Strange but True: Turning a Material Upside Down Can Sometimes Make It Softer

Through the combined effect of two properties inherent to certain types of crystal, flexoelectricity and piezoelectricity, researchers at the ICN2 led by ICREA Prof. Gustau Catalán, in collaboration with researchers from the Autonomous University of Barcelona (UAB), have found that polar materials can be made more or less resistant to dents when they are turned upside down… or when a voltage is applied to switch their polarisation. 

 

Published this week in Advanced Materials, this research points to the future development of “smart mechanical materials” for use in smart coatings and ferroelectric memories.

 

Ferroelectric material  with varying hardness  

Credit: Universitat Autònoma de Barcelona

The ICN2 Oxide Nanophysics Group led by ICREA Prof. Gustau Catalán has recently published the latest findings from their research line on flexoelectricity in Advanced Materials. 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). Highlighted in the journal’s frontispiece, 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 localised flexoelectric polarisation caused by the mechanical stress gradient of the indentation, on the one hand, and the piezoelectric polarisation inherent in polar crystals, on the other. If the two polarisations run parallel, overall polarisation 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 polarisation 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 and/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.

 

Citation: Kumara Cordero-Edwards, Neus Domingo, Amir Abdollahi, Jordi Sort, Gustau Catalan. Ferroelectrics as Smart Mechanical Materials. Advanced Materials, 2017. DOI: 10.1002/adma.201702210