Cupronickel understudies ITO

Monday 4th June 2012
Copper wires can be built in bulk and then "printed" on film to conduct current, transparently. Credit: Benjamin Wiley, Duke Chemistry

As certain materials like gold and silver find researchers discovering more economic substitutes for electronic applications, researchers at Duke University create a new set of flexible, electrically conductive nano wires by combining copper and nickel. The cupronickel wires, in the form of a film, conducts electricity, even under conditions that would break down electron transfer in plain silver and copper nanowires, claim the researchers.

Researchers at Duke University have created a new set of flexible, electrically conductive nanowiresby combining copper and nickel.

The cupranickel nanowires, in the form of a film, conducts electricity even under conditions that break down the transfer of electrons in plain silver and copper nanowire, claim the researchers.

Because films made with copper-nickel nanowires are stable and are relatively inexpensive to create, they are an attractive option to use in printed electronics, products like electronic paper, smart packaging and interactive clothing, said Benjamin Wiley, an assistant professor of chemistry at Duke. His team describes the new nanowires in a NanoLetters pape.

Benjamin Wiley, (left) assistant professor of chemistry at Duke said since films made with copper-nickel nanowires are stable and are relatively inexpensive to create, they hold potential in printed electronicsapplications and products like electronic paper, smart packaging and interactive clothing. 

The new copper-nickel nanowires are the latest nanomaterial Wiley's lab has developed as a possible low-cost alternative to indium tin oxide, or ITO. This material is coated on glass to form the transparent conductive layer in the display screens of cell phones, e-readers and iPads.

At $600 - $800/kg Indium is an expensive rare-earth element. Most  is mined and exported from China, which is cutting exports, causing an indium's price increase. Indium tin oxide (ITO)  is deposited as a vapour in a relatively slow, expensive coating process, which adds to the cost.  In addition, the film is brittle,  a major reason why signature pads at grocery store checkout lines eventually fail and why there is not yet a flexible, rollable iPad.

Last year, Wiley's lab created copper nanowire films that can be deposited from a liquid in a fast, inexpensive coating process. The conductive films are much more flexible than current ITO film. Copper is also one-thousand times more abundant and one-hundred times cheaper than indium. One problem with copper nanowire films, however, is that they have an orange tint not be desirable in a display screen and copper-based films oxidise  when exposed to air, turning old penny green.

Nickels, however, rarely  greens. Inspired by the U.S. five-cent nickel piece, Wiley wondered if hecould prevent oxidation of the copper nanowires by adding nickel. He and his graduate student, Aaron Rathmell (right)  developed a method of mixing nickel into the copper nanowires by heating them in a nickel salt solution.

"Within a few minutes, the nanowires become much more grey in color," Wiley said.

Rathmell and Wiley then baked the new nanowires at various temperatures to test how long they conducted electricity and resisted oxidation. Tests show  the copper-nickel nanowire films would have to sit in air at room temperature for 400 years before losing 50% of their electrical conductivity. Silver nanowires lost half their conductivity in 36 months under the same conditions and plain copper nanowires only lasted 3 months.

While the copper-nickel nanowires stack up against silver and copper alone, they will not replace ITO in flat-panel displays any time soon, Wiley said, explaining that, for films with similar transparency, copper-nickel nanowire films cannot yet conduct the same amount of electricity as ITO. "Instead, we're currently focusing on applications where ITO can't go, like printed electronics," he said.

The greater stability of cupronickel nanowires makes them a better alternative to both copper and silver for applications that require a stable level of electrical conductivity for more than a few years, important for certain printed electronics applications, Wiley said.

Printed electronics combine conductive or electronically active inks with the printing processes that make magazines, consumer packaging and clothing designs. Low cost and high speed of these printing processes make them attractive for the production of solar cells, LEDs, plastic packaging and clothing.

A Durham, NC startup company, NanoForge Corp, which Wiley co-founded has begun manufacturing copper-nickel nanowires to test in these and other potential applications.

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