Demon Materials

Maxwell’s Demon, the watchful agent that, by allowing fast atoms to pass and not slow ones, was proposed to defy thermodynamics. Though the Demon has shed more light on information theory than it has provided free usable energy, I find the notion of passing one thing, and not another, captivating.

Thermoelectric devices either produce electricity from a thermal gradient, or a thermal gradient from electricity. They are great since they have no moving parts; just put them in place, attach wires, and voila! Their limitation, the reason they aren’t everywhere instead of more cumbersome compressors, is because finding a material that passes electrons, and does not pass heat, is tough. So tough, in fact, that, as I understand it, the periodic table doesn’t contain elements that could produce a bulk material with useful efficiency. So, as cool as they are in concept, I have for years forgone hope that they would ever become mainstream.

But. Enter Xiao Shen and colleagues at Vanderbilt University who have found a novel crystalline structure that may improve the differential between thermal and electrical transmission (see also their Nature Communication). We clearly have a way to go before we learn if this will yield a commercially useful material, but these discoveries make me smile. And, I really want a fridge without a compressor.

In the same vein, Aaswath Raman and colleagues at Stanford have made some advance on a different differential. Carbon dioxide in the atmosphere absorbs infrared radiation from the Earth’s surface, heats up, and re-radiates heat back to the surface. At the same time, the atmosphere is transparent to other wavelengths setting the stage for heat to arrive here, but to not leave. This is good since it makes our world the lovely, tomato-growing, swim-suit-friendly place it is, instead of the cryo-cooler it would otherwise be. (Of course, there have been some recent difficulties with extra carbon dioxide production of late, but I digress.)

Though atmospheric carbon dioxide and water vapor block much of the infrared spectrum, there is a window of transparency between about 8 and 13 micrometers. And, since space is very cold relative to Earth’s surface, an object that loses heat in this band should move towards thermal equilibrium with space and cool down. So, let’s build such a material, attach it to something we want to cool down and let it rip. Yay! Instant, free cooling! Oh yes, one thing: most things cool down at night because they lose infrared energy to space through this window. It is during the day when this would be most useful, and during the day the sun is shining and heating things up too. What we want is something that does the opposite of carbon dioxide: something that reflects visible, short-wave light, and emits infrared in the infrared transparency window.

Well, Raman and colleagues appear to have done just this, and achieved about 40 watts worth of cooling per square meter in full sun with no energy consumption. So long as this doesn’t need too much hafnium, we might be in business!

Is Civilization cool or what?