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Polymer composites with high energy density have been of great interest to scientists for a long time, and this accounts for a great wealth of experimental data that are available for interpretation. One of the important phenomena is the variation rate of the permittivity of composites, which always deviates from the linear increase and rises steeply toward the high concentration. Interestingly, the critical transition concentration is always located in the range of 20 vol. %–25 vol. %, but still far less than the common percolation threshold of composites. Through the analysis of the local electric field distribution and surface polarized charges distribution, the phenomenon is successfully interpreted as the effect of the electrostatic interactions between the adjacent polarized particles. Two specified composite configurations (BCC and FCC) are designed, and their dielectric properties have been systematically studied with the help of the finite element method and the phase field method. The simulation results indicate that the strength of the particles' interaction is short-range effective and depends closely on the gap distance between adjacent particles. When the gap distance is smaller than 0.8 times the radius of the particles, which is tantamount to the concentration larger than 20 vol. % and 25 vol. % for BCC and FCC, respectively, the electrostatic interaction of the particles is rather significant, resulting in an additional contribution on the variation of the dielectric properties of composites and vice versa. The synchronous variation tendency between the simulation results and experimental measurements demonstrates the effect of electrostatic interaction on the dielectric properties of composites. [ABSTRACT FROM AUTHOR]

Atmospheric pressure needle-to-plane negative corona discharges have been studied considering the effect of photoionization in electrode gap spacing from 2.9 to 3.9 mm. Trichel pulses, spatial distributions of electron density and electric field, pulse duration time, and pulse frequency under different gap spacing were investigated in this work. It is found that peak value and duration time of the first Trichel pulse decrease with the increase of gap spacing. The movement of charged particles is faster in narrow gap spacing, which leads to higher pulse frequency. Moreover, the effect of photoionization on negative corona discharge under different gap spacing is also studied by numerical simulation. It is found that electric field, as well as electron density, is weakened due to the effect of photoionization. Besides, the magnitude of Trichel pulses, duration time of the first pulse, and pulse frequency are relatively small when taking the effect of photoionization into consideration. These results are of great importance to fully understand the mechanism of negative corona discharge under different conditions. [ABSTRACT FROM AUTHOR]

For practical application, studying the mechanism governing the macroscopic dielectric properties of polymer composites at low concentration is of great significance. A three-dimensional (3D) representative model with randomly dispersed grains in polymer matrix has been established and calculation results of effective permittivity of BT/PVDF composites with finite element method (FEM) show good consistency with experimental measurements, which demonstrate that the distribution of electric field in ceramic particles/polymer composites at low concentration can be approximated by the electrostatic field. Furthermore, the relationship between the internal electric field and dielectric properties of composites has been established. The effect of grain size in submicron and its dielectric constant to the effective permittivity of composites are fully investigated. The former shows of negligible significance compared with the evident effect of volume fraction, while the latter presents saturation as increasing the permittivity ratio between ceramic particle and polymer. [ABSTRACT FROM AUTHOR]

Recent advances in the electrospinning technique offer significant opportunities for improved fiber reinforced polymer composites of interest for capacitor applications. Classical effective medium approximation may fail to reach exact solutions due to the intricately inhomogeneous distribution of the depolarizing field in fibers with arbitrary orientation degrees. In this work, a three-dimensional finite element method (FEM) model has been developed to in-depth study the effects of fiber alignment on the dielectric response in fiber/polymer composites. A quantitative relationship between the effective permittivity of composites and the alignment degree of fiber has been initially established. Furthermore, we define a parameter of the effective angle which has the equivalent contribution on the effective permittivity with the actually distributed multiple alignment angles of fibers in practical composite materials. The calculation results with FEM show good agreement with the experiment measurements delivered recently. [ABSTRACT FROM AUTHOR]

