1:30pm - 2:00pmInvited
Lead-free Na½Bi½TiO3-based Piezoceramic Composites
Technical University Darmstadt, Germany
In the past two decades, the environmental toxicity imposed by the use and disposal of lead-based piezoelectrics led to strong research efforts. The good piezoelectric properties make (1-x)Na1/2Bi1/2TiO3-xBaTiO3 (NBT-xBT) solid solution a key candidate as a low-cost lead-free alternative to lead containing materials. One of the major issues limiting the use of this material is the thermal depolarization. Zhang et al.  recently demonstrated that the inclusion of semiconductor ZnO particles into the NBT-6BT matrix leads to a strong increase in the depolarization temperature (Td).
The composites of NBT-xBT with ZnO inclusions were prepared by conventional solid state synthesis. An increase in Td of ~40‒60 oC was observed for the composites with NBT-6BT and NBT-9BT. In the case of composites with non-ergodic relaxor NBT-6BT, the inclusion of ZnO enhances the volume content of the polar phase at room temperature as evidenced by Nuclear Magnetic Resonance of sodium (23Na NMR). This is attributed to the residual thermal stresses arising from the mismatch in the coefficient of thermal expansion of the constituent phases. The delayed thermal depolarization behavior of the composites is rationalized on the basis of two competing mechanisms - an increase in the transition temperature from ferroelectric to relaxor state (TF-R), thereby enhancing Td, and a stress induced shift in depolarization-temperature that results in a broadened depolarization behavior.
ZnO inclusions were also found to exert a clamping effect that elastically restricts the ability for domain wall movement. A two-fold increase in Qm was observed for the composites with NBT-6BT. Akin to other hard piezoelectrics, a decrease in the saturation polarization and total strain was observed.
 J. Zhang, Z. Pan, F.-F. Guo, W.-C. Liu, H. Ning, Y.B. Chen, M.-H. Lu, B. Yang, J. Chen, S.-T. Zhang, X. Xing, J. Rödel, W. Cao, and Y.-F. Chen, Nat. Commun. 6, 6615 (2015).
2:00pm - 2:30pmInvited
The Impact of Local Structure on Macroscopic Properties of ABO3 Perovskite Relaxor
1University of Wollongong, Australia; 2Pennsylvania State University, United States; 3Xi'an Jiaotong University, China
Relaxor materials with ABO3 perovskite structure have been actively studied for the potential optical, dielectric and piezoelectric applications. Relaxors show remarkable properties, such as high dielectric constant, low hysteresis and frequency dependent dielectric properties, being associated with their unique local structure: the existence of polar nano regions (PNRs), a nanoscale inhomogeneity that coexists with normal ferroelectric domains .
The contribution of these local structures has been theoretically modeled to be the origin of the ultrahigh dielectric and piezoelectric activities of relaxor based perovskite ferroelectrics, accounting for 50-80% of their respective room temperature values . Based on the paradigm, recent developments have experimentally confirmed that modest changes in the polarizability of PNRs and/or local structure, can be regarded as “seeds” to further enhance the dielectric and piezoelectric properties of ABO3 perovskite relaxor based solid solutions. The modified polycrystalline ceramics have been shown to exhibit ultrahigh dielectric and piezoelectric properties, compared to their non-modified counterparts, being on the order of >10,000 and >1000pC/N, respectively. The relationship between local structure and macroscopic properties has been established, try to understand the impact of local structure on dielectric and piezoelectric properties, to explore high performance ferroelectric materials for sensor and ultrasound transducer applications.
 L. E. Cross, “Relaxor ferroelectrics,” Ferroelectrics 76, 241 (1987).
 F. Li et al, “The origin of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution crystals,” Nature Communications, 7, 13807 (2016).
2:30pm - 3:00pmInvited
KNN-based Lead-free Piezoelectric Ceramics: Giant Piezoelectric and Physical Mechanism
Sichuan University, China
(K,Na)NbO3 (KNN)-based lead-free piezoceramics have been investigated extensively in the past ten years due to its large d33 and high TC. In particular, a series of high performance (d33=490~570 pC/N) KNN-based ceramics have been attained in our research group by constructing R-T phase boundary. And the physical mechanisms of the giant piezoelectricity have been explored by TEM, PFM and so on. The structural origin of its high piezoelectric performance can be attributed to a hierarchical nanodomain architecture, where the local structure inside nanodomains comprises a coexistence of R and T nanotwins. The physical origin can be attributed to low domain wall energy and nearly vanishing polarization anisotropy, facilitating easily polarization rotation among different states. We believe that the giant piezoelectricity and the deep exploration about physical mechanism will greatly accelerate the development of KNN ceramics.
