Electrifying improvement in copper conductivity

Xiao Li, a materials scientist at PNNL, holds samples of highly conductive metal wires created on the patented ShAPE platform. Photo: Andrea Starr/Pacific Northwest National Laboratory.
Xiao Li, a materials scientist at PNNL, holds samples of highly conductive metal wires created on the patented ShAPE platform. Photo: Andrea Starr/Pacific Northwest National Laboratory.

A team of materials scientists at the US Department of Energy (DOE)’s Pacific Northwest National Laboratory (PNNL) has found that a common carbon compound produces remarkable performance enhancements when mixed in just the right proportion with copper to make electrical wires. It’s a phenomenon that defies conventional wisdom about how metals conduct electricity.

These findings, reported in a paper in Materials & Design, could lead to more efficient electricity distribution to homes and businesses, as well as more efficient motors to power electric vehicles and industrial equipment.

Keerti Kappagantula and her colleagues at PNNL discovered that graphene, single layers of the same graphite found in pencils, can enhance an important property of metals called the temperature coefficient of resistance. This property explains why metal wires get hot when electric current runs through them.

Researchers want to reduce this resistance while enhancing a metal’s ability to conduct electricity. For several years they have been asking whether metal conductivity can be increased, especially at high temperatures, by adding other materials to it. And if yes, can these composites be viable at commercial scale?

Now, they’ve demonstrated that they can do just that, using a PNNL-patented advanced manufacturing platform called ShAPE (Shear Assisted Processing and Extrusion). When the research team added 18 parts per million of graphene to electrical-grade copper, they found that its temperature coefficient of resistance decreased by 11% without decreasing electrical conductivity at room temperature. This is relevant for the manufacturing of electric vehicle motors, where an 11% increase in the electrical conductivity of copper wire winding translates into a 1% gain in motor efficiency.

“This discovery runs counter to what’s generally known about the behavior of metals as conductors,” said Kappagantula. “Typically, introducing additives into a metal increases its temperature coefficient of resistance, meaning they heat up faster at the same current levels compared to pure metals. We are describing a new and exciting property of this metal composite where we observe enhanced conductivity in a manufactured copper wire.”

Previously, the research team had performed detailed structural and physics-based computational studies to explain the phenomenon of enhancing the electrical conductivity of metals using graphene.

In this study, they showed that the solid-phase processing used to extrude the composite wire leads to a uniform, near pore-free microstructure punctuated with tiny flakes and clusters of graphene that may be responsible for decreasing the composite’s coefficient of resistance.

“We showed that flakes and clusters must both be present to make better conductors for high-temperature operations,” Kappagantula said.

When used for any industrial application, the new copper-graphene composite wires will provide great design flexibility, according to the research team. “Anywhere there’s electricity, we have a use case,” Kappagantula said.

For example, coiled copper wire forms are used in the core of electric motors and generators. Today’s motors are designed to operate within a limited temperature range because their electrical conductivity drops dramatically when they get too hot. With the new copper-graphene composite, motors could potentially be operated at higher temperatures without losing conductivity.

Likewise, the wiring that brings electricity from transmission lines into homes and businesses is typically made of copper. As the population density of cities increases, demand for power follows suit. A composite wire that is more conductive could potentially help meet that demand with efficiency savings.

“This technology is a beautiful solution for copper wiring in high-density urban settings,” Kappagantula said.

The research team is continuing its work to customize the copper-graphene material and measure other essential properties, such as strength, fatigue, corrosion and wear resistance – all of which are crucial to qualify such materials for industrial applications. For these experiments, the research team is manufacturing wires that are about the thickness of a US penny (1.5mm).

This story is adapted from material from Pacific Northwest National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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