0 kV/μs to 15.0 kV/μs, the maximum interturn voltage stress increases by about three times. Different grounding methods have a considerable influence on the voltage stress to ground distribution of winding, and the minimum voltage stress to ground is achieved when a 1/2 grounding system is adopted.