TECHNOLOGY FOR CHANGE

Are New Plastics the Key to Cooling PCs?

Salvatore Salamone | Date: 03-17-10 | Comments

MIT researchers have produced plastic with thermal conducting properties that might be exploited for removing heat away from computer CPUs. While the technique is experimental, it offers an alternative to methods that infuse plastic with materials to achieve similar properties.

In the movie �The Graduate,� Mr. McGuire had one word for Ben Braddock (aka the very young Dustin Hoffman), just one word: �Plastics.�

Well, fast-forward 43 years to current times, and it turns out plastics might be the definitive word when it comes to removing heat from PCs and in other applications that require a material with thermal conductivity but electrical isolation.

Unfortunately, we�re not talking about all plastics. The plastic material that might help improve a PC�s energy efficiency has only been produced in very limited quantities in the lab by researchers at MIT.

In particular, the MIT 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. The research was reported in a Nature Nanotechnology Letter published earlier this month.

In addition to the conductivity boost, the process used by the researchers 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�s CPU.

Alignment Is the Key

The trick to achieving the material�s interesting thermal conductivity properties has to do with the way the plastic is formed. In a two-step process, researchers developed a technique to slowly draw a thin-thread, polyethylene fiber out of a solution. The polymer is then heated again to stretch it further.

Using this process, polymer molecules line up within the material. In contrast, when this material is normally formed, these molecules have a structure that resembles a chaotic tangled mess.

Conductive plastic�s aligned polymer molecules (left) vs. non-conductive plastic�s jumbled molecules (right). Source: MIT

This alignment of the polymer molecules results in a fiber about 300 times more thermally conductive than normal polyethylene along the direction of the individual fibers, according to the MIT researchers.

The team noted that the high thermal conductivity could make such fibers useful for dissipating heat in many applications where metals are now used, such as solar hot water collectors, heat exchangers and electronics equipment.

The researchers noted that the high level of conductivity distinguished this material from other thermally conductive plastics being developed using other techniques. For instance, some work in this field has centered on adding other materials, such as carbon nanotubes, to the mix. However, these materials have achieved only modest increases in conductivity because the interfaces between the two kinds of material tend to add to thermal resistance. The MIT researchers noted that the interfaces scatter heat, thus limiting the improvement.

With this new method developed to draw fibers out of a solution, the material�s conductivity was enhanced so much that it was actually better than that of about half of all pure metals, including iron and platinum.

Moving Forward

The results achieved so far already represent the highest thermal conductivity ever seen in any polymer material, according to MIT. Already, the degree of conductivity they produce, if such fibers could be made in quantity, 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 the coils on the back of a refrigerator or in an air conditioner), cell phone casings or plastic packaging for computer chips.

Other applications might be devised that take advantage of the material�s unusual combination of thermal conductivity with light weight, chemical stability and electrical insulation.

So far, the team has just produced individual fibers in a laboratory setting. The challenge going forward is to scale production up to what the researchers call �a macro scale,� which will allow the production of sheets of material with the same properties.