Study of flexible nanodielectric materials (FNDMs) with high permittivity is one of the most active academic research areas in advanced functional materials. FNDMs with excellent dielectric properties are demonstrated to show great promise as energy-storage dielectric layers in high-performance capacitors. These materials, in common, consist of nanoscale particles dispersed into a flexible polymer matrix so that both the physical/chemical characteristics of the nanoparticles and the interaction between the nanoparticles and the polymers have crucial effects on the microstructures and final properties. This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures. Possible reasons for several persistent issues are analyzed and the general strategies to realize FNDMs with excellent integral properties are summarized. The review further highlights some exciting examples of these FNDMs for power-energy-storage applications.

Compared to electrochemical energy devices such as batteries and supercapacitors, dielectric film capacitors have greater power densities and faster charging and discharging rates and are the essential components in power electronics. [4][5][6] Dielectric polymers possess unique features in comparison to their ceramic counterparts, including high breakdown strength, low dielectric loss, facile preparation, and graceful failure mechanism, which make them the materials of choice for scalable high-energy-density capacitors. [7][8][9][10][11] More recently, there is an urgent demand for dielectric materials capable of operating efficiently at elevated temperatures, e.g., 150 °C, in advanced electronics, electrified vehicles, and aerospace power systems. However, dielectric polymers are limited to relatively low working temperatures. [11][12][13][14][15] For example, the operation temperature of biaxially oriented polypropylene (BOPP), the industrial benchmark dielectric polymer, is well below 105 °C under the applied electric fields. [15] A variety of innovative approaches, including the incorporation of wide bandgap inorganic fillers, [16][17][18] deposition of ceramic coatings onto polymer films, [19][20][21] addition of high-electronaffinity molecular semiconductors, [22] and utilization of multilayer-structured films, [23][24][25] have been developed to improve the high-temperature capacitive performance of dielectric polymers. While these approaches are effective in hindering electrical conduction and reducing energy loss at high fields and elevated temperatures, the energy densities of the current high-temperature dielectric composites are limited (below 4 J cm −3 in most cases) owing to relatively low dielectric constant (K) values of the fillers, such as ≈3.5-4 of SiO 2 and boron nitride nanosheets (BNNSs) [16,26] and ≈7.9-10 of Al 2 O 3 . [26] On the other hand, the direct introduction of high-K inorganic fillers, such as TiO 2 with a K of 110 (ref. [27]) and BaTiO 3 with a K of ≈3000 (ref. [28]), into dielectric polymers with the goal of increasing the energy density has yielded very high energy loss and largely reduced chargedischarge efficiency (η) with increasing applied field and temperature. [29,30] For instance, at an applied field of 400 MV m −1 , the η of the polyimide composites with 1 vol% BaTiO 3 nanofibers is only 55% at 150 °C versus 92% at 25 °C. [30] Herein, we present High-energy-density polymer dielectrics capable of high temperature operation are highly demanded in advanced electronics and power systems. Here, the polyetherimide (PEI) composites filled with the core-shell structured nanoparticles composed of ZrO 2 core and Al 2 O 3 shell are described. The establishment of a gradient of the dielectric constants from ZrO 2 core and Al 2 O 3 shell to PEI matrix gives rise to much less distortion of the electric field around the nanoparticles, and consequently, high breakdown strength at varied temperatures. The wide bandgap Al 2 O 3 shell creates deep traps in the composites and thus yields ...

The development of the Internet of Things has brought new challenges to the corresponding distributed sensor systems. Self-powered sensors that can perceive and respond to environmental stimuli without an external power supply are highly desirable. In this paper, a self-powered wind sensor system based on an anemometer triboelectric nanogenerator (a-TENG, free-standing mode) and a wind vane triboelectric nanogenerator (v-TENG, single-electrode mode) is proposed for simultaneously detecting wind speed and direction. A soft friction mode is adopted instead of a typical rigid friction for largely enhancing the output performance of the TENG. The design parameters including size, unit central angle, and applied materials are optimized to enhance sensitivity, resolution, and wide measurement scale. The optimized a-TENG could deliver an open-circuit voltage of 88 V and short-circuit current of 6.3 μA, corresponding to a maximum power output of 0.47 mW (wind speed of 6.0 m/s), which is capable of driving electronics for data transmission and storage. The current peak value of the a-TENG signal is used for analyzing wind speed for less energy consumption. Moreover, the output characteristics of a v-TENG are further explored, with six actual operation situations, and the v-TENG delivers fast response to the incoming wind and accurately outputs the wind direction data. As a wind sensor system, wind speed ranging from 2.7 to 8.0 m/s can be well detected (consistent with a commercial sensor) and eight regular directions can be monitored. Therefore, the fabricated wind sensor system has great potential in wireless environmental monitoring applications.

Space charge formation in polymeric materials can cause some serious concern for design engineers as the electric field may severely be distorted, leading to part of the material being overstressed. This may result in material degradation and possibly premature failure at the worst. It is therefore important to understand charge generation, trapping, and detrapping processes in the material. Trap depths and density of trapping states in materials are important as they are potentially related to microstructure of the material. Changes in these parameters may reflect the aging taken place in the material. In the present paper, characteristics of charge trapping and detrapping in low density polyethylene (LDPE) under dc electric field have been investigated using the pulsed electroacoustic (PEA) technique. A simple trapping and detrapping model based on two trapping levels has been used to qualitatively explain the observation. Numerical simulation based on the above model has been carried out to extract parameters related to trapping characteristics in the material. It has been found that the space charge decaying during the first few hundred seconds corresponding to the fast changing part of the slope was trapped with the shallow trap depth 0.88 eV, with trap density 1.47 × 1020 m−3 in the sample volume measured. At the same time, the space charge that decays at longer time corresponding to the slower part of the slope was trapped with the deep trap depth 1.01 eV, with its trap density 3.54 × 1018 m−3. The results also indicate that trap depths and density of both shallow and deep traps may be used as aging markers as changes in the material will certainly affect trapping characteristics in terms of trap depth and density.

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