Application of graphene-based materials

Postprint (published version

Effects of carbon nanotubes/graphene nanoplatelets hybrid systems on the structure and properties of polyetherimide-based foams

Foams based on polyetherimide (PEI) with carbon nanotubes (CNT) and PEI with graphene nanoplatelets (GnP) combined with CNT were prepared by water vapor induced phase separation. Prior to foaming, variable amounts of only CNT(0.1–2.0wt%) or a combination of GnP(0.0–2.0 wt %) and CNT (0.0–2.0 wt %) for a total amount of CNT-GnP of 2.0 wt %, were dispersed in a solvent using high power sonication, added to the PEI solution, and intensively mixed. While the addition of increasingly higher amounts of only CNT led to foams with more heterogeneous cellular structures, the incorporation of GnP resulted in foams with ¿ner and more homogeneous cellular structures. GnP in combination with CNT effectively enhanced the thermal stability of foams by delaying thermal decomposition and mechanically-reinforced PEI. The addition of 1.0 wt % GnP in combination with 1.0 wt % CNT resulted in foams with extremely high electrical conductivity, which was related to the formation of an optimum conductive network by physical contact between GnP layers and CNT, enabling their use in electrostatic discharge (ESD) and electromagnetic interference (EMI) shielding applications. The experimental electrical conductivity values of foams containing only CNT ¿tted well to a percolative conduction model, with a percolation threshold of 0.06 vol % (0.1 wt %) CNTPostprint (published version

Effects of graphene nanoplatelets and cellular structure on the thermal conductivity of polysulfone nanocomposite foams

Polysulfone (PSU) foams containing 0–10 wt% graphene nanoplatelets (GnP) were prepared using two foaming methods. Alongside the analysis of the cellular structure, their thermal conductivity was measured and analyzed. The results showed that the presence of GnP can a ect the cellular structure of the foams prepared by both water vapor induced phase separation (WVIPS) and supercritical CO2 (scCO2) dissolution; however, the impact is greater in the case of foams prepared by WVIPS. In terms of thermal conductivity, the analysis showed an increasing trend by incrementing the amount of GnP and increasing relative density, with the tortuosity of the cellular structure, dependent on the used foaming method, relative density, and amount of GnP, playing a key role in the final value of thermal conductivity. The combination of all these factors showed the possibility of preparing PSU-GnP foams with enhanced thermal conductivity at lower GnP amount by carefully controlling the cellular structure and relative density, opening up their use in lightweight heat dissipatorsPostprint (published version

Polyetherimide foams filled with low content of graphene nanoplatelets prepared by scCO2 dissolution

Polyetherimide (PEI) foams with graphene nanoplatelets (GnP) were prepared by supercritical carbon dioxide (scCO2) dissolution. Foam precursors were prepared by melt-mixing PEI with variable amounts of ultrasonicated GnP (0.1–2.0 wt %) and foamed by one-step scCO2 foaming. While the addition of GnP did not significantly modify the cellular structure of the foams, melt-mixing and foaming induced a better dispersion of GnP throughout the foams. There were minor changes in the degradation behaviour of the foams with adding GnP. Although the residue resulting from burning increased with augmenting the amount of GnP, foams showed a slight acceleration in their primary stages of degradation with increasing GnP content. A clear increasing trend was observed for the normalized storage modulus of the foams with incrementing density. The electrical conductivity of the foams significantly improved by approximately six orders of magnitude with only adding 1.5 wt % of GnP, related to an improved dispersion of GnP through a combination of ultrasonication, melt-mixing and one-step foaming, leading to the formation of a more effective GnP conductive network. As a result of their final combined properties, PEI-GnP foams could find use in applications such as electrostatic discharge (ESD) or electromagnetic interference (EMI) shieldingPostprint (published version

Low density polycarbonate-graphene nanocomposite foams produced by supercritical carbon dioxide two-step foaming. Thermal stability

The thermal stability of low density polycarbonate-graphene nanocomposite foams prepared by supercritical carbon dioxide two-step foaming was investigated. Unfilled polycarbonate foams showed improved thermal stabilities when compared to the unfoamed polycarbonate, as the cellular structure of foams effectively slowed down the heat transfer process. Comparatively, polycarbonate foams with larger cells exhibited the highest delays in the early stage of thermal decomposition. Low density polycarbonate-graphene nanocomposite foams (relative densities between 0.07 and 0.28) displayed even higher thermal stabilities, with enhancements of up to 70 ºC in terms of the onset of decomposition when compared to the unfilled PC, which was attributed to a combination of a heat transfer reduction promoted by the cellular structure and the presence of the dispersed graphene nanoplatelets, which acted as a physical barrier to the release of volatile decomposition products.Peer ReviewedPostprint (author's final draft

Enhanced electrical conductivity in graphene-filled polycarbonate nanocomposites by microcellular foaming with sc-CO2

