Thursday, February 25, 2010

Pristine Nano Graphene Platelets Produced by Supercritical Fluid Process

Nanotek Instruments, Inc. (Dayton, OH) scientists Aruna Zhamu and Bor Z Jang have developed a supercritical fluid process for mass producing nano graphene platelets. The process produces pristine or non-oxidized nano graphene platelets (NGPs) that are highly conductive.

According to U.S. Patent Application 20100044646, the process comprises: (i) subjecting a graphitic material to a supercritical fluid at a first temperature and a first pressure for a first period of time in a pressure vessel and then (ii) rapidly depressurizing the fluid at a fluid release rate sufficient for effecting exfoliation of the graphitic material to obtain the NGP material.

Conductive NGPs can be used as a conductive additive in transparent electrodes for solar cells or flat panel displays (e.g., to replace expensive indium-tin oxide), battery and supercapacitor electrodes, and nanocomposite for electromagnetic wave interference (EMI) shielding and static charge dissipation, etc.

The process is superior to many prior art processes in several aspects. Prior art processes based on graphite intercalation/oxidation and high-temperature exfoliation did not allow for a good control over the oxygen content of the resulting GO or NGP platelets. The supercritical fluid process is capable of producing pristine NGPs that have never been exposed to oxidation.

In another commonly used prior art approach, the graphite oxide dispersed in an aqueous solution was reduced with hydrazine, in the presence of a polymer, such as poly (sodium 4-styrenesulfonate). This process led to the formation of a stable aqueous dispersion of polymer-coated graphene platelets. In some applications of NGPs, however, a polymer coating may be undesirable. Furthermore, the commonly used reducing agent, hydrazine, is a toxic substance.  Conventional processes of preparing graphitic oxide (GO) nano sheets that included chemical exfoliation typically were extremely tedious. Such a long process is not amenable to the mass production of conductive nano graphene platelets. The supercritical fluid process is capable of producing NGPs with no or little impurity.

The supercritical fluid process can obviate the need for washing and rinsing the platelets (which was required in the prior art solution approach to the exfoliation of GO and/or subsequent chemical reduction). The supercritical fluid process is fast and environmentally benign. The presently  process is capable of producing ultra-thin NGPs, including those that are single graphene sheets. This process allows for concurrently attaching a desirable functional group to the resulting NGPs (e.g., by simply introducing a desirable chemical species, such as SO3, into the supercritical fluid). This is a powerful approach to varying the dispersibility or solubility of NGPs in a solvent.

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