A graphene coating has been used to boost thermal conductivity of the common plastic polyethylene terephthalate (PET) by up to 600 times. This new result from an international team of physicists and engineers could substantially increase the use of PET and other plastics in technologies such as solid-state lighting and electronic chips, where the ability to conduct heat is essential.

PET is a widely used plastic that will be familiar to anyone who has bought a bottle of water or soft drink. It is low-cost, strong, durable and recyclable, and it can be moulded into just about any shape. Fibres of the plastic are also used to make fabrics such as polar fleece. While PET's low thermal conductivity makes it ideal for warm clothing, its inability to transfer large amounts of heat precludes its use in electronics and other devices where getting rid of heat is important.

Graphene is a sheet of carbon just one atom thick, and has an exceptionally high thermal conductivity of about 2000–5000 W/mK near room temperature – compared with about 0.2 W/mK for PET. Graphene's thermal conductivity will drop when it is placed on a substrate, because heat-carrying lattice vibrations are scattered by interactions with the substrate. However, the thermal conduction of the graphene layer will still remain high, relative to most other materials.

Graphene flakes

Now a team led by Alexander Balandin at the University of California, Riverside and Konstantin Novoselov at the University of Manchester has used graphene flakes to create films just a few microns thick onto a thin PET substrate. The researchers then showed that the presence of the graphene gives the composite material a much greater thermal conductivity than PET alone.

The researchers used a non-contact optothermal Raman technique for their thermal measurements. In this method, the micro-Raman spectrometer is used as a sort of thermometer to measure temperature changes in the sample, and the laser that performs the Raman measurements is also used to heat the sample. The technique was developed in Balandin's lab, where it was used to discover the exceptionally high thermal conductivity of graphene in 2008 (see "Graphene continues to amaze").

Team member Hoda Malekpour, a PhD student in Balandin's group, was responsible for making Raman measurements. Balandin explains: "Our results reveal that the thermal conductivity of PET increases by up to 600 times when it is coated with the graphene laminate films." This gives the laminates a similar thermal conductivity to metals such as iron and lead, approaching that of silicon.

Drastic improvement

Balandin adds: "The thermal conductivity of PET on its own is very low – in the 0.15–0.24 W/mK range at room temperature – and other plastic materials are also poor conductors of heat. This drawback prevents plastics from being employed in many applications that could benefit from their low cost, durability and light weight. Our work proves that a few micron-thick graphene layers deposited on plastic films can drastically improve the way they conduct heat, and so now make such applications possible."

The team, which includes scientists from Riverside, Manchester, Bluestone Global Tech in New York and Moldova State University, used a fairly simple theoretical model in this work to explain how the thermal conductivity of graphene laminates depends on graphene flake size and impurity concentrations. "We would now like to develop a more detailed model based on multi-scale simulations of heat transport in grapheme, to optimize its use as a coating material in thermal management applications," says Balandin.

The research is described in Nano Letters.