Apr 2 2018
Plastics are exceptional insulators, implying that they can effectively capture heat—a property that can be beneficial in an article like a coffee cup sleeve. However, this insulating quality is less beneficial in products like plastic casings for mobile phones and laptops, which could get overheated, partially due to the fact that the coverings trap the heat produced by the devices.
(Image credit: Chelsea Turner/MIT)
At present, a group of engineers from MIT has created a polymer thermal conductor—a plastic material which, though counterintuitively, functions as a heat conductor, dissipating heat instead of insulating it. The innovative polymers, which are flexible and lightweight, have the ability to conduct 10 times as much heat conducted by commercially available polymers.
Traditional polymers are both electrically and thermally insulating. The discovery and development of electrically conductive polymers has led to novel electronic applications such as flexible displays and wearable biosensors. Our polymer can thermally conduct and remove heat much more efficiently. We believe polymers could be made into next-generation heat conductors for advanced thermal management applications, such as a self-cooling alternative to existing electronics casings.
Yanfei Xu, Postdoc - MIT’s Department of Mechanical Engineering
Stretching Spaghetti
An in-depth investigation of the microstructure of an average polymer reveals the reason behind the ability of the material to trap heat very easily. At the microscopic scale, polymers are formed of long chains of monomers, or molecular units, connected end to end. These chains are usually tangled in a spaghetti-like ball. Heat carriers find it very hard to pass through this disorderly structure and have a propensity to get trapped within the polymeric knots and snarls.
However, scientists have made efforts to change these natural thermal insulators into conductors. In the field of electronics, polymers would contribute a distinctive combination of characteristics since they are chemically inert, flexible, and lightweight. Polymers are also electrical insulators, implying that they do not conduct electricity, and hence can be used to prevent short-circuiting of devices such as mobile phones and laptops in the hands of users.
In the recent past, different teams have designed polymer conductors, including Chen’s team, which devised a technique to develop “ultradrawn nanofibers” from a standard sample of polyethylene in 2010. The technique involves stretching the disordered, messy polymers into ordered, ultrathin chains - very similar to untangling a string of holiday lights. Chen discovered that the resultant chains allowed heat to skip easily along and pass through the material, and also that the polymer had the ability to conduct 300 times as much heat as ordinary plastics.
However, the insulator-turned-conductor could dissipate the heat only in one direction, that is, along the length of each polymer chain. Heat could not move between polymer chains due to weak Van der Waals forces—forces typically attracting two or more molecules close to one another. Xu was surprised to think whether a polymer material can be made to scatter heat in all directions.
Xu devised this study as an effort to engineer polymers that have high thermal conductivity by engineering intermolecular and intramolecular forces at the same time—a technique that she believed would allow effective movement of heat between and along the polymer chains.
Eventually, the researchers synthesized a heat-conducting polymer called polythiophene, a kind of conjugated polymer commonly used in various electronic devices.
Hints of Heat in all Directions
Xu, Chen, and members of Chen’s lab collaborated with Gleason and her lab members to devise an innovative method to engineer a polymer conductor by adopting oxidative chemical vapor deposition (oCVD), which involves directing two vapors into a chamber and onto a substrate, where they interact to form a film. “Our reaction was able to create rigid chains of polymers, rather than the twisted, spaghetti-like strands in normal polymers,” stated Xu.
Here, Wang made the oxidant to flow into a chamber, together with a vapor of monomers—individual molecular units which, upon being oxidized, get modified into the chains called polymers.
“We grew the polymers on silicon/glass substrates, onto which the oxidant and monomers are adsorbed and reacted, leveraging the unique self-templated growth mechanism of CVD technology,” stated Wang.
Wang synthesized relatively large-scale samples, where the dimensions of each sample measured 2 cm2, comparable to the size of a thumbprint.
“Because this sample is used so ubiquitously, as in solar cells, organic field-effect transistors, and organic light-emitting diodes, if this material can be made to be thermally conductive, it can dissipate heat in all organic electronics,” stated Xu.
The thermal conductivity of each sample was measured by the researchers using time-domain thermal reflectance—a method in which a laser is shot onto the material to heat up its surface, and subsequently, the drop in its surface temperature is monitored by measuring the material’s reflectance as the heat spreads into the material.
“The temporal profile of the decay of surface temperature is related to the speed of heat spreading, from which we were able to compute the thermal conductivity,” stated Zhou.
Typically, the polymer samples could conduct heat at around 2 W/m/K—nearly 10 times faster than what could be achieved by traditional polymers. At Argonne National Laboratory, Jiang and Xu discovered that polymer samples looked almost isotropic, or uniform. This indicates that the characteristics of the material—for instance, its thermal conductivity—should also be almost uniform. Subsequent to this reasoning, the researchers estimated that the material should have the ability to conduct heat equally well in all directions, thereby enhancing its heat-dissipating ability.
In the future, the researchers will continue to investigate the basic physics behind polymer conductivity, and also ways to allow the material to be used in electronics and other products, for example, films for printed circuit boards, and casings for batteries.
We can directly and conformally coat this material onto silicon wafers and different electronic devices. 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.
Yanfei Xu, Postdoc - MIT’s Department of Mechanical Engineering
The U.S. Department of Energy–Basic Energy Sciences and the MIT Deshpande Center partially supported this study.
Xu and a group of postdocs, graduate students, and faculty have reported their research outcomes in the Science Advances journal on March 30, 2018. The group includes Xiaoxue Wang, who contributed equally to the study along with Xu. The other researchers in the group were Jiawei Zhou, Bai Song, Elizabeth Lee, and Samuel Huberman; Zhang Jiang, physicist at Argonne National Laboratory; Karen Gleason, associate provost of MIT and the Alexander I. Michael Kasser Professor of Chemical Engineering; and Gang Chen, head of MIT’s Department of Mechanical Engineering and the Carl Richard Soderberg Professor of Power Engineering.