Ting Zheng

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.

Large d33 in (K,Na)(Nb,Ta,Sb)O3-(Bi,Na,K)ZrO3 lead-free ceramics

To protect the environment and human health, it is necessary to develop high-performance lead-free piezoceramics to replace the lead-based ones in some electronic devices. Here we report first a large piezoelectricity in (K,Na)NbO3-based lead-free piezoceramics prepared by the conventional solid-state method. The rhombohedral–tetragonal phase boundary is observed in the ceramics with a composition of 0.04 ≤ x ≤ 0.06. Those ceramics with 0.01 ≤ x ≤ 0.06 possess a good comprehensive performance of d33 (380–460 pC N−1) and TC (170–287 °C). Moreover importantly, a peak d33 of ∼460 pC N−1 is shown in the ceramic with x = 0.04, which is superior to all other reported results of KNN-based ceramics, including the reported results by Saito et al. (Nature, 2004, 432, 84). We believe that such a material system is a very promising candidate for potassium–sodium niobate piezoceramics.

New Potassium–Sodium Niobate Ceramics with a Giant d33

For potassium-sodium niobate, poor piezoelectric properties always perplex most researchers, and then it becomes important to attain a giant piezoelectricity. Here we reported a giant piezoelectric constant in (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-xBi0.5Ag0.5ZrO3 lead-free ceramics. The rhombohedral-tetragonal phase boundary was shown in the ceramics with 0.04<x≤0.05, and then the ceramic with x=0.0425 possesses a giant d33 of ∼490 pC/N. We also discussed the physical mechanisms of enhanced piezoelectricity. As a result, such a research can benefit the sustainable development of (K,Na)NbO3 materials.

Potassium–sodium niobate lead-free ceramics: modified strain as well as piezoelectricity

The obvious conflicts between large piezoelectricity and high strain could be solved by developing new phase boundaries in potassium–sodium niobate materials. Here, we have solved this problem by extensive experimental researches and induced a larger strain as well as a higher piezoelectricity in (K, Na)NbO3. Large converse piezoelectric coefficient (d*33 = 599–1553 pm V−1) and high strain (0.18–0.46%) were achieved, which are the highest values reported to date in potassium–sodium niobate, suggesting that such a system is a promising lead-free candidate for electromechanical actuator applications. In addition, high d33 values of 400–490 pC N−1 have also been attained in the ceramic due to its rhombohedral–tetragonal phase boundary, as well as its composition.

Strong Piezoelectricity in (1 – x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1–ySnyO3 Lead-Free Binary System: Identification and Role of Multiphase Coexistence

Here we report a strong piezoelectric activity in (1 - x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free ceramics by designing different phase boundaries. The phase boundaries concerning rhombohedral-orthorhombic-tetragonal (R-O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were attained by changing BKZS and Sn contents and then were identified by the X-ray diffraction patterns as well as temperature-dependent permittivity and ν1 Raman modes associated with BO6 perovskite octahedron. A high strain (strain = 0.21-0.28% and d33* = 707-880 pm/V) and a strong piezoelectric coefficient (d33 = 415-460 pC/N) were shown in the ceramics located at the multiphase coexistence region. The reported results of this work are superior to that (d33* ∼ 570 pm/V and d33 ∼ 416 pC/N) of the textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics [Nature 2004, 432, 84]. We believe that the material system of this work will become one of the most promising candidates for piezoelectric actuators.

Multi-scale thermal stability of niobate-based lead-free piezoceramics with large piezoelectricity

Growing environmental concerns are pushing the development of lead-free piezoceramics with both outstanding piezoelectric properties and reasonable thermal stability. Herein, we realized a large piezoelectric coefficient d33 of 430 pC N−1 in 0.96(K0.4Na0.6)(Nb0.96Sb0.04)O3–0.04Bi0.5K0.5Zr0.85Sn0.15O3 (KNNS–BKZS) polycrystals by constructing a rhombohedral–tetragonal (R–T) phase boundary. Investigations of the in situ thermal stability of the piezoelectric properties on multiple scales reveal that the micro-scale piezoelectric response is much more stable compared to the macro-scale response, indicating the significant role of extrinsic contributions from domain wall movements. These findings demonstrate the relationship between multi-scale properties and domain structures, revealing that the high piezoelectricity is attributed to nano-domains at the R–T phase boundary.

