Controlling the electrical conductivity by altering the temperature is certainly nothing new. Semiconductors and metals show relatively small changes in resistivity with heating, while much larger variations are possible by warming over metal-insulator and superconducting transitions. However, a team of researchers from MIT have recently taken another approach, and exploited liquid-solid phase changes to control the thermal and electrical conductivities of a material [Zheng et al., Nature Comm (2011) doi:10.1038/ncomms1288].
The team used a percolated composite material; in this case graphite flakes suspended in hexadecane. Hexadecane has a melting point close to 18 oC, so while the composite is warm, the solid flakes are freely suspended in the liquid. However, as the hexadecane begins to crystallize upon cooling, needle-like crystals push the graphite flakes together. The result is a drop in the electrical resistance of two orders of magnitude, and an increase in the thermal conductivity by a factor of three.
The large change in electrical compared to thermal conductivity is the result of the contrast between the electrical and thermal properties of hexadecane and graphite. While the electrical conductivities between graphene and hexadecane are wildly different, their thermal properties are much similar.
The change in conductivities appears to be completely reversible, after the first cooling and warming cycle. The initial cycle differs, as before the hexadecane is first cooled, the flakes are distributed in small clusters. During freezing the clusters are pushed together forming a percolation network, which they then remain in during further cycles.
Although the authors of the paper have focused on graphite flakes in hexadecane, they point out that this approach could be replicated in other materials. As such the properties of the composite could be tuned. The key requirements are that the solid flakes can remain suspended in the liquid, and that the liquid is able to crystallize on cooling.
Prof Gang Chen is confident that their approach to controlling conductivity will lead to a number of applications, such as “resettable fuses for electrical circuits, thermal storage materials for buildings, and solar thermal plants.”
While the graphene-hexadecane composite experiences a phase change at around 18 oC, the team are now hoping to extend their approach “to other materials at different temperature ranges” by using such materials as “ionic liquids and molten salts”. They will also be “optimizing structures to achieve more contrast”.


Stewart Bland

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