An inexpensive, lightweight new fabric could help keep cars, spacecraft, and machinery cool in the summer and warm when the temperatures dip in winter. Detailed in the journal Device, the new “thermal cloak” does not need any power, and could reduce the energy needed for cooling and heating.
Besides making cars more bearable to get into in sweltering heat or frigid cold, this electricity-free fabric cover could have a key benefit for electric cars. Exposure to extreme temperatures degrades battery performance and shortens their lifespan. The thermal cloak could help reduce that damage.
“The thermal cloak is like clothes for vehicles, buildings, spacecrafts, or even extraterrestrial habitats to keep cool in summer and warm in winter,” says Kehang Cui, a materials scientist at Shanghai Jiao Tong University and author on the paper.
The cloak has two layers, each with different physical and chemical properties. The outer layer reflects the sun’s heat back into the atmosphere, while the inner layer reflects heat from the car back towards the vehicle.
Cui and colleagues made the outer layer out of silica fibers coated with flakes of hexagonal boron nitride, a material that is the thickness of just a few atoms. The silica fibers reflect visible light while the boron nitride reflects ultraviolet light, so the materials together reflect 96 percent of the sunlight that hits the fabric. The outer layer also absorbs heat from its surroundings and emits it as infrared radiation, which further lowers the temperature under the cloak.
The team braided and wove the fibers to make a fabric that is laminated on the inner layer. This inner layer is made of an aluminum alloy that traps any heat that’s in the space around it and keeps it under the cloak.
To demonstrate how the cloak could withstand extreme conditions of aerospace, the researchers heated it to 800°C and immersed it in liquid nitrogen. They also blasted it with acid and a blow torch to show. The material did not lose its performance.
The team also made a full-size cloak and tested it outdoors in Shanghai by covering an electric car with it. During the summer, the cabin temperature of an uncovered car reached over 50°C at mid-day, but that of a cloak-covered car reached only about 23°C, 27°C lower than the uncovered car and almost 8°C lower than the temperature outside the car. During winter nights, the cloak kept the car almost 7°C warmer than the outside air.
Because it is made from inexpensive materials, the cloak should be easy to make on large commercial scale.
Source: Huaxu Qiao et al, Scalable and durable Janus thermal cloak for all-season passive thermal regulation, Device, 2023.
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