Turning the heat back on?
Shiladitya Ghosh guides you through a novel way to manipulate heat
A researcher at the Massachusetts Institute of Technology (MIT) has, through his self-devised technique, managed to successfully exercise control over heat the same way we enjoy control over light through mirrors and lenses (most disappointingly falling short of developing a giant heat ray, for the time being). This new technique, which utilises semiconducting alloy crystal nanostructures and similar materials, manipulates the physical separation between atoms so that they correspond to the wavelengths of heat phonons.
To elucidate how heat and sound phonons are different, Martin Maldovan, the researcher in question, directs us to consider their associated frequencies: sound phonons are on the order of kilohertz, while heat phonons vibrate in the terahertz frequencies. Sounds simple enough.
In developing this technique, the initial step was to manipulate the heat phonons’ operational frequency so it would be reduced and similar to the range of sound phonons, described as “hypersonic heat”. For this to be possible, germanium nanoparticles of a particular range of sizes were integrated with silicon alloys, with the resultant crystals having the desired atomic separation needed for this hypersonic heat. Furthermore, the frequency range was narrowed through the creation of thin films on the edges of the material, utilising optical principles to ‘concentrate’ the frequencies of the phonons. Not nearly as simple.
As a result, about 40% of the total flow of heat is not only streamlined to a range of several hundred gigahertz (considerably closer to kilohertz than terahertz was!), but also focussed to travel in one general direction, similar to the flow of photons in lasers. That is definitely no mean feat!
With this end-product, the heat phonons can be controlled in a variety of ways using analogs of sound-controlling phononic crystals that havealready been developed. Some potential important applications highlighted by Maldovan are the improvement of thermoelectric devices (which output electricity from temperature gradients), and the development of thermal ‘diodes’ which, crucially, “could be useful in energy-efficient buildings in hot and cold climates” – now that’s definitely something I’d honestly appreciate and be very grateful for.
Perhaps the fanciest application he mentions is the creation of invisibility cloaks for heat: for objects to become perfectly undetectable by heat sensors and the like. Honestly though, I can only foresee how something like that can be misused in the future for non-peaceful purposes on international scales. Nonetheless, this is merely the first breakthrough of this nature in heat-manipulating materials; soon, we may well be channelling the excess heat in the atmosphere back out into space to counteract global warming. Or building that giant heat ray, of course. Just in case we get embroiled in The War of the Worlds.
