Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Review pubs.acs.org/CR

Potassium−Sodium Niobate Lead-Free Piezoelectric Materials: Past, Present, and Future of Phase Boundaries Jiagang Wu,* Dingquan Xiao, and Jianguo Zhu Department of Materials Science, Sichuan University, Chengdu 610064, China 4.5.9. Sintering Aids and Sintering Atmosphere 4.6. Extrinsic Factors 4.6.1. Density vs Electrical Properties 4.6.2. Grain Morphology vs Electrical Properties 4.6.3. Domain Structure vs Electrical Properties 5. Relationship between Phase Boundaries and Piezoelectricity 6. Applications of KNN-Based Materials 6.1. Piezoelectric Energy Harvesting 6.2. Piezoelectric Actuator 7. Outlook and Future Work 7.1. Designing New Phase Boundaries 7.2. Analyzing Phase Compositions 7.3. Physical Origin of High Piezoelectricity 7.4. Intrinsic Characteristics of Phase Boundaries 7.5. Investigating the Stability of Piezoelectricity 7.6. Other Issues 8. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Background of Phase Boundaries 3. Characterization Tools of Phase Boundaries 3.1. High-Resolution Synchrotron X-ray Diffraction 3.2. Neutron Diffraction 3.3. Transmission Electron Microscopy 3.4. Spectroscopic Techniques 3.4.1. Raman Spectroscopy 3.4.2. Infrared Spectroscopy 3.5. Theoretical Methods 3.6. Combined Tools 4. Type and Construction of Phase Boundaries 4.1. Orthorhombic−Tetragonal Phase Boundaries 4.1.1. Li+ Substitution 4.1.2. Li+ and Sb5+ Substitution 4.1.3. Li+ and Ta5+ Substitution 4.1.4. Li+, Sb5+, and Ta5+ Substitution 4.1.5. Addition of ABO3 4.1.6. Addition of ABO3 Multicomponents 4.1.7. Ratio of K/Na 4.1.8. Other Ions 4.2. Rhombohedral−Orthorhombic Phase Boundaries 4.3. Rhombohedral−Tetragonal Phase Boundaries 4.4. Dopant Engineering 4.4.1. Doping with Equipollent Metals 4.4.2. Doping with Aliovalent Metals 4.5. Supplementary Tools 4.5.1. Sintering Temperature 4.5.2. Reactive Template Grain Growth Method 4.5.3. Spark-Plasma Sintering 4.5.4. Nonstoichiometry 4.5.5. Two-Step Sintering 4.5.6. Single Crystals 4.5.7. Thin Films 4.5.8. Poling Conditions

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1. INTRODUCTION High-performance piezoceramics are some of the most important and widely used materials, and as such research into this topic is at the forefront of high-technology advanced materials.1−19 Since the 1950s, (PbZr1−xTixO3, PZT)-based ceramics have been given much attention due to their excellent piezoelectric constants (d33 ≈ 200−750 pC/N) and high Curie temperatures (TC = 180−320 °C) (see Figure 1) and are currently applied in many electronic devices (e.g., sensors, actuators, ultrasound transducers, etc.).4,5,7−11,20,21 However, the lead content (∼60 wt %) of PZT raises environmental concerns during their preparation, process, and even disposal.4,5,7−11,20,21 For maintaining socially sustainable development, regions and countries have put a great deal of effort into the research and development (R&D) of lead-free piezoelectric materials, and moreover, the related laws and regulations have been legislated or improved.22−34 For example, the European Union passed the Restriction of Hazardous Substances (RoHS)

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Received: December 1, 2014

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DOI: 10.1021/cr5006809 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. d33 and TC of PZT-based ceramics.

Figure 2. Publications on lead-free piezoceramics in refereed journals for the time range from 2004 to 2014 (collected from ISI Web of Science with the keywords “lead-free” and “piezoelectric”).

