A collaborative research performed by scientists from UC Riverside, Moldova State University, and Graphenea demonstrates that a method of reducing graphene oxide to graphene via a high-temperature treatment that increases thermal conductivity along the film direction, while decreasing it across the film. The scientists stress the potential of using this method for thermal management applications, such as fillers in thermal interface materials or flexible heat spreaders for cooling electronics.
The research shows that thermal conductivity of GO can be majorly increased (nearly 30 times) by bringing GO to a high temperature during a reduction process. It appears that GO, when heated to 1000°C, turns to reduced GO (rGO) that has a high thermal conductivity along the sheet plane. In contrast, thermal conductivity perpendicular to the sheet shows an opposite trend, decreasing with thermal treatment.
The scientists observed that the film swelled when heated, its thickness increasing more than four times. Dissecting the film and looking from the side, the researchers discovered that the rGO is infiltrated by air pockets longitudinal to the film. Hypothesizing that the air pockets provide thermal insulation across the film, the scientists developed a theoretical framework which supports this finding at several different annealing temperatures. Heat flows uninhibited in the film plane, however, and heat transfer is further enhanced by the strengthening of carbon bonds and the chemical purification induced by the annealing.
The anisotropy of heat conductivity in the resulting rGO is very high even when compared to high-quality graphite. As such, this new material holds much promise for thermal management applications where one would want to remove excess heat along one direction while shielding from heat along the perpendicular direction.