Animals have made use of camouflage to hide from one another for almost as long as eyes have been around to spot them. Humans, being copycats, have made extensive use of camouflage tricks they have seen in nature by applying concealing colouration to everything from clothing to tanks. A way to thwart camouflage, though, is to employ infrared-viewing technology to look for the heat emitted by an otherwise-camouflaged object. Designing something that can prevent this, and can thus carry camouflage into the infrared, has proved tricky. But Coskun Kocabas of the University of Manchester, in England, thinks he can do it.
Giving thermal invisibility to an object whose own temperature and that of its surroundings are constant is not too hard. But maintaining that cloaking as either the object or its surroundings heats up or cools down is tricky. Dr Kocabas thought he might be able to do this using graphene, a material composed of a single layer of carbon atoms.
Pure graphene is almost transparent to infrared, but Dr Kocabas suspected he could tune this transparency by drenching a stack of graphene layers in an ion-rich liquid. Graphene’s optical properties depend on the ability of its electrons to move freely around the atomic sheet. That, in turn, depends on the fact that not all the energy states which electrons could occupy in that sheet are, in fact, occupied. If the electrons in the sheet could be locked in place, however, it would become opaque—reflecting infrared rather than passing it, and thus hiding any source of heat behind it. And the electrons can indeed be so locked, by running a current through the ion-rich liquid. This drives extra electrons into the graphene, filling up all its energy states and eliminating mobility.
Dr Kocabas tested this idea and, as he reports in Nano Letters, he found that it works. When the team used a battery to apply a potential difference of three volts to the ion-soaked graphene layers, the stack became opaque to infrared. And when Dr Kocabas used a thermal camera to record what happened when he pressed his hand against the back of the stack, the answer was “nothing”. The camera could not see his hand through the stack. When he reduced the voltage to zero, however, his hand became visible within a second.
At least as important as this was the finding that applying between zero and three volts allowed some of the heat shed by his hand to be seen. That is what makes the system tuneable. So, after this first test, he arranged for thermocouples, a type of sensor, to measure the changing temperature of the stack’s environment. He then used a computer to convert that information into a voltage appropriate to the task of allowing the graphene layers to blend in with the changing thermal background. This worked.
Given these successful tests, Dr Kocabas thinks that his ion-drenched graphene layers do indeed have the potential to act as thermal camouflage. Though still a long way from the battlefield, they may eventually find their way into military applications where masking heat signatures is, quite literally, a matter of life and death.
This article appeared in the Science and technology section of the print edition under the headline "Heat shield"
