Jun-Wei Zha

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.

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.

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.

Functionalized graphene–BaTiO3/ferroelectric polymer nanodielectric composites with high permittivity, low dielectric loss, and low percolation threshold

The fabrication and dielectric properties of a novel multi-component high-k composite system consisting of poly(vinylidene fluoride), surface-functionalized graphene nanosheets and BT nanoparticles (fRGO–BT/PVDF) were investigated. The fRGO nanosheets were prepared through the π–π stacking of polyaniline and GO following in situ hydrazine reduction. The fRGO–BT/PVDF nanocomposites were fabricated by a solution casting and hot-pressing approach. SEM results confirm that fRGO and BT are well dispersed within the PVDF matrix. The dielectric properties of the binary fRGO/PVDF nanocomposites exhibit a typical percolation transition with the percolation threshold of 1.49 vol%. This type of nanocomposite, co-filled with conductive graphene nanosheets and high-k ceramics, shows a high kr (65) and a relatively low dielectric loss (tan δ = 0.35) at a high frequency of 1 MHz. Meanwhile, the dielectric properties of the fRGO–BT/PVDF nanocomposites show temperature independent behavior over a wide temperature range. These flexible, high-k fRGO–BT/PVDF nanocomposites are potential flexible dielectric materials for use in high-frequency capacitors and electronic devices.

Surface-Functionalized MWNTs with Emeraldine Base: Preparation and Improving Dielectric Properties of Polymer Nanocomposites

A comparative study of the dielectric properties of poly(vinylidene fluoride) (PVDF) based nanocomposites with pristine multiwalled carbon nanotubes (MWNTs) and surface-modified MWNTs with core/shell structure (denoted as MEB) as fillers, was reported. Compared with MWNTs/PVDF composites, the MEB/PVDF composites exhibited lower loss tangent and higher dielectric permittivity. It is suggested that the conductive/nonconducting core/shell structure of the MEB filler is the main cause of the improved dielectric properties. Percolation based MWNTs networks is in charge of the improvement of dielectric permittivity, and the nonconducting emeraldine base layer of the MEB filler supports the low loss tangent and low conductivity in the MEB/PVDF composites.

Improved Thermal Conductivity and Flame Retardancy in Polystyrene/Poly(vinylidene fluoride) Blends by Controlling Selective Localization and Surface Modification of SiC Nanoparticles

The effect of selective localization of silicon carbide (SiC) and polystyrene (PS)-coated SiC (p-SiC) nanoparticles on the thermal conductivity and flame retardancy of immiscible PS/poly(vinylidene fluoride) (PVDF) blends has been systematically studied. The scanning electron microscopy (SEM) images reveal that SiC and p-SiC nanoparticles have different selective localizations in the PS/PVDF blends. The melting and crystallization behaviors of the PVDF component investigated by using differential scanning calorimetry are consistent with the SEM results. To reduce the volume fraction of fillers in the composites, a cocontinuous structure of PS/PVDF has also been built up. The cocontinuity window for PS/PVDF blends is ∼30-70 vol % according to the selective solvent dissolution technique. The selective localization of SiC in the PVDF phase of the PS/PVDF 70/30 blends produces a slightly higher thermal conductivity than that of p-SiC in the PS phase of the PS/PVDF 30/70 blends. However, the composites with selective localization of p-SiC exhibit the best combined properties of thermal conductivity and flame retardancy.

Size-dependent low-frequency dielectric properties in the BaTiO3/poly(vinylidene fluoride) nanocomposite films

Effects of inorganic nanoparticles size and thermal agitation on dielectric properties of BaTiO3/poly(vinylidene fluoride) (BT/PVDF) nanocomposite films at low frequency were studied. The dielectric properties of the BT/PVDF nanocomposite films with three kinds of diameters of BT nanoparticles loading at 50 vol. % were studied in a wide frequency range from 10−2 Hz to 107 Hz by two measured processes. A significant low-frequency dielectric permittivity increase and the difference in dielectric properties between two measured processes were discussed. Interfacial polarization, crystal phase effect, and thermal agitation are considered to analyze the significant increase and difference in dielectric behaviors.

Positive piezoresistive behavior of electrically conductive alkyl-functionalized graphene/polydimethylsilicone nanocomposites

Piezoresistive nanocomposites using alkyl-functionalized graphene (G-ODA) as a conducting filler and polydimethylsilicone (PDMS) as the polymer matrix were prepared and their piezoresistivity behavior was investigated. One-pot synthesis of G-ODA from graphite oxide and octadecylamine improved its dispersion in nonpolar xylene and PDMS with low surface free energy. Results show that the graphene nanosheets were homogeneously dispersed in the PDMS matrix and an ultra-low percolation threshold (0.63 vol%) of the composites was obtained. The G-ODA/PDMS composites with 1.19 vol% content of G-ODA show a remarkable positive piezoresistivity of high sensitivity (R/R0 > 400 under the pressure of 1.2 MPa), excellent repeatability, small hysteresis, and long-term durability. Under uniaxial compression, the resistance of the composites exponentially increased with the pressure. The resistance–pressure curves remain nearly unchanged after 1000 loading–unloading cycles. The results suggest that the G-ODA/PDMS nanocomposites provide a new route toward fabrication of soft piezoresistive sensors with high performance.

High performance hybrid carbon fillers/binary–polymer nanocomposites with remarkably enhanced positive temperature coefficient effect of resistance

Aiming to enhance the positive temperature coefficient (PTC) effect of resistance, immiscible polymer blends [ultra-high molecular weight polyethylene (UHMWPE)/polyvinylidene fluoride (PVDF) = 4 : 1] based composites containing hybrid fillers [carbon nanotubes (CNTs) and carbon black (CB)] were explored. The conductive fillers were premixed with UHMWPE by melt-mixing and then PVDF was introduced into the blends. The preferential distribution of conductive fillers in the UHMWPE phase was desirably observed. Besides decreasing the electrical resistivity of the single UHMWPE based PTC materials, the addition of PVDF with much higher melting point could improve the temperature range of the PTC materials, which is important for their potential applications. A remarkable synergetic effect arising from the combination of CB and CNTs with different geometric structures and aspect ratios on improving the PTC behavior was demonstrated. By introducing 0.5 vol% CNTs into the 4 vol% CB filled UHMWPE0.8–PVDF0.2 composites, the initial resistivity decreased by about two orders of magnitude and the PTC intensity (PTCI) increased by about 30%. Owing to the 3-dimensional conductive networks provided by tube-shaped CNTs and spherical CB and the high viscosity of the UHMWPE matrix, favorable PTC repeatability was also achieved.

Increased electroaction through a molecular flexibility tuning process in TiO2–polydimethylsilicone nanocomposites

Flexible polymer materials with obvious electrostriction characteristics display a significant potential for application as novel potential actuators in the future. We report advanced TiO2–polydimethylsilicone (TiO2–PDMS) nanocomposites with electroaction that is effectively increased through a molecular flexibility tuning process. The increase in the electromechanical sensitivity (by 550%) and actuation strain (by 230%) under a low electric field in low elastic modulus TiO2–PDMS composites originates from the flexibility tuning process by the introduction of dimethylsilicone oil (DMSO). The DMSO is miscible with PDMS resulting in a uniform composition at the molecular level, which can significantly decrease the elastic modulus of the dielectric elastomer composites from 820 kPa to 95 kPa. The experimental results are interpreted using the swelling elastomers theory. It suggests that reducing the elastic modulus could be a good strategy to improve the actuation performance with a low electric field.