A tiny coaxial nanocable has been created that is much smaller than a human hair. The cable could be used as a transmission line for radio frequency signals or to build next-generation energy storage, a tantalizing prospect for wind and grid developers.
The nanocable was developed through recent graphene-research techniques and has a higher capacitance than previously reported microcapacitors. The cable is a solid copper wire surrounded by a thin sheath of insulating copper oxide similar to cable television coaxial cables. In the case of TV cables, a third layer is copper, but in the nanocable it is a thin layer of carbon measuring just a few atoms thick. The cable is about 100 nanometers, or 100 billionths of a meter wide.
The cable can be used to build capacitors, solid-state energy-storage devices. They use a positive and negative electrical conductor separated by a thin layer of insulation. This separation creates an electrical potential that increases as the charges increase and the distances between them decreases. The proportion between charge density and separating distance is called capacitance, and it’s the standard measure of efficiency of a capacitor. “The capacitance is up to 143 μf/cm2, better than the best previous results from microcapacitors,” says study co-author Jun Lou, associate professor of mechanical engineering and materials science at Rice University.
“When people make graphene, they usually want to study the graphene –single-atom thick carbon layer–and they aren’t very interested in the copper,” says Lou. “It’s just used as a platform for making the graphene.” It may be possible to build a large-scale energy-storage device by arranging millions of the tiny nanocables side by side in large arrays, says Lou.
Rice University researchers made the discovery by chance, intending to make pure copper wires covered with carbon. The techniques for making the wires, which are just a few nanometers wide, are well established because the wires are often used as “interconnects” in state-of-the-art electronics. Liu used chemical vapor deposition (CVD) to cover the wires with a thin coating of carbon. The CVD technique is also used to grow sheets of graphene on films of copper. WPE