“This is an Accepted Manuscript of an article published by Taylor & Francis Group in Journal of Adhesion Science and Technology 01/2016, available online: http://www.tandfonline.com/10.1080/01694243.2015.1137700"Electrically conductive polycarbonate (PC) foams containing a low concentration of graphene nanoplatelets (0.5 wt.%) were produced with variable range of expansion ratio by applying a high-pressure batch foaming process using sc-CO2. The structure of the foams was assessed by means of SEM, AFM and WAXS, and the electrical conductivity was measured in the foam growing direction. Results showed that electrical conductivity of PC composite foams remarkably increased when compared to that of non-foamed PC composite, with both the electrical conductivity and the main cell size of the foams being directly affected by the resultant expansion ratio of the foam. This interesting result could be explained by the development of an interconnected graphene nanoparticle network composed by increasingly well-dispersed and reoriented graphene nanoplatelets, which was developed into the solid fraction of the foam upon foaming by sudden depressurising of the plasticised CO2-saturated PC preform. Some evidences of morphological changes in the graphene nanoplatelets after foaming were obtained by analysing variations in graphene’s (0 0 2) diffraction plane, whose intensity decreased with foaming. A reduction of the average number of layers in the graphene nanoplatelets was also measured, both evidences indicating that improved dispersion of graphene nanoparticles existed in the PC composite foams. As a result, foams with a proper combination of low density and enhanced electrical conductivity could be produced, enabling them to be used in applications such as electromagnetic interference shielding.Peer ReviewedPostprint (author's final draft

Effects of graphene nanoplatelets on the morphology of polycarbonate-graphene composite foams prepared by supercritical carbon dioxide two-step foaming

Low density polycarbonate foams containing different amounts of graphene nanoplatelets with variable cellular morphologies were prepared using a supercritical carbon dioxide two-step foaming process, which consisted of the dissolution of supercritical CO2 into moulded foam precursors and their later expansion by double contact restricted foaming. The effects of the processing conditions and graphene content on the cellular morphology of the obtained foams were investigated, showing that the addition of increasingly higher amounts of graphene nanoplatelets resulted in foams with increasingly smaller cell sizes and higher cell densities, due on the one hand to their effectiveness as cell nucleating agents and on the other to their platelet-like geometry, which limited CO2 loss during foaming due to a barrier effect mechanism. Especially significant was the addition of 5 wt.% graphene nanoplatelets, as the high concentration of graphene limited CO2 escape and cell coalescence during expansion, enabling to obtain highly expanded microcellular foams.Peer ReviewedPostprint (published version

Enhancing the electrical conductivity of polyetherimide-based foams by simultaneously increasing the porosity and graphene nanoplatelets dispersion

Significant improvement in electrical conductivity of graphene nanoplatelets-filled polyetherimide (PEI) foams was achieved by simultaneously increasing the porosity and graphene nanoplatelets dispersion. Foams were prepared by means of water vapor-induced phase separation using a concentration of graphene nanoplatelets (GnP) between 1 and 10 wt%. To obtain two sets of foams having different density and porosity, PEI's concentration in N-methyl pyrrolidone (NMP) solvent prior to foaming was set at 15 and 25–30 wt%, respectively. High-power sonication was applied to GnP-NMP suspension before PEI's addition for the foam series with higher porosity (15 wt% PEI). All foams were later characterized in terms of cellular structure, thermal stability, dynamic-mechanical properties, and electrical conductivity. A notable enhancement in electrical conductivity was observed with foaming, especially when increasing the porosity and applying sonication, with foams reaching values as high as 1.7 × 10-1 S/m while maintaining the thermal stability and mechanical performancePostprint (author's final draft

Recent advances in carbon-based polymer nanocomposites for electromagnetic interference shielding

Carbon-based nanoparticles have recently generated a great attention, as they could create polymer nanocomposites with enhanced transport properties, overcoming some limitations of electrically-conductive polymers for high demanding sectors. Particular importance has been given to the protection of electronic components from electromagnetic radiation emitted by other devices. This review considers the recent advances in carbon-based polymer nanocomposites for electromagnetic interference (EMI) shielding. After a revision of the types of carbon-based nanoparticles and respective polymer nanocomposites and preparation methods, the review considers the theoretical models for predicting the EMI shielding, divided in those based on electrical conductivity, models based on the EMI shielding efficiency, on the so-called parallel resistor-capacitor model and those based on multiscale hybrids. Recent advances in the EMI shielding of carbon-based polymer nanocomposites are presented and related to structure and processing, focusing on the effects of nanoparticle’s aspect ratio and possible functionalization, dispersion and alignment during processing, as well as the use of nanohybrids and 3D reinforcements. Examples of these effects are presented for nanocomposites with carbon nanotubes/nanofibres and graphene-based materials. A final section is dedicated to cellular nanocomposites, focusing on how the resulting morphology and cellular structures may generate lightweight multifunctional nanocomposites with enhanced absorption-based EMI shielding propertiesPostprint (author's final draft

Multifunctional foams made of polymer nanocomposites

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