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Are Diamonds the Coolest?

It’s not every day you get to hold a diamond whose size is on the order of inches (or cm, for those of you that require simpler math). But there it was, and when I touched it to ice, my fingers went cold in no time.

This surreal flirtation with a vaunted Girl’s Best Friend (thanks to what has to be one of the most successful marketing campaigns ever) came not as I lounged in the VIP room of some swanky event, baubles ablaze, but in the rather pedestrian setting that is the Sensors Expo exhibit hall. I was sitting with Element Six (don’t worry, DeBeers hasn’t lost control of this aspect of diamondry – this is one of their divisions) discussing diamonds’ role in cooling.

We’ve actually covered their diamonds before in the contest of EUV windows. This time the agenda was completely different: They had samples and simple demos showing the effects of diamond being the best known cooling substance. Which is weird: we’ve all experienced thermally-conductive materials before (even if stopping short of licking a flagpole), but the speed at which this stuff works is uncanny. And that’s specifically because most of us have never handled diamonds of this size and shape before, and even if we were lucky enough to be around a multi-carat rock, we typically don’t geek out and say, “Ooo, let me dip that in this here ice water – feel what happens!” Definite date-killer.

In this case, I was able to watch a typical metal coin slowly carve its way through a piece of ice and then repeat with a diamond wafer. The coin looks like it’s working hard to melt the ice (because it is). The diamond goes through as if it were butter.

We normally associate good thermal conductivity with good electrical conductivity, and that’s not an accident: electrons that move easily can also ferry away heat easily. But diamond is an insulator, and it works differently: through phonons. Phonons are a quasi-particle invented to deal with physical vibration, especially when it comes to crystal lattices.

With Rube Goldberg-style mechanical linkages, if there’s lots of play in the joints, then when you move lever A which is loosely connected to rod B and thence sloppily to rod C which is poorly connected to the peg on wheel D, your initial movement gets attenuated by the time it gets to the wheel, and lots of the energy is lost. You work hard just using up that play.

Crystals are the same way: the tighter the bond, the more efficiently they transmit vibration. And this vibration can carry heat away. Of course, imperfections can mess that all up, causing reflections and other interference effects that result in lower thermal conductivity, so purity of crystal helps.

Graphene is the second-best conductor, but it only conducts in the x and y directions, since it’s not bonded to the layer below it. And even so, these are only sp2 bonds. Diamonds have tighter sp3 bonds in x ,y, and z directions, which accounts for its current reign as the King of Cool.

For certain applications, these guys are trying to get diamond inserted under your dice: they say a 10 °C reduction in temperature can double device lifetime. This isn’t likely going to help extremely price-sensitive devices like memories, but from a system standpoint, they say that, overall, costs can come down by more than the added price of using diamond. Specific target apps are RF, GaN power devices, and LED lighting.

But before you go running to Jared, there’s new news afoot. A shadow may be appearing over diamond’s cooling primacy. The Navy Research Lab (NRL) has announced that they’ve found a material with better thermal conduction than diamond. But before you go running to the NRL, bear in mind – this material hasn’t actually been tested yet. This is the work of “ab initio” simulation. Such simulations are what I refer to as a “first principles” approach, where you model things like atoms and electrons and such at a low level and then build stuff with them to simulate what will happen. This is done regularly for electronic analysis; using it for thermal analysis is apparently newer.

In the case of boron arsenide (BAs), its calculated simulated thermal conductivity is on the same scale as – and possibly higher than – diamond. And the reason is due to an effect that isn’t usually considered: the scatterings that can impede heat dissipation apparently have a hole at certain frequencies; vibrations at those frequencies can carry heat away extraordinarily effectively.

Or so the simulations say. Before you can indulge in the new Cooler than Cool, they have to actually build the stuff and test it and then, if it works, commercialize it. Or, rather, since this is a navy lab, spin it out and commercialize it.

So diamond is safe for now, but it might just be having a few 3:00 AM tossing-and-turning sessions over the next few years as it stresses over whether it will remain the coolest thing in town.

You can find more about Element Six’s cooling diamonds here; you can find the NRL announcement here.

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