KNN–NTK composite lead-free piezoelectric ceramic

Abstract

A (K,Na)NbO3-based lead-free piezoelectric ceramic was successfully densified. It exhibited an enhanced electromechanical coupling factor of kp = 0.52, a piezoelectric constant d33 = 252 pC/N, and a frequency constant Np = 3170 Hz m because of the incorporation of an elaborate secondary phase composed primarily of KTiNbO5. The ceramic's nominal composition was 0.92K0.42Na0.44Ca0.04Li0.02Nb0.85O3–0.047K0.85Ti0.85Nb1.15O5–0.023BaZrO3–0.0017Co3O4–0.002Fe2O3–0.005ZnO, abbreviated herein as KNN–NTK composite. The KNN–NTK ceramic exhibited a dense microstructure with few microvoids which significantly degraded its piezoelectric properties. Elemental maps recorded using transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM–EDS) revealed regions of high concentrations of Co and Zn inside the NTK phase. In addition, X-ray diffraction patterns confirmed that a small portion of the NTK phase was converted into K2(Ti,Nb,Co,Zn)6O13 or CoZnTiO4 by a possible reaction between Co and Zn solutes and the NTK phase during a programmed sintering schedule. TEM studies also clarified a distortion around the KNN/NTK interfaces. Such an NTK phase filled voids between KNN particles, resulting in an improved chemical stability of the KNN ceramic. The manufacturing process was subsequently scaled to 100 kg per batch for granulated ceramic powder using a spray-drying technique. The properties of the KNN–NTK composite ceramic produced using the scaled-up method were confirmed to be identical to those of the ceramic prepared by conventional solid-state reaction sintering. Consequently, slight changes in the NTK phase composition and the distortion around the KNN/NTK interfaces affected the KNN–NTK composite ceramic's piezoelectric characteristics.

Core–Shell Lead–Free Piezoelectric Ceramics: Current Status and Advanced Characterization of the Bi1/2Na1/2TiO3–SrTiO3 System

“…Research in lead‐free piezoceramics has focused mostly in tailoring the electromechanical properties by chemical substitution or microstructure optimization. Recent work, however, has shown the potential of multiphase structures such as ceramic/ceramic composites and core–shell materials . This contribution focuses on the core–shell approach as it provides an alternative towards the improvement of piezoelectric properties for large signal applications.…”

Potassium Sodium Niobate-Based Lead-Free Piezoelectric Multilayer Ceramics Co-Fired with Nickel Electrodes

“…Furthermore, studies of multilayer KNN-based lead-free piezoelectric ceramics with silver-palladium electrodes have been reported [74,75]. Moreover, many studies on the application of KNN-based ceramics and improving the piezoelectric properties of KNN-based ceramics have been reported [76,77,78,79,80,81,82,83,84,85,86,87,88,89]. These activities indicate that a KNN-based multilayer ceramic structure is widely considered to be a good candidate for lead-free piezoelectric devices.…”

Piezoelectric Properties and Microstructure of (K,Na)NbO3– KTiNbO5 Composite Lead-Free Piezoelectric Ceramic

“…The resistivity of the Li-doped KNN ceramic is 6.0 × 10 7 Ω cm [10], whereas that of the KNN-NTK composite ceramic is 3.6 × 10 10 Ω cm. This KNN-NTK composite ceramic was polarized under a high voltage of 6 kV/mm.…”

“…In our present study [10,11], after due consideration of the dielectric constant, we combine KNN with the KTiNbO 5 (NTK) phase, which has a layered structure as shown in Figure 1 and is not piezoelectric material. With this approach, we prepared and densified a KNN-NTK composite ceramic that exhibits enhanced piezoelectric properties; notably, a planar-mode electromechanical coupling coefficient k p = 0.52, which is close to the highest value previously reported for KNN-based composite lead-free piezoelectric ceramics [8,[12][13][14][15].…”