Polymers are usually the go-to material for thermal insulation. Think of a silicone oven mitt, or a Styrofoam coffee cup, which are made at the best of trapping heat.
Now MIT engineers have flipped the picture of the standard polymer insulator, by fabricating thin polymer films that conduct heat-an ability normally associated with metals.
The results of the study, published in the journal Nature Communications may spur the
"We think this result is a "Step to stimulate the field," says Gang Chen, the Carl Richard Soderberg Professor of Power Engineering at MIT, and a senior co-author on the paper. Yanfei Xu, along with Daniel Kraemer, Bai Song, Jiawei.
Chen's co-authors include lead author Yanfei Xu Zhou, James Loomis, Jianjian Wang, Migda Li, Hadi Ghasemi, Xiaopeng Huang, and Xiaobo Li from MIT, and Zhang Jiang of Argonne National Laboratory.
In 2010, the team reported success in fabricating thin fibers of 300 times more thermally conductive than normal polyethylene, and about as conductive as most metals. Their results, published in Nature Nanotechnology, include the attention of various industries, including manufacturers of heat exchangers, computer core processors, and even race cars.
It soon became clear that, in order for polymer conductors to work for any of these applications, the materials would have to be scaled up from ultrathin fibers.
"At that time we said, rather than a single fiber, we can try to make a sheet, "Chen says.
The researchers did not come to terms with fabricate heat-conducting sheets of polymer, but they did so to custom-build an apparatus to test the material's heat conduction, as well as develop images of the material's microscopic structures.
Normally, the microscopic structure of polyethylene and polymer resembles a spaghetti-like tangle of molecular chains.
Xu and her colleagues look for ways to untangle polyethylene's molecular knots, to form parallel chains along which heat can better conduct. To do this, they dissolved in a solution that prompted the coiled chains to expand and untangle. A custom-built flow system further untangled the molecular chains, and spit out the solution on a liquid-nitrogen-cooled plate to form a thick film, which was placed on a roll-to-roll drawing machine that heated and stretched the film until it was thinner than plastic wrap.
The team then built an apparatus to test the film's heat conduction. While most polymers conduct heat at 0.1 to 0.5 watts per meter per Kelvin, the new polyethylene film measured around 60 watts per meter per Kelvin. (Diamond, the best heat-conducting material, comes in at 2,000 watts per meter per kelvin, while ceramic measures about 30, and steel, around 15.) As it turns out, the team's film is two orders of magnitude more thermally conductive
To understand why these engineered polyethylene films have been used for unusually high thermal conductivity, the team conducted X-ray scattering experiments at the US Department of Energy's Advanced Photon Source (APS) at the Argonne National Laboratory.
"These experiments, at one of the world's most bright synchrotron X-ray facilities, allow us to take a look at the nanoscopic details within the individual's fibers stretched film, "Jiang says.
By imaging the ultrathin films, the films exhibiting better heat conduction consisted of nanofibers with less randomly coiled chains, versus those in common polymers, which resemble tangled spaghetti.
"This dream work came true in the end," Xu says.
Better Researchers Are Looking For Ways to Make Better Better Polymer Heat Conductors,
Zhou points out that the team's polyethylene film conducts heat along the length of the fibers that make up the film. Such a unidirectional heat conductor could be useful in carrying heat away in a specified direction, such as laptops and other electronics. But ideal, he says the film should dissipate heat more effectively in any direction.
"If we have an isotropic polymer with good heat conductivity, then we can easily replace it with a good heat conductivity conductive materials, "Zhou says. "We're looking into better heat conduction in all three dimensions."
Researchers tune material's color and thermal properties separately
Yanfei Xu et al, Nanostructured polymer films with metal-like thermal conductivity, Nature Communications (2019). DOI: 10.1038 / s41467-019-09697-7
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New polymer films conduct heat instead of trapping it (2019, Apr 29)
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