Xiang Lv

The structural origin of enhanced piezoelectric performance and stability in lead free ceramics

Lead-based piezoelectric materials are currently facing global restrictions due to their lead toxicity. Thus it is urgent to develop lead-free substitutes with high piezoelectricity and temperature stability, among which, potassium-sodium niobate [(K,Na)NbO3, KNN] has the most potential. It is very difficult to simultaneously achieve high piezoelectric performance and reliable stability in KNN-based systems. In particular, the structural/physical origin for their high piezoelectricity is still unclear, which hinders property optimization. Here we report the achievement of high temperature stability (less than 10% variation for electric field-induced strain from 27 °C to 80 °C), good fatigue properties (stable up to 106 cycles) as well as an enhanced piezoelectric coefficient (d33) of 525 pC N−1 in (1 − x)(K1−yNay)(Nb1−zSbz)O3–xBi0.5(Na1−wKw)0.5HfO3 (KNNS–BNKH) ceramics through manipulating the rhombohedral–tetragonal (R–T) phase boundary. The structural origin of their high piezoelectric performance can be attributed to a hierarchical nanodomain architecture, where the local structure inside nanodomains comprises R and T nanotwins. The physical origin can be attributed to low domain wall energy and nearly vanishing polarization anisotropy, facilitating easy polarization rotation among different states. We believe that the new breakthrough will open a window for the practical applications of KNN-based ceramics.

Identification of Phase Boundaries and Electrical Properties in Ternary Potassium-Sodium Niobate-Based Ceramics

A large piezoelectric constant (d33) of ∼480 pC/N was attained in new ternary (1-x-y)K0.5Na0.5Nb0.96Sb0.04O3-xBaSnO3-yBi0.5Na0.5ZrO3 ceramics by forming rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary using the variations of x and y, and such a phase boundary was successfully confirmed by the convergent beam electron diffraction (CBED) patterns. For (1-x)K0.5Na0.5Nb0.96Sb0.04O3-xBaSnO3, the orthorhombic (O) phase is well-maintained for 0 ≤ x ≤ 0.015, and both the R and T phases can be introduced to (0.99-y)K0.5Na0.5Nb0.96Sb0.04O3-0.01BaSnO3-yBi0.5Na0.5ZrO3 with y = 0.025-0.04 by simultaneously tailoring their compositions (x and y); then, R-O-T multiphases can be well-established. The CBED patterns strongly support the existence of R-O-T multiphases in the ceramics with y = 0.035. When the phase transitions endure from O to R-O-T, their piezoelectric activity endures a leapfrog development from ∼165 to ∼480 pC/N. In the region of the R-O-T phase boundary, a large d33 of ∼480 pC/N was attained in the ceramics with x = 0.01 and y = 0.035. In addition, the ceramics with x = 0.01 and y = 0.04 possess a high strain of ∼0.274% due to the multiphases coexistence. According to the variations of dielectric and ferroelectric properties, the enhancement in εr and Pr plays a part in the improved d33 except for the R-O-T phase boundary. We believe that the (K, Na)NbO3 ternary systems can be used to promote piezoelectric activity by forming new phase boundaries.

Role of antimony in the phase structure and electrical properties of potassium–sodium niobate lead-free ceramics

In the past ten years, antimony has been reported to strongly affect the developments in the piezoelectric properties of (K,Na)NbO3 (KNN) lead-free ceramics, that is, its enhanced piezoelectric activity is closely related to the doped antimony as well its content. In this work, we clarified the role of Sb5+ in the construction of a phase structure and the enhancement of electrical properties of a pure KNN ceramic. Research has shown that doping with Sb5+ can simultaneously move their orthorhombic–tetragonal phase transition temperature (TO–T) and rhombohedral–orthorhombic phase transition temperature (TR–O) forward to room temperature, benefiting the formation of three types of phase boundaries. The coexistence of rhombohedral and orthorhombic phases was established in the Sb5+ composition range of 0.07–0.09 by this regulation. In addition, their grain sizes were determined by the Sb5+ content, that is, the optimum Sb5+ content (x ≤ 0.05) induces grain growth, and their grain sizes become considerably smaller when the compositions deviate from x > 0.05. More importantly, their electrical properties could be also tuned by changing the Sb5+ content. Their dielectric, ferroelectric, and piezoelectric properties are strongly dependent on the antimony content, whereas the strain behavior is mainly ascribed to the multi-phase transition region as well as the structural change of phase transitions. As a result, this work would help to further understand the underlying physical origin for enhanced electrical properties in alkali niobate ceramics.

Composition design and electrical properties in (1-y)(K0.40Na0.60)0.985Li0.015(Nb1−xSbx)O3-yBi0.5Na0.5ZrO3 lead-free ceramics

To realize the enhancement in piezoelectric activities, the composition-induced phase boundaries in (1-y)(K0.40Na0.60)0.985Li0.015(Nb1−xSbx)O3-yBi0.5Na0.5ZrO3 lead-free ceramics were designed and fabricated by the conventional solid-state method. We presented the evolutions of their phase structure, microstructure, and electrical properties with the change of Sb5+ and Bi0.5Na0.5ZrO3 contents. A rhombohedral-tetragonal phase boundary was successfully built in the composition region of 0.04 ≤ x ≤ 0.09 (y = 0.025) and 0.025 ≤ y ≤ 0.035 (x = 0.06), and then the desirable piezoelectric coefficients and bipolar strains (e.g., d33∼390 pC/N, kp∼0.45, Smax∼0.2%, and TC∼250 °C) were simultaneously induced. We think that this may provide a direction of designing high-performance (K,Na)NbO3-based ceramics.

