Scientists have successfully applied pressure combined with a twisting shear to permanently reshape silicon, a crucial component in electronic devices. This manipulation leads to changes in silicon’s microstructure, resulting in various material phases that exhibit distinct and potentially beneficial characteristics.
In 1999, Valery Levitas moved from Europe to the United States, bringing along a rotational diamond anvil cell.
Levitas and his research team continue to utilize an advanced version of this specialized tool, which compresses and shears materials between two diamond surfaces. This allows them to observe in real-time the effects of their manipulations and to verify their theoretical models. They investigate how crystal structures are altered, whether new useful properties emerge, and whether the shearing modifies the pressure required for creating new material phases.
Levitas, a distinguished engineeering professor at Iowa State University, describes this research as sitting at the crossroads of mechanics, physics, materials science, and applied mathematics.
One of the recent discoveries made by Levitas and his colleagues reveals that silicon undergoes unique phase changes when subjected to significant and permanent deformations through pressure and shearing, making it particularly important for electronic applications.
The findings were published in the scientific journal Nature Communications. The lead authors include Levitas and Sorb Yesudhas, a postdoctoral researcher at Iowa State, who played a pivotal experimental role. Additional contributors are Feng Lin, formerly of Iowa State; K.K. Pandey, who has since moved to the Bhabha Atomic Research Centre in India; and Jesse Smith from the High-Pressure Collaborative Access Team at Argonne National Laboratory in Illinois, where in situ X-ray diffraction experiments were conducted.
The research received funding from the U.S. National Science Foundation, U.S. Army Research Office, Iowa State University, and the U.S. Department of Energy.
The team recognizes that while many studies have examined silicon’s changes under high-pressure conditions, there has been limited exploration of silicon under pressure paired with plastic shear deformation. They tested three different particle sizes of silicon — measuring 1 millionth of a meter, 30 billionths of a meter, and 100 billionths of a meter — to the distinctive strains produced by the rotational diamond anvil cell.
According to the researchers, these “plastic strain-induced phase transformations are entirely new and hold the potential for numerous discoveries.”
In one experiment conducted at room temperature using silicon particles that were 100 billionths of a meter in size, they found that applying a pressure of 0.3 gigapascals — a standard pressure measurement unit — along with plastic deformations, changed silicon’s “Si-I” crystal phase into “Si-II.” In contrast, this transformation typically initiates at a much higher pressure of 16.2 gigapascals.
“This means we have reduced the pressure by a factor of 54!” the authors noted.
This is regarded as a significant experimental achievement by Levitas.
“We aim to lower transformation pressures,” he explained. “Therefore, we operate in a realm that other researchers often overlook — very low pressures.”
Levitas also emphasized that the goal of altering material’s form is not about changing their size or shape.
“The essential factor is modifying the microstructure,” Levitas clarified. “This microstructural change is what leads to phase transformations.”
The varied crystal lattice structures associated with different phases — the study examined seven silicon phases — offer different characteristics that could be advantageous for various industrial applications.
“Using this technique, it’s feasible to obtain desired nanostructured pure phases or combinations of phases (nanocomposites) with optimal electronic, optical, and mechanical properties,” the researchers commented.
This method could be of particular interest to industry.
“Attempting to achieve these phase transformations at extremely high pressures is not practical for industry,” Levitas noted. “However, by employing plastic deformations, we can access these traditionally high-pressure attributes and applications at much lower pressures.”
After 20 years of contemplation and theorizing about these materials, Levitas expressed that he anticipated silicon’s unique behavior under the stress of the rotational diamond anvil cell.
“If I hadn’t believed that phase transformations could occur at lower pressures, we wouldn’t have pursued these experiments,” he remarked. “These findings validate many of our theoretical predictions while also posing new theoretical challenges.”
