Most polymers – materials made of long, chain-like molecules – are very good insulators for both heat and electricity.
But an MIT team has now found a way to transform the most widely used polymer, polyethylene, into a material that conducts heat just as well as most metals, yet remains an electrical insulator.
The process causes the polymer to conduct heat very efficiently in just one direction – unlike metals, which conduct equally well in all directions. This may make the new material especially useful for applications where it is important to draw heat away from an object, such as microprocessor.
The key to the transformation was getting all the polymer molecules to line up the same way, rather than forming a chaotic tangled mass, as they normally do. The team did that by slowly drawing a polyethylene fibre out of a solution using the finely controllable cantilever of an atomic-force microscope, which they also used to measure the properties of the resulting fibre.
This fibre was about 300 times more thermally conductive than normal polyethylene along the direction of the individual fibres, said the team’s leader, Gang Chen, the Carl Richard Soderberg professor of Power Engineering and director of MIT’s Pappalardo Micro and Nano Engineering Laboratories.
Chen explained that most attempts to create polymers with improved thermal conductivity have focused on adding in other materials, such as carbon nanotubes, but these have achieved only modest increases in conductivity because the interfaces between the two kinds of material tend to add thermal resistance.
’The interfaces actually scatter heat, so you don’t get much improvement,’ Chen said. But, using this new method, the conductivity was enhanced so much that it was actually better than that of about half of all pure metals, including iron and platinum.
Producing the new fibres, in which the polymer molecules are all aligned instead of jumbled, required a two-stage process, explained graduate student Sheng Shen. The polymer is initially heated and drawn out, then heated again to stretch it further. ’Once it solidifies at room temperature, you can’t do any large deformation,’ Shen said, ’so we heat it up twice.’
Even greater gains are likely to be possible as the technique is improved, said Chen, noting that the results achieved so far already represent the highest thermal conductivity ever seen in any polymer material.
If such fibres could be made in quantity, they could provide a cheaper alternative to metals used for heat transfer in many applications, especially ones where the directional characteristics would come in handy, such as heat-exchanger fins (like as the coils on the back of a refrigerator or in an air conditioner), cell-phone casings or the plastic packaging for computer chips.
So far the team has only produced individual fibres in a laboratory setting, but Chen is hoping to scale up the process to produce whole sheets of material with the same properties.