I love light and the different ways we can control it. It’s a good time for me because we are really in a golden age of light control. We can manipulate it to see details that would otherwise be invisible. We can route it around objects so that they are invisible. Light is made to stand still and dance on the pointed end of pins.
All this control of light is indirect and comes through our control of materials with which the light interacts. Now researchers have created a material that modifies its properties so that its infrared appearance is hotter or colder than the object it encloses. In other words, hot objects may appear cold, or cold objects may appear hot – it’s infrared camouflage.
It’s all about those electrons
So, are you making some infrared camo gear yourself? The basic procedure is to control efficiency with a material that can emit infrared radiation. Take gold as an example. Gold is an almost perfect metal: it has a high conductivity and does not absorb infrared radiation as easily. That means it will reflect incoming radiation; that’s why emergency blankets have a thin gold coating: the gold reflects your own infrared radiation back to you to keep you warm.
On the other side of the scale you have something like the black soot of a fire. Soot is not very reflective, so the background infrared radiation is not scattered by the soot. But when placed on a heating plate, it glows in the infrared. Gold, on the other hand, does not radiate at these wavelengths.
The efficiency with which a material emits (or absorbs) light is described by its emissivity, which scales from zero to one. Gold is very close to zero, while a perfect emitter (like soot) is close to unity.
Emissivity is the knob the researchers turn to control the infrared appearance of their material. For this they use the unusual properties of graphene. Graphene is a single layer of carbon atoms, all arranged in a hexagonal pattern. It’s not quite a metal; it’s more of a semiconductor like the silicon that the CPU is made of that power the device you’re reading this on. The difference is that silicon only starts to conduct when you apply a measurable voltage to it, while graphene starts to conduct as soon as the voltage is not zero (this is called a zero band gap semiconductor). This means that graphene always has electrons available to conduct, but not so many. And if you take manufacturing defects into account, the conductivity of some graphene layers can be quite poor.
But as with other semiconductors, the conductivity can be changed by adding impurities that contain extra electrons. For example, adding a small amount of phosphorus to silicon will give it a higher conductivity because phosphorus has spare electrons to give up.
A salty blanket
The researchers made a thick graphene layer (about 100 to 150 graphene sheets thick) that was very conductive. As a result, the emissivity of the layer is quite high. Under that layer they placed a thin layer of ionic liquid. The ionic liquid is essentially a liquid salt (not a salt dissolved in water). The liquid salt was sealed in place by a thin layer of transparent plastic and the assembly was placed on a thin gold electrode.
Under normal conditions, the salt layer sits outside the graphene layer, which absorbs and emits infrared light. However, if a voltage is applied between the graphene layer and the gold layer, the ions in the liquid move into the graphene, giving it a higher conductivity. As a result, it stops absorbing and emitting infrared radiation and becomes an infrared reflector. In fact, the transition from absorber to reflector is controllable. Different voltages result in different conductivity, so the emissivity of the graphene layer varies continuously from its maximum to its minimum value (0.8 to 0.35).
What does that mean? Imagine placing this on a hot plate. When the emissivity is set to the minimum value, the hob appears to have the same temperature as its surroundings: it is hidden. But you can also go the other way: if the emissivity is increased, the hob will appear hotter than it otherwise would.
At first, this seems counterintuitive: If the layer glows hotter than the thing covering it, the material is definitely actively cooling the hot plate. I’d say this must be true on the face of it. But in practice it depends on a number of factors: the temperature of the object depends on the efficiency of its infrared radiation, but also on its conduction to the environment and convection (the mass movement of warm liquid, such as air, which dissipates heat ) . The cover could simply change the balance between these different heat transfer mechanisms rather than actually cooling the covered object.
Seeing is believing
The researchers show many photos and videos of their material that camouflages hot objects. They show that the material is flexible, so it could be worn. And, equally important, it is vacuum compatible.
It may come as a surprise, but keeping things cool or warm in vacuum chambers and in space is a real challenge. Under normal circumstances, convection carries most of the heat away from a hot object – which is why radiators are so good at keeping homes warm. In a vacuum there is nothing to carry the heat. So ultimately the only way to control the temperature is to change how efficiently heat is radiated from surfaces. And now we have a material that can do this. I think that’s probably the most important application.
Of course, we don’t know how physically robust the material is or how many emission transitions it can do before it stops working. But, assuming these unpack and the ionic liquid isn’t too expensive or toxic, I’d imagine this showing up pretty quickly in specialized applications.
Nano letters2018, DOI: 10.1021/acs.nanolet.8b01746