Research into new materials sees transformations at the atomic level

When manufacturing techniques transform metals, ceramics, or composites into a technologically useful form, understanding the mechanism of the phase transformation process is critical to shaping the behavior of these high-performance materials. However, it is difficult to see these transformations in real time.

A new study in the journal Nature, led by Professor Guangwen Zhou of the Department of Mechanical Engineering at Thomas J. Watson College of Engineering and Applied Science and the Materials Science Program at Binghamton University, uses transmission electron microscopy (TEM) to observe the transformation oxide-metal at the atomic level. Of particular interest are mismatch dislocations that are always present at interfaces in multiphase materials and play a key role in determining structural and functional properties.

Students of Zhou Xianhu Sun and Dongxiang Wu are the first co-authors of the article (“Dislocation-induced stop-and-go kinetics of interfacial transformations”). Sun recently completed his doctoral thesis and Wu is a doctoral candidate. Other contributors are Lianfeng Zou, MS ’12, PhD ’17, now a professor at Yanshan University, and doctoral student Xiaobo Chen; Professor Judith Yang, Visiting Assistant Research Professor Stephen House, and Postdoctoral Fellow Meng Li of the Swanson School of Engineering at the University of Pittsburgh; and scientist Dmitry Zakharov from Functional Nanomaterials Centera science office of the U.S. Department of Energy (DOE) Ease of use at Brookhaven National Laboratory.

By using the advanced technique, Zhou said, “Manufacturers can be able to control the microstructure and properties of current materials and design new types of materials.” There is a certain practical importance to this research, but there is also a fundamental meaning.

The experiments tested the transformation of copper oxide into copper. Direct observation of such interface transformation at the atomic scale is difficult because it requires an ability not only to access the buried interface, but also to apply chemical and thermal stimuli to drive the transformation.

By using environmental TEM techniques capable of introducing hydrogen gas into the microscope to drive the oxide reduction while simultaneously performing TEM imaging, the research team was able to atomically monitor the interfacial reaction. Surprisingly, the researchers observed that the transformation of copper oxide to copper occurs intermittently because it is temporarily halted by mismatching dislocations, a behavior similar to a stop-and-go process regulated by fires. traffic.

“This is unexpected, because common sense accepted by the materials research community is that interface dislocations are the locations to facilitate transformation rather than retard it,” Zhou said.

To understand what was at work, Wu developed computer codes to explain what they were witnessing in the experiments. This back-and-forth process between experiments and computer modeling helped the team understand how misfit dislocations control the long-range transport of atoms needed for phase transformation.

“This iterative looping process between experiments and computer modeling, both at the atomic level, is an exciting aspect for materials research,” Zhou said.

The fundamental information could prove useful for designing new types of multiphase materials and controlling their microstructure, which can be used in various applications such as load-bearing structural materials, electronic manufacturing, and catalytic reactions for clean energy generation and environmental sustainability.

After collecting the initial data at Binghamton, Sun and the research team repeated the experiments on Pitt’s and Brookhaven’s equipment, which have different capabilities.

“It’s a collaborative work. Without the facilities at Brookhaven Lab and the University of Pittsburgh, we can’t see what we need to see,” Sun said. “Also, in the later stages of analyzing my data, I discussed the results with Judy, Meng, and Dmitri several times. I remember when we finished the first draft and sent the manuscript to Dmitri, he told me that maybe we should include some equations to confirm our observed results, and he sent some relevant literature, so now we can show that these calculations agree with our experimental results.

Yang also called the search a “really nice partnership” that brought together the best people from Binghamton, Pitt and Brookhaven.

“The ability to use advanced tools is one of the things that underpins new science, as illustrated here,” she said. “Brookhaven has an exceptional microscope that can withstand environmental stresses at higher pressures than the one we have at the University of Pittsburgh, and it has higher analytical capability. But the one from the University of Pittsburgh is a good high-resolution transmission electron microscope that can accept gas, it’s a more robust microscope. There is also more research time available.

She used an analogy to explain why it’s important to see chemical reactions happening in real time: “When you buy fish and it’s packaged, you can’t understand much about that fish as opposed to seeing fish in a real environment.”

Because DOE National Laboratories can offer state-of-the-art instruments and high-caliber expertise that complement what is available in academia and high-tech industry, they can help researchers – especially those at the start of their careers – to take their work to the next level, in most cases for free.

Zakharov said he was happy to have played a part in this materials research: “The power of the technique is that it is a direct method to see all these dislocations and phase transformations. You can control the reaction and you can go back and forth to observe the behavior of these dislocations in the interfaces. There is no other technique with such direct observation.

Sun – who now works at Lawrence Berkeley National Laboratory, also a DOE national laboratory – is happy that this research is finally being published.

“I started analyzing this data in March 2018, so it took almost five years to complete this work,” he said. “It’s hard, but it’s worth it.”

This work was supported by the US Department of Energy Basic Energy Science. The research used the electron microscopy facility at the Center for Functional Nanomaterials and the Scientific Data and Computing Center at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy’s Office of Basic Energy Sciences. This research also used environmental TEM from the University of Pittsburgh with the support of a major research instrumentation award from the National Science Foundation.

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