Graphene and its derivatives have been widely applied as conductive fillers for the fabrication of high-permittivity (high-k) polymer composites. Materials with high dielectric constants and low dielectric losses are extensively employed in embedded capacitors and energy storage devices, while those with high constants and high losses are promising in energy dissipation applications, such as electromagnetic interference (EMI) shielding. In this research, the full potential of graphene/polymer composites for high-k applications is explored by the chemical/physical modification of graphene, the appropriate selection of graphene structure, and the design of materials morphologies.

Chlorine-doped reduced graphene oxide (rGO)/ poly (vinylidene fluoride) (PVDF) composites with except...[ Read more ]

Graphene and its derivatives have been widely applied as conductive fillers for the fabrication of high-permittivity (high-k) polymer composites. Materials with high dielectric constants and low dielectric losses are extensively employed in embedded capacitors and energy storage devices, while those with high constants and high losses are promising in energy dissipation applications, such as electromagnetic interference (EMI) shielding. In this research, the full potential of graphene/polymer composites for high-k applications is explored by the chemical/physical modification of graphene, the appropriate selection of graphene structure, and the design of materials morphologies.

Chlorine-doped reduced graphene oxide (rGO)/ poly (vinylidene fluoride) (PVDF) composites with exceptional dielectric constants and low loss tangents are fabricated for energy storage applications. Graphene oxide (GO) sheets are doped with chlorine by mixing thionyl chloride into the GO dispersion, resulting in the formation of both charge transfer complexes and covalently bonded chlorine on GO sheets. Ionic Cl functions as an electron acceptor, i.e. p-type dopant, and an optimal chlorination gives rise to a maximum ionic Cl, the highest charge carrier density, and the highest electrical conductivity of GO. The composites deliver an exceptional dielectric constant of 364 with a low dielectric loss of 0.077 at 1 kHz. Synergistic effects arising from chlorination are identified, including the much enhanced electrical conductivity of Cl-GO sheets by more than 3 orders of magnitude through introducing charge-transfer complexes, the improved interfacial interactions between the fillers and the PVDF matrix through hydrogen bonds, and the transformation of PVDF to β-phase with an inherently high dielectric constant due to dipolar interaction.

High-k micro-sandwich composites consisting of alternating polydopamine functionalized rGO-polyurethane (PU) and boron nitride (BN) nanosheets-PU layers are developed for energy storage applications. Highly oriented rGO-PU composites are initially fabricated by freeze casting/drying, giving rise to exceptional dielectric constants with relatively high dielectric losses and low breakdown strengths. The losses are significantly suppressed while the dielectric strengths are restored after the incorporation of electrically insulating BN-PU barrier layers among the aligned rGO-PU skeleton in the second freeze casting/drying routine. The transverse ligaments connecting the conductive rGO-PU layers are effectively removed by the BN-PU barrier layers, effectively blocking current leakage in the transverse direction. The micro-sandwich composites deliver a remarkable maximum dielectric constant of 1084 with a low dielectric loss of 0.091 at 1 kHz. Taking advantage of the blocked current leakage and the incorporation of high-dielectric-strength BN-PU barrier layers, rGO-PU/BN-PU micro-sandwich composites present a much enhanced energy storage density of 22.7 J/cm³, which is 37 and 168 folds larger than those of the neat PU and rGO-PU composites, respectively.

In an effort to explore high-k composites for energy dissipation, high-performance EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites are synthesized by drop coating of PEDOT:PSS on cellular-structured, freestanding GFs. The GF/PEDOT:PSS composites possess an ultralow density of 18.2 × 10^-3 g/cm³ and a high porosity of 98.8%, as well as an enhanced electrical conductivity by almost 4 folds from 11.8 to 43.2 S/cm after the incorporation of the conductive PEDOT:PSS. The composites deliver remarkable EMI shielding performance with a shielding effectiveness (SE) of 91.9 dB and a specific SE of 3124 dB·cm³/g. The excellent electrical conductivities of composites arising from both the GFs with three dimensionally interconnected conductive networks and the conductive polymer coating, as well as the left-handed composites with absolute permittivity and/or permeability larger than one give rise to significant microwave attenuation by absorption.