For the first time, researchers at Washington State University (WSU) have watched a material change its crystal structure in real time.
While exposing a sample of silicon to intense pressure – through the impact of a plastic projectile traveling at nearly 12,000mph – the researchers documented the silicon’s transformation from its common cubic diamond structure to a simple hexagonal structure. At one point, they could see both structures as the shock wave traveled through the sample in less than half a millionth of a second.
Their discovery is dramatic proof of a new concept for discerning the make-up of various materials, from impacted meteors to body armor to iron at the center of the Earth.
Until now, researchers have had to rely on computer simulations to follow the atomic-level changes of a pressure-induced structural transformation, said Yogendra Gupta, professor and director of the WSU Institute of Shock Physics. This new method now provides a way to actually measure the physical changes and to see if the simulations are valid.
"For the first time, we can determine the structure," Gupta said. "We've been assuming some things but we had never measured it." Writing in Physical Review Letters, the researchers say their findings already suggest that several long-standing assumptions about the pathways of silicon's transformation "need to be re-examined".
"We're making movies. We're watching them in real time. We're making nanosecond movies."Yogendra Gupta, Washington State University
The discovery was made possible by a new facility, the Dynamic Compression Sector at the Advanced Photon Source, located at the Argonne National Laboratory. Designed and developed by WSU, the sector is sponsored by the US Department of Energy (DOE)'s National Nuclear Security Administration, whose national security research mission includes fundamental dynamic compression science. The Advanced Photon Source synchrotron, funded by the DOE's Office of Science, produced high-brilliance x-ray beams that passed through the test material and created diffraction patterns that the researchers could then use to monitor the crystal as it changed its structure in as little as five billionths of a second.
"We're making movies," explained Gupta. "We're watching them in real time. We're making nanosecond movies."
Stefan Turneaure, lead author of the paper and a senior scientist at the WSU Institute for Shock Physics, said that the researchers exposed silicon to 19 gigapascals, nearly 200,000 times atmospheric pressure. The researchers accomplished this by firing a half-inch plastic projectile into a thin piece of silicon on a Lexan backing. While x-rays hit the sample in pulses, a detector captured images of the diffracted rays every 153.4 nanoseconds – the equivalent of a camera shutter speed of a few millionths of a second.
"People haven't used x-rays like this before," said Turneaure. "Getting these multiple snapshots in a single impact experiment is new."
"What I'm very excited about is we are showing how the crystal lattice, how this diamond structure that silicon starts out with, is related to this ending structure, this hexagonal structure," said Gupta. "We can see which crystal direction becomes which crystal direction. Stefan has done a great job. He's mastered that. We were able to show how the two structures are linked in real time."
This story is adapted from material from Washington State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.