Move Over Silver....alternatives To Silver Inks

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Move Over Silver … This article is based off the NanoMarkets report Silver Inks and Pastes for Printable Electronics: 20082015

Alternatives to Silver Inks Page | 1 NanoMarkets predicts that by 2015, 184.5 million ounces of silver will be used by the electronics industry (see report, Silver Inks and Pastes for Printable Electronics: 2008-2015). This isn’t surprising, given that silver is more conductive than copper, gold or any other element; even its oxide is conductive. Unfortunately, silver prices often reflect demands completely unrelated to availability, causing squeamishness among industrial users. Even without the drama of the Hunt Brothers infamous 1980 attempt to manipulate the market, it is prudent to consider lower-cost alternatives to silver. These could be in any form from other minerals to silver alloys and even novel production methods. Alternatives -- Materials In some applications, such as medical implants, silver can be replaced with substitute materials such as titanium or biocompatible polymers. But in industrial applications where conductivity is important, the first alternative that comes to mind is usually copper. Pure Copper Initially, copper may seem like an ideal substitute for silver. While it has 95 percent of silver’s conductivity, it’s only one percent of its price. What’s not to like? Plenty, as it turns out. Copper oxide, the verdigris that makes the Statue of Liberty green, is non-conducting. While it does protect the underlying metal from oxidizing further, the challenge of oxidation remains at any scale, if there is a chance of exposure to air. Worse, at a small enough size, copper powder is so reactive, it can ignite. This may be fun in high school chem lab, but a disaster for large-scale manufacturing. Formulations have been created that reduce the chances of oxidation, but they add to the price. Silver-plated copper inks remain a potential option for printed RFID tags, especially in situations where economics dictate that they can barely be priced above paper barcodes (such as inventory-tracking of inexpensive parts and goods). One solution is the creation of copper nanoparticles. According to Vivek Subramanian, associate professor of electrical engineering and computer sciences at the University of California, Berkeley, copper nanoparticles are as conductive as silver, but suffer from the oxidation problem, and therefore require sintering in nitrogen or formalin gas. Subramanian’s team studies the metallic, semiconductor, and insulating nanoparticles for use in printable inks.

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Alloys As Cecil Adams once wrote in his Straight Dope column, one of the more infamous copper-silver alloys was the mixture in Tyco Brahe’s artificial nose. Supposedly, Brahe’s purpose was to get a more flesh-like color, but lower cost may have been a factor too, as it often is with silver alloys. Carbon-silver inks, for example, were developed in response to the 1980s price hikes, and are still in use today. They are a balance of trade-offs: While they are lower cost, they are also less conductive than silver alone. In addition, the R&D cost to create the perfect admixture is not inconsiderable. Carbon/Graphite The allotropes of carbon have such contrasting identities, the element could almost star as a science fiction superhero. Carbon itself is non-conductive, as is its best-known form, diamond; but as graphite, it is conductive enough to be placed high on the list of silver alternatives. According to Michigan State Chemical Engineering professor Lawrence Drzal, graphites have the potential to be competitors to traditional conductive additives like copper and silver. His lab has fashioned xGnP Exfoliated Graphite NanoPlatelets, which as monolayer coatings have a conductivity similar to that of ITO. They are currently being commercialized by XG Sciences, Inc. Carbon Nanotubes Carbon nanotubes may prove to be the ultimate in good news/bad news. IBM Fellow Phaedon Avouris and Columbia University professor Tony Heinz just won the Julius Springer Prize for Applied Physics for demonstrating the possibilities of nanotubes as transistors and logic circuits, along with their optical properties, which open up new realms for nanophotonics. But while carbon nanotubes may thrill physicists and chemists, the medical community is considerably less impressed. Last May, Ken Donaldson of the University of Edinburgh published a report in Nature Nanotechnology [1] that showed mice exposed to carbon nanotubes exhibited tissue damage similar to the effects of asbestos inhalation. The research suggests that far from being benign configurations, carbon nanotubes may be potential cancer agents. Calls for regulation are already being promulgated, what this will ultimately mean for large-scale manufacturing is yet unknown, but it does reduce some of the brave new world optimism. Alternatives -- Methods Another approach makes perfect sense in a world becoming ever more conscious of limited natural resources: simply use less silver, as in smaller particles. Silver and other Nanoparticles Nano-sized silver and other conductive inks can be ink-jetted, without the risk of clogging nozzles, thus saving both material costs and production runs. According to silver ink supplier Cabot, the cost savings of ink-jet over screen printing could be considerable, given the potential reduction in thickness of the ink jet layer. In Cabot’s example, the company compared an 8

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micron screen-printed layer to an 0.4 micron ink jet printed layer, to calculate it would only cost 0.76 for ink jet printing versus $15.12 for screen printing. Nanoparticle-sized inks also can be cured at much lower temperatures, thus opening up a wider range of substrates, including lower-cost plastics. For example, last year Bayer Materials Science (BMS) introduced BayInk, which the company touts as having a cure temperature compatible with plastics. BMS also suggests that BayInk would be ideal for RFID, which is no coincidence since this is the major new market for conductive inks. Inks can also be sintered. In this process, particles are bonded together at just below the melting point. The great advantage is that they are joined, rather than merely touching, greatly increasing the inter-particle contacts. This is one of the ways that using smaller particles improves “conductivity efficiency.” Remember that in essence, inkjet printing is depositing dots. But current can only flow between dots where they touch. As with polka dots, larger dots are surrounded by proportionately more “white space,” where current cannot flow. With nanoparticles, there are many more, smaller dots, closer together, and thus more dots coming into contact with more surface area of other dots—for the same, or less, volume of silver, you get greater conductivity. Still, Drzal cautions, while the intrinsic conductive properties of individual particles can be very high, once they are assembled into a composite, the contact resistance—where the particles come together—can become high as well. As a result, conductivity requirements of the bulk materials can be lower. Unfortunately, there are also human health and environmental concerns with nanosilver particles, since they are small enough to be inhaled and could also possibly penetrate human skin. While their toxicity to bacteria is more welcome, it is still a concern, since it is possible nanosilvers could also kill beneficial bacteria. Worse, surviving generations of bacteria could potentially grow resistant. Given these concerns, regulatory agencies in many countries will likely examine the use of nanosilvers, which could affect both the cost, and even the possibilities for manufacturing nanosilver-based products. While there are many silver alternatives already in the marketplace, they are not without concerns. Fortunately, the price of silver will continue to drive innovative solutions for replacements. At the moment, however, the quest for a material that is cheaper than silver, as or nearly as conductive, and doesn’t oxidize leaves you, in the words of Drzal, “Without too many choices.” Bibliography [1] Nature Nanotechnology 3, 423 - 428 (2008) Published online: 20 May 2008 For additional information about NanoMarkets and its full listing of reports and services please visit us on the web at www.nanomarkets.net or emailing us at sales ” at”nanomarkets.net.

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