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Engineers develop new polymer films that conduct heat

ANI | Updated: Apr 30, 2019 14:56 IST

Washington DC [USA], Apr 30 (ANI): Polymers are known for their thermal insulation- a process of insulating against transmission of heat. However, engineers have recently fabricated thin polymer films that conduct heat, an ability associated with metals.
The study was published in Nature Communications.
In experiments, engineers found the films, which are thinner than plastic wrap, which conduct heat better than many metals, including steel and ceramic.
The study may spur the development of polymer insulators as lightweight, flexible, and corrosion-resistant alternatives to traditional metal heat conductors, for applications ranging from heat dissipating materials in laptops and cellphones, to cooling elements in cars and refrigerators.
"We think this result is a step to stimulate the field. Our bigger vision is, these properties of polymers can create new applications and perhaps new industries, and may replace metals as heat exchangers,” said Gang Chen, senior co-author of the study.
In 2010, the team reported success in fabricating thin fibre of polyethylene that was 300 times more thermally conductive than normal polyethylene, and about as conductive as most metals.
The results drew the attention of various industries, including manufacturers of heat exchangers, computer core processors, and even race cars.
It soon became clear that for polymer conductors to work for any of these applications, the materials would have to be scaled up from ultrathin fibres (a single fibre measured one-hundredth of the diameter of a human hair) to more manageable films.
"At that time we said, rather than a single fibre, we can try to make a sheet. It turns out it was a very arduous process,” said Chen.
The researchers not only had to come up with a way to fabricate heat-conducting sheets of polymer, but they also had to custom-build an apparatus to test the material's heat conduction, as well as develop computer codes to analyse images of the material's microscopic structures.
In the end, the team was able to fabricate thin films of conducting polymer, starting with a commercial polyethylene powder.
Normally, the microscopic structure of polyethylene and most polymers resemble a spaghetti-like tangle of molecular chains. The heat has a difficult time flowing through this jumbled mess, which explains a polymer's intrinsic insulating properties.
Yanfei Xu, lead author of the study, and her colleagues looked for ways to untangle polyethylene's molecular knots, to form parallel chains along which heat can better conduct.
To do this, they dissolved polyethylene powder 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 onto a liquid-nitrogen-cooled plate to form a thick film, which was then 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 around 0.1 to 0.5 watts per meter per kelvin, Xu found the new polyethylene film measured around 60 watts per meter per Kelvin.
It turned out that the team's film is two orders of magnitude more thermally conductive than most polymers, and also more conductive than steel and ceramics.
To understand why these engineered polyethylene films have such an unusually high thermal conductivity, the team conducted X-ray scattering experiments.
"These experiments allow us to see the nanoscopic details within the individual fibres that make up the stretched film," said Zhang Jiang, one of the researchers.
By imaging the ultrathin films, the researchers observed that the films exhibiting better heat conduction consisted of nanofibres with less randomly coiled chains, versus those in common polymers, which resemble tangled spaghetti.
"This dream work came true in the end," Xu said.
Going forward, the researchers are looking for ways to make even better polymer heat conductors, by both adjusting the fabrication process and experimenting with different types of polymers. (ANI)

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