3:00pm - 3:15pmOral
(K, Na)NbO3-Based Lead-Free Piezoelectric Materials: Giant Property, Structure and Physical Mechanism
National University of Singapore, Singapore
Piezoelectric materials with direct conversion between mechanical and electrical energy find remarkably wide application in all electronic devices. Almost all are based on toxic lead-based compounds, which are soon to be banned from use in many regions of the world. Hence, there is an urgent need for replacement by lead-free materials. However, so far, lead-free piezoelectrics have not been able to provide competitive performance to the lead-based materials. Key to the academic problem is a lack of fundamental understanding on the actual mechanisms involved at the microscopic (unit cell) level. While it is understood that giant responses occur near structural phase boundaries, there is at present no atomistic understanding of the origin of the response. New materials have therefore been synthesized largely by informed trial and error.
Recently we has achieved notable breakthroughs in improving the properties of (K, Na)NbO3 (KNN) based pizioceramics (525 pC/N in (K,Na)(Nb,Sb)3- Bi(Na,K)HfO3 and 550 pC/N in (K,Na)(Nb,Sb)3-BiFeO3−BiNaZrO3) through phase boundary engineering, and simultaneously improved the stability (temperature/electric-field stability and fatigue). Electron microscopy has indicated a hierarchical structure of nanodomains. Convergent-beam electron diffraction and atomic-resolution polarization mapping by Z-contrast imaging reveals the intimate coexistence of rhombohedral (R) and tetragonal (T) phases inside nanodomains, that is, a structural origin for the R–T phase boundary in the present KNN system. Hence, the physical origin of high piezoelectric performance can be deduced. We believe these breakthroughs in property and new understanding of piezoelectric behaviour are valuable to eradicate lead-based products from the marketplace, with concomitant environmental and commercial benefits worldwide.
Ting Zheng,#Haijun Wu,# et al, Energy and Environmental Science, Advance Article, 2017. (#equally contribution)
Bo Wu,# Haijun Wu,# et al, Journal of the American Chemical Society, 2016, 138 (47), pp 15459–15464. (#equally contribution)
3:15pm - 3:30pmOral
Lead-free (K, Na)(Sb, Nb)O3-x(Bi, Na, K)ZrO3 Thin Films with Large Piezoelectric Coefficients Derived from Chemical Solution Modified with Combinational Stabilizing Agents
1Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; 2Department of Chemistry, National University of Singapore, Singapore; 3Department of Mechanical Engineering, National University of Singapore, Singapore; 4NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore
As a promising lead-free piezoelectric ceramic, potassium sodium niobate (KNN) system has received intensive attention to replace the currently market-dominating lead-based piezoelectric ceramic, due to the increasing serious concern of the lead toxicity. Although high performance piezoelectric properties have been reported in KNN-based bulk ceramics by constructing morphotropic phase boundary (MPB) with complex compositions, the excellent piezoelectric properties have not been achieved in KNN-based thin films with the same MPB compositions due to the great technical challenge in controlling the complex and volatile compositions for thin films. Moreover, the stress due to substrate constraint under different film thicknesses also changes the film’s crystal structure and shifts the film’s MPB.
In this work, we demonstrate the outstandingly large effective piezoelectric strain coefficient d33 of 184.0 pm V-1 and voltage coefficient g33 of 39.4 mm V N-1 from macro scale characterization in a solution-derived lead-free piezoelectric thin film with a complex composition of (K, Na)(Sb, Nb)O3-x(Bi, Na, K)ZrO3 (KNSN-BNKZx, 0.01x 0.07). The KNSN-BNKZx films with different thicknesses (0.5-2.0 μm) are prepared by an ethanolamine-diethanolamine-ethylenediaminetetraacetic acid modified-chemical solution deposition method, and the excellent piezoelectric properties are obtained in KNSN-BNKZ0.05 film with the thickness of 2.0. With the effective suppression of volatile compositional loss by introducing appropriate combinational chemical agents in the precursor solution, phase transitions from orthorhombic to rhombohedral to tetragonal and the effect of the residual stress under different film thicknesses are clearly observed in the KNSN-BNKZx thin films, with theoretical supports from the first principle simulation. Our results demonstrate a valuable strategy for realizing high-performance piezoelectric properties in thin films with volatile and complex MPB compositions under stress condition.
 Y. Wang, K. Yao, M. Sharifzadeh Mirshekarloo, F. E. H. Tay, J. Am. Ceram. Soc. 99, pp. 1631–1636, 2016.