In the search to create materials that can withstand extreme radiation, Yanwen Zhang, a researcher at the Department of Energy's Oak Ridge National Laboratory, says that materials scientists must think outside the box.
For example, when considering next-generation structural materials for nuclear reactors, "we cannot limit ourselves according to conventional alloys, or to the knowledge normally only applied to metals," she said.
Since 2014 she has directed an Energy Frontier Research Center, called Energy Dissipation to Defect Evolution, or EDDE, funded by the Department of Energy's Office of Science and led by ORNL. In its six years with Zhang at the helm, EDDE has significantly advanced understanding of how to create materials that can better withstand the effects of radiation, resulting in fewer and smaller flaws. During that time, she and her collaborators have hypothesized, confirmed, and begun to quantify how fine-tuning the chemical complexity of an emerging class of materials, called single-phase concentrated solid-solution alloys, or SP-CSAs, can increase their resistance to radiation, pressure and heat.
The knowledge gained by EDDE could help engineers build more durable reactors for future nuclear electricity production, which the International Atomic Energy Agency predicts could grow by up to 50% over the next 10 years. In Zhang's view, directing the center has also been an opportunity to create a useful foundation - a cookbook of sorts - for other researchers seeking to design structural alloys with unique properties.
It's analogy she often uses to show students how learning scientific foundations can help them create the materials they dream up: "If you like cooking and you want something sweet, spicy, salty, all those, then you know what to add," Zhang said. "If you know the similar thing for various alloy properties, if we can actually find out what are the controlling factors…is there a way we could see the big picture?"
Seeking out the big picture
Zhang's approach to solving problems by looking for underlying factors started when she was growing up in Beijing. Though her future career path wasn't clear at young age, she had a knack for mathematics and physics. "When I grew older, I realized that those subjects were easier for me because I can see the rules," Zhang said.
This natural inclination pushed her to major in solid state physics at the prestigious Beijing Normal University, where she stayed for a master's in materials science and engineering. She then pursued two doctorate degrees - one in nuclear physics from the Lund Institute of Technology in Sweden, which emphasized experimental research, and one in materials science and engineering from Beijing Normal University, which was a modeling effort. Thankful for her multi-faceted training, Zhang credits her dual background for some of her current success.
"It makes it easy to communicate with the modeling people and with the experimental people, and to grab different techniques relatively quickly," Zhang said. "Different techniques have strengths and limitations, so you can see the bigger picture if you use the combined strengths of multiple techniques."
Different types of ion-beam analysis, for example, harness light or heavy charged particles to help researchers pinpoint how various types of defects are created in a crystal of a material. As a postdoctoral fellow at Sweden's Uppsala University, advancing ion-beam analysis techniques was one of Zhang's main research areas. Later, as an assistant professor at Uppsala, her focus shifted to defects in semiconductor materials.
"Ion-solid interactions produce defects, so it's all connected," Zhang said.
It was during her time as an assistant professor - when she was already recognized for her work in ion-beam physics - that Zhang traveled to Brazil for a conference on ion beam modification of materials. There she stayed late after a talk by a well-known materials science researcher from Pacific Northwest National Laboratory, William Weber. The two became collaborators, and in 2002, they married.
In 2003, Zhang was hired at PNNL, where she began work on fundamental science projects focusing on ion-beam physics and radiation effects. She also assisted visiting researchers using ion-beams at the Environmental Molecular Sciences Laboratory, a DOE user facility.
At PNNL, she won both a 2005 DOE Early Career Award and a 2005 Presidential Early Career Award for Scientists and Engineers, or PECASE, the highest honor bestowed by the U.S. government on early-career scientists and engineers. The PECASE honored her contributions to ion-beam physics and ion-solid interactions in materials, her commitment to STEM outreach and especially her development of a novel technique to measure electronic stopping, the way atomic particles slow down as they pass through materials.
A twofold opportunity
Five years later, ORNL and the University of Tennessee offered Zhang an opportunity to combine her love for teaching and research through a joint-faculty position in ORNL's Materials Science & Technology Division and UTK's Department of Materials Science and Engineering.
Zhang accepted a challenge to put together an Energy Frontier Research Center out of ORNL in 2013, and helped choose energy dissipation to defect evolution as its topic. ORNL's strengths in materials science - especially in growing high-quality, complex alloys and in radiation effects research - made the lab a good place to base the center.
Part of the idea behind the EFRC, Zhang said, was inspired by properties of complex ceramics - materials Zhang has studied throughout her career. One specific ceramic, called a pyrochlore, was special because similar-sized atoms in its ordered lattice structure could easily switch locations, making it resistant to the radiation damage that occurs when charged particles displace atoms in a material's structure.
The hypothesis was that single-phase concentrated solid-solution alloys, or SP-CSAs, a new class of alloys made of two to five elements all in high concentrations, would reflect this quality because all of the atoms in an SP-CSA are randomly arranged. Conventional alloys, alternatively, are typically made of one or two main elements with impurities.
"We thought, we had a material that's stable in this random arrangement," Zhang said. "This material should fall into that category that should be radiation resistant." Fine-tuning the elemental concentrations or replacing certain elements (i.e., adjusting the sugar, spice and salt), she and her colleagues thought, could help them learn how radiation energy dissipates in the material and allow them to increase its radiation tolerance even more.
The original idea proved correct. Years of research dedicated to understanding SP-CSAs on the level of atoms and electrons has revealed some of the specific ways that altering chemical complexity changes the material's properties. EDDE research shows that, by adjusting elements with both partially filled and nearly full electron bands, one can tailor the local chemical order and concentration. As a result, alloy properties can be significantly modified for technological applications.
A review of the results, published in the journal Materials Research Society Bulletin in 2019, summarizes much of the center's work and is a point of pride for Zhang. A more recent paper that she co-authored proposes a model for how chemical complexity can be used to suppress radiation damage.
Asking good questions
The center, which is in its final year, has brought together dozens of researcher partners from three national laboratories and several universities worldwide. "We have many energetic young minds - post-docs, students and early career scientists. They work with people who have been very knowledgeable in their own fields…so the balance is really good," Zhang said. "The common ground is curiosity."
Inspired by young researchers asking good questions and coming up with new solutions, Zhang hopes her research will pave the way for applied work and open the doors for translating the knowledge to other material systems.
Zhang's research is supported by the DOE Office of Science.
