A Breakthrough in Pb-free Piezoelectrics

--A new Pb-free system surpasses PZT

Ph.D. student Wenfeng Liu and Xiaobing Ren, the director of MMRC have recently discovered a non-Pb piezoelectric material that exhibits an ultrahigh piezoelectric coefficient d₃₃ of 560~620pC/N. It is the first time over the past 50 years that a non-Pb system outperforms “the icon of piezoelectrics”, the famous PZT (lead zirconate titanate). This result may bring about important consequences for the huge piezoelectric industry that has relied on PZT for decades.

Over half a century, a substantial portion of our industry and daily life has relied on a kind of piezoelectric material called PZT, which can convert electrical energy to mechanical energy (and vice versa) with high efficiency. Applications of PZT materials range from almost all commodities of our daily life like mobile phones, personal computers, gas igniters, to high-tech products like atomic force microscopes. However, this important pillar of our society is now facing an enormous challenge – the legislations of many countries have banned the use of toxic Pb in commercial products. PZT, which contains more than 50% Pb in weight, is nevertheless exempted temporarily from the “black list” because there has been no substitute over the last decades: all existing non-Pb piezoelectrics are much inferior to PZT in piezoelectric performance. However, this precarious situation may change overnight when a Pb-free piezoelectric material comparable with the PZT is discovered and the huge industry of piezoelectrics and the related applications will be significantly affected.

In the present work, Liu and Ren discovered a new non-Pb piezoelectric system Ba(Ti0.8Zr0.2)O3-(Ba0.7Ca0.3)TiO3, which has an ultrahigh piezoelectric coefficient d33 of 560~620pC/N, even outperforming the best PZT. It is the first time over the past 50 years that a non-Pb system manifests such level of piezoelectricity. More importantly, they have revealed the origin of such a high piezoelectricity. The new theory indicates that there is no necessary relationship between high piezoelectricity and toxic Pb; any system exhibiting a “tricritical morphotropic phase boundary” will be able to produce high piezoelectricity. This may become an important guideline for seeking other Pb-free systems with even better piezoelectricity. They further predict that the single crystal form of their material may exhibit a giant piezoelectricity of d33=1500~2000pC/N.

The importance of this work is three-fold. First, the discovery of high piezoelectricity in a Pb-free system may ignite the transformation of the whole piezoelectric industry and related huge industry to Pb-free materials. Second, the new theory may guide researchers to discover newer and even better Pb-free materials that will eventually accomplish transformation into Pb-free piezoelectrics. Third, the advent of high-performance Pb-free piezoelectric materials may remove the current legislative exemption of PZT and thus may alter the whole piezoelectric industry in a big way.

This important paper will be published online in Physical Review letters in December 11th, 2009.