A research group at the University of Tokyo announced that it developed a new elastic material with a high electrical conductivity for the first time in the world.
The group is led by Takao Someya, associate professor at School of Engineering at the University of Tokyo. The new material makes it possible to attach large electronic circuits with organic transistors to free-form surfaces, thus creating various new applications for organic transistors, the group said.
"The elasticity of the new material enhances the mechanical resistance to bending," Someya said.
A paper about this development was posted in the Aug 7th issue of "Science Express," an online magazine of advance reports from the US journal "Science."
The new material, mainly consisting of single wall carbon nanotubes (SWNT), an ionic liquid and an elastic resin, is a black rubber-like substance. It has an electrical conductivity of 57S/cm, which is much higher than that of the commercially available conductive rubber (0.1S/cm).
When stretched, the new material can be 1.38 times longer than its original length at the maximum, without sacrificing its conductivity. When the material is wired in a mesh pattern, it can be extended up to 2.34 times longer by the effect of deformation of the mesh pattern, the group said.
Thus far, there has been a conductive rubber manufactured by mixing carbon nanotubes with resin, but its conductivity was only 10S/cm and it could only be extended by 1.1 times at the most.
According to Someya, the key point in the latest development is that the group "discovered, for the first time in the world, a combination of an ionic liquid and an elastic resin that is soluble in the liquid." Specifically, the group used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) for the ionic liquid, and the Daiel-G801, which is a kind of fluorine resin, for the elastic resin.
In general, ionic liquids are known to dissolve SWNTs without causing aggregation. It is estimated that, if a resin can be dissolved in an ionic liquid, then SWNT can be evenly dispersed in the resin, thus resulting in a high conductivity. Until now, researchers had been unable to find an elastic resin that was soluble in an ionic liquid.
Another key point is the adoption of extremely long SWNTs. The SWNTs used in the material measure 2-4mm per piece and only 3nm in diameter. This is equivalent to a machine-sewing thread with a diameter of 0.3mm that measures 200-400m.
"It is highly likely that the long SWNTs play an important role in enhancing the conductivity and the elasticity," Someya said. "We assume that the SWNTs are tangled together like spaghetti and they are extended in that state. We think this is why the conductivity does not change even when the material is stretched."
Someya's group developed an "electronic artificial skin," which can be attached on free-form surfaces with the use of a wiring formed in a mesh pattern, in 2005. At that time, the wiring could only be extended to 1.25 times longer than its original length.
The major difference from the wiring made of the latest material is that the previous wiring could only be extended by the effect of deformation of the mesh structure whereas the latest wiring extends by itself.
"Nonelastic materials have a risk of breakage when they are hit by a pointed object, etc, even if they are bendable," Someya said. "For flexible devices, elasticity is a very important factor."
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