Machine learning has shown its great potential in the accelerated discovery of advanced materials in the field of computational molecular design. High-temperature polymer dielectrics are urgently required with the emerging applications of energy-storage dielectric film capacitors under high-temperature conditions. Here, we demonstrate the successful prediction of polymers with a high dielectric constant (ɛ) and high glass transition temperature (Tg) using a Bayesian molecular design model. The model is trained on a joint data set containing 382 computed ɛ values using density functional perturbation theory and experimentally measured Tg values of ∼7000 polymers to build relative quantitative structure–property relationships and identify the promising polymers with specific desired range of dielectric constant and glass transition temperature. From the hypothetical polymer candidates, ten promising polymers are proposed based on their predicted properties and synthetic accessibility score for high-temperature dielectric film capacitors' application. Moreover, 250k novel polymer structures are generated with the model to support future polymer informatics research. This work contributes to the successful prediction of high-temperature polymer dielectrics using machine learning models. [ABSTRACT FROM AUTHOR]

Highlights: A bilayer dielectric film is prepared via coating boron nitride nanosheets (BNNSs) by solution casting on the surface of polyethylene terephthalate (PET) film. The BNNS layer acts as the efficient barrier layer to suppress the charge injection, thereby making the surface-modified PET films exhibit excellent breakdown strength and electrostatic energy storage performance. The surface coating methods are accessible and suitable for large-scale roll-to-roll process production of dielectric films. The further electrification of various fields in production and daily life makes it a topic worthy of exploration to improve the performance of capacitors for a long time, including thin-film capacitors. The discharge energy density of thin-film capacitors that serves as one of the important types directly depends on electric field strength and the dielectric constant of the insulation material. However, it has long been a great challenge to improve the breakdown strength and dielectric constant simultaneously. Considering that boron nitride nanosheets (BNNS) possess superior insulation and thermal conductivity owing to wide band gap and 2-dimensional structure, a bilayer polymer film is prepared via coating BNNS by solution casting on surface of polyethylene terephthalate (PET) films. By revealing the bandgap and insulating behavior with UV absorption spectrum, leakage current, and finite element calculation, it is manifested that nanocoating contributes to enhance the bandgap of polymer films, thereby suppressing the charge injection by redirecting their transport from electrodes. Worthy to note that an ultrahigh breakdown field strength (~ 736 MV m−1), an excellent discharge energy density (~ 8.77 J cm−3) and a prominent charge–discharge efficiency (~ 96.51%) are achieved concurrently, which is ascribed to the contribution of BNNS ultrathin layer. In addition, the modified PET films also have superior comprehensive performance at high temperatures (~ 120 °C). The materials and methods here selected are easily accessible and facile, which are suitable for large-scale roll-to-roll process production, and are of certain significance to explore the methods about film modification suitable for commercial promotion. [ABSTRACT FROM AUTHOR]

Polymer-based dielectrics are widely applied in the fields of electronics and electrical power systems due to the high breakdown strength, excellent flexibility, and unique self-healing capability. However, the low stored energy density and unsatisfactory heat management of polymer dielectrics hinder the development of polymer-based film capacitors. In this research, artificial composites with improved capacitive energy storage and thermal conductivity are fabricated by blending the two-dimensional semiconductive MXene sheets with a polyimide (PI) matrix. Remarkably, a PI based composite with 1 wt. % MXene sheets increases its dielectric permittivity from ≈3.27 to ≈3.53 and enhances its discharged energy density from ≈1.93 to ≈2.38 J/cm3 while maintaining its low dielectric loss of <0.02 and relatively high charge–discharge efficiency of >80%. Meanwhile, a high in-plane thermal conductivity of 0.418 W m−1 K−1 is achieved for PI/MXene composites with 5 wt. % MXene. In addition, the distribution of temperature field inside the composites has been investigated by a finite element method. These results represent a strategy in polymer dielectrics to achieve simultaneous high energy density and thermal conductivity, which may also have potential for applications in high temperature environments. [ABSTRACT FROM AUTHOR]

