A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Recent development in lead-free perovskite piezoelectric bulk materials

Stoichiometric and nonstoichiometric (Bi05Na05)TiO3 (BNT) ceramics were prepared by a conventional ceramic fabrication process This study revealed that the high conductivity of BNT ceramics is associated with Bi vaporization during sintering An x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480 °C in BNT ceramic as well as BNT single crystals In addition, the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant were determined to be 187, approximately 300, and 325 °C, respectively, from the temperature dependences of dielectric properties using unpoled and poled specimens The piezoelectric properties of all vibration modes and the temperature dependences of the piezoelectric properties were measured using fully poled BNT ceramics It was also revealed that BNT ceramics exhibit three thermal depoling processes at Td, between Td and TR-T, and between TR-T and Tm 

Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics

Developing eco-friendly high-performance piezoceramics without lead has become one of the most advanced frontiers in interdisciplinary research. Although potassium sodium-niobate {(K,Na)NbO3, KNN} based ceramics are believed to be one of the most promising lead-free candidates, the relatively inferior piezoelectric properties and strong temperature dependency have hindered their development for more than 50 years since being discovered in the 1950s. It was not until 2014 that our group initially proposed a new phase boundary (NPB) that simultaneously improved the piezoelectric properties and temperature stability of non-textured KNN-based ceramics to the level of partly lead-based ceramics. The NPB has been then proved by some researchers and believed to pave the way for "lead-free at last" proposed by E. Cross (Nature, 2004, 432, 24). However, the understanding of the NPB is still in its infancy, leaving many controversies, including the phase structure and physical mechanisms at the NPB as well as the essential difference when compared with other phase boundaries. In this context, we systematically summarized the origin and development of the NPB, focusing on the construction, structure and intrinsic trait of the NPB, the effects of the NPB on the performance, and the validity and related incipient devices of the NPB. Particularly, we concluded the phase structure and domain structure locating at the NPB, analyzed the physical mechanisms in depth, proposed the possible methods to further improve the performance at the NPB, and demonstrated the validity and scope of the NPB as well as the device application. Finally, we gave out our perspective on the challenges and future research of KNN-based ceramics with NPB. Therefore, we believe that this review could promote the understanding of the NPB and guide the future work of KNN-based ceramics.

/pdf/emerging-new-phase-boundary-in-potassium-sodium-niobate-2gjb40rkrw.pdf

Emerging new phase boundary in potassium sodium-niobate based ceramics

Phase structure and properties of sodium bismuth titanate lead-free piezoelectric ceramics

Microwave dielectric properties of ZnAl2O4–TiO2 spinel-based composites

Novel high dielectric constant and low-loss tetragonal tungsten bronze dielectrics Ba5LnMgNb9O30 (Ln=La, Nd, Sm, Gd and Yb)

A novel forming method–aqueous gelcasting was used to assist conventional solid-state method to prepare 45 wt% Ba0.6Sr0.4TiO3–55 wt% MgO (BSTM) ceramic powders. For comparison purpose, conventional solid-state method was also used to synthesize the same powders. It is found that the BSTM ceramic powders prepared by our method are more uniform with smaller and more sphere-like particles than those prepared by the conventional method, whereas the phase compositions are completely the same no matter what powder preparation method is adopted. In addition, the BSTM green samples and ceramics from powders prepared by our method are more uniform than those from powders prepared by the conventional method. Moreover, it is found that using this novel method to prepare BSTM ceramic powders could not only improve the dielectric properties of the BSTM ceramics, but also decrease their sintering temperature. Aqueous gelcasting–assisted solid-state method could be a novel promising method to prepare high performance ceramic powders in the future.

Ba0.6Sr0.4TiO3–MgO Ceramic Powders with Uniform Microstructures Prepared by Aqueous Gelcasting‐Assisted Solid‐State Method

This invention relates to bismuth natrium titanate group lead-free piezoelectric ceramics, its goal is to reduce coercive field and improve piezoelectricity. Basic material mol ratio and component expression is (1-x-y)(Bi1/2Na1/2)TiO3-x(Bi1/2K1/2)TiO3-yKNbO3, in the formula 0<=x<1,0<y<0.2, 0<(x+y)<1, and its preparation method is that (1) Bi2O3, Na2CO3, (COOK)2.H2O, TiO2, Nb2O5 are prepared by chemical measuring ratio of the formula as materials, pure alcohol is medium ball mill. (2) Powder material after ball mill is dried and pre-burned for 2 hours at 800-850 degree C, and then adhesive is added for forming. (3) They are sintered 2-3 hours at 1100-1180 degrees C. (4) Silver electrode is prepared on the ceramics piece after sintered, then it is polarized for 10-15 minutes under voltage of 4-5 kV in 30-60 degrees C silicone oil. The d33 of the compound in this invention can reach 190pC/N, Kp can reach 0.37, and its technique is stable, so it is suitable to piezoelectric oscillator and ultrasonic transducer fields.

Bismuth-sodium titanate base lead-free piezoelectric ceramic

The crystal structure and piezoelectric properties of ternary ceramics x(Bi1∕2Na1∕2)TiO3–y(Bi1∕2K1∕2)TiO3–zKNbO3 (abbreviated as BNT-BKT-KN100x∕100y∕100z) system have been investigated. Effects of BKT and KN on the crystalline structure were examined. The morphotropic phase boundary (MPB) phase diagram of BNT-BKT-KN ternary system was determined by x-ray diffraction patterns from sintered specimens. Near the MPB, BNT-BKT-KN82∕16∕2 ceramics had good performances with piezoelectric constant d33=215pC∕N which reached a maximum value in this ternary system and planar electromechanical coupling factor kp=35%. Besides, BNT-BKT-KN87∕10∕3 ceramics had good performances with piezoelectric constant d33=138pC∕N and electromechanical coupling factor kp=38% which attained a maximum value in this ternary system.

Morphotropic phase boundary and piezoelectric properties of (Bi1∕2Na1∕2)TiO3–(Bi1∕2K1∕2)TiO3–KNbO3 lead-free piezoelectric ceramics

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