Gradient-Acceptor-Doping Breaks the Piezoelectric Trade-Off in Lead-Free (K, Na)NbO3 Ceramics.

(K,Na)NbO3 based ceramics are considered to be one of the most promising lead-free ferroelectrics replacing Pb(Zr,Ti)O3. Despite extensive studies over the last two decades, the mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics has not been fully understood. Here, we combine temperature-dependent synchrotron x-ray diffraction and property measurements, atomic-scale scanning transmission electron microscopy, and first-principle and phase-field calculations to establish the dopant–structure–property relationship for multi-elements doped (K,Na)NbO3 ceramics. Our results indicate that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures with low-angle polar vectors are responsible for the high dielectric and piezoelectric properties. This work explains the mechanism of the high piezoelectricity recently achieved in (K,Na)NbO3 ceramics and provides guidance for the design of high-performance ferroelectric ceramics, which is expected to benefit numerous functional materials.

Evolution of textured microstructure of Li-doped (K,Na)NbO3 ceramics prepared by reactive templated grain growth

Excellent piezoelectric properties can be obtained near the polymorphic phase transition (PPT) region in (K, Na)NbO3 based piezoceramics. The enhanced piezoelectric properties resulted from the 18-fold spontaneous polarization directions caused by the coexistence of orthorhombic and tetragonal phases. In this study, the various polarization directions derived from frequently changing Ta-O bonds in Li-, Ta-, and Sb-modified (K, Na)NbO3 ceramic were directly observed by extended X-ray absorption fine structure. More than three peaks were observed and represented the nearest neighbor Ta-O bonds because of the coexistence of tetragonal and orthorhombic phases as well as Ta displacements along the [001]c and [110]c directions. Hence, the domains rotated easily and responded actively to an external electric or force field. Large Pr and optimized piezoelectric properties were obtained near the PPT region.

In this letter the authors report the observation of double hysteresis loops in Cu-doped K0.5Na0.5NbO3 (KNN) ceramics. Unlike other ferroelectric titanates (e.g., BaTiO3), aging is not required for the ceramic to exhibit the double-loop-like characteristics. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions-Cu2+ and O2− vacancies along the polarization direction after the diffuse tetragonal-orthorhombic phase transition of the ceramic. Because of the low migration rates of defects, the defect dipoles remain in the original orientation during the P-E loop measurement, providing a restoring force to reverse the switched polarization. The defect dipoles also provide “pinning” effects in the normal piezoelectric activities. As a result, the ceramic becomes “hardened,” exhibiting an extraordinarily high mechanical quality factor (2500), while the other piezoelectric properties remain reasonably good: electromechanical coupling coefficients kp=39%, kt=47%, and piezoelectric coefficient d33=82pC∕N.

Properties of Li- and Ta- modified (K,Na)NbO3 piezoceramics with the base composition near the orthorhombic-tetragonal phase boundary have been investigated with respect to variation of temperature, stoichiometry, compositional homogeneity, atmosphere of thermal treatment, and dopants (Ba, Mn). Although the influence of the most of the abovementioned factors has already been reported in the literature, the paper focuses on several aspects of the composition—property relationships that still remain controversial or poorly elucidated. In particular, we show that improvement of compositional homogeniety in these ceramics emphasizes the instability of piezoelectric response with respect to variation of temperature in the vicinity of the orthorhombic-tetragonal phase transition. Ba dopant is shown to suppress conductivity in ceramics sintered in air, though it makes conductivity more sensitive to variation of the oxygen partial pressure. Mn dopant is shown to suppress conductivity and strongly reduce the influence of the oxygen partial pressure on conductivity of the ceramics. Finally, we show that chemical modifications to the ceramic composition, such as Mn dopant or variation of nonstoichiometry affect the piezoelectric response mainly by the shift of the orthorhombic-tetragonal phase transition temperature.

Lead free piezoelectric materials are being intensively investigated in order to substitute lead based ones, commonly used in many different applications. Among the most promising lead-free materials are those with modified NaNbO3, such as (K, Na)NbO3 (KNN) and (Ba, Na)(Ti, Nb)O3 (BTNN) families. From a ceramic processing point of view, high density single phase KNN and BTNN ceramics are very difficult to sinter due to the volatility of the alkaline elements, the narrow sintering temperature range and the anomalous grain growth. In this work, Spark Plasma Sintering (SPS) and high-energy ball milling (HEBM), following heat treatments (calcining and sintering), in oxidative (O2) atmosphere have been used to prepare single phase highly densified KNN (“pure” and Cu2+ or Li1+ doped), with theoretical densities ρth > 97% and BTNN ceramics (ρth ~ 90%), respectively. Using BTTN ceramics with a P4mm perovskite-like structure, we showed that by increasing the NaNbO3 content, the ferroelectric properties change from having a relaxor effect to an almost “normal” ferroelectric character, while the tetragonality and grain size increase and the shear piezoelectric coefficients (k15, g15 and d15) improve. For KNN ceramics, the results reveal that the values for remanent polarization as well as for most of the coercive field are quite similar among all compositions. These facts evidenced that Cu2+ may be incorporated into the A and/or B sites of the perovskite structure, having both hardening and softening effects.

