New Thin Film Insulator Material for e-Readers and Transistors
Materials scientists at Johns Hopkins have found a new use for sodium beta-alumina, a compound that has until now been thought of as an electrical conductor. The researchers have found a way to use it as a thin film insulator. By orienting the compound in a different way, the material blocks the flow of electricity, but can induce large electric currents elsewhere. The material could have important usages in transistor technology as well as in screen-enabled devices such as electronic books.
A paper, published in the November issue of Nature Materials,describes sol–gel-beta-alumina films as transistor gate dielectrics with solution-deposited zinc-oxide-based semiconductors and indium tin oxide (ITO) gate electrodes. Sodium beta-alumina (SBA) has high two-dimensional conductivity, owing to mobile sodium ions in lattice planes, between which are insulating layers. SBA can provide high capacitance perpendicular to the planes, while causing negligible leakage current owing to the lack of electron carriers and limited mobility of sodium ions through the aluminium oxide layers.
“This form of sodium beta-alumina has some very useful characteristics,” said professor Howard E. Katz, who supervised the 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, 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.”
Sodium Beta-Alumina Film 100 Atoms Thick
The material’s thinness and transparency of the (on the order of 100 atoms thick) make it perfect for use in e-book readers, which depend on see-through screens and portable power sources, Katz said. He also notes that potential transportation applications include Head Up Display instrument readouts embedded in the windshield of an aircraft or automobile.
Sodium beta-alumina’s application as an insulator was a surprise, Katz said. The compound has been known for decades, and traditionally has been used to conduct electricity; it has been considered as a possible battery component. SBA 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 said. “The material acts as an insulator instead of a conductor. Our team was the first to exploit this discovery.” The researchers have filed for international patent protection for the discovery.
