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Wednesday, 24 April 2019

Crack propagation is asymmetric in polar materials

The ICN2 Oxide Nanophysics Group, led by ICREA Prof. Gustau Catalán, has published in Physical Review Letters how, due to flexoelectricity, cracks in ferroelectrics (switchable polar materials) propagate more easily in the polar direction than in the opposite.

Fracture physics is a central field of study in materials science. In the case of piezoelectric materials, due to their ability to generate strains if subjected to a voltage and vice versa, microcracks are usual and shorten the lifespan of the devices in which they are used. Researchers therefore look for ways in which fractures can be prevented, although sometimes they can also be used to our advantage. For instance, controlled cracking has been proposed as a mechanism for device nanopatterning.

Fracture fronts concentrate the maximum deformation that a solid can withstand, so flexoelectricity (polarization induced by deformation gradients) plays a key role. A recent study published in Physical Review Letters shows that crack-generated flexoelectricity acts facilitating or hampering crack propagation, depending on the polarization axis of the material. This study has a couple of important implications. It is the first to experimentally demonstrate that crystal fracture is not symmetric: cracks travelling in the polar direction are measurably longer than those traveling against. Second, since the polarity of a ferroelectric can be switched by voltage, voltage can serve as a tool to manage the propagation of cracks in polar materials, either to mitigate fatigue (the weakening and subsequent breakage of the material), or to promote fracture-based patterning schemes.

The research was led by ICREA Professor and Group Leader Gustau Catalan, from the ICN2 Oxide Nanophysics Group. The first author of the article is Dr Kumara Cordero-Edwards, who is a former member of the Group and nowadays is developing her research at the Université de Genéve (Switzerland). The work counted with the participation of other researchers from the KTH-Royal Institute of Technology (Sweden) and ICREA Prof. Jordi Sort, from the Physics Department at the UAB.

Article reference:
Kumara Cordero-Edwards, Hoda Kianirad, Carlota Canalias, Jordi Sort, and Gustau Catalan. Flexoelectric Fracture-Ratchet Effect in Ferroelectrics. Phys. Rev. Lett. 122, 135502. DOI: https://doi.org/10.1103/PhysRevLett.122.135502