Multiscale Structure Engineering for High-Performance Pb-Free Piezoceramics

Abstract

ConspectusThe increasing world energy crisis drives humans to harvest the energy in nature as much as possible without heavily damaging the environment. However, most of the energy in nature cannot be used directly. Therefore, pursuing technologies or matter that can directly interconvert different energies has been one of the most cutting-edge fields in science and technology. Such a magic ability exists in true-life piezoelectric materials that generate charge when being given a force and vice versa, rendering them highly promising for energy harvesting and conversion because of the tremendous mechanical energy on the earth, such as tide energy. Thus, piezoelectric materials, represented by the lead zirconate titanate (Pb(Zr, Ti)O3, PZT) family, have been largely used in various traditional and burgeoning fields, such as electronic information, biomedical treatment, and wearable flexible electronic devices, and are one of the most important favorites in multidisciplinary fields. However, Pb is highly toxic. Driven by environmental protection and concern for human health, Pb-free piezoceramics are rapidly being developed to hopefully replace Pb-based ones. In particular, the renewal of Restriction of the use of certain Hazardous Substances (e.g., RoHS 2), issued by the European Union, declared that replacement of PZT “...may be scientifically and technologically practical to a certain degree...”. Therefore, developing high-performance Pb-free piezoceramics has become more urgent than ever. In this context, some Pb-free piezoceramics, represented by potassium sodium niobate ((K, Na)NbO3, KNN), barium titanate (BaTiO3, BT), bismuth barium titanate ((Bi, Na)TiO3, BNT), and bismuth ferrite (BiFeO3, BFO), stand out because of their unique or similar traits. However, several key challenges, including an inferior overall performance compared to Pb-based counterparts and an unclear structure–property relationship from multiscale viewpoints, have severely hindered the development of Pb-free piezoceramics for a long time.Pb-based piezoceramics possess decent performance due to the strategy of phase boundary engineering, which inspired the researchers to pursue it in Pb-free counterparts. In the last 10 years, our group has been aiming at the new phase boundary (NPB) and new physical phenomenon in Pb-free piezoceramics. This Account presents our recent contributions to the development of Pb-free piezoceramics concerning the good performance and emerging phenomenon. First, we introduce the construction of the NPB in KNN-based piezoceramics by emphasizing the role of some key elements (i.e., Bi, Sb, Zr, and Hf). Then, we summarize the effects of the NPB on KNN- and BT-based ceramics and the new physical phenomenon in BNT-based ceramics. The NPB boosts the piezoelectric properties and temperature stability of KNN- and BT-based piezoceramics, comparable to some Pb-based piezoceramics. Combining the NPB and the multilayer ceramics substantially enhances the temperature stability of the piezoelectric constant. A new physical phenomenon of the nanoscale bubble domains with polar topologies is for the first time revealed in BNT-based ceramics, showing potential applications for nonconventional and high-density nonvolatile memories. In particular, we emphasize structure engineering from multiscale viewpoints including the local, microscopic, mesoscopic, and macroscopic structure (e.g., lattice structure, ferroelectric domains, and phase structure). Finally, we provide perspectives on the future developments of Pb-free piezoceramics toward practical applications.