Rethinking heat for new types of chip cooling
Several new types of cooling are emerging with lasers and phonons to tackle the challenge of heat in powerful AI datacentre chips.
Sandia National Laboratories in the US is working with Minnesota-based startup Maxwell Labs on laser-based photonic cooling. This may seem counter intuitive, as lasers are used to heat things up. But they can be used at a particular light frequency matched with a very small, very pure target of a specific element. For example lasers are used in some quantum sensors and computers to hold individual atoms at ultra-low temperatures.
This can be used for GPUs in an AI datacentre if the laser light can be focused on small, localized hot spots.
“About 30 to 40 percent of the energy data centres use is spent on cooling,” said Raktim Sarma, the lead Sandia physicist on the project. He added that in some communities, the amount of water needed can strain local resources. “We really only have to cool down spots that are on the order of hundreds of microns,” said Maxwell Co-Founder and Chief Growth Officer Mike Karpe.
The idea is to use a photonic cold plate to either replace or complement water and air based cooling systems, which also allows for the resulting extracted heat in the form of light to be recycled and turned back into electricity.
The Maxwell cold plate would be a light-based variation, designed with gallium arsenide features a few microns in size to channel the laser light to localized hot spots. Because laser light will heat up impurities, erasing any cooling effect, the cold plate needs to have extremely pure, thin layers of crystalline gallium arsenide as the epitaxial layers,
Modelling indicates a laser-based cooling system can keep chips colder than water-based systems. A research agreement will see Maxwell Labs will generate the technical designs and Sandia build the devices.
Upgrading phonons
Another approach is to use a new type of phonons, or acoustic waves, to carry thermal energy out of the chip.
A team at the University of Virginia (UVA) are using hexagonal boron nitride (hBN) to move heat like a beam of light.
“We’re rethinking how we handle heat,” said Patrick Hopkins, professor of mechanical and aerospace engineering and Whitney Stone Professor of Engineering at UVA. “Instead of letting it slowly trickle away, we’re directing it. This could change how we design everything from processors to spacecraft.”
Instead of relying on slow-moving heat vibrations called phonons, the team used hyperbolic phonon-polaritons (HPhPs) that can carry heat at extraordinary speeds.
The team’s method transforms heat into tightly channeled waves that travel efficiently across long distances. Heating a tiny gold pad sitting on the hBN turns the energy into fast-moving polaritonic waves that instantly carried the heat away across and away from the interface between the gold and hBN.
“This method is incredibly fast,” said Will Hutchins, a mechanical and aerospace engineering Ph.D. candidate at UVA (above). “We’re seeing heat move in ways that weren’t thought possible in solid materials. It’s a completely new way to control temperature at the nanoscale.”
Meanwhile in New York, other researchers are also using these hyperbolic phonon-polaritons.
The team at the Advanced Science Research Centre at the CUNY Graduate Centre (CUNY ASRC) at ETH in Zurich are using a thin layer of graphene sandwiched between two hexagonal boron nitride (hBN) slabs.
“One major challenge is that exciting and detecting phonon-polariton waves is both expensive and inefficient, typically involving costly mid-infrared or terahertz lasers and near-field scanning probes,” said Qiushi Guo, a professor with the CUNY ASRC’s Photonics Initiative. “We wanted to explore whether we could emit phonon-polaritons using just an electrical current, similar as how semiconductor lasers or LEDs work.”
Graphene is well known for its high electron mobility at room temperature. When encapsulated by hBN slabs, its mobility is further enhanced due to surface passivation and reduced impurities. “This means that when a current passes through the graphene encapsulated by hBN slabs, electrons in graphene can be accelerated to very high speeds and efficiently scatter with HPhPs in hBN,” said Guo.
This achieved the emission of HPhPs when applying a modest electric field of just 1 V/µm to the graphene, highlighting the efficiency of HPhP electroluminescence. The study provides the first experimental demonstration of exciting phonon polariton waves exclusively through electrical methods.
Specifically, the team identified two possible pathways for HPhP emission. “When the electron concentration in graphene is low, HPhPs are emitted through interband transitions. However, at higher electron concentrations, HPhP emission occurs through both interband transitions and intraband Cherenkov radiation in graphene”, said former Caltech postdoc Iliya Esin, now an assistant professor of physics at Bar Ilan University, Israel.
During the HPhPs electroluminescence, hot electrons in graphene rapidly lose their excess kinetic energy for more efficient heat dissipation in electronic devices, says Guo.
www.cuny.edu; www.virginia.edu; www.sandia.gov; www.mxllabs.com;
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