It may sound counterintuitive, but researchers at MIT have used plastic, commonly known as a material for heat insulator, to develop the next-generation heat conductor.
“Nowadays heat dissipation is an increasingly critical challenge for integrated devices that continue to miniaturize towards nanoscale,” said Yanfei Xu, postdoc in MIT’s Department of Mechanical Engineering and co-lead author of the study.
The challenge was made apparent in the wake of mass recall of Samsung’s Galaxy Note 7 after reports of explosion due to overheating in 2016, and companies have invested in developing a smarter heat dissipation technique for rechargeable devices.
According to Xu, unlike the traditional materials for heat conductors like metals and ceramics, the industry needed a lightweight, flexible and economic material that would be as efficient at dissipating heat.
The study is published in Science Advances.
The Next-Generation Heat Conductor
Plastics are also known as manufactured polymers, made from long chains of monomers, or molecular units, linked end to end. Because these chains are often tangled, heat gets trapped within the polymeric snarls and knots, making plastics great heat insulators.
But the MIT researchers saw something different. Plastics are lightweight, flexible and chemically inert, meaning they do not conduct electricity, and can therefore be used to prevent devices like laptops and cellphones from short-circuiting in their users’ hands. If they were only better at dissipating heat, plastics could be the next-generation heat dissipating, not heat trapping, material.
In 2010, Gang Chen, head of MIT’s Department of Mechanical Engineering and the Carl Richard Soderberg Professor of Power Engineering, and his team of researchers invented a method to stretch the messy, disordered polymers into ultrathin, ordered chains.
They found that the resulting chains enabled heat to skip easily along and through the material, and that the polymer conducted 300 times as much heat compared with ordinary plastics.
But the insulator-turned-conductor could only dissipate heat in one direction, along the length of each polymer chain. Heat couldn’t travel between polymer chains.
Hearing about the team’s development, Xu wondered whether this polymer material could be made to scatter heat away, in all directions.
In a joint effort with a team of postdocs, graduates and faculty, including Chen and Karen Gleason, associate provost of MIT and the Alexander I. Michael Kasser Professor of Chemical Engineering, Xu and Xiaoxue Wang, a doctoral candidate in the MIT’s School of Engineering and co-lead author of the study, developed a method that would enable efficient heat transport along and between polymer chains.
They used oxidative chemical vapor deposition (oCVD), whereby monomers and oxidants are directed into a vacuum chamber and onto a substrate, where the polymerization takes place. And a thin, uniquely oCVD-grown polymer film with more ordered structure is formed, allowing heat to transport both along and across the more extended and ordered chains.
“We obtained a more extended and ordered chain structure with good intermolecular interactions, compared to the random coil, messy ‘spaghetti-like,’ structure in conventional polymers,” said Wang.
Testing the Material
The team produced relatively large-scale samples, each measuring two square centimeters — about the size of a thumbprint.
To test how efficiently their new polymer dissipated heat, they measured each sample’s thermal conductivity using time-domain thermal reflectance — a technique in which they shoot a laser onto the material to heat up its surface and then monitor the drop in its surface temperature by measuring the material’s reflectance as the heat spreads into the material.
On average, the polymer samples conducted heat at about 2 watts per meter per kelvin — about 10 times faster than what conventional polymers can achieve.
The team found that their polymer samples appeared nearly isotropic, or uniform, suggesting that the material’s properties, such as its thermal conductivity, should also be uniform.
In line with this finding, the team reasoned that the material should conduct heat equally well in all directions, increasing its potential to dissipate heat.
The Next Step
The team will continue exploring the fundamental physics behind polymer conductivity, as well as ways to enable plastics to be used in electronics and other products, such as casings for batteries and films for printed circuit boards.
“We can directly and conformally coat this material onto silicon wafers and different electronic devices,” Xu said in a statement. “If we can understand how thermal transport [works] in these disordered structures, maybe we can also push for higher thermal conductivity. Then we can help to resolve this widespread overheating problem, and provide better thermal management.”