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Herein we report a rapid, chemical free and low temperature method for high throughput production of graphene by exfoliation of graphite oxide using focused solar radiation. The simple method of preparation, easy scalability, cost effectiveness and high electrical conductivity (1.5 Â 10 3 S m À1 ) of solar graphene hold great promise for true practical applications.

A solution-free green method using focused solar electromagnetic radiation is used to synthesize graphene and graphenemultiwalled carbon nanotube (MWNT) composite. Stable nanofluids are prepared by dispersing the nanomaterials in polar base fluids. Thermal conductivity of the nanofluids improves with graphene-MWNT nanocomposites as additives, which could be due to prevention of restacking of graphene sheets by MWNT along with a synergistic effect of intrinsic high thermal conductivity of graphene and MWNT. The advantage of the present synthesis method in particular to nanofluids application is that the presence of oxygen functional groups resulting from prefunctionalized MWNT rules out the need for functionalizing the hybrid composite again, thereby preserving the high thermal properties of graphene. Thermal conductivity enhancement of 9.2% and 10.5% is obtained with graphene and graphene-MWNT nanofluids in de-ionized water at room temperature for 0.04% volume fraction. The high thermal transport characteristics of graphene-MWNT composite nanofluids is ascribed to the high aspect ratio of MWNT and graphene, which in turn can form tightly bonded clusters and, by suppressing the interface resistance, can become excellent additives to attain high thermal conductivity. Further, an enhancement in heat-transfer coefficient of 193% at Reynolds number 2000 for 0.02% volume fraction of aqueous graphene-MWNT nanofluids suggests the potential application of the present hybrid material-based nanofluids in cooling circuits.

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