Thermal image of an electric heating plate, with all but the bottom section covered with a new aerogel film developed by researchers at the Chinese Academy of Sciences. The film effectively hides the hot object from infrared detection. [Image: American Chemical Society]
As long as scientists have been improving thermal imaging systems for military and commercial purposes, other scientists have sought new coatings and materials to make objects invisible to heat-seeking cameras. A team based in China has developed an inexpensive, flexible film that the researchers say renders the objects it covers virtually invisible in infrared light (ACS Nano, doi: 10.1021/acsnano.8b08913).
The film, developed by Xuetong Zhang's group at the Chinese Academy of Sciences, has the consistency of an aerogel—an extremely low-density solid that looks like a frozen cloud. This particular aerogel consists of two familiar main ingredients: DuPont's Kevlar, a synthetic fiber with high tensile strength, and polyethylene glycol (PEG), a water-soluble polymer with many applications.
The burgeoning world of nanostructures and metamaterials has led to numerous schemes for using static nanopatterns to hide thermal emitters from infrared detectors, but such patterns cannot be fine-tuned, according to the researchers. Some dynamic methods for controlling infrared emissivity consume a fair amount of electricity or respond slowly to temperature changes. Plain old thermal blankets can be thick and heavy.
Constructing the aerogel
To create the aerogel, Zhang's team dissolved Kevlar into dimethyl sulfoxide (DMSO), resulting in a nanofiber solution. The researchers subsequently solidified the material, rinsed off the remaining DMSO and freeze-dried the solids. Finally, the scientists allowed the aerogel to absorb PEG, a phase-change material that can store heat, and shaped it into a film attached to a protective waterproof layer.
Adjusting the percentage of Kevlar by weight in the nanofiber solution significantly changed the pore size and mechanical properties of the resulting film. The optimal thickness and concentration was a film 150 μm thick, made of a solution of 2 percent Kevlar nanofibers by weight; that film had a tensile strength of 1.27 MPa. The film survived 20 rounds of folding and unfolding, and it can also be rolled up.
To test the aerogel film's thermal properties, the researchers exposed it to a simulated outdoor cycle of day–night lighting variations and found that its temperature rose more slowly in mock sunlight than that of the target material underneath the film. Infrared imaging showed that one, three and five layers of the Kevlar–PEG aerogel transmitted significantly less heat than the same number of layers of aerogel that did not contain PEG.
Zhang is currently a Royal Society fellow at University College London, U.K.