Electrical tree is a common phenomenon in XLPE cable fault, and there may be voltage fluctuation during cable operation. However, there are few reports on the influence of voltage amplitude on the growth morphology and partial discharge characteristics after electrical tree is initiated. In this paper, the difference of growth morphology and partial discharge characteristics under different voltage amplitudes after electrical tree initiated was studied. The experimental results show that when the applied voltage is higher, the electrical tree morphology tends to develop into a cluster tree, and when the applied voltage is lower, the electrical tree morphology tends to develop into a dendritic tree or a single branched tree. The maximum partial discharge intensity test at higher voltage can be used to predict breakdown fault, but it is difficult to predict the growth of electrical tree at lower voltage through partial discharge intensity. The maximum discharge interval of electric tree becomes longer with the increase of voltage applied time, and the intermittent discharge is more obvious. The discharge interval of electric tree is closely related to the growth of electrical tree. The intermittent partial discharge occurs only when the growth of electrical tree is stagnant, and the partial discharge hardly stagnates when the electrical tree is growing.

With the development of portable electronic devices, there is an increasing demand for thermal management materials with high thermal conductivity and electrical insulation. As a new thermal conductive filler with high conductivity and insulating property, fluorinated graphene (FG) has received extensive attention from the scientific community. In this paper, the preparation method of FG and the relationship between its structure and properties were summarized, the thermal conducting mechanism of FG was analyzed especially, and the latest research progress of FG as functional filler composite modified polyimide (PI) was systematically described. The problems and challenges were summarized and prospected.

In this paper, the aggregation properties of molecular chain as well as permittivity of 10 kinds of polyimides containing carbonyl group or sulfone group were characterized. Moreover, microscopic parameters such as theoretical density, free volume fraction, polarizability density, and dipole moment density were calculated based on density functional theory (DFT) and molecular dynamics theory (MD). The results show that the partially crystallized polyimides show relatively high theoretical density and low free volume fraction (FFV), which indicates that the measured results are in good agreement with the theoretical calculations. Compared with using polarizability density, using dipolar moment density, which is highly correlated with the orientation polarization, provides a more accurate prediction of the size of the polyimide dielectric constant in relation to the molecular structure in tests in the frequency range of kHz .

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.

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.

Polyimide is widely used in high-tech fields because of its excellent performance characteristics. In recent years, the new display technology represented by smartphones with foldable screen has greatly promoted the development of optically transparent polyimide materials. In order to understand the technical condition and future development direction of polyimide patents of foreign companies, this work took KOLON, SKC and their related companies as the research object, and deeply analyzed their polyimide film patents applied in China during 2017 to 2022, including the number of applications, disclosure condition, field distribution, etc. The technical route and layout characteristics of transparent polyimide patents were focused, and the structural formula, preparation methods, key performance of representative transparent polyimide films were respectively introduced in detail. The current technological challenges and future breakthrough directions were also summarized.

The development in the field of electronics and microelectronics has greatly promoted the research on thermally conductive polymer film materials. However, there is a lack of systematic study on both the analyzing technology of thermal conductivity and the testing methods suitable for polymer films. The light/laser flash analysis (LFA) is the most representative method in transient analysis technique. In this paper, the basic principle, testing conditions and application scope of LFA method were systematically introduced. Thermally conductive polyimide films were taken as the representative examples to analyze the key factors on the testing results in detail, including the film thickness, surface quality, pretreatment conditions, instrument parameters, and data analyzing and processing. In addition, the application of LFA method for testing thermal conductivity along different directions of polymer film materials was further discussed. The results show that the thickness accuracy, surface quality, and transparency of the samples will affect the test results, and the pretreatment methods such as gold sputtering or graphite spraying can effectively improve the test accuracy. In addition, the parameter setting and data processing of the testing instrument will affect the shape of temperature-time curve and the fitting results. Thus, it is necessary to reasonably select the appropriate testing conditions according to the characteristics of polymer film samples.

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.

Polyimide (PI) has attracted wide attention in the field of new high-temperature energy storage dielectric materials because of its excellent properties. Compared with inorganic/PI composite dielectric materials, all-organic PI composite dielectric materials can maintain excellent mechanical properties while obtaining high dielectric permittivity and high energy storage density. This paper firstly discussed the key parameters affecting the energy storage characteristics of polymer dielectric materials, including dielectric permittivity, dielectric loss, breakdown field strength, energy storage density, charge/discharge efficiency, and heat resistance. And then introduced the key factors and development trends affecting the energy storage characteristics of all-organic PI composite dielectric materials from physical blending and chemical blending, respectively. Finally, the problem of how to effectively improve the high-temperature energy storage characteristics of all-organic PI composite dielectric materials was summarized and the future development direction was proposed.

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.