Unwanted heat is a big problem in modern electronics based on conventional silicon circuits – and the problem is getting worse as devices become ever smaller and more sophisticated. Graphene could be ideal for use as a filler material in TIMs for carrying away heat because pure graphene has a large intrinsic room-temperature thermal conductivity that lies in the 2000–5000 Wm–1K–1 range. These values are higher than those of diamond, the best bulk crystal heat conductor known.

TIMs are applied between a heat source – for example a computer chip – and a heat sink and play a crucial role in cooling down devices. Conventional TIMs are generally filled with thermally conducting metal particles, and have thermal conductivities in the 1–5 Wm–1K–1 range at room temperature. A high volume fraction (of more than 50%) of filler particles is usually needed to achieve such conductivities.

Ideally, for practical applications, researchers would like to make TIMs with thermal conductivities of about 25 Wm–1K–1. Such materials would be used to not only efficiently cool down digital electronic components but in energy applications as well – for example to prevent solar cells from overheating – and in next-generation high-power-density communication devices.

Alexander Balandin and colleagues have now succeeded in increasing the thermal conductivity of a routinely employed industrial epoxy-resin-based TIM, or "grease" as it is better known in the industry, from around 5.8 Wm–1K–1 to a record 14 Wm–1K–1. The filler particles in this case consist of an optimized mixture of graphene and few-layer graphene and the volume fraction of the carbon-based material in the epoxy is very low at just 2%.

The researchers prepared their own graphene and few-layer graphene using an inexpensive and simple liquid-phase exfoliation technique. This is a high-yield method that can easily be scaled up to industrial levels.

Smaller 'Kapitza' thermal interface resistance

According to the team, it is the presence of single and bilayer graphene together with thicker graphitic multilayers that enhance the thermal conductivity of the composite to the values observed. "The excellent performance of graphene in this respect – compared with, say carbon nanotubes, for instance – probably comes thanks to the smaller 'Kapitza' thermal interface resistance between graphene and the base matrix material," Balandin told nanotechweb.org. "Graphene simply couples to the matrix material better."

The results show that graphene and few-layer graphene flakes are more efficient filler materials for increasing the thermal conductivity of TIMs than conventionally used fillers, such as alumina particles. The new graphene-based fillers are also different to previously tested materials, like carbon nanotubes or graphitic nanoplatelets because these materials only weakly couple to the matrix.

Exploiting nanoscale effects

Balandin says that he has been studying the thermal properties of nanostructures – including extremely thin films and nanowires – for nearly 15 years now. "My motivation was to exploit nanoscale effects to control the propagation of phonons – vibrations of the crystal lattice responsible for heat conduction in many materials," he explained.

The team now plans to work with industry engineers to develop the next generation of TIMs, which could well be based on graphene. "These would have to meet the specific requirements of different applications," said Balandin.

The current work was reported in Nano Letters.