Imagine your smartphone expanding up to twice its size if you unfold it. Wouldn’t it be wonderful? There is a researcher and a professor at Hanyang University who brought this world a step closer to such technology. Kim Hak-sung (Mechanical Engineering)’s recent paper “UV-assisted flashlight welding process to fabricate silver nanowire/graphene on a PET substrate for transparent electrodes” revealed a new progress in technology to weld silver nanowire onto PET (polyethylene terephthalate, a thermoplastic polymer) substrates.
In order to actualize a foldable smartphone and commercialize it, the flexible part must be both transparent and durable. Although there has been a decade of research in the field to discover such technology, one has yet to be found. The two main obstacles were, first, to keep the wires laid in a knit-like organization without raising the electronic resistance through time. The issue here was while the flexible display is folded and unfolded repeatedly, mechanically placed silver nanowires are slowly detached from the substrate. A substrate is a substance or layer that underlies something, or on which some process occurs. This leads to bigger resistance, as the road of a same amount of electricity can move is technically reduced. If resistance increases more than a certain level, the display malfunctions, making the entire device useless.
Another obstacle was not being able to weld the nanowires to the substrate. This is because silver nanowire melts at 300 degrees Celsius while the PET substrate melts at 150 degrees. “Not even experts in the fields believed me when I told them I could weld silver nanowire onto the PET substrate,” chuckled Kim. Using PET substrate is also the key to manufacturing cheap and flexible displays, as a thin ceramic substrate, no matter how thin they are, inevitably cracks after repeated use.
(Photo courtesy of Kim)
Kim solved both dilemmas by welding silver nanowires onto PET substrate, using flashlight sintering. Flashlight sintering uses a lamp filled with Xenon gas, a highly inert gas due to its structure. Kim drew this idea from skin care technology called Intense Pulsed Light (IPL) where they use a flashlight instead of a laser to burn moles or wrinkles without damaging the skin. “I wondered, Can the polymer substrate and the nanowire work as the skin and the mole?” mentioned Kim. With academic interest, Kim researched further during his post-doctorate degree at UCLA.
By welding the silver nanowire instantly with the light, it reduces tech-time and therefore reduces the manufacturing price. Moreover, Kim added a layer of graphene to the network of wires to further enhance the conductivity. Graphene is a form of carbon, consisting of a single layer of carbon atoms in a hexagonal lattice. Although it is a form of carbon, the thickness being one atom makes it look transparent. Welded silver nanowire with a layer of graphene prevents the resistance from raising even more, extending the lifetime of the display.
The hardest part of wielding such results was the skepticism. “Because the IPL technology did not exist in Korea, I was often scoffed at by others. So, I had to make my own devices as there were no research funds,” reminisced Kim. For about two years after Kim started making progress and received a big government project, not a lot of people believed that silver nanowire welding was even possible. Now, thanks to Kim, we will soon be able to see foldable smartphones.
Kim So-yun dash070@naver.com
Photos by Kang Cho-hyun