Anisotropic heat conductivity in graphene films

Reducing graphene oxide to graphene via a high-temperature treatment increases thermal conductivity along the film direction, while decreasing it across the film. Scientists from UC Riverside, Moldova State University, and Graphenea see the potential of this new method in thermal management.

Graphene possesses an extremely high thermal conductivity along the sheet plane, making it a candidate for thermal management applications, for example as a filler in thermal interface materials, or as a flexible heat spreader for cooling today's demanding electronics. One of the most researched forms of graphene for industrial applications is graphene oxide (GO), a material produced with scalable and rather inexpensive methods. Having demonstrated excellent mechanical properties, GO has trailed behind other forms of graphene in electrical performance and in thermal conductivity. Current research shows that thermal conductivity of GO can be increased nearly 30 times by bringing GO to a high temperature during a reduction process.

The research team, led by Prof. Alexander Balandin of the Nano-Device Laboratory at University of California Riverside, in their paper entitled “Strongly Anisotropic Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films Annealed at High Temperature” recently published in Wiley's Advanced Functional Materials, present the discovery 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. This seemingly confusing conclusion is elegantly explained with detailed characterization and theoretical support.

Image: Sketch of air pockets impeding heat transport perpendicular to the graphene sheets, and a SEM image of the same.

The first observation that the scientists made was 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 such obtained rGO is exceptionally 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.