A piezoelectric material produces electricity in response to mechanical stress, and vice versa. The effect is eminently useful, making possible devices from microphones to microscopes. At present, the most commonly used industrial piezoelectric is lead zirconate titanate, which unfortunately contains lead — now a banned element in electronics in many countries. A promising lead-free alternative is the ceramic (Na,K)NbO3, or KNN, which has a strong piezoelectric response when implanted, or atomically ‘doped’, with tantalum or antimony. Tantalum, however, is expensive, and antimony toxic — lithium is cheaper and safer, but lithium-doped KNN conventionally has a much weaker piezoelectric response. Jing-Feng Li and Ke Wang from Tsinghua University in Beijing, China,1 have now demonstrated that the piezoelectric response of Li/KNN can be enhanced significantly without introducing other dopants.

Their method relies on the effect of applying a high electric field across the material. Referred to as ‘poling’, this process increases the piezoelectric response by aligning the electrical domains in the material. The use of poling for Li/KNN, however, has previously been somewhat ineffective. Li and Wang found success by poling their Li/KNN sample twice, letting it age for two months in between. This increased the piezoelectric coefficient — a measure of the piezoelectric response — by a factor of 1.7, equaling the performance of KNN doped with tantalum or antimony.

Fig. 1: Transmission electron microscopy image of a ferroelectric domain pattern in a lithium-doped KNN piezoceramic.

The procedure worked only on KNN samples doped with lithium to a concentration sufficient to create two co-existing phases with different crystal structures (Fig. 1). The researchers concluded that the aging process allowed doping-related defects in the KNN structure to ‘drift’ under the influence of the internal electric field induced by the first poling event. This created a defect-induced electrical polarization, which was then rotated by the second poling process to align with other domains, increasing the total electric polarization and therefore the piezoelectric response.

The technique is attractive for its simplicity, but it also opens up a new approach for the development of high-performance piezoelectrics. “We have shown that the engineering of defect domains in polycrystalline piezoelectrics using poling and aging can be as important as doping, adding another tool to the community’s tool box,” says Li.