In order to study the influence of typical defects at the silicone rubber/cross-linked polyethylene (SR/XLPE) composite interface of cable terminal on the electric field, temperature field, and stress field distribution, a 10 kV cable termination simulation model was established by using COMSOL Multiphysics simulation software. The electric-thermal-force field distribution with metal particles, semiconducting particles, and scratch defects on the composite interface was simulated. The results show that the electric field, temperature field, and stress field on the interface defect area are distorted to different degrees when metal particles or semi-conductive particles exist on the composite interface of cable terminal, and the influence of metal particles on the physical field distribution at the interface is more obvious. For the air gap defect caused by the scratch of XLPE at the truncation of outer semiconductive layer, it is found that the electric field at the interface air gap defects is distorted and local hot spots are generated. The coating of silicone grease on the interface can obviously improve the distortion of electric field and temperature field at the scratch, but the stress distribution in the scratch area is not uniform due to the filling of silicone grease, and the stress is high at both ends and is low in the middle.

Modified boron nitride (BN)/polyimide (PI) composites were prepared by in-situ polymerization using surface modified BN with terminal group of amino, carboxyl, and hydroxyl and unmodified BN nanosheets as fillers. The effects of BN modified by amino (BN-NH₂), carboxyl (BN-COOH), and hydroxyl (BN-OH) and BN on the thermal conductivity of the composites at different temperatures were investigated. The results show that thermal diffusivity of the composite increases with the increase of the mass fraction of BN-NH₂. With the increasing of BN-COOH and BN-OH, the thermal diffusivity of the composites increases at first and then decreases, reaches to the peak when the mass fraction of fillers is 2%. At 200 ℃, the thermal diffusivity of BN-NH₂/PI reaches to the maximum when the mass fraction of fillers is 5%, while that of BN-COOH/PI and BN-OH/PI reaches to the maximum at when the mass fraction of fillers is 2%, and BN-COOH/PI shows the highest thermal diffusivity. Therefore, surface modification by amino is beneficial for BN/PI composites to obtain a higher thermal conductivity at high fillers content, while carboxyl modification is the best choice to obtain the highest thermal conductivity.

Biaxially oriented polypropylene (BOPP) film is a mainstream dielectric material used in power capacitor, there is still a large gap between thermal stability performance of imported BOPP films and that of domestic films. In order to systematically investigate the thermal stability of domestic and imported BOPP film, as well as to reveal the thermal ageing mechanism, we selected typical domestic and imported BOPP film samples, and conducted vacuum thermal ageing test on them for 1 000 h under 80℃ and 120℃, respectively. The changing law of molecular structure, aggregation structure, and electrical properties of the films before and after thermal ageing was compared and analyzed via infrared spectroscopy (FTIR), optical microscope observation, X-ray diffraction (XRD), dynamic thermomechanical analysis (DMA), dielectric spectroscopy, and direct current breakdown strength. The results show that compared with that of imported BOPP films, the crystallinity of domestic films is lower and the force between molecular chains is weaker. The breakdown field strength of domestic films is 5.7% lower than that of imported films. Under the effect of long-term thermal ageing, both BOPP films have undergone recrystallization, including the increase of crystallinity and lamellae size in the crystal region and the increase of free volume and the molecular chains loss scale in the amorphous region. Meanwhile, the change of the aggregate structure leads to the increase of the dielectric loss and the decrease of the breakdown strength at low probability zone of the film after ageing. Moreover, compared with domestic films, the stronger inter-chain forces in imported films effectively inhibit the recrystallization process, and decrease the generation and development of electrical weakness, which makes the imported BOPP films have better long-term thermal stability.

The performance of domestic corona-resistant polyimide film as well as the corona-resistant film magnet wire was compared with that of the similar imported products, and then the reliability of domestic corona-resistant film magnet wire in the application process was discussed. The results show that the conventional properties of domestic corona-resistant film and its magnet wire are comparable to those of similar imported products, and the corona resistance performance and initial partial discharge voltage of domestic corona-resistant film magnet wire are better than those of similar imported products. Domestic corona-resistant film magnet wire has excellent manufacturability in the coil forming process, which meets the technical requirements of inverter traction motor applications.

In order to study the effect of silane coupling agent modification on the thermomechanical and dielectric properties of SiO₂/PI composite and its internal mechanism, we established the composite model of pure PI, SiO₂/PI and SiO₂/PI with 6% and 12% of the grafting density of silane coupling agent on the surface of SiO₂. The solubility parameters, interaction energy, glass transition temperature, Young's modulus, shear modulus, mean square displacement, free volume fraction, relative permittivity, and dielectric strength of the four models were calculated. The results show that the silane coupling agent modification significantly improves the thermomechanical and dielectric properties of the composite material, and the graft density has an obvious effect on the modification effect. The SiO₂/PI composite system with 6% of grafting density of silane coupling agent has the best thermomechanical properties, while maintains a lower relative permittivity and a higher dielectric strength. In addition, the two systems grafted by silane coupling agent have smaller free volume fraction and mean square displacement, as well as larger solubility parameters and interaction energy. It is indicated that limiting the movement of molecular chains and improving the compatibility between SiO₂ and PI matrix are the keys to improving the thermomechanical and dielectric properties of composite materials.

