Hong Kong Polytechnic University

About: Hong Kong Polytechnic University is a education organization based out in Hong Kong, China. It is known for research contribution in the topics: Computer science & Tourism. The organization has 29633 authors who have published 72136 publications receiving 1956312 citations. The organization is also known as: HKPU & PolyU.

High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates

Abstract: Ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) have great potential for applications in micro-electromechanical devices and high-charge storage capacitors. In order to realize these applications, it is highly desirable to substantially improve the dielectric constants of such ferroelectric polymers. Up to now, much work has focused on the preparation of 0–3-type composites based on polymers and ceramics of high dielectric constant, and the resultant composites usually possess a relatively high dielectric permittivity (about 100). Nevertheless, the high volume fraction (> 50 vol%) of ceramics, which was necessary to achieve the high dielectric constants, presents a number of limitations, in terms of high weight, low flexibility, and poor mechanical performance, as a result of the weak matrix-filler bonding and agglomeration of ceramic nanoparticles. Furthermore, most ceramics of high dielectric constant are lead-based, and potentially harmful to our health. To overcome the limitations of ferroelectric polymer/ceramic composites, very promising work has been carried out recently based on percolation theory, in which a small volume fraction of some conductive filler was added to the polymer matrix to achieve a high dielectric constant, thus preserving the mechanical flexibility of the polymer. For example, Dang et al. fabricated a new poly(vinylidene fluoride)/carbon-nanotube composite, with a percolation threshold of 8 vol%, possessing a dielectric constant of 600 (the dielectric loss value, tan d, is about 2) at 1000Hz. For a P(VDF-TrFE-CFE)/carbon-nanotube system, the dielectric constant increased from 57 to 102 (tand! 0.36) at 100Hz, by inclusion of only 2wt% (1.2 vol%) carbon nanotubes. A dielectric constant as high as 56 was observed in a PVDF/acetylene-black composite, when the acetylene-black concentration was in the neighborhood of the percolation threshold (about 1.3 vol%). Recently, Panda et al. reported that, for PVDF/Ni composites, a high effective dielectric constant of 2050 (tand1⁄4 10) at 100Hz was observed near the percolation threshold of 27 vol%. Along this line, in this communication, we propose a novel nanocomposite system consisting of poly(vinylidene fluoride) and exfoliated graphite nanoplates (PVDF/xGnPs). The xGnPs were selected as the conductive filler, because of their good electrical and thermal conductivity, high mechanical strength, and more importantly, large aspect ratio and unique layered structure with nanoscale thickness, which give advantages in the formation of a large number of parallel-board microcapacitors with low filler loading. Moreover, functional groups, such as C–O–C, C–OH, and C––O, existing on the surface of graphite nanoplates, can promote the interaction between the PVDF and the graphite nanoplates, leading to good dispersion of xGnPs in thematrix. It is well known that the homogenous dispersion of conductive fillers in a polymer matrix is a critical factor in achieving a high-performance nanocomposite. Therefore, it was expected that a much lower volume fraction of xGnP in the PVDF/xGnP nanocomposite could result in a greater increase in dielectric permittivity. xGnPs were obtained from subjecting natural graphite flakes to acidic intercalation, rapid thermal treatment, and ultrasonic powdering, in sequence (see S1 in Supporting Information). Natural graphite flakes (see S2 in Supporting Information), a naturally abundant and low-cost carbon-based material, are composed of parallel carbon layers. Carbon atoms within the graphite layers connect to each other to form six-member rings through strong covalent bonds, while the parallel carbon layers are joined together by weak van der Waals force. Such a structure makes it possible to intercalate some small molecules into the interlayer space of graphite. In the present study, natural graphite flakes were first converted to graphite intercalation compounds (GICs), through intercalation and chemical oxidation in the presence of concentrated H2SO4 andHNO3.When heated at high temperatures, because of the volatilization of the mixed acid, the GICs could be expanded up to a few hundred times along the direction perpendicular to the carbon-layer plane of the intercalated graphite, so that expanded graphite (EG), which is a worm-like material (see the scanning electron microscopy (SEM) image, shown in Fig. 1a), could be obtained. It is also important to note that some functional groups could be introduced to the graphite during the preparation of the GICs and EG. After ultrasonic treatment, the graphite worms were fragmented into exfoliated graphite nanoplates with diameters of 0.5–25mm and thicknesses of 20–60 nm (see S3 in Supporting Information), as shown by the SEM image in Figure 1b. C O M M U N IC A TI O N www.advmat.de