Wednesday, August 22, 2018 by Edsel Cook
What do you think of a thin sheet that can trap almost all of the infrared light that hits it? Well, how about draping the sheet on things that need to be hidden from the heat-hunting electronic eyes of aerial drones? Wisconsin-based researchers reported creating a new material that can turn objects and people invisible to infrared sensors, an article on the University of Wisconsin-Madison news site reported.
Whereas optical cameras use visible light, infrared cameras seek out the heat given off by living things and machines. These sensors allow aerial drones to see their targets in the darkest of nights or through the thickest fog bank.
University of Wisconsin-Madison (UW Madison) researchers found a way to reset that twisted game of hide-and-seek. Their new “cloaking material”allows objects to become invisible to those sensors. The material is also light enough to be carried and used by people.
“What we have shown is an ultrathin stealth sheet,” explained Hongrui Jiang, a professor of the university’s electrical and computer engineering department. “Right now, what people have is much heavier metal armor or thermal blankets.”
Jiang’s team described their invention in a paper published in the journal Advanced Engineering Materials. (Related: Researchers develop thin film that can turn heat from electronics into energy.)
Warm objects give off heat as infrared light, which is visible to infrared cameras. There are existing technologies that can mask heat, but Jiang said that his team’s cloaking material is a step above those in terms of cost, usability, and weight.
The sheet is about as thick as 10 pages of paper put together. This millimeter-thin material can absorb 94 percent of the infrared light that strikes its bristling surface. The effect is to turn the warm object invisible to infrared detectors that are looking for the heat that normally radiates from its target.
An important characteristic of the cloaking material is its ability to absorb medium and long wavelengths of infrared light. These wavelengths happen to be the same as the light that radiates from objects of 98.6 degrees Fahrenheit (37 degrees Celsius), which is better known as the body temperature of a normal human being.
To further fool infrared cameras and the aerial drones that mount them, the stealth sheet can be fitted with electronics that generate heat. The heat sources would be shaped in such a way as to look like something else.
“You can intentionally deceive an infrared detector by presenting a false heat signature,” Jiang suggested regarding his team’s invention. “It could conceal a tank by presenting what looks like a simple highway guardrail.”
Black silicon gives the cloaking sheet its ability to trick heat-seekers. A modified version of silicon, the black version is covered with numerous nanowires that all point upward.
Any light that enters this thick forest is only able to bounce between the vertical nanowires. It is unable to escape the black silicon, which absorbs its energy much more efficiently than its vanilla semiconductor self.
Black silicon absorbs visible light, which is why it is used in solar cells to improve the efficiency. But it is also very effective at capturing infrared light.
Jiang’s team was the first to take advantage of black silicon’s infrared-absorbing properties. They modified the material by greatly increasing the height of the nanowires, making them more effective at imprisoning light.
The UW Madison researchers used silver nanoparticles to etch the silicon. Silver also absorbs infrared light.
Additionally, the backing of the sheet is filled with tiny air channels. These channels help regulate the heat of the cloaking sheet.
Wondering what other stealth materials can hide your important gear from sight? Check out Science.news.