NASA is using carbon nanotubes to cut thermal resistance, aiming to improve heat flow between chips and heatsinks. Li’s alternative uses a form of nanotube called carbon nanofibres in the interface. With two [plain] pieces of silicon, the interface resistance is about 2cm²K/W which can be improved with grease or solder. People are shooting for less than 0.5cm²K/W; and 0.2 for the next generation. We can achieve 0.2 already and it is under-optimised.”
The NASA interface relies on the incredible thermal conductivity of carbon nanotubes. “There is a lot of interest in this thermal conductivity. There is literature to say it is as high as diamond. But it is highly anisotropic,” said Li. Along the length of the tube it has diamond-like thermal conductivity, but heat does not pass into the side of the tube easily. “If nanotubes are packed together like spaghetti, thermal conductivity is very poor.