With the improvement of ecological environment, transmission line faults caused by birds are also increasing. Among them, the tripping accident of transmission lines caused by bird has become an important factor threatening the safe operation of power grid. Based on this, we explored the influence of the thickness and length of insulating sheath on the breakdown voltage and surface flashover voltage respectively through tests. Then the configuration method of the length and thickness of insulating sheath was put forward. Finally, a real test platform simualting the the flashover of bird droppings was established to verify the feasibility of the configuration method. The results show that when the bird droppings penetrate the entire air gap, the breakdown voltage of the insulating sheath is a quadratic function of its thickness. The surface flashover voltage is proportional to the flashover distance. The configuration method proposed in this paper can provide guidance for the application of insulating sheath in practical engineering and has engineering practical value.

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.

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.

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.

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.

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.

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.

A semi-alicyclic polyimide resin (TFCPI) and the derived colorless and transparent polyimide film (TFCPI-0) were prepared by one-step high-temperature polycondensation using 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (H-PMDA) and a fluoro-containing aromatic diamine 2,2′-bis(trifluoromethyl)benzidine (TFMB). Then, a series of colorless and transparent polyimide/SiO₂ composite films with different loading amounts of nano-sized silica fillers were prepared via mechanical blending method by mixing TFCPI matrix with dispersion liquid of colloidal nano-silica/N,N-dimethylacetamide (DMAC). The results show that the loading amounts of nano-silica in the composite films can reach to 25%. The derived composite film TFCPI-25 shows the optical transmittance at the wavelength of 450 nm (T₄₅₀) and yellow index (b*) values of 87.5% and 1.56, respectively, which are slightly inferior to those of TFCPI-0 (T₄₅₀=88.5%, b^*=0.91). The 5% weight loss temperatures (T_5%) and glass transition temperature (T_g) of TFCPI-25 is at the same level with that of TFCPI-0. However, the storage modulus of TFCPI-25 at 50℃ (E′₅₀) and its linear coefficient of thermal expansion (CTE) in the range of 50-250℃ is 4.86 GPa and 30.9×10^-6 K^-1, respectively, which are obviously superior to those of TFCPI-0 (E′₅₀=3.02 GPa, CTE=39.6×10^-6 K^-1).

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.