Flexoelectricity is a property of specific materials that exhibit an electrical charge induced by a strain. The ICN2 Oxide Nanophysics Group led by ICREA Prof. Gustau Catalán has recently published the latest findings from their research line on flexoelectricity. PhD student Kumara Cordero-Edwards is the lead author of this work, carried out in collaboration with researchers from the Autonomous University of Barcelona (UAB). The article outlines how the indentation toughness of polar crystals can be manipulated in such a way that they become easier or harder to dent from a given direction.
This is the result of the interaction between the localized flexoelectric polarization caused by the mechanical stress gradient of the indentation, on the one hand, and the piezoelectric polarization inherent in polar crystals, on the other. Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress.
If the two polarization run parallel, overall polarization is going to be very strong. This carries a higher energy cost, which makes the act of indentation itself more difficult. But if we turn the material over, the flexoelectric effect of the knock will be acting in the opposite direction to the spontaneous piezoelectric effect, making total polarization weaker and indentation correspondingly easier.
But the observations of our researchers did not end there. In the case of a particular subset of piezoelectric materials, ferroelectrics, it is not even necessary to physically turn the material upside down; we can simply apply an external voltage to flip its polar axis.
These effects were observed not only for forceful indentations or perforations, but also for the gentler, non-destructive pressures delivered by the tip of an atomic force microscope. Aside from potential applications in smart coatings with switchable toughness, these effects could one day be used as a means of reading ferroelectric memories by touch alone.