Piezoelectric crystals treated with an AC electric field can become transparent. Photo: Bo Wang/Penn State.
Piezoelectric crystals treated with an AC electric field can become transparent. Photo: Bo Wang/Penn State.

Using an AC (alternating current) rather than a DC (direct current) electric field can improve the piezoelectric response of a crystal, meaning how much it deforms in response to the electric field. Now, an international team of researchers show that cycles of AC fields can also make the internal crystal domains bigger and the whole crystal transparent.

"There have been reports that the use of AC fields could significantly improve the piezoelectric responses – for example by 20% to 40% – over DC fields, and the improvements have always been attributed to the smaller internal ferroelectric domain sizes that resulted from the cycles of AC fields," said Long-Qing Chen, professor of materials science and engineering, professor of engineering science and mechanics, and professor of mathematics at Penn State. "About three years ago, Dr. Fei Li, then a research associate at the Materials Research Institute at Penn State, largely confirmed the improvement of piezoelectric performances from application of AC fields. However, it was not clear at all how the internal ferroelectric domains evolved during AC cycles.

"Our group does mostly computer modeling, and more than a year ago we started looking into what happens to the internal domain structures if we apply AC fields to a ferroelectric piezoelectric crystal. We are very curious about how the domain structures evolve during AC cycles. Our computer simulations and theoretical calculations did show an improved piezoelectric response, but our simulations also demonstrated that the ferroelectric domain sizes actually got bigger during AC cycles rather than smaller as reported in the literature."

Piezoelectric materials generate electric charges when a mechanical force is applied to them and deform or change shape when an electric field is applied. The researchers investigated lead magnesium niobate-lead titanate (PMN-PT), a commercially available piezoelectric material. Their computational results were unexpected, because most people in the piezoelectric community believed that the smaller the domains are, the higher the piezoelectric response.

Domains within a crystal are areas where the electric dipoles or electric polarization arrange themselves along the same direction. Before the dipoles or polarization in a PMN-PT crystal are all aligned in the same direction by an electric field, there are many tiny domains with polarization along different directions.

The simulations produced by Chen and his colleagues showed that, as cycles of AC electric fields are applied to the crystal, these domains realign, becoming fewer and larger. After several AC cycles, the domains are large and in layers.

"The simulation results were in contradiction to reports in the literature," said Chen. "We needed to dig deeper to see if reality agrees with our simulation results."

Researchers at Xi'an Jiaotong University in China then grew their own PMN-PT crystals and carefully examined the domain configurations in their samples using various experimental characterization techniques under different AC cycling conditions. They confirmed the computational predictions from Penn State that the domains actually become larger during AC cycles.

The larger domain size and the layered structure also suggest that a ray of light projected onto the crystal would be unimpeded and shine right through, meaning the crystal would be transparent. This was also confirmed by the Chinese researchers. They found that AC-treated crystals not only possess ultrahigh piezoelectricity, but are also highly transparent after their surfaces are carefully polished. In the past, crystals like this have always been opaque.

The researchers report these findings in a paper in Nature. They say that "the work presents a paradigm to achieve an unprecedented combination of properties and functionalities through ferroelectric domain engineering, and the new transparent ferroelectric crystals reported here are expected to open up a wide range of hybrid device applications, such as medical imaging, self-energy-harvesting touch screens and invisible robotic devices."

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.