Polyimide (PI) is a class of high-performance polymer materials containing π-conjugated imide ring in the main chain, and its size of the band gap is one of the main factors directly affecting the thermal stability, optoelectronic properties, dielectric properties and other properties of the material. The electrically powered diamine group and the electrically absorbing dianhydride group in the conventional PI molecular structure determine that the value of band gap is nearby 3.0 eV, which directly affects its performance in the field of high-temperature energy storage, high-frequency communication, electrical insulation, etc. Because of the excellent structural designability of PIs, the band gap of PIs can be adjusted by modulating the monomer combination/chain segment structure/space structure, and the above properties of PIs can be optimized. In this paper, based on the main research progresses of PI bandgap modulation reported in recent years, the main strategies of PI bandgap modulation were elaborated from the perspectives of polymer structure and adjusting polymerization process, respectively, and the difficult problems faced in PI bandgap modulation were discussed with the example of its application in the field of dielectric energy storage. Finally, the future development direction of PI bandgap modulation was discussed on the basis of current research status of PI bandgap modulation.

A series of resins with ≥30% of solid content and about 4 000 mPa·s of viscosity and their polyimide films were prepared by introducing flexible 4,4′-oxobisphthalic anhydride (ODPA) or twisted non-coplanar structure 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (BPDA) into the molecular structure of homophthalic tetracarboxylic acid dianhydride-4,4′-diaminodiphenyl ether (PMDA-ODA), and capped with a capping agent. The properties of the films were characterized by tensile testing machine, breakdown voltage tester, TMA, DMA, TGA, and cross-cut tester. The results show that when the solid content of the semi-rigid structure PMDA-ODA resin increases to 30% and the viscosity reduces to about 4 000 mPa·s, the film could not be formed due to the low molecular weight, and the PI film with good electromechanical properties can be prepared through introducing flexible monomer to improve the the polymer flexibility and using norbornene diacetic anhydride (NA) for capping. When continuing to increase the resin solid content and increase the proportion of flexible monomers, the electromechanical properties, heat resistance, ahd the adhesion on the copper sheet of the film decrease. The films with 30% of solid content, about 4 000 mPa·s of viscosity, 30% mole fraction of ODPA, and end-capped with NA show better electromechanical properties, heat resistance, and adhesion, which can be used as a impregnating varnish for insulation protection on the surface of substrate.

Photosensitive polyimide (PSPI) has a unique role in semiconductor packaging, among them, ester-type PSPIs containing acrylic acid derivatives are widely used in industry. Generally, the PSPIs are synthesized by the well-established chloride method, but chloride ions are introduced during the synthesis process, which negatively affects the reaction apparatus, resin purification, and the environment. For the development of green and reliable synthesis methods, we explored the reaction process for the preparation of hydroxyethyl methacrylate PSPI by polyisoimide using 4,4′-diaminodiphenyl ether (ODA) and 4,4′-biphenyl ether dianhydride (ODPA) as the polymerized monomers. The effect of solid content, solvent, and reaction temperature on the gel formation during the synthesis process of PSPI was discussed. The photolithographic and thermo-mechanical properties of PSPI prepared by the polyisoimide method were investigated. The results demonstrate that the PSPI prepared by this method has good photolithographic patterning ability (for a film with thickness of 4.5 μm, circular holes can be opened at a diameter of 15 μm, and the film retention rate is ≥90%) and excellent thermal properties and mechanical properties (T_d5%=471℃, T_g=297℃, and tensile strength is 127.5 MPa). There are no Cl-ions introduction during synthsis process, and the process is green and friendly, and simplified with a short reaction cycle. The method was extended to the preparation of other four PSPI systems, and the isomerization method is found to be universal, and this efficient and green synthesis method provides theoretical guidance for the preparation of high-performance PSPI photoresists in laboratories and enterprises.