Effect of Domain Size on the Coercive Field of Orthorhombic (Li,K,Na)NbO3 Ceramics

Mimicking the scheme of incorporating La3+ to Pb(Zr1−xTix)O3, Ca2+ is used to substitute Na+ in the lead-free NaNbO3 compound, with A-site vacancy introduced to maintain charge neutrality. The anticipated relaxor behavior is expected to suppress the remanent polarization and improve the energy storage properties of NaNbO3-based ceramics. Specifically, (Na1−2xCax)NbO3 (x = 0.01, 0.02, 0.04, 0.08) ceramics were prepared with the solid-state method, and their structures and electric properties were investigated. X-ray diffraction and transmission electron microscopy reveal the existence of minor amount of CaNb2O6 second phase. Polarization vs. electric field hysteresis loop measurements verify the suppression of remanent polarization in compositions of x ≤ 0.04. The temperature-dependent dielectric tests indicate that both the relaxation and diffuseness parameters monotonically increase with Ca2+ content. The results demonstrate that the introduction of a smaller donor dopant and charge-compensating vacancies on the A-site in NaNbO3 is an effective strategy to disrupt the long-range dipole order.

For applications in high power ultrasonic transducers, both high piezoelectric coefficients d 33 and high mechanical quality factor Q m are essential. In this work, acceptor-doped Pb(Zr0.26Sn0.26Ti0.48)1−x Fe x O3−x/2 ferroelectric ceramics with compositions near the morphotropic phase boundary were prepared by routine solid-state reaction. By introducing Fe3+ ion into the perovskite crystal lattices, defect dipoles resulting from Fe3+ ions and oxygen vacancies are formed, pinning extrinsic domain wall motion, and hence increasing Q m but reducing d 33. It was found that substitution with 0.5 mol% Fe3+ resulted in high d 33 (390 pC/N) and high Q m (540). The nonlinear dielectric responses of these ceramics were evaluated on the basis of the Rayleigh law. Both Rayleigh coefficients, $$\alpha$$ and $$\kappa_{\rm{init}},$$ are reduced by doping with the acceptor ion Fe3+, indicating that the intrinsic contribution and reversible domain wall mobility are both reduced.

The hardening of K0.5Na0.5NbO3 (KNN) ceramics with the addition of Ag2O and CuO and the crystal-orientation behavior of KNN and CuO-added KNN powders under a high magnetic field were investigated. Ag2O reduced the degree of compositional deviation of KNN ceramics and increased their density. Additionally, the piezoelectric characteristics were improved by the addition of Ag2O. CuO functioned as a sintering aid for KNN, and the CuO-added KNN ceramics increased the grain size and mechanical quality factor by up to approximately 1200. The simultaneous addition of Ag2O (0.01 mol) and CuO (0.01 mol) to KNN resulted in hardening with a mechanical quality factor of 1400 owing to the synergistic contributions of each additive. Under a high magnetic field of 5 T, the KNN powder and CuO (0.01 mol)-added KNN powder exhibited b- and c-axes orientation parallel to the magnetic field, which is expected to enhance the piezoelectric properties through crystal orientation.

The piezoelectric properties in some bismuth layer‐structured ferroelectric (BLSF) ceramics were investigated focusing on a mechanical quality factor, Qm. Many BLSF compositions with various Curie temperatures, Tc, were selected in this study, and the Qm values of (33) mode were plotted as a function of their Curie temperature, Tc. The Qm increased with increasing the Tc in the low‐Tc compositions below 500°C. It is considered to relate with the domain wall pinning because the coercive field Ec also increased with increasing the Tc. However, in the high‐Tc compositions above 750°C, the Qm decreased with increasing the Tc, because of the difficulties in poling treatment due to the Ec. On the other hand, high Qm values exceeding 10,000 were obtained in the intermediate‐Tc compositions between 500 and 750°C. These values are quite high as the piezoelectric ceramics. The key point of the high Qm in these compositions is the poling process at high temperature of 300°C. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(12): 39–45, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10185

Emerging high-power piezoelectric applications demand the development of piezoelectric materials featuring both a high mechanical quality factor (Qm) and a large piezoelectric coefficient (d33). However, it is widely accepted that an increase in d33 is usually accompanied with a decrease in Qm, and vice versa. Herein, a multiscale regulation strategy is proposed to improve Qm and d33 simultaneously from the perspectives of phase structure, ferroelectric domains, and lattice defects. A well-balanced combination of electromechanical performances with Qm = 726, d33 = 502 pC N-1, kp = 0.69, tan δ = 0.0024, and TC = 267 °C was obtained. Through structural characterization, it was observed that the morphotropic phase boundary and enhanced dispersion behavior lead to a lowered energy barrier, which contributes to polarization rotation and enhances piezoelectric performance. At the same time, the excellent piezoelectric performances also benefit from the highly oriented domain structure and small domain size after high-temperature poling. Furthermore, the segregation of Ba2+ causes A-site defects in the crystal lattice, accompanied with an increase in oxygen vacancies, which maintains the hardening effect of the ceramics. This study proposes a multiscale regulation strategy, providing insights for the design of high-power piezoelectric ceramics with high d33 and Qm.

(1 − x)(Na0.5Bi0.5)0.94Ba0.06TiO3–xSmAlO3 [(1 − x)BNBT6–xSA] lead-free piezoelectric ceramics were synthesized by conventional solid-state method. Effects of composition on the micro-structure and electrical properties of (1 − x)BNBT6–xSA ceramics were investigated. X-ray diffraction data shows that (1 − x)BNBT6–xSA ceramics form the pure perovskite phases, and the ceramics have the morphotropic phase boundary when x ≤ 0.0100. At room temperature, the electrical properties of the BNBT6 ceramics have been improved with the addition of appropriate SA. At room temperature, (1 − x)BNBT6–xBA ceramics at x = 0.0100 has the highest remnant polarization (P r = 47 μC/cm2), the highest piezoelectric coefficient (d 33 = 179 pC/N), the highest planar coupling factor (k p = 0.296), the higher dielectric constant (ɛ r = 1154) and higher mechanical quality factor (Q m = 131).

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

High electromechanical performance with spark plasma sintering of undoped K0.5Na0.5NbO3 ceramics