Ultrahigh energy-storage potential under low electric field in bismuth sodium titanate-based perovskite ferroelectrics

Relaxor ferroelectrics are receiving an increasing amount of attention because of their superior energy-storage density. Due to environmental concerns, lead-free alternatives are highly desirable, with bismuth sodium titanate highlighted for its energy-storage applications. Here, we realized an enhancement in energy-storage performance with a recoverable energy density (Wrec) of 2.42 J cm−3 (low electric field of E = 143 kV cm−1) in {Bi0.5[(Na0.8K0.2)0.90Li0.10]0.5}0.96Sr0.04(Ti0.975Ta0.025)O3 ceramics by a hot-pressed sintering (HPS) method, which is greatly superior to the reported perovskite ceramics under similar electric fields. In addition, excellent fatigue and thermal stabilities (variation of Wrec ≤ 0.047% after 105 cycles and Wrec > 2 J cm−3 over 25–175 °C) can be observed. The HPS method greatly increases the dielectric breakdown strength (DBS ∼ 143 kV cm−1) because of a denser structure consisting of large and small grains, which is superior to those (78–97 kV cm−1) of spark plasma sintering (SPS), conventional air sintering (CAS), and MnO aids sintering (AS) methods. In addition to the contribution of the enhanced breakdown strength, the ultrahigh energy-storage density is also due to the almost complete RE to FE transition resulting from strain, polarization, and current density versus electric field (S–E, P–E, j–E) loops. Interestingly, a giant strain of 0.65% can also be found by the HPS method. In particular, a conceptual model based on the nature of relaxor ferroelectrics is employed to understand the excellent energy-storage properties observed in this work. We believe that the findings in this work may provide future tips and guidance for this direction of study.

High unipolar strain in samarium-doped potassium–sodium niobate lead-free ceramics

In this work, a high unipolar strain has been developed in the 0.9675(K0.48Na0.52)(Nb0.915Sb0.035Ta0.05)O3–0.0325(Bi1−xSmx)0.5(Na0.82K0.18)0.5ZrO3 (KNNST-B1−xSxNKZ) ceramics by introducing Sm, and the composition dependence of their phase structures and electrical properties is also discussed. The addition of Sm3+ can change the phase structure of the ceramics by simultaneously shifting TR–O and TO–T to a higher temperature, and we obtained a rhombohedral–orthorhombic–tetragonal coexistence phase (R–O–T) with x = 0, a coexistence phase having orthorhombic and tetragonal (O–T) phases with 0.05 ≤ x ≤ 0.60 and an orthorhombic phase with 0.8 ≤ x ≤ 1.0. In addition, the doping with Sm3+ can greatly enhance the unipolar strain of the ceramics without significantly sacrificing its TC, and a high unipolar strain (∼0.28%) was observed in the ceramics with x = 0.20. More importantly, a large Smax/Emax of ∼833 pm V−1 was also observed in the ceramics with x = 0.20 under a low applied electric field of 1.8 kV cm−1. We believe that such a high unipolar strain can benefit the practical applications of actuators.

An Alternative Way To Enhance Piezoelectricity and Temperature Stability in Lead-Free Sodium Niobate Piezoceramics

To further balance the relationship between piezoelectricity and temperature stability, the (0.975 - y)NaNbO3- yBaTiO3-0.025BaZrO3 ( y = 0-0.20) ceramics are developed by constructing a wide tetragonal phase region. Effects of BaTiO3 on the relationships among phase structure, electrical properties, and temperature dependence are investigated. With increasing BaTiO3 contents, the ceramics endure the structural evolutions from orthorhombic phase to tetragonal phase, and then to relaxor cubic phase. A wide tetragonal phase zone of 24-180 °C can be realized in the ceramics with y = 0.08, together with an enhanced piezoelectric coefficient d33 = 215 pC/N. Intriguingly, excellent temperature stability of unipolar strain ( Suni) and piezoelectric coefficient ( d33) are observed in the ceramics with y = 0.08 within 20-180 °C. This work provides an alternative way to enhance piezoelectricity and temperature stability in lead-free piezoceramics.

High-Performance 0-3 Type Niobate-Based Lead-Free Piezoelectric Composite Ceramics with ZnO Inclusions

Because of their high toxicity, lead-based materials in electronic devices must be replaced by lead-free piezoelectric materials. However, some issues remain that hinder the industrial applications of these alternative ceramics. Here, we report the construction of a 0-3-type ceramic composite (KNNS-BNKZ: xZnO), where the Sb-doped ZnO submicronic particles were randomly distributed throughout the potassium-sodium niobate-based ceramic matrix. In this (K,Na)NbO3 (KNN)-based ceramic composite, superior temperature stability, excellent piezoelectric properties, and a high Curie temperature were simultaneously achieved. The unipolar strain varied from +20 to -16% when the temperature was increased from 23 to 200 °C in KNNS-BNKZ: xZnO with x = 0.75. By increasing the ZnO content from x = 0 to x = 5.0, the Curie temperature was increased from 227 to 294 °C. More importantly, the piezoelectric coefficient remained high ( d33 = 480-510 pC/N) for a wide range of compositions, x = 0.25-1.0. Transmission electron microscopy (TEM) experiments showed that the compensatory electric fields generated by the Sb-doped ZnO submicronic particles were responsible for the improved temperature stability. The high piezoelectricity was due to the existence of nanodomains, which were clearly observed in the TEM experiments. The results presented in this work clarify some of the physical mechanisms in this KNN-based ceramic composite, thus advancing the development of lead-free ceramics.