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Rutgers discovery could revolutionize how computers stay cool

Moore’s Law states that the number of transistors on an integrated circuit — what most people refer to as computer chips — will double every year. 

As the number of transistors on a chip increase so does the amount of heat generated by the computer chip. Removing excessive heat is an ongoing problem, but a recent discovery by a team based partly at Rutgers has found a new solution to this issue.

Using graphene – a single molecular layer of carbon – on a substrate of hexagonal boron nitride has proven to be extremely efficient at removing heat from computer chips, said Eva Andrei, an author of a paper published last year in the Proceedings of the National Academy of Sciences.

“One of the main issues in computers (central processing units) is the transistors generate heat, and when the device heats up, it no longer operates properly,” said the professor of the Department of Physics and Astronomy.

How are computers normally cooled?

Desktop computers and laptops use fans or water-cooling systems to remove excess heat from their CPUs, Andrei said. This type of solution, while adequate for personal computers, is not that efficient, so her team looked at removing heat directly from the chips that go inside the CPU.

“These solutions are kind of marginal at this point so people got more and more interested in going to on-chip solutions where you have refrigeration right on the chip, so you have the ability to carry away the heat,” she said. “That’s where thermoelectrics come in.”

Existing methods for cooling computers include diffusion cooling, convective cooling and thermoelectric cooling, she said.

Diffusion cooling uses a material like copper to carry heat away. Convective cooling uses fans or liquids to remove heat. Neither of these methods are particularly efficient, she said, and convective cooling relies on mechanical parts which are prone to breaking.

“In thermoelectric cooling, you use electrical current to carry away the hot electrons from the heat source and replace with cold electrons,” she said. “You cool with current, and that is what we did on the chip.”

What is boron nitride?

Andrei said her team’s counterparts in Japan initially discovered that using a material called boron nitride would help graphene transfer heat at a much more efficient rate than existing materials.

“It’s a very good insulator … it does not interfere with the electronic properties of graphene as most other substrates would,” she said. “So we could carry the heat away in a much more efficient manner than having to scatter off the substrate.”

To put a cooling system on a computer chip, graphene is placed onto a substrate, Andrei said. This allows a heat transfer process to occur through the graphene, which removes the heat generated by the chip from the area.

Normally a material like silicon dioxide would make up the substrate, but this is a relatively rough material, she said. Boron nitride is a much smoother substance.

If the heat transfer process was similar to sledding down a hill, existing thermoelectric conductors on computer chips would be like a grassy, rocky hill. While the sled – in this case, heat – will still go from the top of the hill to the bottom, the process is not smooth or efficient.

Andrei’s team essentially put ice on the hill, allowing for a much smoother – and faster – ride.

She said using boron nitride has created a heat transfer process nearly twice as efficient as the current leading industry material.

“This chip is designed to cool, to carry away heat, which is a major problem in the electronics industry,” she said. “And if you combine it with transistors you can make a CPU where you can cram in more components in the same volume without burning itself out.”

Boron nitride is not a new or innovative material. Cosmetic manufacturers actually sell it in makeup kits – boron nitride crystals are used to make glitter, she said.

Commercially available boron nitride generally consists of defective crystals though, and would not be usable in computer chips.

“Some groups actually purchase large amounts of this material,” she said. “The quality doesn’t come close (but) one can get it commercially in all sorts of places. The boron nitride has to be good quality for it to perform.”

How does this cooling process work?

One of the aspects of this combination of materials is it can efficiently perform both active and passive cooling at the same time, Andrei said.

Active cooling occurs when an electric current flows through a pair of conductors, removing heat from the area, while passive cooling relies on heat transferring to the graphene itself directly, without using an electric current.

In other words, an electrical current flowing through graphene carries heat away, while the thermal properties of graphene itself also allow heat to transfer through it.

“We’ve been studying the properties of graphene for some time, and graphene being two-dimensional, it’s extremely sensitive to the environment,” Andrei said. “In order to realize the wonderful properties that two-dimensional materials have you have to think about isolating them from external influences, so one of these things is what do you put them on.”

Chips with the graphene/hexagonal boron nitride cooling system can be as small as 500 micrometers, or roughly one-millionth the thickness of a piece of paper, she said.

What's next?

Andrei said her team is looking to improve upon boron nitride by examining other materials in an effort to find an even more efficient one.

“We were pretty excited about (this discovery), we didn’t think that this was such a big deal until we saw the reactions,” she said. “It’s pretty exciting, usually we look at fundamental properties and they are pretty far from applications, but it’s pretty cool when you can present something that will become useful.”

Nikhilesh De is a correspondent for The Daily Targum. He is a School of Arts and Sciences senior. Follow him on Twitter @nikhileshde for more.

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