Polyimide (PI) is a class of insulating materials with great potential in the field of high-voltage and high-power device encapsulation, which has excellent insulating properties. However, its intrinsic thermal conductivity is low, which is easy to cause localized overheating and damage to the device. In this paper, the thermal conductivity of polyimide materials was improved by doping with thermal conductive fillers. PI/c-BN and PI/h-BN composite films were prepared by in situ polymerization with cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN) as filler, and the effects of different content of c-BN and h-BN on the thermal conductivity, thermal stability, and mechanical properties of the composite films were investigated. The results show that the thermal conductivity of the composite films increases with the increase of BN content. The thermal conductivity of PI/c-BN composite films is higher than that of PI/h-BN composite films within the studied content, and the thermal conductivity of PI/40c-BN composite films increases by 1 068% compared to pure PI. The thermal conductivity of PI matrix is improved by c-BN at lower content, and the tensile strength of PI/c-BN composite films still reaches 40 MPa when the filler mass fraction is 30%. In addition, the thermal stability of the composite films is improved.

To develop polyimide (PI)-based composite films with low dielectric constant (D_k), low dielectric loss (D_f), and high temperature resistance at high frequency, a water-soluble polyamic salt (PAAS) was prepared by adding organic base into the polyamic acid, which is the precursor of fluoropolyimide (FPI), and composite films were prepared by compounding low dielectric polytetrafluoroethylene (PTFE) concentrated dispersion with the dried PAAS to form a water dispersion system. The effects of PTFE content on the dielectric, thermal, and mechanical properties of the composite films were studied. The experimental results show that with the increase of PTFE content, the dielectric constant of the composite films decreases continuously, but there will be some effects on the thermal stability and mechanical properties. The dielectric constant of 50%PTFE/FPI reaches the minimum value (D_k=1.5@8.5 GHz), and the dielectric loss of 10%PTFE/FPI in the frequency band of 9.25-10.25 GHz is less than 0.005. The glass transition temperature of the composite films is in the range of 289-297℃, and the thermal decomposition temperature at 5% is higher than 508℃. The thermal expansion coefficient of 40%PTFE/FPI is as low as 59.67×10^-6 K^-1. The tensile strength of 20%PTFE/FPI is 70 MPa, the tensile modulus is 1.59 GPa, and the elongation at break is 7.7%.

With the development of economy and technology, the demand of safer and more stable packaging materials for electronic devices has increased. Traditional polyimide (PI) base materials are extensively used due to their excellent mechanical and heat-resistant properties. In this study, a novel fluorinated polyimide (FPI) porous composite film was developed by introducing fluorine groups and a porous structure into polyimide, along with adding the flame retardant 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Experimental results demonstrate that under 80%RH of ambient humidity and 5% of DOPO mass fraction, the new FPI porous composite film exhibits uniform and regularly distributed pore size, excellent mechanical properties, and lower water absorption. The incorporation of flame retardant DOPO forms a protective phosphorus-containing layer on the film surface, which significantly improves the flame retardancy and hydrophobic properties of film. This research provides a new material with enhanced safety for the packaging of electronic devices, which is expected to play a crucial role in improving the stability and safety of electronic equipment.

Epoxy resin/micron-alumina composites are widely employed as supporting and insulating components in electrical power equipment, and their glass transition temperature and thermal expansion coefficient critically influence the long-term performance of electrical power equipment. Epoxy resin composite dielectrics containing 0%, 20%, 40%, and 60% mass fractions of micron-alumina fillers were prepared in this paper. Their thermal expansion coefficients and glass transition temperatures were calculated through numerical fitting of dielectric constant measurements at high-temperature and high-frequency. The results demonstrate that with the increase of the mass fraction of micron-alumina fillers, the thermal expansion coefficient of composite dielectrics reduces significantly, and the glass transition temperature also showing a decreasing trend. Comparative analysis between calculated values and existing experimental data reveals that the calculated value is consistent with the experimental value, confirming the scientific validity and effectiveness of dielectric spectroscopy as a methodology for evaluating the thermal properties of epoxy composite dielectrics. This approach can serves as an effective supplementary technique to conventional experimental methods for investigating the thermal characteristic of polymer.

