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.

With the miniaturization and light-weighting of electronic equipment, graphite films with high thermal conductivity attract extensive attention. In this paper, a series of nanocomposite films were prepared by copolymerizing p-phenylenediamine (p-PDA) with 4,4′-oxybisbenzenamine (ODA) and 1,2,4,5-benzenetetracarboxylic anhydride (PMDA), and various two-dimensional carbon nanomaterials, including graphene oxide (GO), reduced graphene oxide (rGO), and graphene (GP), were added via mechanical mixing. And then corresponding graphite films were obtained after high-temperature sintering of polyimide films at 2 850℃, the morphology and crystal type of graphite films were analyzed. The results show that graphite films with two-dimensional carbon nanomaterials are flat, they exhibit more flexible, higher degree of graphitization, and larger crystal size, and the lamellar spacing is closer to the ideal graphite. In particular, the horizontal thermal conductivity and vertical thermal conductivity of graphite film containing 2% of GO nanosheets is 1 207 W/(m·K) and 8.38 W/(m·K), respectively. It is concluded that GO nanosheets can induce graphitization of polyimide films under high temperature, and the graphite sheet sintered shows excellent thermal conductivity.

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 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 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 further clarify the key properties of matrix resin in semiconducting shielding materials, three different types of semiconducting shielding materials at home and abroad were selected, and their microstructure characteristic, mechanical properties, electrical properties, and rheological properties were tested. The results show that the volume resistivity of each shielding material is low, and the temperature stability of resistivity is good. The acrylate content of the imported shielding material matrix resin is higher, polarity is greater, and the mechanical toughness is better. At the same time, the pseudoplasticity of the imported shielding material is stronger. In a certain shear rate range, the viscosity of the material can be reduced by increasing the shear rate appropriately, and the energy consumption can be reduced by increasing the fluidity, and then the production efficiency can be improved. Based on the results of the experimental analysis, the polarity or ester content of the material is proposed as a key parameter to reflect the matrix resin in the semiconductive shielding material.

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.

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 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.