This article was updated on the 19th August 2019.
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Carbon nanotubes are superb conductors of heat, infinitesimal in size and are used to prevent overheating in next-generation computing devices or as fillers to enhance the thermal conductivity of insulating materials, such as durable plastics or engine oil.
However, a research team at Rensselaer Polytechnic Institute discovered that the role of nanotubes as thermal superconductors is greatly diminished when mixed with materials such as polymers that make up plastics.
Pawel Keblinski, Assistant Professor of Materials Science and Engineering and Head of Rensselaer’s research team stated: “Carbon nanotubes are superior thermal conductors by themselves, but that doesn’t mean they will exhibit the same level of high conductivity when integrated into other materials”. The Rensselaer team’s research was published by Nature Materials in 2003.
A team of researchers found that 1% of carbon nanotubes added to epoxy and other organic materials increased the thermal conductivity of the newly created composites by two or three times; however, using conventional engineering estimates, Keblinski noted that the conductivity of the composites should have had a 50-fold increase.
Keblinski stated that: “Atoms forming stiff carbon nanotubes vibrate at much higher frequencies than the atoms in the surrounding material. This leads to high interfacial resistance for the heat flow between the tubes and the other elements”.
Energy exchange between two different elements is immediate and plentiful when frequencies in both are similar. Interfacial resistance happens when the frequencies are different, and the heat energy has a difficult time leaping from one element to the next.
To test the magnitude of the problem, Keblinski and his Rensselaer collaborators performed computer simulations on a model nanotube composite. Meanwhile, another research group headed by David Cahill at the University at Illinois at Urbana Champaign heated real carbon nanotubes with a laser. From the rate of cooling, in both the simulation and the physical experiment, the researchers derived the value of the interfacial resistance. In both instances, they found the resistance was so high that it limited the thermal conductivity of the nanotubes.
One way to reduce the interfacial resistance in such nanocomposites is to induce a stronger bond between the nanotube and other materials to make it easier for heat to cross from one element to the next. However, extensive bonding may distort the original nanotube structure that allows the tubes to be a superconductor of heat in the first place.
Keblinski remains optimistic about the use of carbon nanotubes to improve insulating materials. He has said: “By adding a small fraction of carbon nanotubes to such materials, we can still increase the thermal as well as electrical conductivity. So, although we may have to lower our expectations, we have not given up hope quite yet that nanotubes will improve materials for a number of applications”.