Polylactic acid (PLA) is a kind of the most widely studied green biodegradable polymer material and is currently recognized as the most promising industrial application due to its excellent biodegradability, biocompatibility, processability, high mechanical strength, non-toxicity, and no stimulation. However, the disadvantages of slow crystallization rate, low crystallinity, and poor heat resistance of PLA severely limit its application in the fields with higher temperature requirements. This paper reviewed the current research on the heat resistance of PLA inside and outside China, mainly from the crystal type modification, blending modification, and cross-linking modification, and then prospected the future development of high heat resistance PLA.

In order to deeply study the variation rule of cable joint temperature and stress under micro-gap discharge and explore its influence on the cracking of the composite interface of cable joints, we took 110 kV cable intermediate joint as the prototype, established a three-dimensional simulation model combining with the most common air gap position in actual cable joint. Temperature, stress, and interface crack were taken as indexes to measure the damage degree of cable joint, the initial grip force between cable joint and ontology as well as the air gap pressure were taken into consideration, the charge law of temperature and stress of the joint with the change of gap discharge energy, air gap pressure, air gap thickness, and air gap position was calculated by finite element method. The effect of the change of gap discharge energy and air gap pressure on cracking situation of the composite interface of the joint was studied in detail. The results show that with the increase of the gap discharge power, the local temperature and stress near the gap increase rapidly, and the interface cracks appear at a distance from the original air gap and new gap generates. With the increase of the air gap pressure, the gap pressure decreases at first and then increases. The negative air gap pressure can increase the local stress, while the positive air gap pressure can reduce the local stress in a certain range, which leads to the interface cracking on both sides of the gap, results in the continuous expansion of the original gap.

In order to improve the bonding strength of SiO₂ and methyl vinyl silicone rubber (MVSR) matrix, we selected poly(methyl methacrylate) (PMMA) as the graft chain of SiO₂. Molecular simulation was used to calculate the change of binding energy, molecular overlap length, and mean orientation shift. The change law of interface bonding strength between SiO₂ and MVSR matrix under different graft densities and lengths of PMMA short chains and the effects of electric field and temperature on the interface bonding strength were studied, and compared with SiO₂ ungrafted and KH570 grafted. The results show that as a graft chain, PMMA has a better improve effect on the interface bonding strength of SiO₂ and MVSR than the common silicone coupling agent KH570. When the graft density is 13% or 16%, and the graft chain length is 5, PMMA shows the best grafting effect. Under negative electric field, the interfacial bonding strength increases with the increase of the field intensity, while it decreases with the increase of the field intensity under the positive electric field. With the increase of temperature, the bonding strength between SiO₂ and MVSR interface decreases continuously.

The micro-water content in transformer oil is an important factor to measure whether the transformer can operate stably for a long time. Based on multi-frequency ultrasonic detection combined with artificial neural network algorithm, a method for predicting micro-water content in transformer oil was proposed in this study. Firstly, the micro-water content in 210 groups of oils was determined by Carl Fischer titration. Secondly, 210 groups of oil samples were detected by multi-frequency ultrasound to analyze the relationship between micro-water content in oil samples and amplitude and phase signals in multi-frequency ultrasonic data. Finally, the original 242-dimensional multi-frequency ultrasonic data was reduced to 23-dimensional by PCA. Two prediction models for micro-water content in transformer oil based on PCA-GA-BPNN and PCA-PSO-GRNN were established by combining with BPNN and GRNN artificial neural networks as well as GA and PSO optimization algorithms. The prediction results were compared with the actual results. The results show that the forecast accuracy of both models is higher than 90%, which indicates that the method proposed in this study can effectively detect the moisture content in transformer oil.

In order to study the gas production characteristics of ester insulating oils under transformer fault conditions, three ester insulating oils including single ester, natural ester, and synthetic ester insulating oils were selected to analyze their gas production characteristics under simulated low energy discharge conditions. The results show that the hydrogen production ratio of ester insulating oil is higher under low energy discharge, and the production of acetylene is much lower than that of mineral insulating oil, especially the acetylene production of synthetic esters is the lowest. The low energy discharge of mono ester and natural ester insulating oils can be determined by three-ratio, Duval triangle, and Duval pentagon methods. The low energy discharge of the synthetic ester can be determined by the Duval triangle method.

