Helping the hunt for lead-free piezoelectrics

Schematic of the structure of a typical perovskite

Schematic of the structure of a typical perovskite showing the octahedral structure with A cations shown in grey and B cations in green.
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Piezoelectrics are a class of material that generates an electric charge when subject to mechanical stress. Discovered by Pierre and Jacques Curie in 1880, an early application of the phenomenon was in sonar during the First World War. Since then they’ve been applied in a wide range of sectors from watches to automotive and medical applications. PbZr1-xTixO3, or PZT, is one of the most widely used piezoelectrics because of its high performance. A group of scientists have been using GEM and HRPD at ISIS to understanding the relationship between PZT’s structure and its performance. Their research has been published in Nature Communications.

Piezoelectric materials are used in a large variety of applications including actuators, sensors, electromechanical motors and sonar systems. The global market for piezoelectric actuators and motors was projected to reach $12.3 billion in 2014 with an annual growth rate of 13.2% per year, according to a report published in 2010. PZT is one of the most widely used piezoelectric ceramic materials, taking up around 98% of the total actuator market.

PZT belongs to a class of materials called perovskites. These materials form crystal structures which have a formula ABX3, where A and B are cations and X is an anion. The structure is composed of octahedra (polyhedra with eight triangular faces) that are linked together through the anions at the corners; the B cations are inside the octahedra and A cations in the spaces between octahedra.

This apparently simple structure can in fact accommodate a wide range of structural modifications, and these have a significant impact on the properties of the material. This variation means that perovskites can be used in a wide range of applications. But to optimise a particular perovskite, or indeed to find new perovskites with desired properties the link between structure and property needs to be understood at the molecular level. In particular, all current industrial piezoelectrics contain lead, which is toxic, so there is a strong drive to find environmentally friendly alternatives.

Prof David Keen from ISIS, who co-authored the paper, says, “We wanted to establish exactly what it is about the structure of PZT that leads to its high performance. This requires understanding both long- and short-range order, so we used a combination of conventional structural analysis methods and pair distribution function (PDF) studies at ISIS to provide the full picture.  We discovered that the lead ‘A’ cations in the complicated mixed region, where the piezoelectric effect is particularly high, moved – with the benefit of hindsight at least – in a very sensible and sensitive way.”

Mike Glazer, Emeritus Professor at the University of Oxford, who is also a co-author says, "Here we have shown a mechanism that seems to highlight an important ingredient in our understanding of current piezoelectrics and in the wider field of perovskites. This is a significant step towards widening the applications of lead-free piezoelectrics in industry.”

Sara Fletcher

Research date: October 2014

Further Information

N. Zhang, et al. "The missing boundary in the phase diagram of PbZr1-xTixO3." Nature Communications. DOI: 10.1038/ncomms6231

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