Researchers created a "process friendly" technique that would allow microprocessors to cool using carbon nanotubes.
The researchers used organic molecules to form covalent bonds between metal surfaces and the carbon nanotubes; this proved to improve the flow of heat to the nanotubes six-fold, which would allow a microprocessor chip to cool much more quickly, a Lawrence Berkeley National Laboratory news release reported.
"We've developed covalent bond pathways that work for oxide-forming metals, such as aluminum and silicon, and for more noble metals, such as gold and copper," Frank Ogletree, a physicist with Berkeley Lab's Materials Sciences Division, said in the news release. "In both cases the mechanical adhesion improved so that surface bonds were strong enough to pull a carbon nanotube array off of its growth substrate and significantly improve the transport of heat across the interface."
Microprocessors are prone to overheating; this can cause them to no longer have the ability to function as transistors. Modern technology requires microprocessor chips to hold more and work faster than ever before, which is only exacerbating the problem.
"The first challenge is to conduct heat out of the chip and onto the circuit board where fans and other techniques can be used for cooling," the news release reported.
Nanotubes are ideal for conducting heat, but using them for this purpose has been tricky in the past due to high thermal interface resistances within the systems.
"The thermal conductivity of carbon nanotubes exceeds that of diamond or any other natural material but because carbon nanotubes are so chemically stable, their chemical interactions with most other materials are relatively weak, which makes for high thermal interface resistance," Ogletree said. "Intel came to the Molecular Foundry wanting to improve the performance of carbon nanotubes in devices. Working with Nachiket Raravikar and Ravi Prasher, who were both Intel engineers when the project was initiated, we were able to increase and strengthen the contact between carbon nanotubes and the surfaces of other materials. This reduces thermal resistance and substantially improves heat transport efficiency."
The team used reactive molecules to "bridge the carbon nanotube/metal interface - aminopropyl-trialkoxy-silane (APS) for oxide-forming metals, and cysteamine for noble metals." They grew vertically aligned carbon nanotubes on silicon wafers and evaporated aluminum or gold on glass microscope cover slips; this caused the metal films to become "functionalized" allowing them to bond with the carbon nanotubes. This effectively enhanced heat flow.
"You can think of interface resistance in steady-state heat flow as being an extra amount of distance the heat has to flow through the material," Sumanjeet Kaur, lead author of the paper, said in the news release. "With carbon nanotubes, thermal interface resistance adds something like 40 microns of distance on each side of the actual carbon nanotube layer. With our technique, we're able to decrease the interface resistance so that the extra distance is around seven microns at each interface."
The experiment succeeded in strengthening the contact between the metal and carbon nanotubes, but there is still a chance that connection could fail. The researchers' next steps will be to improve the density of the carbon nanotubes and metal surfaces.
"Part of our mission at the Molecular Foundry is to help develop solutions for technology problems posed to us by industrial users that also raise fundamental science questions," Ogletree said. "In developing this technique to address a real-world technology problem, we also created tools that yield new information on fundamental chemistry."
