Environment‐friendly lead‐free piezoelectric ceramics have been studied extensively in the past decade with great progress particularly in systems based on a niobate perovskite compound formulated as (K, Na)NbO3 (abbreviated as KNN). A comprehensive review on the latest development of KNN‐based piezoelectric ceramics is presented in this article, including the phase structure, property enhancement approaches, and sintering processes as well as the status of some promising applications. The phase structure of KNN was reexamined and associated with the effect of chemical modification on its tetragonal‐to‐orthorhombic transition. Then, a special focus is placed on the temperature dependence of piezoelectric properties of KNN‐based ceramics, followed by reviewing the recent approaches devoted to the temperature‐stability enhancement. The processing fundamentals related to the sintering of KNN‐based ceramics are also presented with an emphasis on compositional and microstructural control. Finally, this review introduces several industrial attempts of traditional piezoceramic products using KNN‐based ceramics and the studies on some promising application in authors' laboratory.

The development of lead‐free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead‐free (K,Na)NbO3 (KNN)‐based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficientd33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field‐induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field‐induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature‐dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature‐insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature.

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Abstract:High piezoelectricity of (K,Na)NbO 3 (KNN) lead-free materials benefits from a polymorphic phase transition (PPT) around room temperature, but its temperature sensitivity has been a bottleneck impeding their applications. We find that good thermal stability can be achieved in CaZrO 3 -modified KNN lead-free piezoceramics, in which the normalized strain d 33 * almost keeps constant from room temperature up to 140 o C. In situ synchrotron X-ray diffraction experiments combined with permitivity measurements disclose the occurrence of a new phase transformation under an electrical field, which extends the transition range between tetragonal and orthorhombic phases. It is revealed that such an electrically-enhanced diffused 2 polymorphic phase transition (EED-PPT) contributed to the boosted thermal stability of KNN based lead-free piezoceramics with high piezoelectricity. The present approach based on phase engineering should also be effective in endowing other lead-free piezoelectrics with high piezoelectricity and good temperature stability. IntroductionPiezoelectricity, a phenomenon whereby materials become electrically polarized upon the application of stress or deform in response to electrical stimuli, has been an active research topic since its discovery in 1880 by Pierre and Jacques Curie, because of its scientific interests and abundant applications. For the last half-century, the lead-contained materials, e.g., Pb(Zr,Ti)O 3 (PZT) and Pb(Mg,Nb)O 3 -PbTiO 3 (PMN-PT), have been the icons of piezoelectrics, exhibiting a morphotropic phase boundary (MPB), where plural phases with negligible difference in free energy coexist and strongly enhanced functional properties arise.[1] However, a possible toxicity of lead in PZT and PMN-PT has been raising intense health and environmental concerns; thus, the last decade has witnessed the surging dedication to viable lead-free alternatives. [2][3][4][5] Resembling the principle characteristics of MPB, [4][5][6][7][8][9][10] polymorphic phase transition (PPT) boundary has also been extensively pursued. [2, 11,12] Unfortunately, contrary to the nearly vertical MPB in the well-known PZT and PMN-PT systems, [1] the PPT in lead-free piezoelectrics is always tilted, resulting in unavoidable thermally unstable electromechanical properties. [13][14][15] Weak thermal stability is unacceptable for many industrial applications, even though lead-free piezoelectrics have competitive performance at ambient conditions. To address the issue, two approaches have been adopted so far, i.e., fabricating textured samples, [2] or shifting the PPT temperature T O-T well below room temperature. [13] However, the former confronts the poor reproducibility due to an excessively complex synthesis procedure; while the latter would inevitably sacrifice a large 3 portion of piezoelectric activity. Consequently, a barrier still exists in developing reliable lead-free piezomaterials as alternatives to currently market-dominating lead-based materials.Inspired by the nature of MPB in PZT and PMN-PT...

The temperature dependence of piezoelectric properties (direct piezoelectric coefficient d33, converse piezoelectric coefficient d33(E = 0), strain S and electromechanical coupling coefficient kp) for two niobate‐based lead‐free piezoceramics have been contrasted. 0.92(Na0.5K0.5)NbO3–0.02(Bi1/2Li1/2)TiO3–0.06BaZrO3 (6BZ/2BLT/92NKN) has a morphotropic phase boundary (MPB) between rhombohedral and tetragonal at room temperature and 0.92(Na0.5K0.5)NbO3–0.03(Bi1/2Li1/2)TiO3–0.05BaZrO3 (5BZ/3BLT/92NKN) features an MPB engineered to be located below room temperature. At 30°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 252 pC/N, 230 pm/V, 0.069%, 0.51 for 5BZ/3BLT/92NKN; and 348 pC/N, 380 pm/V, 0.106%, 0.57 for 6BZ/2BLT/92NKN, respectively. With increasing temperature, the piezoelectric properties decrease. At 200°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 170 pC/N, 160 pm/V, 0.059%, 0.36 for 5BZ/3BLT/92NKN; and 181 pC/N, 190 pm/V, 0.061%, 0.39 for 6BZ/2BLT/92NKN. It is found that the electromechanical coupling coefficient has a better temperature stability than the piezoelectric coefficient in the studied system due to a large temperature‐dependent compliance change. The results demonstrate that engineering an MPB is highly effective in tailoring temperature stability of piezoceramics.