Bending the rules: ASU students spearhead graphene-metal inventions

Wonmo Kang’s research group develops ultrastrong, flexible and conductive composites to supercharge the energy industry

 Wonmo Kang, an associate professor of mechanical engineering in the School for Engineering of Matter, Transport and Energy, part of the Ira A. Fulton Schools of Engineering at Arizona State University, works in his lab with students. Pictured left to right: Associate Professor Kang, mechanical engineering doctoral student Uschuas Dipta Das, postdoctoral research scholar Wonjune Choi, and materials science and engineering doctoral student Jiali Yao. Together with his team of students, Kang has recently published a series of high impact papers in top journals. Photographer: Erika Gronek/ASU

In the quest for innovation, from the recently announced iPhone 16 to the latest Tesla Model S, engineers are continuously seeking novel materials to manufacture products that are more compact, lighter and sturdier. 

Among these novel materials, graphene, with its incredibly strong carbon-to-carbon bonds, is one of nature’s most durable materials and leads to highly stable composites when combined with metal. However, most existing graphene-metal composites lack flexibility and often fracture when stretched or bent.

With the support of his team of students, Wonmo Kang, an associate professor of mechanical engineering at the School for Engineering of Matter, Transport and Energy, part of the Ira A. Fulton Schools of Engineering at Arizona State University, created a graphene-coated nickel wire that has the highest combined strength and ductility — the ability to bend or stretch without breaking — in the graphene-metal composite field.

“It’s very difficult to do, and we’re so happy about this result,” Kang says.

Graphene-metal composites lack flexibility largely due to how the materials are mixed. For example, when producing nickel-graphene composites, normally researchers add a small amount of graphene flakes in a large nickel matrix. Imagine a smidgen of breakfast cereal in a bowl full of milk.

While manufacturing composites using this approach makes metals stronger, it can also make them brittle, breaking easily when force is applied at weak nickel-graphene interfaces by a mechanism known as dislocation. Every metal has atomic defects, and when pulled or stretched, atoms move to accommodate the stress. Permanent material deformation and eventually mechanical failure happen when too many atomic defects escape to the surface.

Using a process called chemical vapor deposition, Kang’s group continuously coats nickel with graphene. By fully covering nickel with such a strong material, the whole composite becomes more stable. Most importantly, it can be stretched and bent without breaking because dislocations are tightly bound within the graphene shell.

Taking this approach, Kang’s research group continuously combined copper and graphene to manufacture a wire with 40% more conductivity than copper electrical conductors.

Cutting electricity costs 

Kang tests graphene-copper wires in his laboratory. Photographer: Erika Gronek/ASU

Deciding to use copper or graphene for electrical conductivity is like choosing between being stuck in traffic (electron scattering) on a local road or taking a highway with fewer cars (electron carriers) and a high speed limit (electron mobility). Think about electrons as cars and copper or graphene as roads. 

Copper, which has many electron carriers, is the most widely used metal conductor in history; however, its atomic structure comes with issues that lead to a limited electrical conductivity. Similar to a congested community road, when electric current is applied and electrons travel from one end of a copper conductor to the other, they collide with each other and copper atoms, creating heat in a process known as joule heating.

The increased temperature leads to higher vibrations in the conductor, causing more collisions and stopping many electrons from reaching their final destination — just like how one car accident can stop everybody on the road or cause more collisions.

In contrast, graphene is like a highway with limited cars. As a two-dimensional carbon material, graphene doesn’t have as many electron carriers as copper, which causes a limited conductivity when used as a conductor regardless of electrons moving faster within it.

Kang’s group seeks to merge the best of both worlds, creating wires with exceptionally high conductivity.

“Mobility of electrons in a graphene conductor is three orders of magnitude higher than that of copper,” Kang says. “My group has found a way to combine copper and graphene such that the accessible electrons in the metal can move freely through graphene.”

Kang adds, “Due to their high conductivity, our graphene-copper wires would potentially reduce energy loss during electric transmissions from power plants to users, hence cutting monthly energy costs for a lot of people.”

To share these impressive advancements, Kang worked with his group of students to write a series of high-impact research papers, which were featured in the top most-cited engineering journals including Advanced Functional Materials and Small Methods.

Paving the way for students to be successful 

“My students do almost everything,” Kang says. “I just give them the idea, demonstrate how it would work and let them prove the hypothesis through experiments.”

The research group includes postdoctoral research scholars Chunghwan Kim and Wonjune Choi as well as materials science and engineering doctoral student Jiali Yao and mechanical engineering doctoral student Uschuas Dipta Das. They all played significant but different roles in Kang’s projects.

Kim used his computational skills to develop theoretical models to predict material properties of the composites and design experiments. Then, Choi worked with Yao and Das to run the experiments, analyze data and, with Kang’s support, document the observations in preparation for the papers. The students say they believe their work with Kang has set them up for future success.

“Working on the graphene-metal composites has allowed me to deepen my understanding of theoretical modeling,” Kim says. “Collaborating with the experimentalist on the team strengthened my ability to connect computational theories with real world research and prepared me to dive into my field as an independent researcher.”

As a new doctoral student, Das plans to continue learning from his colleagues and practicing his research skills. His long-term goal is to become a renowned researcher in one of the U.S.’s national laboratories, and he says that working on graphene-metal composites was a great preparation for a successful research career in the near future.

Das is grateful for Kang’s support throughout these projects.

“Professor Kang is a very good mentor,” Das says. “As a new PhD student, I did not know anything, and I made a lot of mistakes. Throughout the process, the professor allowed me to experiment and try new things, and through his mentorship I actually learned something.”

Yao shares the same view. She says that Kang encouraged her to overcome her hesitations and get involved in the lab’s research projects.

“As a newbie, I doubted my research skill set,” Yao says. “With the support of Professor Kang, I feel proud that I contributed to a paper published in a high-impact journal as a new PhD student.”

Kang says he is proud of his team and is thrilled for the attention and recognition the group has received due to the recently published papers. 

“The Army Research Office and other research committees are interested in the mechanical properties of our composites, especially for high load applications,” Kang says. “I’ve been invited to submit a proposal, and I think it’ll go through.”

Every Friday, the team meets to discuss ongoing research projects’ challenges and exchange solution ideas. Choi found the group discussions particularly useful.

 “As a postdoc, I need to find a new job soon, and I feel like the skills I developed and things I learned in our weekly meetings set a solid base for a successful independent research career,” Choi says. “For example, I attended at least one conference per year and made good connections in my field, thanks to Kang and my colleagues.”