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Reactive Silver Ink Allows Reliable Printed Electronics
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Reactive Silver Ink Allows Reliable Printed Electronics

One of the key issues when printing electronic circuits on the fly, is ensuring reliability. You don't wish to be in the situation where you have to go over the new circuitry by hand to look for places the conductive ink has not affixed to the surface properly, not if you are making complex 3D shapes with internal circuitry, and aiming to keep the price as low as possible.

Ideally, the ink should print properly the first time, with no need to check up afterwards. This allows for fully automated 3D printing of complex circuitry, and even 3D cvircuitry, or circuitry on the inside of an object never meant to be opened.

We already have one potential contender, in the form of Vor-ink, a silver/graphene hybrid that is relatively cheap to produce. However, that approach has been the only contender on the market and they are still not commercialised. Now, another lab is trying a similar approach, but with greater flexibility.

University of Illinois materials scientists have developed a different formula reactive silver ink for printing high-performance electronics on ubiquitous, low-cost materials such as flexible plastic, paper or fabric substrates. Literally printing electronics into the magazines you read, and the clothes you wear. It is still too expensive to produce to realistically be used for smart bar codes, but the potential applications are still innumerable, even if throw-away, use-once circuitry is not among them.

Most conductive inks rely on tiny metal particles suspended in the ink. The new ink is a transparent solution of silver acetate and ammonia. The silver remains dissolved in the solution until it is printed, and the liquid evaporates, yielding conductive features.

Jennifer Lewis, the Hans Thurnauer Professor of Materials Science and Engineering, and graduate student S. Brett Walker described the new ink in the Journal of the American Chemical Society.

“It dries and reacts quickly, which allows us to immediately deposit silver as we print,” Walker said.

The reactive ink has several advantages over particle-based inks. It is much faster to make: A batch takes minutes to mix, according to Walker, whereas particle-based inks take several hours and multiple steps to prepare. The ink also is stable for several weeks.

The reactive silver ink also can print through 100-nanometer nozzles, an order of magnitude smaller than particle-based inks, an important feature for printed microelectronics. Moreover, the ink’s low viscosity makes it suitable for inkjet printing, direct ink writing or airbrush spraying over large, conformal areas.

“For printed electronics applications, you need to be able to store the ink for several months because silver is expensive,” Walker said. “Since silver particles don’t actually form until the ink exits the nozzle and the ammonia evaporates, our ink remains stable for very long periods. For fine-scale nozzle printing, that’s a rarity.”

The reactive silver ink boasts yet one more key advantage: a low processing temperature. Metallic inks typically need to be heated to achieve bulk conductivity through a process called annealing. The annealing temperatures for many particle-based inks are too high for many inexpensive plastics or paper. By contrast, the reactive silver ink exhibits an electrical conductivity approaching that of pure silver upon annealing at 90 degrees Celsius.

“We are now focused on patterning large-area transparent conductive surfaces using this reactive ink,” said Lewis.

References

Particle-free silver ink prints small, high-performance electronics
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