21 February 2012
Researchers from the University of California at Riverside, US, say that they have developed a new “thermal interface material” (TIM) that could efficiently remove unwanted heat from electronic components such as computer chips or light-emitting diodes. The material is a composite of graphene and multilayer graphene.
Unwanted heat is a big problem in modern electronic systems that are based on conventional silicon circuits – and the problem is getting worse as devices become ever smaller and more sophisticated. TIMs are positioned between a heat source – such as a computer chip – and a heat sink, and they play a crucial role in cooling devices. Conventional TIMs are generally filled with thermally conducting metal particles and have thermal conductivities in the range 1–5 Wm–1 K–1 at room temperature. A high volume fraction (of more than 50%) of filler particles is usually needed to achieve such conductivities.
Graphene could be ideal for use as a filler in TIMs for carrying away heat because pure graphene has a large intrinsic room-temperature thermal conductivity that lies in the range 2000–5000 Wm–1 K–1. These values are higher than those of diamond, the best bulk-crystal heat conductor known.
Ideally, for practical applications, researchers would like to make TIMs with thermal conductivities of about 25 Wm–1 K–1. Such materials would be used to not only efficiently cool digital electronic components, but also in energy applications – for example, to prevent solar cells from overheating – and in next-generation high-power-density communication devices.
Alexander Balandin and colleagues proposed the use of few-layer graphene as a TIM in 2010. Now, they have 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 about 5.8 Wm–1 K–1 to a record 14 Wm–1 K–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.
According to the team, it is the presence of single and bilayer graphene together with thicker graphitic multilayers that enhances 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 about thanks to the smaller ‘Kapitza’ thermal-interface resistance between graphene and the base matrix material,” says Balandin. “Graphene simply couples to the matrix material better.”
The experiments 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, such as 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. “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 explains.
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,” says Balandin.
The work was reported in Nano Letters.