Piezoelectric materials generate a voltage when subjected to mechanical stress. This turns out to be a very useful property, lending itself to applications from the common microphone and loudspeaker to scientific instruments such as atomic force microscopes. However, the most efficient piezoelectric materials — those that produce the most charge for a given force, like lead zirconate titanate (PZT) — contain lead, which is coming under increasingly tight regulation because of its high toxicity.

While lead-free piezoelectrics do exist, their performance is only a fraction of that of their lead-bearing counterparts. Now, Wenfeng Liu and Xiaobing Ren from Xi’an Jiaotong University in China and the National Institute for Materials Science in Japan have developed a lead-free piezoelectric with performance comparable to the best PZT.1

The researchers focused on ferroelectric materials, which have an electrical polarization that can be reversed by applying an electric field. By tuning their composition and temperature, ferroelectrics can be balanced between two different ferroelectric configurations or phases. A mechanical stress can then tip them into either one phase or the other, thus changing their polarization and inducing a voltage.

Fig. 1: A common feature of high-performance piezoelectric materials is the existence of a ‘triple point’ in the composition–temperature phase diagram.

Ren and Liu studied the phases of PZT and lead-free piezoelectrics as a function of composition and temperature. They found that while the phase diagram of PZT had a ‘triple point’ at which three different phases (cubic paraelectric, rhombohedral ferroelectric and tetragonal ferroelectric) intersect (Fig. 1), the phase diagram of the lead-free piezoelectrics did not. This led them to hypothesize that the high piezoelectricity of lead-based piezoelectrics such as PZT was not due the presence of lead, but instead to the existence of a triple point in the composition–temperature phase diagram.

To test this hypothesis, the team designed a lead-free piezoelectric, BZT-BCT, that included a triple point in its phase diagram. The resulting piezoelectric effect was more than double that of the previous best lead-free result, and comparable even to PZT, confirming their hypothesis. They further estimate that the single-crystal form of BZT-BCT — rather than the ceramic tested — may have a piezoelectric effect that is higher still, by up to a factor of three.

This study opens up a new approach to designing high-performance, non-toxic piezoelectrics. Work remains to be done, however. “One limitation of the new material is its relatively low Curie temperature of 93 °C, above which piezoelectricity disappears,” says Ren, “which restricts the range of applications.” Ren and his team are currently developing new systems with higher Curie temperatures.