Graphene oxide (GO) was produced using acidic graphite oxidation and dispersed within an epoxy matrix using a solvent-based technique, to give nanocomposites containing up to 2 wt% of GO. Transmission and scanning electron microscopy revealed a fine dispersion of graphitic sheets which alters the nanocomposite's fractured surface morphology, while Fourier transform infrared spectroscopy revealed an excess of epoxide groups in the system, which are associated with the included GO. These additional moieties react with hardener amine groups and, consequently, displace the reaction stoichiometry away from the optimum. The result of this is a change in the network architecture and, in particular, the introduction of epoxy-terminated branches, which modify the dielectric c relaxation. During post-curing, hydroxyl groups on the GO surface react with residual epoxide groups through etherification reactions, to give a marked increase in the glass transition temperature. These reactions lead to increased interfacial interactions between the GO and the matrix, which contribute to an increase in tensile performance. In addition, post-curing also reduces the defect content within the GO lattice which, in turn, increases the electrical conductivity, dielectric permittivity and low frequency losses of the system. Associated chemical pathways are proposed.

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A synthetic route for the production of graphene oxide is described, in which the commonly used potassium permanganate (KMnO 4 ) is replaced by chromium trioxide (CrO 3 ) as the oxidizing agent. Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy demonstrate that the product is characterized by a reduced level of oxidation and a reduced defect content, compared to conventional graphene oxide (GO). We therefore term the product moderately oxidized graphene oxide (mGO). In comparison with GO, it is shown that when introduced into an epoxy matrix, mGO offers significant potential benefits. These include: excellent compatibility with the epoxy matrix leading to a low percolation threshold for electrical conductivity (* 0.5 vol%); an associated increase in electrical conductivity of about eight orders of magnitude; no adverse influence on the epoxy curing reactions; potentially simplified material processing strategies.

In this study, three different moieties containing amino‐groups were incorporated into graphene oxide (GO) in order to modify interfacial interactions within an epoxy matrix. The GO was functionalized with two bifunctional molecules of different molar masses (d230 and d4000) and a trifunctional (t440). The resulting functionalized GO (fGO) systems were characterized by Raman spectroscopy, thermogravimetric analysis (TGA) and X‐ray photoelectron spectroscopy (XPS), which demonstrated the presence of the functionalizer groups. Analysis of nanocomposites including the above three fGOs by microscopy and differential scanning calorimetry (DSC) demonstrated that the presence of the functionalizer moieties served to improve the platelet distribution within the matrix and to affect the local fGO/matrix interfacial interactions perturbing the epoxy curing reactions; we suggest that these changes stem from the fGO surface structure and the introduction of additional reactive amine nitrogen. X‐ray diffraction revealed reduced graphitic stacking with increased functionalizer molar mass. The electrical conductivity of the fGO‐filled epoxy was enhanced with increasing functionalizer molecular weight, an effect we relate to the influence of this on platelet stacking. The thermal conductivity was, however, adversely affected by all reagents at low fGO contents.

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Nanocomposite materials based on polydimethylsiloxane (PDMS) reinforced by electrospun poly(vinylidene fluoride) (PVDF) nanofibers and barium titanate (BTO) nanoparticles were fabricated and tested as dielectric materials for capacitive energy storage applications. Two types of BTO nanoparticles were examined, prior and after ball milling, to investigate the effect of interfacial area and size on the dielectric properties. The morphology of the produced PVDF nanofibers was evaluated via scanning electron microscopy (SEM) to ensure the optimum electrospinning conditions and verify the incorporation of BTO nanoparticles. The composite systems were analyzed by dielectric spectroscopy, and three dielectric processes were revealed: the dynamic glass-to-rubber transition processes of PDMS and PVDF and an interfacial polarization process. It was observed that the dynamic glass-to-rubber transition process of the PVDF nanofibers strongly depends on the size of the BTO nanoparticles that introduce confinement effects and affect thus the temperature dependence of the relaxation. In addition, as verified by ac conductivity, ball milling reduced the conduction of the nanocomposites by 80%, indicating the increase of the charge carrier trapping area around the BTO nanoparticles. Finally, the developed nanocomposites were tested as dielectric materials for capacitor applications at room temperature conducting charge/ discharge measurements under the influence of a dc electric field, and their discharge performance and efficiency were examined at various dc voltages (50−300 V) and cycle life. Here, experimental evidence regarding the importance of interfacial area on the energy storage performance in nanodielectrics is presented that will aid the development of more efficient energy materials.

Abstract-Nanodielectric composites belong to a type of materials engineered for improved performance in several different fields. Advanced technology devices require a new class of materials, combining suitable electrical and thermal properties, as well as, mechanical performance and easy processing. In nanocomposites, a key parameter is the interaction between the matrix and the nanofiller. In the present work, the effect of filler load of graphene oxide (GO), on the thermal and electrical response of epoxy-based nanocomposites was investigated. The final goal of the project is to develop epoxy nanocomposite systems which will meet the requirements to be utilized as a matrix, in carbon fiber reinforced polymer (CFRP) wind turbine blades with improved lightning protection performance. It was found that a filler load of 3 wt% leads to percolation threshold. The dielectric spectroscopy results confirm the percolation and also show inert dipoles with increasing filler load. The high filler load leads also to lower impulse breakdown strength. The thermal response is affected by the oxygen-based groups of the GO. It was also found that the dispersion/exfoliation method used affects the response of the epoxy resin.

Two different graphitic powders, namely: moderately-oxidized graphene oxide (mGO) synthesized via a chromium-based technique and a commercial edge-oxidized graphene oxide (eGO), were characterized and incorporated into an epoxy resin, suitable for wind turbine blade structural components. Raman spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis revealed low oxygen content, but divergent structural characteristics for both powders confirming the increased basal-plane functionality of mGO compared to the peripherally decorated eGO. It is also shown that the eGO, displays carbon-based impurities. The inclusion of mGO, into the epoxy resulted in an initial glass transition temperature (T g ) increase (~ 5 °C at 4.4 vol.% mGO) but thereafter T g decreased sharply. On the contrary, the inclusion of eGO resulted only in a progressive T g increase. Introduction of just 1 vol.% of eGO deteriorated the tensile strength (~ 15% reduction) of the epoxy, while the strength of the mGO-filled samples was retained. Inclusion of mGO results in a percolation threshold (increase from 4.6 × 10 −16 to 6 × 10 −9 S/cm) at 0.53 vol.%; in contrast, at the same filler content, the eGO-filled systems are characterized by drastically lower conductivity values (3.4 × 10 −16 S/cm). Nevertheless, further analysis indicates similar intrinsic conductivity (~ 10 −6 S/cm) for the two fillers. Finally, the maximum achieved thermal conductivity increase with mGO was 200% (at 9.13 vol.%) compared with the unfilled epoxy, while the respective increase with eGO was 150% (at 18 vol.%).