Polyvinylidene fluoride (PVDF) is widely used in the fields of energy storage, sensors, and energy collection due to its excellent dielectric, piezoelectric, and ferroelectric properties. PVDF has a variety of crystalline phases, which correspond to different molecular conformations and physical properties. At present, there are still controversies about which PVDF crystal phases dominate under certain conditions, the corresponding morphological characteristics of each crystal phase, and the influence on the physical properties of PVDF. This article reviewed the structural characteristics of the three main crystal forms (α, β and γ) of PVDF and the current mainstream characterization methods, and analyzed the difficulties and potential ways to accurately establish the "structure-activity" relationship of PVDF polymorphs. The factors affecting the formation of PVDF polymorphs and the transformation rules were also reviewed.

With the continuous advancement of electronic technology, the oriented polypropylene (BOPP), which is currently the most widely used in commercial applications, cannot meet the requirements of electronic products due to its small energy storage density. Among many dielectric film materials, polyvinylidene fluoride (PVDF)-based nanocomposites have received widespread attention due to their high dielectric constant and good processing properties. In this paper, the theoretical model of dielectric composites was introduced, the main preparation strategies of PVDF-based nanocomposites: filler modification, multi-phase blending, and construction of novel structures, were reviewed systematically, and the development of PVDF-based composite energy storage materials was prospected.

The flash point measured value of transformer oil is significantly affected by atmospheric pressure, therefore the standard flash point value of transformer oil at 101.3 kPa need to be obtained through pressure correction on the measured value. The open-cup and closed-cup flash point value of new 25^# transformer oil and used transformer oil under different air pressure was measured by flash point tester. The results indicate that there is a certain linear relationship between the reciprocal value of flash point (1/T_F-exp) and the logarithm of experimental pressure (lnP). The linear expression of 1/T_F-exp-lnP for each experimental oil sample is determined, and the range of 1/T_F-exp-lnP slope value is (-2.31--1.22)×10^-4 (for the open-cup flash point) and (-6.68--2.32)×10^-5 (for the closed-cup flash point) for different oil samples. The flash point value of the experimental oil sample determined under different pressure (60.0-102.0 kPa) is corrected to the standard flash point value at 101.3 kPa, and it is found that the correction deviation is far less than the deviation caused by using the current standard method on basis of the T_F-exp-P linear relationship. The correction method based on the linear relationship of 1/T_F-exp-lnP has a better correction accuracy and a wider pressure range, which has good practical significance to obtain more accurate corrected flash point value of transformer insulating oil.

A low viscosity, fast-curing single component thermally conductive structural adhesive base glue was prepared by using bisphenol A epoxy resin E124 as the main resin, polythiol as the curing agent, and imidazole as the accelerator, its curing process was discussed by surface drying time and gel time test and DSC test. A thermal conductive structural adhesive was prepared by adding thermal conductive fillers, and the particle size, shape and amount of fillers on the properties were studied. The results show that when the mass ratio of the base glue to two different sizes of alumina, which is m(base glue): m(40 μm Al₂O₃):m(2 μm Al₂O₃)=15:7:3, m(base glue): m(40 μm Al₂O₃):m(5 μm Al₂O₃)=15:7:3, m(base glue):m(40 μm Al₂O₃):m(10 μm Al₂O₃)=15:8:2, the three high thermal conductive structural adhesives could cure at 90℃ in 15 min. Their thermal conductivity is 2.46, 2.59, and 2.42 W/(m·K), adhesive strength is 5.92, 6.46, and 6.49 MPa, viscosity is 63 800, 74 100, and 87 000 mPa·s, respectively, which meets the basic performance requirements of screen printing process and chip package heat dissipation materials.

Gas chromatography is a common way to determine the dissolved gas content in insulating oils, but there is many types of natural insulating oil, and the Ostwald coefficient is not clear. In order to be able to accurately calculate the dissolved gas content in natural insulating oils, the Ostwald coefficient of seven typical dissolved gases in four natural ester insulating oils was measured using double equilibrium method and compared with the Ostwald coefficient in mineral oil. The results show that there is little difference on Ostwald coefficient of the seven dissolved gases in the three natural esters, while the Ostwald coefficient of dissolved gas in the modified esters is slightly greater than that in the natural esters. The Ostwald coefficient of dissolved gas in FR3 has little relative error compared with the available data, which proves that the data obtained by the method has good credibility.