New materials research looks at transformation at the atomic level

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

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

Students Zhou, Xianhu Sun and Dongxiang Wu are the first co-authors of this paper (“Kinetics of dislocation-induced interfacial transformation”). Sun has just finished his PhD thesis, and Wu is a PhD candidate. Other contributors are Lianfeng Zou, MS ’12, PhD ’17, now a professor at Yanshan University, and PhD candidate Xiaobo Chen; Professor Judith Yang, Visiting Research Assistant Professor Stephen House and postdoctoral researcher Meng Li of the University of Pittsburgh’s Swanson School of Engineering; and staff scientist Dmitri Zakharov of Brookhaven National Laboratory (BNL).

Using observational techniques, Zhou said, “manufacturers may be able to control the microstructure and properties of current materials and design new types of materials. There is some practical importance to this research, but there is also fundamental significance. ”

The experiment tested the transformation of copper oxide into copper. Directly observing such interface transformations at the atomic scale is a challenge as it requires the ability not only to access buried interfaces but also to apply chemical and thermal stimuli to drive the transformations.

By using an environmental TEM technique that is capable of introducing hydrogen gas into a microscope to induce oxide reduction while simultaneously performing TEM imaging, the research team was able to atomically monitor the interface reactions. Surprisingly, the researchers observed that the transformation from copper oxide to copper occurred intermittently because it was paused by mismatched dislocations, a behavior similar to the stop-and-go process regulated by traffic lights.

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

To understand what was at work, Wu developed computer code to explain what they saw in the experiment. This back and forth process between experimentation and computer modeling helped the team understand how mismatched dislocations control the long-distance atomic transport required for phase transformations.

“Iterative, iterative processes between experiments and computer modelling, both at the atomic level, are an interesting aspect for materials research,” Zhou said.

Fundamental information has proven useful in designing new types of multiphase materials and controlling their microstructures, which can be used in a wide variety of applications such as load-bearing structural materials, electronic fabrication, and catalytic reactions for clean energy production and environmental sustainability.

After collecting initial data at Binghamton, Sun and the research team repeated the experiment on the equipment at Pitt and Brookhaven, which had different capabilities.

“This is cooperation. Without the facilities at BNL and the University of Pittsburgh, we can’t see what we need to see,” said Sun. “Also, in the final stages of my data analysis, I discussed the results with Judy, Meng, and Dmitri many times. I remember when we finished the first draft and sent the manuscript to Dmitri, he told me that maybe we should enter some equations to confirm our observations, and he sent some relevant literature. So now we can show that the calculations match our experimental results.”

Yang also called the research an “excellent partnership” bringing together the best elements from Binghamton, Pitt, and Brookhaven.

“The ability to use cutting edge tools is one of the things that underpins new science, as exemplified here,” he said. “Brookhaven has an extraordinary microscope that can pick up environmental stresses at higher pressures than we have at the University of Pittsburgh, and has higher analytical capabilities. But the University of Pittsburgh one is a good high resolution transmission electron microscope that can accept gases, it’s a more powerful microscope. There is also more research time available.”

He uses an analogy to explain why seeing chemical reactions occur in real time is important: “When you buy a fish and it’s packaged, there’s only so much you can understand about that fish compared to seeing fish in a real environment. You learn more about the way it lives, about the types of fish, and so on.”

Because US national laboratories offer a free service and complement what is available in universities and the high-tech industry, they can help researchers – especially those still early in their careers – take their work to the next level with state-of-the-art tools.

Zakharov said he was delighted to have been involved in the research of this material: “The strength of this technique is the direct method for viewing all of these dislocations and phase transformations. You can control the reaction, and you can go back and forth to observe how the dislocations in the interface behave. There is no other technique with such direct observation.”

Sun – who now works at Lawrence Berkeley National Laboratory in Berkeley, California – is delighted that the research has finally been published.

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

This work was supported by the US Department of Energy Basic Energy Sciences. The study used the Center for Functional Nanomaterials Electron Microscopy Facility and the Center for Scientific Computing and Data at Brookhaven National Laboratories, supported by the US Department of Energy’s Office of Basic Energy Sciences. This research also used Environmental TEM at the University of Pittsburgh with support through the National Science Foundation Major Research Instrumentation award.

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