Showing papers by "Zhong-Zhen Yu published in 2011"

Tough Graphene−Polymer Microcellular Foams for Electromagnetic Interference Shielding

Abstract: Functional polymethylmethacrylate (PMMA)/graphene nanocomposite microcellular foams were prepared by blending of PMMA with graphene sheets followed by foaming with subcritical CO2 as an environmentally benign foaming agent. The addition of graphene sheets endows the insulating PMMA foams with high electrical conductivity and improved electromagnetic interference (EMI) shielding efficiency with microwave absorption as the dominant EMI shielding mechanism. Interestingly, because of the presence of the numerous microcellular cells, the graphene−PMMA foam exhibits greatly improved ductility and tensile toughness compared to its bulk counterpart. This work provides a promising methodology to fabricate tough and lightweight graphene−PMMA nanocomposite microcellular foams with superior electrical and EMI shielding properties by simultaneously combining the functionality and reinforcement of the graphene sheets and the toughening effect of the microcellular cells.

Enhanced Thermal Conductivity in a Nanostructured Phase Change Composite due to Low Concentration Graphene Additives

Abstract: The liquid−solid phase change enthalpy, crystallization, and thermal conductivity of graphene/1-octadecanol (stearyl alcohol) composite, a nanostructured phase change material, was investigated as a function of graphene content. The thermal conductivity (κ) of the nanocomposite increased by nearly 2.5-fold (∼140% increase) upon ∼4% (by weight) graphene addition while the drop in the heat of fusion (i.e., storage capacity) was only ∼15.4%. The enhancement in thermal properties of 1-octadecanol obtained with the addition of graphene is markedly superior to the effect of other nanofillers such as silver nanowires and carbon nanotubes reported previously in the literature. Boosting the thermal conductivity of organic phase change materials without incurring a significant loss in the heat of fusion is one of the key issues in enabling their practical application as latent heat storage/release units for thermal management and thermal protection.

Enhanced electrical conductivity in polystyrene nanocomposites at ultra-low graphene content.

Abstract: We compared the electrical conductivity of multiwalled-carbon-nanotube/polystyrene and graphene/polystyrene composites. The conductivity of polystyrene increases from ∼6.7 × 10–14 to ∼3.49 S/m, with an increase in graphene content from ∼0.11 to ∼1.1 vol %. This is ∼2–4 orders of magnitude higher than for multiwalled-carbon-nanotube/polystyrene composites. Furthermore, we show that the conductivity of the graphene/polystyrene system can be significantly enhanced by incorporation of polylactic acid. The volume-exclusion principle forces graphene into the polystyrene-rich regions (selective localization) and generates ∼4.5-fold decrease in its percolation threshold from ∼0.33 to ∼0.075 vol %.

Vacuum-assisted synthesis of graphene from thermal exfoliation and reduction of graphite oxide

Abstract: We report a vacuum-assisted method for thermal exfoliation and in situreduction of graphite oxide in large quantity at a temperature as low as 135 °C. The resulting graphene sheets contain only few-layered sheets with an average thickness of 0.9 nm, and their specific surface area (758 m2 g−1) is comparable to that of conventional graphene generated at 1050 °C at atmospheric pressure (700 m2 g−1). The in situ thermal reduction during the exfoliation process was confirmed by the increased C/O atomic ratio compared to that of graphite oxide. The restoration of the graphitic sp2 network makes it highly efficient in improving the electrical conductivity of polymers at a low graphene loading.

Raman study of interfacial load transfer in graphene nanocomposites

Abstract: We tracked the strain-sensitive characteristic Raman G-band shift of graphene platelets in polydimethyl-siloxane (PDMS) nanocomposites revealing the filler-to-matrix interactions. We obtained large debonding strains of ∼7% for graphene in PDMS, with the peak shift rate with strain being ∼2.4 cm−1/composite strain % in comparison to single-walled carbon nanotube composites, where a relatively low rate of ∼0.1 cm−1/composite strain % was obtained, suggesting enhanced load-transfer effectiveness for graphene. A surprising observation was that for large strains (>1.5%) the graphene fillers went into compression under uniaxial tensile deformation and vice versa. We propose that this effect is related to the high mobility of the PDMS chains at room temperature.

Graphene Colloidal Suspensions as High Performance Semi-Synthetic Metal-Working Fluids

Abstract: We report the use of graphene as an additive to improve the lubrication and cooling performance of semisynthetic metal-working fluids (MWFs) used in micromachining operations. Microturning experiments were conducted in the presence of MWFs containing varying concentrations of graphene platelets. Graphene-based MWF formulations performed significantly better as compared to conventional MWFs. Moreover, an analysis of the trends in the cutting forces and cutting temperatures, taken in conjunction with the trends in the wetting ability, thermal conductivity, and kinematic viscosity of the modified MWFs, establishes graphene as a superior additive over both single and multiwalled carbon nanotubes. The superior performance of graphene is attributed to the increased wettability of the cutting fluid that allows for penetration of the graphene platelets into the tool−workpiece interface. Once in that interface, the graphene platelets provide efficient lubrication because of the relative sliding of graphene layers ...

Dispersion of graphene oxide and its flame retardancy effect on epoxy nanocomposites

Abstract: Graphene oxide was prepared by ultrasonication of completely oxidized graphite and used to improve the flame retardancy of epoxy. The epoxy/graphene oxide nanocomposite was studied in terms of exfoliation/dispersion, thermal stability and flame retardancy. X-ray diffraction and transmission electron microscopy confirmed the exfoliation of the graphene oxide nanosheets in epoxy matrix. Cone calorimeter measurements showed that the time to ignition of the epoxy/graphene oxide nanocomposite was longer than that of neat epoxy. The heat release rate curve of the nanocomposite was broadened compared to that of neat epoxy and the peak heat release rate decreased as well.