Bisphenol A epoxy resin is widely used in the field of copper clad laminates. However, its flame retardancy and dielectric properties cannot meet the high-speed development of copper clad laminates. To simultaneously improve the flame retardancy and dielectric properties of epoxy resin, a reactive oligomeric flame retardant (PDDV) based on 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ), 1,4-dichlorobenzene (DCX), and 4-vinylbenzyl chloride (VBC) was synthesized, and its chemical structure was determined by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) characterization. EP/PDDV composites were prepared by physical blending, and their thermal, flame retardancy, and dielectric properties were tested. The results show that when the mass fraction of PDDV is 30%, the char yield of EP/PDDV composites increases to 38.74%, the glass transition temperature (T_g) rises to 138.9℃, the limiting oxygen index (LOI) increases to 55%, UL-94 reaches V-0 grade, the dielectric constant (D_k) is 2.69, and the dielectric loss factor (D_f) is 0.007 91. The comprehensive performance of the composites is the best.

In order to realize the synergistic enhancement of electrical-mechanical properties of epoxy resin for power electronics encapsulation, CaCO₃ oligomer inorganic nanoparticle filler was prepared by solution method, epoxy resin was carried out organic matrix-inorganic filler modification together with organic silicone, and single inorganic modified, single organic modified, and organo-inorganic jointly modified epoxy resin composites were prepared. Taking the pure epoxy resin as a reference, the AC electric strength, DC volume conductivity, surface potential attenuation characteristics of the four epoxy resin composites were investigated, and the internal trap energy levels were calculated from them, while their bending strength and impact strength characteristics were also investigated. The results show that the electrical properties of single inorganic modified, single organic modified, and organic-inorganic co-modified epoxy resin composites are significantly improved, and their electric strength increases by 6.55%, 12.66% and 9.61%, respectively. Their DC volume resistivity is 5 times bigger than that of pure epoxy resin, the surface potential decay rate decreases, the trap density and trap energy increase, but the improvement of electrical properties by the organic-inorganic co-modification do not superimpose compared with single modification. The flexural and impact strengths of the modified epoxy resin composites significantly increase, with exponential growth compared to the pure epoxy resin. The research results provide a reference for the performance improvement of modified epoxy resin composites in the field of power electronics insulation and encapsulation.

To address the challenges of epoxy resins in electrical insulation and flame-retardant applications, a phosphorus-containing bio-based flame-retardant resin—bis(methacryloyloxy-4-hydroxy-3-methoxyphenyl) phenyl phosphate (DGEBDB) was synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and renewable vanillin, and cured materials were prepared through blending it (0%, 25%, 50%, and 75%) with bisphenol A epoxy resin (DGEBA). Their flame retardancy, thermal properties, mechnical properties, and electrical properties were analyzed. The results show that when the mass fraction of DGEBDB is 75%, the limiting oxygen index (LOI) of epoxy bending system increases from 22.6% to 34.2%, and the flame retardant grade achieves V-0 in UL 94 tests. Cone calorimetry reveals that the DGEBDB can reduce the heat release rate (HRR) and total heat release (THR), showing excellent fire suppression. Electrical properties tests show that the electrical properties of the cured epoxy maintain good when the mass fraction of DGEBDB is as high as 75%, which ensures the reliability of DGEBDB in electrical application. Mechanical properties indicate that with the increase of DGEBDB content, the flexural strength and tensile strength increase, indicating an increase in structural integrity and load bearing capacity of the cured epoxy. Therefore, the addition of DGEBDB significantly enhances the flame retardantcy of composite, while maintaining the excellent thermal, mechanical, and electrical properties of epoxy resin, and when the mass fraction of DGEBDB is 25%, the overall performance is the best, which has a better prospect for practical applications.

