ORNL scientists used new techniques to create long lengths of a composite copper–carbon nanotube material with improved properties for use in electric vehicle traction motors. Image credit: Andy Sproles, ORNL
In the pursuit of electric cars with better range and lower cost, researchers have sought improvements to every inch of the vehicle, from a lighter-weight chassis to more energy-dense batteries. ORNL scientists recently contributed a new material to the cause, creating a method to produce a composite that is both lighter and better at conducting electricity than standard electric motor components.
Scientists have long sought to increase the conductive capacity of copper wires without adding extra weight. ORNL researchers turned to carbon nanotubes, or CNTs, which are nanometer-sized tubelike structures made of carbon that are lightweight and 10 times stronger than steel, with exceptional electrical conductivity. Scientists deposited CNTs on flat copper substrates, producing a metal–matrix composite material with better current-handling capacity and tensile strength than copper alone.
Previous attempts by others to join CNTs and copper had resulted in materials that were only micrometers or millimeters long, with limited scalability, or that were longer but performed poorly. To overcome the problem, ORNL researchers deposited single-wall CNTs using electrospinning, a commercially viable method that creates fibers while a jet of liquid speeds through an electrical field. The technique provides control over the structure and orientation of deposited materials, explained ORNL materials scientist Kai Li. In this case, the process allowed scientists to successfully orient the CNTs in one general direction to facilitate the flow of electricity.
The team then used magnetron sputtering, a vacuum-coating technique, to add thin layers of copper film on top of the CNT-coated copper tapes. The coated samples were annealed in a vacuum furnace to produce a highly conductive copper–CNT network.
Using this method, ORNL scientists created a copper–carbon nanotube composite 10 centimeters long and 4 centimeters wide with exceptional properties. Researchers found the composite was able to carry up to 14 percent greater current than pure copper, with tensile strength improved up to 20 percent, making it ideal for ultra-efficient, power-dense electric vehicle traction motors.
The material can be deployed in any on- and off-board component that uses copper — creating more efficient bus bars and connectors, for instance — as well as for applications such as wireless and wired vehicle charging systems.
Project lead Tolga Aytug said that “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. 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.”
The work builds on a rich history of superconductivity research at ORNL, which has produced superior materials to conduct electricity with low resistance. ORNL’s superconductive wire technology invented in the 2000s was licensed to several industry suppliers, enabling such uses as high-capacity electric transmission with minimal power losses.
“Electric motors are basically a combination of metals — steel laminations and copper windings,” said Burak Ozpineci, who leads ORNL’s Vehicle and Mobility Systems Research Section. “To meet DOE’s 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.”
