Lead-free piezoceramics: Status and perspectives
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Introduction European Union (EU) legislation on the restriction of hazardous substances1 as well as an article2 in Nature about 15 years ago provided the triggers for a strong effort into the science and technology of lead-free piezoceramics, with many stakeholders coming into play. While EU legislation restricts the use of lead in piezoelectric devices unless exempted under specific conditions,1 the article in Nature suggests that leadfree compositions may now be available to replace lead in certain piezoceramic applications.2 With the goal to reduce the production and waste disposal of toxic lead, research on lead-free piezoceramics strives to make lead zirconate titanate (PZT) and similar perovskite materials redundant. Shrout and Zhang3 have summarized the complexity of this task in a concise manner. The scope of this endeavor to develop lead-free piezoceramics can be gleaned by considering the history of PZT,4,5 which highlights many years of development. The toxicity of lead, including risks to the environment during mining, processing, and disposal, is the driving force for this research. The article by Bell and Deubzer6 in this issue summarizes the current level of understanding. Legislation in Europe, and in many other parts of the world,7 to restrict and reduce hazardous substances such as lead has proven to be a strong driving force toward research into nontoxic replacements. Review processes and interactions with industry to address a variety
of legislative directives have been developed for different applications. Exemption 7(c)-I, for example, has been recently reviewed and provides an exemption for lead in piezoelectronic devices until July 2021, with applications for further extension of this exemption due to the European commission by January 2020. Bell and Deubzer6 have outlined current legislation and future options. Piezoelectricity describes the creation of an electric charge in response to a mechanical stress. The converse effect is the development of a strain as a function of an applied electric field (Figure 1). The proportionality constant is identical for both cases and is described as the piezoelectric coefficient, d.4,8 Since polarization is a vector and stress a second rank tensor, the piezoelectric constant must be written as a third rank tensor, but can be transformed to a second rank matrix.9 As we are considering the replacement of PZT and of related materials, which are ferroelectrics and are converted to piezoelectrics by a poling process, ferroelectric crystal structures are becoming important. Domains in ferroelectrics (Figure 2) can be considered to be a form of twins that develop at the paraelectric/ ferroelectric phase transition in order to reduce the elastic and electrostatic energies.10 Application of an electric field may lead to lattice extension (intrinsic contribution to the piezoelectric effect), domain-wall movement resulting in a strain contribution (extrinsic contribution to the piezoelectric effect),
Jürgen Rödel, TU Darmstadt, Germany; Tsinghua University, China;
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