Plastics are excellent thermal insulators — a quality that can be an advantage in some applications. But this property is less desirable in products such as plastic casings for laptops and mobile phones, which can overheat. Dr. Xu is a member of a team that has developed a new polymer that works as a heat conductor. The polymer, which is lightweight and flexible, can conduct ten times as much heat as most commercial polymers.
Tech Briefs: What was your motivation for this work?
Dr. Yanfei Xu: The goal was to transform a heat insulator into a heat conductor. Traditional polymers are both electrically and thermally insulating. Heat dissipation is an increasingly critical challenge for integrated devices. It is highly desirable to develop lightweight, flexible, chemically inert, and economical heat conductors in order to replace conventional metals and ceramics.
We sought to understand why the thermal conductivity of polymers is so low. At the microscopic level, polymers are made from long chains of monomers, or repeating units, linked end-to-end. These chains are often tangled in a spaghettilike ball. Heat carriers have a hard time moving through this disorderly structure and tend to get trapped within the polymeric snarls and knots.
Recently, researchers were able to stretch the messy, disordered polymers into ultradrawn, aligned, ordered chains. They found that the resulting chains enabled heat to move along the stretching direction, and that the polymer conducted 300 times as much heat compared with ordinary plastics.
Tech Briefs: How did you make the polymer conductive?
Dr. Xu: We produced a heat-conducting polymer known as polythiophene, a type of conjugated polymer that is commonly used in organic electronic devices. We used oxidative chemical vapor deposition (oCVD) whereby two vapors — an oxidant along with a vapor of monomers — are directed into a chamber and onto a silicon/glass substrate, where they interact and form a film.
Tech Briefs: Could this be used to mount electronic devices?
Dr. Xu: Yes. With electronics, we need thermally conductive interface materials that can transfer heat away from hot spots. Now we can coat an electronic device with our polymer, which would not only transfer the heat, but still be a good electrical insulator. The thermal transport mechanism is still not very clear to us. We need to explore the relationship between structures and thermal transport in order to achieve efficient transport in polymers so we can further engineer highly thermally conductive plastics.
Tech Briefs: Are there other applications?
Dr. Xu: Polymers are flexible, lightweight, and inexpensive. They are used throughout modern technology, including computers and phone casings. It will be a great step forward if we can add high thermal conductivity to these polymers without altering their other properties.
As far as investigating possible applications, we will coat this on different devices, such as casings for batteries, and as a conformal coating for printed circuit boards. We would then test the thermal dissipation performance under real-world operating conditions.