law in 2003, the “Household Electronic Products Recycling Law” was passed in Japan, and the “Electronic Information Product Pollution Control Management Regulation” was established in China in 2006. Although the lead content of piezoelectric devices and some capacitors is currently exempted due to related technical questions, the validity of this exemption treaty will expire in July 2016. As a result, the R&D of lead-free piezoelectrics has become urgent under current international legislature.1−3,14−19,35−285 Among these lead-free candidates, (K,Na)NbO3 (KNN) has become one of the most extensively investigated piezoelectric systems in the past 10 years due to its large d33 and high TC.12,17,97,101,102,104,106 Recently, a number of countries have invested considerable manpower and financial resources into the study of KNN-based piezoelectrics, and some significant breakthroughs have been achieved.12,101,105−118,166,256,260,262 For example, Satio et al. observed a large d33 value (∼416 pC/N) in (K,Na,Li)(Nb,Ta,Sb)O3-textured ceramics fabricated using reactive templated grain growth (RTGG) methods.12 In the meantime, Cross commented that Satio’s work may allow us to be “Lead-free at last”.100 Guo et al.,101,102 meanwhile, also observed enhanced piezoelectricity (d33 = 200−235 pC/N) in KNN ceramics by doping LiNbO3 or LiTaO3 using conventional solid-state methods. In these studies, their d33 values can be promoted to be several times that of a pure KNN ceramic,3 and the involved phase boundaries play a large part in their improved piezoelectricity (refs 12, 16−18, 97, 101, 105−119, 166, 251, 256, 260, and 262). In 2014, we developed new KNN-based material systems with large piezoelectricity (d33 ≈ 490 pC/N) by designing new phase boundaries consisiting of rhombohedral and tetragonal (R−T) phases;106 moreover, this material was able to demonstrate both a large d33 and a high TC through refining of its composition,106−118,256,260 further increasing research interest in KNN-based piezoelectrics. Figure 2 displays statistics related to refereed publications on lead-free piezoceramics from 2004 to 2014 using the keywords “lead-free” and “piezoelectric”. We classified the referenced publications according to the keywords “lead-free piezoelectric”, “alkali niobium-based”, and “construction of alkali niobium-based materials”, as shown in Figure 2. There is a gradually increasing trend in the number of publications concerning alkali niobium-based materials, among which the construction of phase boundaries has become a prominent tool for the improvement of their electrical properties. The

statistical results indicate that alkali niobium-based materials have received a great amount of attention.12,90−285 To better understand this situation, we counted the percentage of the publications according to the data of Figure 2, as plotted in Figure 3a and 3b. Publications on alkali niobium materials make

Figure 3. (a) Publications on alkali niobium-based and lead-free piezoceramics in refereed journals for the time range from 2004 to 2014; (b) publications on lead-free piezoceramics with phase boundary construction and others.

up ∼43.85% of those on lead-free piezoelectrics, and moreover, those concerned with the construction of phase boundaries comprise 53.27%. As a result, phase boundaries play an important role in the development of alkali niobium lead-free piezoelectrics as a whole. In the past, the “morphotropic phase boundary” was also used to realize a major breakthrough in the study of the piezoelectric activity of PZT.4,5 Generally speaking, the piezoelectric activity of KNN-based ceramics is still inferior to that of most PZT materials (see Figure 1), and thus, further breakthroughs must be achieved by extensively replacing leadbased materials in some applications. As mentioned above, the construction of phase boundaries will be key to realizing the enhanced piezoelectricity of KNN compared to PZT; thus, it is necessary to review the developments of phase boundaries in the field of KNN-based materials for further promotion of the piezoelectric activity and to guide our studies. In the past 10 years, researchers have reviewed the advances of KNN-based materials from different viewpoints,16−19,96−98,103−105,266,267 greatly accelerating research progress.16−19,103−120 For example, the relationships between microstructure (domain wall) and properties concerning leadB


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Figure 4. Diagram summarizing the topics in this review.

free piezoelectric materials were first reviewed by Damjanovic et al.,96 and Shrout and Zhang comprehensively analyzed the difference between KNN and PZT.18 Rodel et al. reviewed the phase structure and electrical properties of KNN-based materials by analyzing different processing techniques and chemical modifications,17 and the related applications of leadfree piezoelectrics have been recently summarized.120 Panda reviewed the effects of additives on the electrical properties of KNN-based materials.103 In 2009, Xiao et al. reviewed the composition design and property modification of KNN- and BNT-based ceramics by considering their recent publications.104 Li et al. introduced the developments of lead-free piezoelectric materials in China, and their preparation and electric properties were emphasized.266 More recently, the phase structure (e.g., orthorhombic−tetragonal), property enhancement approaches, and sintering processes as well as some promising applications of KNN-based materials were reviewed by Li et al.105 Finally, Priya et al. published a monograph on lead-free piezoelectric materials in 2012.267 In most reviews, the phase structure mainly refers to the orthorhombic−tetragonal phase boundary or the KNN-based materials are only reviewed as a minor part of a larger field or from different viewpoints. In addition, there are few systematic reviews on the developments of phase boundaries in potassium−sodium−niobate-based materials. As mentioned