New potassium-sodium niobate lead-free piezoceramic: Giant-d33 vs. sintering temperature

The objective of this work is to achieve a giant piezoelectric constant in (K,Na)NbO3-based lead-free ceramics, and then 0.96K0.46Na0.54Nb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5ZrO3 lead-free piezoceramics were designed and prepared by optimizing the sintering temperature (TS). The rhombohedral-tetragonal phase boundary is found in the ceramics sintered at 1070 ∼ 1105 °C and is suppressed when sintered at low TS of 1060 ∼ 1065 °C. The threshold for TS is 1070 °C in terms of their ferroelectric and piezoelectric properties owing to the difference in the phase boundary and the microstructure, and a large d33 of 388 ∼ 465 pC/N could be attained in a wide TS range of 1070 ∼ 1105 °C, benefiting their practical applications because of broad TS. More interestingly, the ceramic sintered at 1075 °C has a giant d33 of ∼465 pC/N. We think that such a giant d33 of this material system can benefit the development of (K,Na)NbO3-based piezoceramics.

Composition-Driven Phase Boundary and Piezoelectricity in Potassium–Sodium Niobate-Based Ceramics

The piezoelectricity of (K,Na)NbO3 ceramics strongly depends on the phase boundary types as well as the doped compositions. Here, we systematically studied the relationships between the compositions and phase boundary types in (K,Na) (Nb,Sb)O3-Bi0.5Na0.5AO3 (KNNS-BNA, A=Hf, Zr, Ti, Sn) ceramics; then their piezoelectricity can be readily modified. Their phase boundary types are determined by the doped elements. A rhombohedral-tetragonal (R-T) phase boundary can be driven in the compositions range of 0.035≤BNH≤0.040 and 0.035≤BNZ≤0.045; an orthorhombic-tetragonal (O-T) phase boundary is formed in the composition range of 0.005≤BNT≤0.02; and a pure O phase can be only observed regardless of BNS content (≤0.01). In addition, the phase boundary types strongly affect their corresponding piezoelectricities. A larger d33 (∼440-450 pC/N) and a higher d33* (∼742-834 pm/V) can be attained in KNNS-BNA (A=Zr and Hf) ceramics due to the involvement of R-T phase boundary, and unfortunately KNNS-BNA (A=Sn and Ti) ceramics possess a relatively poor piezoelectricity (d33≤200 and d33*<600 pm/V) due to the involvement of other phase structures (O-T or O). In addition, the underlying physical mechanisms for the relationships between piezoelectricity and phase boundary types were also discussed. We believe that comprehensive research can design more excellent ceramic systems concerning potassium-sodium niobate.

Effects of site engineering and doped element types on piezoelectric and dielectric properties of bismuth ferrite lead-free ceramics

Poor piezoelectric activity is often observed in BiFeO3 ceramics due to their low resistivity and high coercive field, which can easily result in piezoelectric breakdown before the domains are switched. Here, we attained a high piezoelectricity using a series of bismuth ferrite ceramics substituted by rare earth elements and transition metal elements {e.g., Bi0.925La0.05A0.025FeO3, A: Sm, Yb, Ho, Y, Nd, Pr, Dy, Gd; Bi0.925La0.05Sm0.025Fe0.95M0.05O3, M: Sc, In, Al, Ga, Ni, Co} fabricated using the conventional solid-state method. The influences of site engineering (e.g., Bi site or Fe site) as well as the doped element types on their phase structure, microstructure, and electrical properties have been comparatively analyzed. The ions (e.g., A = Sm, Yb, Ho, and Y) substituting at the Bi site are helpful to attain both a pure phase structure and a relatively good piezoelectricity (d33 ≥ 40 pC N−1) for BFO ceramics, while ion substitutions at the Fe site cannot suppress the formation of impurity phases which results in degraded electrical properties. Both XRD and backscattered electron images fully confirmed the existence of impurity phases (Bi-rich and Fe-rich counterparts) in the ceramics doped by Ga. According to the related experiments, the piezoelectric properties of bismuth ferrite ceramics can be promoted by site engineering as well as the optimization of the element types. This result will point out a way for us to promote the piezoelectric properties of bismuth ferrite ceramics through choosing both suitable doping elements and eliminating impurity phases.

New (1 − x)K0.45Na0.55Nb0.96Sb0.04O3-xBi0.5Na0.5HfO3 lead-free ceramics: Phase boundary and their electrical properties

Here, we reported a high unipolar strain and large piezoelectricity in new (1 − x)K0.45Na0.55Nb0.96Sb0.04O3-xBi0.5Na0.5HfO3 ceramics. The rhombohedral-tetragonal (R-T) phase boundary was constructed in the ceramics with 0.03 < x ≤ 0.05, which shows a large d33 value of ∼419 pC/N. More importantly, a high unipolar strain of ∼0.31% was observed due to the multiphase coexistence. In addition, the piezoelectricity of the ceramics could be effectively enhanced if their compositions are located at the phase boundaries region, where a very low electric field of ∼1.2 kV/mm can readily rotate the R/T domains. We also noticed that the deviation from phase boundary induced by applying an external electric field results in the deterioration of piezoelectricity after the “second-poling” method. We believe that as a potassium-sodium-niobate based material, the ceramics developed in this work may find practical applications in lead-free piezoelectric devices such as actuators and fuel injectors in the future owing to th...