Jack Kenny, Editor11.11.09
Scientists at Xerox have created a new silver ink that they say will pave the way for commercialization and low-cost manufacturing of printable electronics. The ink, which they describe as “a silver bullet,” can be applied to plastics as well as fabrics for conductive circuitry. “This development will aid the commercialization of new applications such as ‘smart’ pill boxes that track how much medication a patient has taken, or display screens that roll up to fit into a briefcase,” said Paul Smith, laboratory manager at the Xerox Research Center of Canada, in Mississauga, ON.
The product was introduced recently at the Printed Electronics Europe Conference in Dresden, Germany.
“For years, there's been a global race to find a low-cost way to manufacture plastic circuits,” Smith said. “We’ve found the silver bullet that could make things like electronic clothing and inexpensive games a reality today. This breakthrough means that the industry now has the capability to print electronics on a wider range of materials and at a lower cost.”
The scientists report that the ink is formulated so that its molecules align precisely in a configuration that is optimal for conducting electricity.
The ink is formulated as an inkjet product, Smith said, “but it’s also available as a powder, so that others can do with it as they wish. It’s suitable for piezo inkjet – very similar to a home printer. It creates lines between 50µ and 40µ.
“Everybody’s goal is that if you are going to print these materials, you’d ideally like to print on flexible substrates,” Smith added. “That’s for all electronic materials. You can print on glass but you end up with limitations. It’s not very robust, not flexible, and it has weight. If you go flexible, you have all these other advantages: You take a lot of weight out of the device, and it’s more robust. In theory you can have it rollable. Potentially you take cost out of the manufacturing because you go roll-to-roll. The printable materials have to have specific properties, sit nicely on the substrate and not spread out.”
In order to function as a conductive material, the ink has to be able to melt. “Silver melts at 1000° C, but a flexible substrate wouldn’t tolerate that temperature,” said Smith. “If people are going to print this conductive ink they should be able to print on flexible substrates, and the ink has to be less than 140° C. We have brought the melting point of the silver down to that temperature. The user can bring the temperature to that level and the particles will melt into one another and form a line. That’s what differentiates this ink from others that are commercially available.”
Lowering the melting point of the silver involves reducing the size of the particles, Smith explains. “If you make very small particles in the nanometer range, the melting point gets lower and lower. We create small particles and make them stable in the ink, so that they don’t settle out. And then they can be jetted out of small inkjet nozzles.”
Xerox has been working on various printed electronics research projects for the past eight years, he said, and on this particular one for a few years. “The challenge is not just the particle size, but making them stable.” The silver particles want to join with their nearest neighbors, he added, but that would create instability in the ink. Stability is important for printing as well as for shelf life. “That’s what has taken time.”
The Xerox Research Center has the advantage of laboratory environments as well as “scale-up” facilities. “It’s one thing to create in the laboratory,” Smith declared. “At Xerox research labs we take the material from the test tube and we have the ability to scale up at our pilot plant, so that when we are creating these materials we have the ability to design them for future use. We have engineers who work on scaling up the project so that we can observe performance at a new level.”
As part of its commercialization initiatives, Xerox is aggressively seeking interested manufacturers and developers by providing sample materials to allow them to test and evaluate potential applications.
“The response has been very pleasing,” said Smith. “A lot of people are interested in a veery wide range of fields. We have developed a suite of materials for developing electronic devices – conductors, semiconductors, dielectrics. Some companies might only need the silver. They are all in unique fields.”
Using the new technology, circuits can be printed just like a continuous feed document without the extensive clean room facilities required in current chip manufacturing. In addition, the scientists have improved their previously developed semiconductor ink, increasing its reliability by formulating the ink so that the molecules precisely align themselves in the best configuration to conduct electricity.
Printable circuits could be used in a broad range of products, including low-cost radio frequency identification tags, light and flexible e-readers and signage, sensors, solar cells and novelty applications including wearable electronics.
Headquartered in Norwalk, CT, USA, Xerox employs 54,000 people worldwide.