High thermal conductivity combined with electrical insulation capability could make polyethylene fibres useful for dissipating heat in many applications where metals are now used, such as solar hot water collectors, heat exchangers and electronics.
Most polymers — materials made of long, chain-like molecules — are very good insulators for both heat and electricity. But an MIT ( Massachusetts Institute of Technology) team has 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, according to an MIT press release.
The key
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.
The new 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 a computer processor chip. The work is described in a paper published on March 7 in Nature Materials .
High conductivity
This fibre was about 300 times more thermally conductive than normal polyethylene along the direction of the individual fibres, says the team's leader, Gang Chen. Chen explains 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 says.
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. Producing the new fibres, in which the polymer molecules are all aligned instead of jumbled, required a two-stage process.
The polymer is initially heated and drawn out, then heated again to stretch it further. Once it solidifies at room temperature, any large deformation is not possible.
