Now it’s conductive; now it’s not

JOHNS HOPKINS (US)—A substance long known to conduct electricity is now being used in a different way to block it, opening up possible new applications in transistor technology and devices such as electronic books.

Johns Hopkins University materials scientists have found a way to produce the conductive compound in the form of an extremely thin, transparent film called solution-deposited beta-alumina. Instead of conducting, it blocks the flow of electricity, but can induce large electric currents elsewhere.

“This form of sodium beta-alumina has some very useful characteristics,” says Howard Katz, a professor of materials science and engineering who supervised the research team. “The material is produced in a liquid state, which means it can easily be deposited onto a surface in a precise pattern for the formation of printed circuits.

“But when it’s heated,” Katz explains, “it forms a solid, thin transparent film. In addition, it allows us to operate at low voltages, meaning it requires less power to induce useful current. That means its applications could operate with smaller batteries or be connected to a battery instead of a wall outlet.”

The discovery is described in the November issue of the journal Nature Materials and appears in an early online edition.

The transparency and thinness of the material (the hardened film is on the order of only 100 atoms thick) make it ideal, Katz says, for use in increasingly popular e-book readers, which rely on see-through screens and portable power sources. Possible transportation applications, he adds, include instrument readouts that can be displayed in the windshield of an aircraft or a ground vehicle.

The emergence of sodium beta-alumina as an insulator was a surprising development, Katz notes. The compound, known for decades, has traditionally been used to conduct electricity and for this reason has been considered as a possible battery component. The material allows charged particles to flow easily parallel to a two-dimensional plane formed within its distinct atomic crystalline arrangement.

“But we found that current does not flow nearly as easily perpendicular to the planes, or in unoriented material,” Katz says. “The material acts as an insulator instead of a conductor. Our team was the first to exploit this discovery.”

The Johns Hopkins researchers developed a method of processing sodium beta-alumina in a way that makes use of this insulation behavior occurring in the thin film form. Working with the Johns Hopkins Technology Transfer staff, Katz’s team has filed for international patent protection for their invention.

Funding for the research was provided by the U.S. Department of Energy, the U.S. Air Force Office of Scientific Research, and the National Science Foundation.

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