Improving the performance of dielectric capacitors is one of the effective methods to meet the requirements of miniaturization and integration of circuit components. How to improve the dielectric properties of dielectric composites has attracted extensive attention of researchers. A series of ZIF-67/PVDF composite films were prepared by flowing spread molding using polyvinylidene fluoride (PVDF) as matrix and metal organic frameworks (MOFs), ZIF-67, as filler. The influence of MOF on the microstructure and macroscopic properties of PVDF composites was investigated. The results show that ZIF-67 can induce PVDF to generate more β phase, promote the polarization of matrix, and improve the mechanical and dielectric properties of the composite. When the mass fraction of filler is 5%, the Young's modulus of ZIF-67/PVDF composite film under 40℃ is 1 200 MPa and the breakdown strength is 139.74 kV/mm, both of them are higher than those of pure PVDF. The dielectric constant of ZIF-67/PVDF composite at 100 Hz is 10.39, which is 35.29% higher than that of pure PVDF film, and the dielectric loss can be kept low.

Aimed at the application requirements of polyimide (PI) film materials in the field of high frequency, polyimides containing semi-alicyclic structure were synthesized by the semi-alicyclic diamine 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindan (DAPI) with different aromatic dianhydrides. And their corresponding films were prepared, the correlation between their structure and properties was explored. The results show that polyimides containing semi-alicyclic structure have good solubility and processability, they are soluble in conventional organic solvents such as NMP, DMF, CHC1₃, etc. The corresponding films had good mechanical and thermal properties, their tensile strength ranges from 64.8 MPa to 82.6 MPa, and their glass transition temperature can exceed 487℃. The dielectric constant (D_k) of the PI films is 2.63-4.62 at 1 kHz-1 MHz and 2.46-2.75 at 10 GHz, respectively. The dissipation factor (D_f) is 0.003 1-0.020 5 at 1 kHz-1 MHz and 0.006 6-0.017 4 at 10 GHz, respectively. In particular, the D_k and D_f of the B-PI(BPADA-DAPI) film at 10 GHz is 2.75 and 0.006 6, respectively. It is indicated that the polyimide films with low dielectric constant and low dielectric loss at high frequency can be obtained by the introduction of semi-alicyclic moieties and bisphenol A groups into the main chains.

Epoxy resin micro-alumina composite materials with micro-alumina mass fraction of 0, 20%, and 40% were prepared. The effect of micro-alumina filler on the dielectric relaxion behavior of epoxy resin at high temperature was analyzed by dielectric test at room temperature and high temperature and fitting calculation of high temperature dielectric spectra. The results show that the effect of micro-alumina on the dielectric relaxion at high temperature is more obviously than that at room temperature. With the introduction of alumina, the optical frequency dielectric constant increases, while the relaxation strength of relaxion process α shows little change. With the increase of mass fraction of filler, the relaxion time of relaxion process α decreases. Besides, the micro-alumina filler significantly increases the relaxion strength of relaxation process δ. However, with the increase of mass fraction of micro-alumina, the relaxation strength of relaxation process δ decreases, while the relaxation time increases continuously.

In the life evaluation of polyimide (PI) film modified by nano TiO₂, a small sample insulation life evaluation method based on gray neural network was proposed to solve the problems such as lack of failure data, life distribution cannot be determined uniquely, high cost, and long test time. Nano TiO₂ modified PI film was prepared by in-situ polymerization method, and the accelerated electrical ageing test was conducted on the films to obtain the failure data. The grey neural network was constructed and trained to obtain the extended data with the similar characteristics and changing rules with the original data. The least square method was used for parameter estimation, and the life evaluation results of different failure data samples, distribution models, empirical cumulative failure functions were analyzed and compared. The different life evaluation schemes before and after extending were analyzed by calculation example. The results show that the combination of Weibull distribution and mathematical expectation formula is more suitable for life evaluation of original sample size.The evaluation of insulation life using expanded failure data has a better effect, and the combination of inverse power function model, lognormal distribution, and median empirical formula is the most suitable for the life evaluation of the extended samples.

In order to solve the problem of agglomeration and poor dispersion of fillers in high-viscosity poly(amic acid) (PAA), we prepared PAA solution with low viscosity by anhydride hydrolysis method using PAA solution of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) under a high solid content. On this basis, boron nitride/polyimide (BN/PI) composite films with the mass fraction of BN ranging from 0 to 40% were prepared by ultrasonic pulverization pretreatment and high-efficiency ball milling mixing process. The dispersion of fillers and the mechanical properties, heat resistance and thermal conductivity of composite films were systematically investigated. The results show that the low viscosity of PAA and the mixing process improve the dispersibility of fillers and have an important impact on the properties of BN/PI composite films. When the mass fraction of filler is 40%, the tensile strength of the composite film is about 140 MPa, the glass transition temperature is 385.2℃. The maximum thermal conductivity is up to 0.741 W/(m·K), which is 338% higher than that of pure polyimide film.