Polymer‐based film capacitors with high breakdown strength and excellent flexibility are crucial in the field of advanced electronic devices and electric power systems. Although massive works are carried to enhance the energy storage performances, it is still a great challenge to improve the energy density of polymer composites under the premise of large‐scale industrial production. Herein, a general strategy is proposed to improve the intrinsic breakdown strength and energy storage performances by blending core‐shell structured methyl methacrylate‐butadiene‐styrene (MBS) rubber particles into a polymer matrix. Good compatibility and uniform dispersion state of MBS particles are observed in the matrix. Polarizing microscopy images show that blended films exhibit clear reduction of crystalline grains with the addition of MBS particles. Accordingly, an increased breakdown strength of 515 MV m−1 and discharged energy density of 12.33 J cm−3 are observed in poly(vinylidene fluoride‐co‐hexafluoropropylene)‐based composite films. Through comprehensive characterizations, it is believed that the superior energy storage performance of composite films is attributed to decreased crystalline grains, improved mechanical properties, and restriction on carrier motion. These results provide a novel design of dielectric polymers for high breakdown strength and discharged energy density applications. [ABSTRACT FROM AUTHOR]

Multilayer structural design (including sandwich structure) has received wide attention due to its complementary advantages of different layers in the field of high energy density dielectric materials. Although multilayer polymer composites with improved breakdown strength and energy density have been prepared in some studies, the key mechanism is not clear. In this work, different kinds of multilayer films consisting of various pure polymer [polyimide, poly(vinylidene fluoride) (PVDF), and polypropylene (PP)] layers were prepared to investigate the underlying mechanism of improved performances by multilayer structural design. Experimental results indicated that the relative position of the multilayer film and electrode had an effect on the result of breakdown strength. Meanwhile, when the layer with high relative permittivity (high-k layer) came in contact with the negative electrode, the thinner the high-k layer was, the more remarkable the enhancement of the multilayer materials breakdown strength was. Finally, a double-layer film of PP and PVDF with a Weibull breakdown strength of 649.31 MV/m, which was 60.83 MV/m higher than that of the commercial PP film, was prepared. The simulation result showed that the coating high-k layer could effectively restrain the local electric field distortion around the defect. This study reveals an effective way to improve the breakdown strength of films, which is of great significance for the improvement of capacitor performance and enhancement of energy storage. [ABSTRACT FROM AUTHOR]

The saturated reactor is used in the DC converter valve to protect thyristors. A fluid-solid coupling heat dissipation model was established for the saturated reactor in this study, which included water cooling, heat conduction, heat convection and heat radiation. By the means of finite element method, the temperature and flow field distribution were calculated, which indicated that the low thermal conductivity of epoxy resin hinders the heat dissipation of the saturated reactor. To research effects of thermal conductivity on heat dissipation, the heat dissipation performance of saturated reactor with different epoxy resins was numerical simulated by using this model. The results showed that the heat dissipation performance of the saturated reactor can be effectively improved by the rise of the thermal conductivity of epoxy resin, but the marginal benefit of increasing the thermal conductivity of epoxy resin is gradually reduced. Meanwhile, high-thermal conductivity epoxy resin could have the longer service life under the same operation conditions. [ABSTRACT FROM AUTHOR]

With the renewable energy application fields of film capacitors gradually expanding, mining film capacitors' data is an important issue. When the aging mechanism and environment are unfamiliar, it will be hard to extract the capacitance's variation law and predict its changes. At the same time, the parallel operation of film capacitor components also brings practical needs and data basis for collaborative prediction. In this work, the capacitance of five film capacitors at room temperature and humidity for 70 days was measured and the characteristics of capacitance variation were analyzed. Then, the capacitance law extraction, single prediction, and collaborative prediction methods for film capacitors were proposed based on the smoothing splines theory. Finally, the effectiveness and application potential of the single prediction and collaborative prediction methods were verified by actual data. The performance of the single prediction and collaborative prediction methods is evaluated by error analysis. The single prediction and collaborative prediction methods proposed in this work can also provide a reference for the prediction and evaluation of other parameters like equivalent series resistance of film capacitors, and help to effectively and accurately explore the aging mechanism. • We measured the capacitance for 70 days at room temperature and humidity and analyzed the characteristics. • Based on smooth splines theory, the law extraction, single prediction, and collaborative prediction methods were constructed. • This work will provide a reference for predicting and evaluating other parameter such as the equivalent series resistance of film capacitors, helping to monitor the state and explore the aging mechanism effectively and accurately. [ABSTRACT FROM AUTHOR]