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Changing the dynamics of bulk materials

Tue, 10/09/2012 - 8:16am

Lawrence Livermore National Laboratory researchers have developed a new bulk material whose physical properties can be dynamically changed by an external signal.

The scientists came up with a method to fabricate mass-producible, graphene-based bulk materials from low-cost, polymer-derived carbon foams by selectively removing carbon atoms from a network composed of both unstructured carbon and graphite nanoplatelets.

"The new technique is inexpensive, scalable, and yields mechanically robust, centimeter-sized monolithic samples that are composed almost entirely of interconnected networks of single-layer graphene nanoplatelets" says Ted Baumann of Lawrence Livermore who developed the synthetic approach.

These graphene bulk materials have an ultrahigh surface area and may thus be used for energy storage systems such as super-capacitors where energy is stored by polarization of the graphene electrode/electrolyte interface.

Graphene bulk material also could be used as an electrically conductive network to support the active material in battery applications. Desalination using capacitive desalination is another emerging field.

The advantage of using bulk materials versus composite materials (made from porous carbon particles and a binder) is their superior stability, which allows for longer lifetimes, higher conductivity (less losses during charging and discharging), and the ability to tune the pore structure.

"This is a potentially game changing concept in materials science," says Juergen Biener, lead LLNL author of the article in Advanced Materials. "Just imagine what you could do with a bulk material with properties you can change dynamically by an external variable. For example, you could switch a bulk material dynamically between a conductive and an insulating state."

The specific surface area of this 3D nanographene bulk material is comparable to that of a free-standing graphene layer, but it has an open porosity that allows rapid mass transport through the material.

Most graphene based bulk materials are made by self-assembly of graphene oxide, which is still very expensive and costs up to several hundred dolloars per gram. At this price, it is not economical to use graphene based bulk materials for energy storage even though they have excellent properties for this application. Biener said. By contrast, the Livermore technique of making graphene based bulk materials is inherently inexpensive (only a few dollars per kilogram), scalable, and yields mechanically robust, centimeter-sized monolithic samples. "That is a major breakthrough toward applications," Biener says.

The group has tested the new technique by making large pieces of the material, and tested actuator and the tunable resistor applications.

Source: Lawrence Livermore National Laboratory

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