Graphene Nano-Chimneys Could Cool Future Circuits

Rice University students have determined that chimney-like structures made of carbon nanotubes could be the key to the future of heat dissipation.

Heat Dissipation Desperation

Heat dissipation can be very problematic for design engineers because of the issues that heat can bring about. For example, when lithium ion batteries get hot they can undergo a destructive cycle called thermal runaway where the increase in battery temperature makes the battery even hotter.
As silicon devices get hot, they can also fail due to thermal runaway because when the temperature of a semiconductor material such as silicon increases, so does the number of free electrons. These free electrons reduce the resistance of the semiconductor which allows for more current to flow (and this leads to a further increase in temperature).

To combat heat problems, designers must incorporate methods of removing heat by exploiting the three main forms of heat transfer: conduction, convection, and radiation.

Conventional cooling methods typically rely on bulky pieces of material - Image courtesy of FancyCrave

The main problem with heat removal technologies is their size and energy costs. Removing heat from devices such as CPUs and GPUs typically relies on large heavy heatsinks with either fans or water pump systems for cooling. While this does the job, the weight of the device and the energy consumption can be significantly increased.

This is why many engineers try to use ICs and components that use as little energy as possible (less energy consumed = lower temperature). Therefore, one way to keep devices cool is for component producers to design components that can efficiently remove their heat without the need for external cooling (or at least, lower cooling requirements). Components with the ability to cool themselves could be achieved with a new idea that has been theorized by students from Rice University.

Carbon Chimneys

Carbon structures such as graphene and nanotubes have incredible heat dissipation capabilities (with researchers showing how carbon nanotubes might be used to help dissipate heat in future semiconductors). However, no matter how good your heat dissipating materials are, they will always be as bad as the weakest link (this is why materials such as thermal paste are needed between heatsinks and semiconductors).
The weakest link typically is either a filling material (such as the epoxy found in IC packages) or air, both of which are very good insulators of heat. So to help with heat dissipation, this air/material gap needs to be changed. This is exactly what researchers from Rice University have done using computer simulations to create carbon nano-chimneys that can funnel heat efficiently.

Carbon nanotube chimney - Image courtesy of Rice University

Carbon nanotubes grown from graphene have great hydrogen trapping properties but not heat transfer properties. When a tube grows from graphene, heptagonal rings are formed which can dissipate phonons (quasiparticles waves that are the mechanism responsible for heat transfer through a material). The dissipation of phonons is what reduces the ability of the nanotube to dissipate heat efficiently. Researchers instead removed a number of carbon atoms from the graphene sheet which creates cones between the carbon nanotube and graphene sheet.

This conical structure greatly improves heat transfer as it spaces out the heptagonal carbon structures, allowing phonons to travel more easily. Simulation of the conical structures shows that 20-angstrom nano-chimneys are just as efficient as carbon nanotubes in heat transfer and 40-angstrom nano-chimneys are 20% more effective than carbon nanotubes.

"Our interest in advancing new applications for low-dimensional carbon – fullerenes, nanotubes, and graphene – is broad. One way is to use them as building blocks to fill 3D spaces with different designs, creating anisotropic, nonuniform scaffolds with properties that none of the current bulk materials have." – Boris Yakobson, in a press release from Rice University

Summary

While the demonstrations were in simulation only, it does show that carbon nanostructures will have a big role in future devices. If the requirement for growing the cones is the removal of a few carbon atoms then these structures could be easily created with the use of probe tools with the ability to move individual atoms.

Devices are becoming faster and more powerful, encouraging engineers to develop more efficient cooling systems. If these chimneys are as good as they claim and scientists continue to develop better heat dissipation technology, heatsinks could become a device of the past.