A kind of silicone encapsulant with excellent thermal conductivity, self-leveling properties, mechanical properties, and flame retardancy was prepared by employing end vinyl silicone oil with different viscosity as matrix, containing hydrogen silicone oil as cross-linking agent, and spherical alumina as thermal conductive and reinforcement filler. The effects of ratio of end vinyl silicone oils with different viscosity, molar ratio of active hydrogen and vinyl, filler ratio and filling amount on the performance of silicone encapsulant were investigated. The results show that the silicone encapsulant has the best performance when the PDMS-1 and PDMS-2 vinyl end silicone oil are mixed at a mass ratio of 1∶1 as the matrix, the molar ratio of active hydrogen and vinyl is 1.2, and the ratio of spherical alumina with different particle sizes m(5 μm)∶m(15 μm)∶m(50 μm)=2∶5∶3. When the total filling amount of different sizes of spherical alumina reaches 500 parts, the silicone encapsulant has up to 1.50 W/(m·K) of thermal conductivity, up to 1.70 MPa of tensile strength, FV-0 of vertical combustion grade, and excellent operation and processing performance as well as dispersing stability, and its comprehensive performance is excellent.

Meta-aramid insulation paper is widely used in transformers and high-voltage bushings due to its excellent dielectric and mechanical properties, but its low thermal conductivity limits its application in high thermal load environments. To enhance the thermal conductivity of nano-filler/aramid insulation paper, this study employed simulation-guided experimental design to calculate the thermal conductivity of aramid simulation models. Three nano-fillers with excellent thermal conductivity properties, namely nano SiO₂ (KH570), nano TiO₂, and nano C₃N₄, were screened out. The mechanical and insulation performance of laboratory composite aramid paper hand sheets were compared, and the optimal filling amount was determined. Further, the influence of nano-fillers on the thermal conductivity of composite aramid paper was explored, and their inherent thermal conduction mechanism was analyzed. The results show that the optimal addition amounts of three nano-fillers are: nano SiO₂ (KH570) at 10%, nano TiO₂ at 4%, nano C₃N₄ at 4%. Through thermal conductivity testing, nano C₃N₄ is determined to be the optimal nano-filler, and the composite aramid paper with 4% of nano C₃N₄ showing an increase in elastic modulus by 135%, Young's modulus by 198%, electric strength by 60.24%, volume resistivity by 3 713%, and thermal conductivity by 304.31% compared with pure aramid paper.

Under the high temperature conditions (90, 105, 120, and 135°C), accelerated ageing tests of nitrile rubber (NBR) were conducted in hot air, hot oil, hot air compression, and hot oil compression. The ageing mechanisms were investigated through Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM), and the effects of ageing temperature, time, and deformation conditions on the ageing behavior of NBR were revealed. The results indicate that crosslinking, oxidation, and chain-breaking reactions occur during the ageing process of NBR, and the crosslinking reaction is the predominant reaction. After ageing at 120°C, grooves appear on the surface of NBR, and after ageing at 135°C, defects such as holes and damages appear on the surface. In the early stage of ageing, transformer oil has a suppressive effect on the increase of permanent deformation under compression. In the later stage of ageing, transformer oil plays a promote role for the decrease of tensile strength, and the higher the temperature, the more obvious the effect.

The stator winding insulation of 10 kV three-phase asynchronous motor will subject to thermal, electrical, environmental, and mechanical stresses during operation, which will generate insulation defects and lead to partial discharge (PD). It is very difficult to obtain the reference voltage waveform in on-line monitoring condition, and the loss of reference phase makes it impossible to use PRPD pattern in PD recognition. To solve these problems, three kinds of PD models, e.g. internal discharge, slot discharge, and corona discharge, were made in the laboratory conditions, and the PD signals were collected. The polar coordinate method was used to convert PRPD pattern into circular polar coordinate pattern to solve the problem of reference phase loss. The results indicate that different types of PDs have different polar coordinate pattern characteristic distributions, the internal discharge has symmetrical distribution in the first and third quadrants. Slot discharge and corona discharge have unsymmetrical distribution, and the amplitude of the third quadrant is much larger than that of the first quadrant. The slot discharge has larger point cluster discharge concentration range and wider discharge shape than corona discharge. The characteristic parameters such as skewness, kurtosis, and discharge proportion are obviously different in different discharge types. With the increase of voltage level, the distribution and characteristic parameters of polar coordinate pattern will change obviously.

