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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.

Trifluorophenyl‐functionalized multi‐walled‐carbon‐nanotube/poly(vinylidene fluoride) (TFP‐MWNT/PVDF) nanocomposites are fabricated by employing a wet‐chemistry route. The modified MWNTs are observed to form a well‐dispersed, structurally random nanophase within the polymer matrix (see figure). The TFP‐MWNT/PVDF nanocomposite exhibits enhanced dielectric permittivity when the content of TFP‐MWNT is close to the percolation threshold.

Fe [Co¯Fe¯], if one considers the moment of the cobalt ion being reduced due to the stabilization of the lowest Kramers doublet. [16] The slow rise of the virgin magnetization is consistent with a pinning-type magnet with no reversible region. [2] However, the S-type increase with field of the initial (virgin) magnetization after ZFC is not that expected for a random distribution of super-paramagnets, and this anomaly may be a consequence of the presence of a mean dipolar field acting on the particles, which depends on the direction and magnitude of the magnetization of surrounding particles. [18] We, therefore, think that the large coercive field reflects the ªintrinsic anisotropy of the particles enhanced by the inter-particle dipolar fieldsº. To test this hypothesis, we examine a sample (7.5 % CoFe 2 O 4 ±SiO 2 , annealed at 800 C) consisting of 3.2 nm particles with varying separation between particles. These particles are super-paramagnetic at room temperature and the ZFC±FC magnetization measurements in 5 Oe indicate a blocking temperature of 55 K (see supplementary material S2, available from the author). The super-paramagnetism is exemplified by the absence of remanance magnetization and coercivity above 55 K, while a wide hysteresis loop with a remanance magnetization of 50 % of that at saturation is observed and the coercivity is 13.5 kOe at 2 K (see supplementary material S3, available from the author). We also note that the remanance in this case is as expected for the model of Stoner for a random oriented polycrystalline magnetic sample. [19] Further work is in progress on samples prepared at different dilution of spinel and different annealing temperatures between 800 C and 1100 C in order to elucidate the effective dipolar field as a function of size and inter-particle distances. We can only note presently that the size of the particle, its blocking temperature and coercive field decrease on lowering both the concentration and sintering temperature. The lack of evidence for the presence of c-Fe 2 O 3 in the composite does not rule out the model of Skomski and Coey for the enhancement of magnetic hardness in composites consisting of hard and soft magnetic materials. This remains to be verified. [20] We have observed an unusually high magnetic hardness, characterized by a 20 kOe coercive field at 2 K, for CoFe 2 O 4 particles of a size of 12 nm in amorphous SiO 2 prepared by the sol±gel method and annealed at 1000 C. The coercivity reflects the intrinsic anisotropy enhanced by dipolar fields. Its high value for smaller particles tends to confirm this hypothesis. ExperimentalThe sample was prepared by the sol±gel method as follows: Co(NO 3 ) 2´6 H 2 O (2.18 g) and Fe(NO 3 ) 3´9 H 2 O (6.06 g) were dissolved in 4.5 g of water acidified with nitric acid (0.03 M) as a catalyst. To this solution tetraethoxysilane (TEOS, 10.6 g), dissolved in 7 g of methanol, and formamide (2.25 g), as a modifier, were added. After gelation (approximately 2 h at 40 C) and ageing (24 h), the sample was dried at 40 C f...

In this letter, the dielectric properties of the untreated multiwall carbon-nanotubes∕poly(vinylidene fluoride) (MWNT∕PVDF) composites are studied. Towards low frequencies, the dielectric constant of a composite with about 2.0vol% of MWNT increases rapidly and the value of the dielectric constant is as high as 300. However, by a calculation, the percolation threshold of the MWNT∕PVDF composites is only 1.61vol% (0.0161 volume fraction) of MWNT. Both the large aspect ratio and the high conductivity of the MWNT may lead to the low percolation threshold of the MWNT∕PVDF composites. For the percolation composite, the dielectric loss value is always less than 0.4, irrespective of the frequency. Therefore, the experimental results suggest that the dielectric properties of MWNT∕PVDF composites may be improved significantly without the chemical functionalization to carbon nanotubes.

Dielectric properties of poly(vinylidene fluoride) (PVDF) based nanocomposites filled with surface hydroxylated BaTiO(3) (h-BT) nanoparticles were reported. The h-BT fillers were prepared from crude BaTiO(3) (c-BT) in aqueous solution of H(2)O(2). Results showed that the dielectric properties of the h-BT/PVDF nanocomposites had weaker temperature and frequency dependences than that of c-BT/PVDF nanocomposites. Meanwhile, the h-BT/PVDF composites showed lower loss tangent and higher dielectric strength. It is suggested that the strong interaction between h-BT fillers and PVDF matrix is the main reason for the improved dielectric properties.

Authors are grateful to Mrs. F. Garnier and P. Gemeiner for the SEM investigations and Raman measurements, and Mr. P. Haghi-Ashtiani for the TEM characterizations. J.-K.Y. also gratefully acknowledges the financial support of China Scholarship Council.

With the development of flexible electronic devices and large-scale energy storage technologies, functional polymer-matrix nanocomposites with high permittivity (high-k) are attracting more attention due to their ease of processing, flexibility, and low cost. The percolation effect is often used to explain the high-k characteristic of polymer composites when the conducting functional fillers are dispersed into polymers, which gives the polymer composite excellent flexibility due to the very low loading of fillers. Carbon nanotubes (CNTs) and graphene nanosheets (GNs), as one-dimensional (1D) and two-dimensional (2D) carbon nanomaterials respectively, have great potential for realizing flexible high-k dielectric nanocomposites. They are becoming more attractive for many fields, owing to their unique and excellent advantages. The progress in dielectric fields by using 1D/2D carbon nanomaterials as functional fillers in polymer composites is introduced, and the methods and mechanisms for improving dielectric properties, breakdown strength and energy storage density of their dielectric nanocomposites are examined. Achieving a uniform dispersion state of carbon nanomaterials and preventing the development of conductive networks in their polymer composites are the two main issues that still need to be solved in dielectric fields for power energy storage. Recent findings, current problems, and future perspectives are summarized.