(K,Na)NbO₃‐Based Lead‐Free Piezoceramics: Fundamental Aspects, Processing Technologies, and Remaining Challenges

Abstract

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.

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…153 Considerable research has been performed in BT-based materials or KNN-based materials characterized by presence of PPBs. 29,150,154 Alternatively, research has also focused considerably on systems with presence of a MPB-like ‡ ‡ transition such as in lead-based piezoelectrics or in some BNT-based materials. 10,29,150,151,154,155 The origin of maximized properties at interferroelectric phase boundaries has been commonly attributed to a FD transverse instability of that ease its reorientation due to increased multiplicity of polarization states, reduced free energy anisotropy, and softening of the crystal lattice.…”

“…29,150,154 Alternatively, research has also focused considerably on systems with presence of a MPB-like ‡ ‡ transition such as in lead-based piezoelectrics or in some BNT-based materials. 10,29,150,151,154,155 The origin of maximized properties at interferroelectric phase boundaries has been commonly attributed to a FD transverse instability of that ease its reorientation due to increased multiplicity of polarization states, reduced free energy anisotropy, and softening of the crystal lattice. 42,150,[156][157][158][159][160] Transverse instabilities have been pointed out in several works as the key for the enhancement of dielectric and electromechanical properties.…”

“…[121][122][123][124][125][126][127][128][129][130][131][132][133][134][135][136][137][138][139]258,[275][276][277][278] The KNN-based family of materials is also widely considered as a potential candidate to replace PZT. 10,29,154,279 pm/V at 2 kV/mm. These exceptional properties were achieved through a complex texturing process.…”

“…[286][287][288][289] Additional problems arise also during sintering due to an extremely narrow sintering window in terms of temperature and dwelling time [290][291][292] , early activation of surface diffusion that reduces the driving force for densification 273 , and high volatilization of K + above 1000 °C. 154,288 The processing of KNN-based materials remains an open research area in which new techniques such as pressure assisted sintering, spark plasma, and microwave sintering, and others are being investigated for developing reproducible and dense materials with high functional properties. 154,155 A survey of 33 as a function of depolarization temperature (either or ) for the most relevant lead-free material families is introduced in Figure 3.2 (a).…”

“…154,288 The processing of KNN-based materials remains an open research area in which new techniques such as pressure assisted sintering, spark plasma, and microwave sintering, and others are being investigated for developing reproducible and dense materials with high functional properties. 154,155 A survey of 33 as a function of depolarization temperature (either or ) for the most relevant lead-free material families is introduced in Figure 3.2 (a). family is characterized by an electric field range in which their applicability is more suited.…”