The discovery could have important implications.
Could this explain how lightning is formed?
A new study has found an interesting and, perhaps, important property of ice. It appears to generate electricity when it is bent.
Water ice is abundant on Earth, being one of the most widespread solids on the planet. Given water's importance to life, and ice's importance to the Earth's climate, we have also studied the hell out of it.
"Despite the ongoing interest and large body of knowledge on ice, new phases and anomalous properties continue to be discovered, suggesting that our understanding of this ubiquitous material is incomplete," researchers from the Catalan Institute of Nanoscience and Nanotechnology (ICN2) explain in their new study.
One thing we do know about ice in its most common form on Earth (hexagonal ice – Ih) is that it is not piezoelectric. This is the phenomenon where certain materials, such as crystals and ceramics, produce electricity due to mechanical stress, for example, through a collision.
"Despite the polarity of individual water molecules, common ice Ih is not piezoelectric, due to the geometric frustration introduced by the so-called Bernal–Fowler rules: two hydrogen protons must be adjacent to each oxygen atom, but there can only be one hydrogen proton between two oxygen atoms," the team explains.
"As a result, in contrast to the oxygen atoms, which are arranged in a hexagonal lattice, the hydrogen atoms do not exhibit long-range order, resulting in randomly oriented water dipoles and, thus, no macroscopic piezoelectricity."
And yet, ice is known to generate electricity due to mechanical stress in nature, though we weren't entirely sure how. For example, we know that collisions between ice particles can build up charge in clouds, which can discharge as lightning. Now, after a series of experiments conducted by bending ice and analyzing the results, the team suggests an answer.
“During our research, the electric potential generated by bending a slab of ice was measured. Specifically, the block was placed between two metal plates and connected to a measuring device," ICREA Professor Gustau Catalán, leader of the Oxide Nanophysics Group at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), said in a statement. "The results match those previously observed in ice-particle collisions in thunderstorms."
According to the team, the flexoelectric effect in ice puts it on par with electroceramic materials such as titanium dioxide, used inside advanced technologies such as capacitors and sensors. More importantly, they have found a new explanation for how charge could build up in clouds and eventually produce lightning.
"We discovered that ice generates electric charge in response to mechanical stress at all temperatures. In addition, we identified a thin ‘ferroelectric’ layer at the surface at temperatures below -113 °C (160K). This means that the ice surface can develop a natural electric polarization, which can be reversed when an external electric field is applied—similar to how the poles of a magnet can be flipped," Dr Xin Wen, a member of the ICN2 Oxide Nanophysics Group and one of the study's lead researchers, added.
"The surface ferroelectricity is a cool discovery in its own right, as it means that ice may have not just one way to generate electricity but two: ferroelectricity at very low temperatures, and flexoelectricity at higher temperatures all the way to 0 °C."
The team plans to continue research into the effect, including whether new electronic devices could be produced that use ice as an active material, which could be produced in and suited to cold environments.
The study is published in Nature Physics.
