The properties of Carbon nanotubes (CNTs) include high strength, low weight, and excellent thermal and electrical conductivities. This makes them suitable materials for applications including reinforcement materials, energy storage and conversion devices, and electronics. Despite their potential, it has proven challenging to incorporate them onto plastic substrates for fabricating flexible CNT-based devices.
The breakthrough from Japan involves coating a polypropylene (PP) film with an MWNT film about 10μm thick and then exposing it to a mW UV laser. The result is a conductive wiring made of a combination of MWNT and PP. The research is detailed in Scientific Reports.
In a statement, project lead Associate Professor Dr. Takashi Ikuno said: “This process enables the easy ‘drawing’ of wiring and flexible devices for wearable sensors without the need for complex processes.”
The researchers attributed the formation of these wires to the difference in the thermal conductivities between the MWNT and the PP film.
As the MWNT/PP film is exposed to the laser, the high thermal conductivity of the MWNT layer causes the heat to spread along the length of the wire, resulting in high temperatures at the MWNT–PP interface and lower temperatures elsewhere in the PP film.
The team added that directly below the laser, where temperatures are the highest, the PP diffuses into the MWNT film to form a thick PP/MWNT composite, while a thin PP/MWNT layer is formed at the edges of the laser where temperatures are relatively low.
The proposed method is also said to allow for the fabrication of carbon wires with different resistance values within the same process by changing the irradiation conditions, which eliminates the need for additional steps.
Exposing the PP/MWNT film to high laser energies, achieved either by low scanning speeds, a high number of laser exposures, or the use of a high-powered laser, produces thicker wires with a higher concentration of MWNTs. Consequently, the lower resistivity of MWNT and the thicker wire lowers the resistance per unit length of the wire.
By precisely controlling the exposure of the MWNT/PP film to laser light, the researchers fabricated MWNT wires with a wide range of resistance values, from 0.789kΩ/cm to 114kΩ/cm. Furthermore, the wires were highly flexible and maintained their resistance, even when bent repeatedly.
The team added that with its simplicity, efficient utilisation of CNTs, and the capability to create high-quality wires, the new method has the potential to realise large-scale manufacturing of flexible carbon wiring for flexible sensors and energy conversion and storage devices.
“We expect the process cost to be significantly reduced compared to that for conventional methods. This, in turn, will contribute to the realisation of low-cost flexible sensors that are expected to have wide applications in large quantities,” said Dr Ikuno.
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