Multi-scale mining of the spatio-temporal coupling relationship of dissolved gases in oil is helpful to improve the prediction accuracy of dissolved gases in oil and provide a reliable theoretical basis for transformer operation and maintenance decisions. Thereby, a multi-scale fusion prediction method for dissolved gases in transformer oil considering spatio-temporal coupling information was proposed in this study. Firstly, the Res2Net was used to extract the multi-scale time characteristics of the dissolved gas data in oil, and the periodic time feature information of the characteristic gas under different frequencies was captured. Secondly, the implicit relationship between characteristic gases was captured by calculating mutual information, the correlation between different gases was described in the form of topological graphs, and the spatial information features were extracted by using graph convolutional neural network (GCN). Finally, multi-scale temporal information and spatial information were fused and spliced, and temporal convolution network (TCN) was used to predict the dissolved gas in oil. The proposed method was validated using online oil chromatography monitoring data from a 500 kV transformer. The results show that compared with the traditional prediction method, the Res2Net-GCN-TCN model can effectively improve the prediction accuracy of dissolved gas content in oil, and the average prediction accuracy is 98.68%.

Bushing is an important component of oil-immersed transformers, and its internal insulation moisture, ageing, and development of some defects after long-term operation will cause distortion of local electric field, which seriously threatens the safe operation of bushing. In this paper, the variation patterns of dielectric parameters of the aged and damped oil-immersed paper under different test temperature environment were obtained, and a dynamic dielectric parameter calculation model was established. Meanwhile, a simulation model was established on the basis of actual structure of a 252 kV/1 250 A bushing. Electro-magnetic-thermal coupling multiphysics were used to conduct simulation analysis, and the transient electric field distribution and distortion severity inside the bushing under three typical defect conditions were obtained. The results indicate that when the end screen of bushing is unreliable grounding (resulting in floating potential), electric field distortion occurs at the contact side of bushing and end screen, and the voltage grading effectiveness of capacitive core is weakened. When there is wrinkle on the capacitance screen of bushing, the maximum electric field intensity at the wrinkle regions increases proportionally with the bending curvature. Notably, the field strength inversion phenomena are observed at the distortion zones when the capacitance screen reach specific bending curvature. The bubbles attached on the end screen surface have greater influence on the electric field distribution than bubbles dispersed in transformer oil. Such surface-attached bubbles are prone to produce partial discharge.

To solve the problem that it is difficult to diagnose the moisture defects of cable joints with the existing methods, this paper proposes a method for diagnosing the moisture defects of cable joints based on frequency modulated continuous wave (FMCW) and time reversal (TR). Firstly, a distributed parameter model of cable was established. The FMCW method was employed to capture multi-frequency reflected signals from impedance mismatches at defects and joints. These inversion signals were injected into a test cable model to derive an energy curve, and the energy curve is served as a diagnostic spectrum for cable defects. Subsequently, the impedance discontinuity point detection was carried out for the simulation cable models with different end-loads and intermediate joints. Finally, joint defect diagnosis was carried out for the real 750 m 10 kV power cable and 2 km 10 kV power cable. The results show that the method proposed in this paper can not only accurately determine the location and characteristic of the impedance discontinuity points in cable, but also improve the distance resolution of the defect location peak. The joint positioning peaks of "positive first and then negative" and "negative first and then positive" can respectively represent normal and damp cable joints. Therefore, the method proposed in this paper can accurately detect the moisture defects of cable joints and has a good engineering application prospect.