The remarkable mechanical properties of carbon nanotubes arise from the exceptional strength and stiffness of the atomically thin carbon sheets (graphene) from which they are formed. In contrast, bulk graphite, a polycrystalline material, has low fracture strength and tends to suffer failure either by delamination of graphene sheets or at grain boundaries between the crystals. Now Stankovich et al. have produced an inexpensive polymer-matrix composite by separating graphene sheets from graphite and chemically tuning them. The material contains dispersed graphene sheets and offers access to a broad range of useful thermal, electrical and mechanical properties. Individual sheets of graphene can be readily incorporated into a polymer matrix, giving rise to composite materials having potentially useful electronic properties. Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects1,2,3; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties4,5,6,7,8. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite9 and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold10 of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes11; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications12. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Graphene-based composite materials

Graphene sheets offer extraordinary electronic, thermal and mechanical properties and are expected to find a variety of applications. A prerequisite for exploiting most proposed applications for graphene is the availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. Here we report that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization. This discovery has enabled us to develop a facile approach to large-scale production of aqueous graphene dispersions without the need for polymeric or surfactant stabilizers. Our findings make it possible to process graphene materials using low-cost solution processing techniques, opening up enormous opportunities to use this unique carbon nanostructure for many technological applications.

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Processable aqueous dispersions of graphene nanosheets

The reduction of graphene oxide

Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

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Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors

Flexible graphene films were prepared by the filtration of water-soluble noncovalently functionalized graphene sheets with pyrenebutyrate. The work presented here will not only open a new way for preparing water-soluble graphene dispersions but also provide a general route for fabricating conducting films based on graphene.

Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets

Unilamellar colloids of graphite oxide (GO) were prepared from natural graphite and were grown as monolayer and multilayer thin films on cationic surfaces by electrostatic self-assembly. The multilayer films were grown by alternate adsorption of anionic GO sheets and cationic poly(allylamine hydrochloride) (PAH). The monolayer films consisted of 11−14 A thick GO sheets, with lateral dimensions between 150 nm and 9 μm. Silicon substrates primed with amine monolayers gave partial GO monolayers, but surfaces primed with Al13O4(OH)24(H2O)127+ ions gave densely tiled films that covered approximately 90% of the surface. When alkaline GO colloids were used, the monolayer assembly process selected the largest sheets (from 900 nm to 9 μm) from the suspension. In this case, many of the flexible sheets appeared folded in AFM images. Multilayer (GO/PAH)n films were invariably thicker than expected from the individual thicknesses of the sheets and the polymer monolayers, and this behavior is also attributed to folding...

http://dns2.asia.edu.tw/~ysho/YSHO-English/1000%20WC/PDF/Che%20Mat11,%20771.pdf

Layer-by-Layer Assembly of Ultrathin Composite Films from Micron-Sized Graphite Oxide Sheets and Polycations

An approach has been developed to construct functional expressions for calculating the effective mechanical characteristics of the undermined rock massif during its repeated mining, taking into account the disturbance of the different-type massif continuity and the time passed after the undermining. The approach was developed as applied to the conditions of the Starobin potash salt deposit. It is based on introducing special correction factors into the expressions for mechanical characteristics of the massif. At the same time, the state of the undermined massif area is considered in the continuum model approximation. It is shown that one of the most important factors to be considered when constructing the functional dependence for mechanical characteristics of the undermined layered massif is to take into account the mutual slippage of layers and their lamination related to it, because the strength characteristics such as bonding strength and internal friction coefficient mainly change when the massif is undermined. The algorithm for calculating the mechanical properties of the undermined massif proposes the use of correction factors that take into account the heterogeneity of the rock massif; lamination and slippage of the contacting layers; changes in the properties of the undermined massif with variation of the depth of repeated mining; changes in the properties resulting from the technological disturbance of the massif initial equilibrium state (primary undermining, time passed since the primary undermining).

Calculation of the effective mechanical characteristics of the undermined rock mass

The disadvantages of the traditional description of a mechanical engineering product are considered, which are based on features focused on single, typical and group operations of the technological process. It is proposed to design and manufacture individual elements of parts and the product as a whole from the standpoint of operation and monitoring of its condition using the modular approach in engineering and technology. The representation of parts by sets of modules makes it possible to identify the locating, working and binding modules of the surfaces and on their basis to build a unified classification of parts focused on various stages of the product life cycle. The fundamentals of the locating theory are updated, taking into account the revision of the existing description from theoretical mechanics by means of the position of 3 coordinate points; for the transition to a new description from the theory of machines and mechanisms by depriving of object of locating the degrees of freedom (mobility) in the technical system (design, technological, measuring one). It is shown how the design of the production technology of the product and the measurement of its characteristics is carried out in a system that includes equipment and facilities as mechanisms, the closing link of which are locating items.

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Sergey A. Chizhik

Papers

Layer-by-Layer Assembly of Ultrathin Composite Films from Micron-Sized Graphite Oxide Sheets and Polycations

Calculation of the effective mechanical characteristics of the undermined rock mass

Scientific and methodological foundations of the locating theory actualization based on the degrees of freedom analysis of the locating object