How Graphene Could Improve the Efficiency of Electric Motors

Researchers at the U.S. Department of Energy’s (DOE) Pacific Northwest National Laboratory have found that adding graphene to copper wiring can improve the performance of electric motors and other applications.

Graphene is comprised of pure carbon and extracted from graphite; it is known for being a light but durable material which can be utilized in various applications.

Materials scientist Keerti Kappagantula and her colleagues at Pacific Northwest National Laboratory (PNNL) discovered that by adding graphene to copper, it was possible to improve the metal’s temperature coefficient of resistance – a concept which explains why metals like copper get hot when electric current runs through them.

If too high a temperature is reached, performance issues can occur. This is one of the concerns with electric motors, a common user of copper wiring.

Understanding this issue, Kappagantula and her colleagues have been researching ways to reduce the temperature coefficient of resistance while also enhancing the ability of a metal to conduct electricity which can provide further performance improvements. The key question in their research has been whether the addition of other materials to a metal could increase metal conductivity, and if that could be done at high temperatures.

Adding graphene to the metal was the answer to achieving both aspects.

What are the Benefits of Using Graphene?

The researchers at PNNL found that adding 18 parts per million of graphene to electrical-grade copper reduced the temperature coefficient of resistance by 11% without decreasing electrical conductivity at room temperature. Maintaining or even improving electrical conductivity can benefit efficiency.

It was noted in the PNNL press release announcing findings of the research that for electric motors used in electric vehicles, an 11% increase in electrical conductivity of copper wire winding translates into 1% gain in motor efficiency. And the more efficient the motor can be, the less energy it drains from the vehicle’s battery.

Keerti Kappagantula shows off the highly conductive copper-graphene composite wiring she developed with her colleagues.

According to Kappagantula, graphene has very high electrical conductivity compared to copper. “So, using it as an additive in copper to improve its conductivity is beneficial to create pathways for electrons to zoom through during energy transport,” she said in an interview with Power & Motion.

This will benefit copper electrical wiring used in electric motors as well as that used to distribute energy throughout homes and businesses.

She said the biggest benefit provided by use of graphene with copper wiring will be the ability to achieve higher electrical conductivity at elevated operating temperatures. This could help overcome current challenges with electric motors which have to be kept within a certain operating temperature range. If they get too hot, electrical conductivity drops and performance is negatively impacted.

However, with the addition of graphene in the copper it could be possible to maintain conductivity even if higher operating temperatures are reached.

To date, Kappagantula said electric motor designers have had to deal with copper losses as a given issue in designing a motor. As a way of overcoming this, they have enhanced their thermal management solutions to lower temperatures within motors. “Our composites can decrease thermal losses, which can provide additional design freedom for motor designers,” she said.

Kappagantula also noted that it could be possible to improve the mechanical properties of copper through the addition of graphene which could offer further performance benefits as well.

Xiao Li, a materials scientist, holds samples of highly conductive metal wires created on the patented Shear Assisted Processing and Extrusion platform.

Overcoming Preconceived Notions of Metal Additives

The integration of additives into metal typically increases its temperature coefficient of resistance. But the copper-graphene composite wiring developed by the PNNL researchers demonstrates the opposite is actually possible.

Solid Phase Processing Offers Alternative for Metals Manufacturing

Production of metals and composites has typically required the melting of metals which are then heat treated, forged, rolled or have various other processes done to them, all of which can be energy intensive.

Solid phase processing (SPP), however, uses high shear strain to apply mechanical energy to a metal to create friction heat which then deforms it. Not only does this reduce the amount of energy needed to produce a metal or composite but it also enables a more tailored microstructure to improve their properties.

Pacific Northwest National Laboratory (PNNL) has applied this concept to its patented ShAPE, or Shear Assisted Processing and Extrusion, method which brings together an extrusion machine, tooling and processing techniques to manufacture wire, bar and tubes.

It works by pressing a metal feedstock against an extrusion die within the rotating head of the extrusion machine. This creates friction to generate heat to deform the feedstock, softening it to extrude through the rotating die. Advanced sensors and controls help to monitor and regulate the process.

PNNL states on its website that a combination of linear pressing and rotational mixing cause extreme deformation to occur, enabling the creation of material chemistries and structures not possible with conventional extrusion machines.

In addition, it can extrude materials faster while using less energy.

Kappagantula and her colleagues had previously conducted detailed structural- and physics-based computational studies to demonstrate how the addition of graphene could actually enhance the electrical conductivity of metals. This research showed the use of solid-phase processing was a key enabler to doing so as it allowed for the creation of a microstructure on the copper wiring which has tiny flakes and clusters of graphene.

Having both flakes and clusters of the material is necessary to achieve conductivity at higher temperatures.

Kappagantula said she and her colleagues tried adding graphene to copper in bulk scales as many other researchers over the years have done. “However, the persistent issues they reported were seeing porosity in composites microstructure, or graphene agglomerating in carbon chunks, both of which are detrimental to electrical and mechanical properties of the composites,” she explained.

“Our approach to manufacturing these wires using a solid phase processing technique via the shear assisted processing and extrusion (ShAPE) platform avoids both these issues through a combination of composite design and process window design,” she said. “We make copper-graphene composites with a good distribution of graphene and minimal porosity in the copper microstructures. The interfaces between copper and graphene in the composites we manufactured are very conducive to energy carrier transport.”

Using a testbed designed at PNNL which measures electrical properties with a high degree of precision and accuracy, the research team was able to validate its copper-graphene composite wiring can provide improved conductivity.

Watch PNNL's video below to see the ShAPE platform in action.

The Next Steps for Bringing Copper-Graphene Composite Wires to the Market

Kappagantula sees her team’s findings benefiting a range of applications in which electricity is used, including electric motors and electrical wiring for buildings. For the latter, a more conductive copper wire could help better meet the increasing demand for electric power in cities – which are expected to continue growing in size over the coming years – while doing so more efficiently.

The team is working on further customization of the copper-graphene material as well as measuring other necessary properties including strength, fatigue, corrosion and wear resistance. Understanding these will help ensure it can be used in various market applications.

In addition, Kappagantula said the team is scaling up its manufacturing capabilities as well as working with motor wire manufacturing facilities to determine how the insulation coating typically seen for electric motor wires will affect the composites’ electrical performance.

“There is always more research to pursue in this area, where we are constantly looking for other, better process windows and composite formulations that result in even higher electrical performance improvements,” she concluded.

Aditya Nittala measures copper wire conductivity using specialized equipment developed at Pacific Northwest National Laboratory.

Editor’s Note: Power & Motion's WISE (Workers in Science and Engineering) hub compiles our coverage of workplace issues affecting the engineering field, in addition to contributions from equity seeking groups and subject matter experts within various subdisciplines.