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Nanotubes boost copper's current capacity by 14%

28 September, 2020

US scientists have created a composite material that, they say, can boost the electrical current capacity of copper wires by 14%, while improving their mechanical properties – such as strength and weight – by up to 20% compared to pure copper. The researchers, from the US government’s Oak Ridge National Laboratory (ORNL), add that the material could be used in any component that uses copper, resulting, for example, in more efficient busbars, smaller connectors and ultra-efficient, high-power-density motors.

The work is part of a programme aimed at reducing barriers to the wider adoption of electric vehicles, including cutting the costs of ownership and improving the performance and life of components such as motors and power electronics.

“Electric motors are basically a combination of metals – steel laminations and copper windings,” points out Burak Ozpineci, manager of ORNL’s Electric Drive Technologies programme and leader of its Power Electronics and Electric Machinery group. “To meet Department of Energy’s Vehicle Technologies Office 2025 electric vehicle targets and goals, we need to increase power density of the electric drive and reduce the volume of motors by eight times, and that means improving material properties.”

To produce their lighter conductive material with improved performance, the ORNL researchers deposited and aligned carbon nanotubes (CNTs) on flat copper substrates. This resulted in a metal-matrix composite material with a higher current-handling capacity and better mechanical properties than copper by itself.

Incorporating CNTs into a copper matrix to improve conductivity and mechanical performance is not a new idea. CNTs have the attractions of being lightweight, strong and electrically conductive. But previous attempts to create composites have been limited to samples that are only micrometres or millimetres in length, and have limited scalability – or to longer lengths that performed poorly.

The ORNL team decided to experiment with depositing single-wall CNTs using electrospinning, a technique that creates fibres by passing a jet of liquid through an electric field. It allows the structure and orientation of the deposited materials to be controlled, explains Kai Li, a researcher in ORNL’s Chemical Sciences Division. The process allowed the researchers to orient the CNTs in one general direction to enhance the flow of electricity.

The team then used a vacuum coating technique known as magnetron sputtering to add thin layers of copper film on top of the CNT-coated copper tapes. The coated samples were then annealed in a vacuum furnace to produce a highly conductive copper-CNT network by forming a dense, uniform copper layer, allowing the copper to diffuse into the CNT matrix.

The US-developed composite material uses nanotubes the improve the electrical and physical properties of copper and could boost the performance of motors and many other electrical components
Photo: Andy Sproles/ORNL, US Department of Energy

Using this method, the ORNL scientists created a copper-carbon nanotube composite that is 10cm long and 4cm wide and offers “exceptional properties”.

“By embedding all the great properties of carbon nanotubes into a copper matrix, we are aiming for better mechanical strength, lighter weight and higher current capacity,” says the project’s lead investigator, Tolga Aytug. “Then you get a better conductor with less power loss which, in turn, increases the efficiency and performance of the device. Improved performance, for instance, means we can reduce volume and increase the power density in advanced motor systems.”

As well as enhancing the performance and properties of electric motors, the new composite could also could have benefits in other applications where efficiency, mass and size are key factors, Aytug adds. The improved performance can be achieved using commercially viable techniques, opening up new possibilities for advanced conductors in a wide range of electrical and industrial applications.

The ORNL team is also exploring the use of double-walled CNTs and other deposition techniques, such as ultrasonic spray coating, coupled with a roll-to-roll system, to produce 1m-long samples.

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