Inking a silver solution - particle-free ink for printed electronics
A particle-free silver ink for printed electronics applications may have several advantages over conventional inks.
According to the researchers, at the University of Illinois, USA, the new ink – which has a conductivity equivalent to that of pure silver (6.25x105 S/cm) – can be made up in minutes, rather than the several hours for particle-based inks, and remains stable for months. The team claims it can be printed through 100nm nozzles, allowing finely detailed features to be printed; and that its low viscosity (2mPa•s) makes it suitable for direct ink writing, inkjet printing and airbrushing. The technology was found to have a low annealing temperature (90ºC) and can be used on inexpensive plastics and paper.
The ink is a transparent solution of silver acetate and ammonia, with a little formic acid to lower its pH and introduce formate anions to reduce the silver complex formed. It has been synthesised using a modified Tollens’ process – the silver acetate was dissolved in aqueous ammonium hydroxide, and the formic acid was then titrated into the solution, which was mixed thoroughly.
The ink first reacts when the labile ammonia ligands that stabilise the complex evaporate and are allowed to dry at room temperature and pressure. The silver particles are formed only after patterning, as evaporation takes place. So far the ink has been tested on materials including polyimide substrates, PET, cellophane, cellulose acetate, glass and synthetic and natural fibres, the results show particularly good adhesion with the substrates.
Although it is ammonia-based, Professor Jennifer Lewis, who led the research, says the ink is no more toxic than other commercially available silver inks, and that its use would be subject to the same requirements for adequate ventilation and personal protective equipment that are standard for any other ammonia-based products and printed electronics materials.
Also, while no cost analysis has been carried out on it yet, she believes the ink will be cost-competitive.
Although still at the development stage, there appears to be no reason why the ink could not be used commercially. Professor Lewis says, ‘For processes such as inkjet printing and airbrush spraying, electronics manufacturers would not need to change their production line equipment. We are however modifying the ink for other processes, such as screen printing, that are more common in the printed electronics industry.
‘The ink could be used immediately, if we found an industrial partner interested in scaling up the synthesis process.’