Tikalon Blog by Dev Gualtieri

Graphene-Copper Heat Sinks

March 26, 2014

Diamond has a very high room temperature thermal conductivity, 22 W/(cm-K), which is five times greater than that of the excellent thermal conductor, copper.[1] If you want to go to extremes, diamond formed exclusively from the isotope ¹²C has a theoretical thermal conductivity at 80 kelvin of about 2000 W/(cm-K).[2] Isotopic purity has this effect, since it reduces the scattering of phonons, the carriers of heat in solids.

Diamond's high thermal conductivity is a consequence of its strong covalent bonding which is efficient at coupling the lattice vibrations responsible for heat transfer across large volumes of a crystal. This type of bonding is present, also, in the carbon single layer sheets of graphene, which has a thermal conductivity of about 50 W/(cm-K).[1]

A cubic zirconia gemstone.

Cubic zirconia resembles diamond since it has a nearly identical refractive index, but it has a much smaller thermal conductivity.[3]

(Photo by Gregory Phillips, via Wikimedia Commons).

Diamond is sometimes used as a heatsink material for high power semiconductor devices, such as power transistors and laser diodes. This is now easier to do with the availability of synthetic diamonds. Although the cost of diamond is high, the cost of the devices protected is often high enough to make the use of diamond economical.

It would be useful to exploit the high thermal conductivity of graphene, but the problem here is that atomically thick graphene is generally placed on a substrate, rather than being used as a substrate itself. A team of scientists from the University of California, Riverside (Riverside, California), the University of Manchester (Manchester, UK), and Bluestone Global Tech (Wappingers Falls, New York) have overcome this limitation by demonstrating that deposition of graphene on copper increases copper's thermal conductivity.[4-5] Their results are reported in a recent issue of Nano Letters.[4]

Deposition of graphene on copper is a surprisingly well developed technology. A 2009 study demonstrated growth of centimeter-sized patches of graphene on copper by chemical vapor deposition using methane.[6] In that study, most of the deposited area was single layer graphene with about 5% of the area being multiple layers. Since carbon has a low solubility in copper, the process was self-limiting.[6]

Since I'm an experimentalist, I enjoy images of how experiments are conducted.

Here, copper foil is being placed into a fixture prior to chemical vapor deposition of graphene.

(University of California, Riverside, image.)[5]

In the experiments reported in Nano Letters, a single atomic layer of graphene was deposited by chemical vapor deposition on both sides of 9 μm thick copper foils. The resultant composites showed an enhanced thermal conductivity at room temperature of up to 24%.[4-5] This was surprising, since the graphene added just two atomic layers to a copper substrate having thousands of atomic layers of copper.

As they found, it wasn't the graphene itself adding to the thermal conductivity, but a change in the microstructure of the copper. The high temperature chemical vapor deposition of graphene stimulated grain growth in the copper foil (see figure). Larger grains lead to better thermal conductivity.[4-5] This effect was more pronounced for thinner copper foils, so there's the possibility for the formation of highly thermally conductive layers at thenanometer level.[5]

Photomicrographs of graphene layers on copper: Copper before thermal processing (left), copper after thermal processing (center), and copper after graphene deposition (right). The large copper grains are apparent. (University of California, Riverside, image.)[5]

The research team plans to investigate graphene on nanometer thick copper substrates and develop a theoretical model for the enhancement.[5] Says Alexander A. Balandin, Director of the UC-Riverside Nano-Device Laboratory,

"This enhancement of copper's ability to conduct heat could become important in the development of hybrid copper — graphene interconnects for electronic chips that continue to get smaller and smaller."[5]

This research was funded by the National Science Foundation, the STARnet Center for Function Accelerated nanoMaterial Engineering, and the Defense Advanced Research Projects Agency (DARPA).[5]

Graphene-Copper Heat Sinks

References: