Unique constraints affect phase transformations in silicon, a vital material for electronics

When Valery Levitas left Europe in 1999, he brought a rotating diamond anvil cell with him to the United States. He and his group of researchers still use a highly advanced version of this compression and torsion tool to compress and shear materials between two diamonds to see in situ, in real-world experiments, what happens and to verify the researchers’ theoretical predictions.

How, for example, do crystal structures change? Does this produce new, potentially useful properties? Does shearing change the high pressure that must be applied to create new material phases?

“This is research ‘at the intersection of advanced mechanics, physics, materials science and applied mathematics,'” wrote Levitas, the Anson Marston Distinguished Professor of Engineering at Iowa State University and Murray Harpole Chair in Engineering.

One of the latest discoveries by Levitas and his collaborators is that silicon, an important material for electronics, exhibits unusual phase transformations when squeezed and sheared to large, plastic, or permanent, deformations.

The newspaper Nature Communications The results were recently published. Corresponding authors are Levitas and Sorb Yesudhas, a postdoctoral research associate in aerospace engineering at Iowa State University and principal experimenter. Co-authors are Feng Lin, formerly of Iowa State University; KK Pandey, formerly of Iowa State University, now at the Bhabha Atomic Research Center in India; and Jesse Smith, of the Collaborative High-Pressure Access Team at Argonne National Laboratory in Illinois, where the group performed in situ X-ray diffraction experiments.

The researchers acknowledge that many studies have been conducted on silicon changes under high pressure, but not on silicon under pressure and plastic shear deformation. In this case, they subjected three sizes of silicon particles (1 millionth of a meter, 30 billionths of a meter, and 100 billionths of a meter) to the unique stresses of the rotating diamond anvil cell.

“Such ‘plastic stress-induced phase transformations are entirely different and promise many discoveries,” the researchers wrote.

An experiment performed at room temperature on silicon samples 100 billionths of a meter in diameter showed that pressures of 0.3 gigapascals, a common unit of pressure measurement, and plastic deformations transformed the crystalline phase called “Si-I” of silicon into “Si-II”. Under high pressure only, this transformation begins at 16.2 gigapascals.

“The pressure is reduced by a factor of 54,” the authors write.

“This is a revolutionary experimental discovery,” Levitas said.

“One of our goals is to reduce transformation pressures,” he said. “So we’re working in an area that other researchers generally ignore: very low pressures.”

Moreover, he added, the goal of the researchers’ material deformations is not to change the shape or size of the material samples.

“The key thing is to change the microstructure,” Levitas explained. “That’s what drives the changes that produce the phase transformations.”

The different crystal structures of the different phases (this paper studies seven phases of silicon) offer different properties that could be useful in real industrial applications.

“It is possible to obtain the desired nanostructured pure phases or mixture of phases (nanocomposites) with optimal electronic, optical and mechanical properties,” the researchers write.

This is a technique that could be of interest to industry.

“Working with very high pressures for these phase transformations is not practical for industry,” Levitas said. “But with plastic deformation, we can get these traditionally high-pressure phases, properties and applications at very modest pressures.”

After 20 years of thinking and theorizing about these material questions, Levitas said he expected an unusual response from silicon to the stresses in the rotating diamond anvil cell.

“If I didn’t expect phase transformations at low pressure, we would never have checked,” he said. “These experiments confirm several of our theoretical predictions and also open up new challenges to the theory.”