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.

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.

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.

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.

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 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.

The key power equipment with the oil-pressboard insulation winding structure is prone to appear the problem of acetylene content exceeding the standard when putting into operation, and then partial discharge and arcing phenomena are produced. Therefore, the point discharge and arcing characteristics of oil-pressboard insulation were studied under switching impulse voltage and superimposing power frequency electrical stress. The discharge and arcing characteristics of different point defects were compared and analyzed, and the influence of the superimposed phase of impulse voltage and power frequency voltage was discussed. The results show that the arcing process of the defect model containing pressboard after breakdown can divide into high-frequency re-arcing stage, stable arcing stage, and re-arcing stage before the voltage restored. Among them, the high-frequency re-arcing stage has higher arcing frequency and lower recovery voltage. Under the switching impulse voltage, the pure oil gap defect model shows multiple breakdown, and the oil-pressboard composite insulation defect model shows that the pressboard breakdown directly and the oil pressboard breakdown successively, and the defect model type affects the discharge characteristics and arcing rule. The superposition phase affects the discharge characteristics and arcing law by affecting the electrical stress magnitude.

In order to analyze the influence of natural ester insulating oil on the insulating properties of power transformers, at first, the winding wave processes of a 110 kV natural ester insulating oil and a mineral insulating oil power transformers with the same insulation structure were calculated, and the longitudinal insulation margins of the two transformers were obtained. And then, the winding potential with linear distribution under power frequency and the nonlinear winding potential calculated by wave process were used as excitation sources respectively, the composite electric field of the two insulating oil transformers main insulation were simulated, and their distribution characteristics were compared and analyzed. At last, the main insulation margins of the two insulating oil transformers were calculated by using the whole-region electric flux line scanning method, and in view of the weak main insulation strength of natural ester insulating oil transformer, the structure was improved and the improvement effect was analyzed. The results show that through adding insulation paper boards and adjusting the size of oil ducts, the minimum insulation margin of the improved natural ester insulating oil transformer increases by 15.8%, which meets the requirements of main insulation strength, which provides a reference for developing large natural ester insulating oil transformer and optimizing its structure.