Zhi-Min Dang

Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage

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.

Carbon nanotube composites with high dielectric constant at low percolation threshold

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.

Improving Dielectric Properties of BaTiO3/Ferroelectric Polymer Composites by Employing Surface Hydroxylated BaTiO3 Nanoparticles

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.

Improved Dielectric Properties of Nanocomposites Based on Poly(vinylidene fluoride) and Poly(vinyl alcohol)-Functionalized Graphene

In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 10⁷ Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.

Influence of silane coupling agent on morphology and dielectric property in BaTiO3/polyvinylidene fluoride composites

Surface of BaTiO3 particle was chemically modified using silane coupling agent (KH550) in order to improve its compatibility with polyvinylidene fluoride (PVDF) matrix polymer, and therefore, expectable microstructure and dielectric property of the BaTiO3/PVDF composites were acquired. Infrared spectra reveal an obvious interaction between BaTiO3 and PVDF induced by the addition of KH550 coupling agent, and the interaction was also confirmed by the observation of morphology of fractured surface of the BaTiO3/PVDF composite when the concentration of KH550 is around 1.0wt%. Crystal lattice parameter of BaTiO3 in the composite was also changed because of the interaction. Finally, increased dielectric constant in the PVDF matrix composite with BaTiO3 treated by 1.0wt% KH550 was found.

1D/2D Carbon Nanomaterial‐Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications

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.

Significantly enhanced low-frequency dielectric permittivity in the BaTiO3/poly(vinylidene fluoride) nanocomposite

To disclose an effect of ceramic particle size on dielectric properties of BaTiO3/ poly(vinylidene fluoride) (BT/PVDF) composites at low frequency, the dielectric properties of the BT/PVDF composites loaded with different diameter BT particles at the same volume concentration were studied at wide frequency ranges from 10−1to107Hz. A significant low-frequency dielectric permittivity increase in the BT/PVDF nanocomposite was noticed, which was not reported previously. The authors considered that the marked aggrandizement of interface and dipole polarization at low frequency is charged with the significant increase in dielectric permittivity.

Dielectric behavior and dependence of percolation threshold on the conductivity of fillers in polymer-semiconductor composites

Polymer-semiconductor PVDF∕LNO (polyvinylidene fluoride∕Li doped NiO) composites were fabricated via simple blending and hot-molding technique. The dielectric behavior of such composites was studied over broad frequency. The results revealed the dependence of percolation threshold on the conductivity of LNO filler in the composites. And the conductivity of the LNO fillers played an important role on the dielectric properties and critical exponents of the PVDF∕LNO composites. High dielectric constants and low conductivities of the composites were observed near the percolation threshold. Finally, critical exponents were also used to explain the experimental results, and provided useful information for understanding the resultant dielectric properties.

Fabrication and dielectric properties of advanced high permittivity polyaniline/poly(vinylidene fluoride) nanohybrid films with high energy storage density

Energy storage capacitors have been the focus of increasing attention due to their advantages such as handiness, high efficiency and environment friendliness. To obtain the dielectric capacitor composites, poly(vinylidene fluoride) (PVDF) and conductive polyaniline (PANI) were selected as the polymer matrix and the filler respectively. The influences of volumetric fraction of PANI (fPANI), preparation processing and frequency on dielectric properties were studied. The results showed when the fPANI was up to 0.05 (higher than the percolation threshold fc = 0.042), the dielectric permittivity of the hybrid film was as high as 385 (at 103 Hz), the breakdown strength was 60 MV m−1, and the energy storage density was 6.1 J cm−3, which was three times higher than that of neat PVDF. Moreover, the dielectric permittivity was frequency-independent even for the hybrid films with the fPANI approaching the fc. SEM and XRD revealed that the PANI can be dispersed uniformly in the PVDF matrix, and that the PVDF existed mainly as typical β-crystal PVDF for our preparation method. Percolative theory and microcapacitor modeling were employed to explain these results. This route was demonstrated to be effective to prepare the high energy density capacitor material used in the wide frequency range.

Influence of aspect ratio of carbon nanotube on percolation threshold in ferroelectric polymer nanocomposite

Multiwall carbon nanotube (MWNT) with different aspect ratio (AR) was dispersed into ferroelectric polyvinylidene fluoride (PVDF) to fabricate the MWNT/PVDF nanocomposites. An increase of dielectric constant with increasing the AR value was observed in the MWNT/PVDF composites. Meanwhile, the result showed that the change of percolation threshold displayed the same rule with that of critical exponent, and both of them increased with increase of AR in some extent, which provides an effective and simple way to describe the dielectric transition of practical composites in the neighborhood of percolation threshold with the scaling theory.