above, the improved piezoelectricity of such a material should be largely due to the construction of phase boundaries. As a result, we focused our attention on the design and construction of different phase boundaries (e.g., rhombohedral−orthorhombic (R−O), orthorhombic−tetragonal (O−T), rhombohedral− tetragonal (R−T), etc.) in potassium−sodium niobate materials, the relationships between phase boundaries and piezoelectricity are particularly emphasized, and some suggestions have been addressed. This review has been organized as shown in Figure 4. In the first part, we give a general introduction to the background and characterization methods of phase boundaries for elucidating the phase structure−performance relationships. In the second part, we focus on the design and construction of phase boundaries in potassium−sodium niobate materials by chemical modifications. Special emphasis is placed on the compositioninduced phase boundaries by analyzing the chemical modifications in detail, and dopant engineering is also mentioned. It is worth noting that we observed the largest piezoelectricity ever reported by developing new phase boundaries. In addition, supporting tools for further enhancing piezoelectricity are also discussed in this review. In the third part, we illuminate the relationships between phase boundaries and piezoelectric activity, discuss some existing challenges, C


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Figure 5. Intrinsic difference between (a) polymorphic phase transition (PPT) and (b) morphotropic phase boundary (MPB)17,200

Figure 6. Phase transition behavior of K0.48Na0.52Nb0.98Sb0.02O3 ceramics.

kind of phase boundary is thought to be an MPB,286 which is only dependent on the compositions and independent of the measurement temperatures, as shown in Figure 5b. However, the phase boundaries (e.g., R−O and O−T) in KNN-based materials were considered as the intrinsic characteristics of “polymorphic phase transition (PPT)”,120,131 and their electrical properties are very sensitive to not only the compositions but also the temperatures (see Figure 5a). According to advances in KNN materials, there are three kinds of existing phase transitions (Figure 6), corresponding to the rhombohedral−orthorhombic phase transition temperature (TR−O), the orthorhombic−tetragonal phase transition temperature (TO−T), and the rhombohedral−tetragonal phase transition temperature (TR−T).17,18 Figure 6 shows the phase transition behavior of K0.48Na0.52Nb0.98Sb0.02O3 ceramics, measured at −150−550 °C and f = 10 kHz. One can see that TR−O, TO−T, and TC are clearly shown according to the temperature dependence of the corresponding dielectric peaks. At room temperature, the R−O or O−T phase boundary could be formed in KNN by shifting TR−O or TO−T to room temperature using additives such as LiNbO3,101,160,161,166−178 Li(Ta/Sb)O3,102,133,163−165,182−203 Li(Ta−Sb)O3,132,151,156,157,204,206−214 (Bi0.5A0.5)TiO3 (A = Na+, K+, Li+),215−218 BiBO3 (B = Al3+, Fe3+, Sc3+),225,230,252 SrZrO3,138 Sb5+,251 BaZrO3,397 Ta5+,254 etc. In contrast, the phase boundary corresponding to R−O or O−T possesses the PPT characteristic,97,155,251,264,265 depending on not only the

suggest possible methods for further improving piezoelectricity, and provide some conclusions.

2. BACKGROUND OF PHASE BOUNDARIES The modification of phase boundaries is a powerful tool for the promotion of electrical properties of piezoelectric materials, i.e., the involved phase boundaries and their corresponding types are largely responsible for the enhancement in piezoelectric activity, regardless of whether a material is lead based or lead free (refs 4, 5, 11, 16−19, 35, 70, 76, 96−120, and 286−315). A number of experimental (refs 70, 287−298, 300, 301, 304, 305, 307, 309, 311, and 312) and theoretical methods (refs 20, 299, 301, 303, 306, 308, 310, and 313−315) have been used to investigate the relationships between phase boundaries and the piezoelectric activity of a material. It is generally accepted that a high degree of alignments of ferroelectric dipoles can be driven by a large amount of thermodynamically equivalent states under a driving electric field during the poling process, easily generating enhanced electrical properties.70,286−311 The construction of morphotropic phase boundaries (MPB) in PZT is considered a classic case in the field of piezoelectric materials,286,289,293,305 wherein the crystal structure changes abruptly and the piezoelectricity is maximized for compositions at MPBs.286,289,293,305 For example, enhanced piezoelectric activity was observed in PZT materials with a Zr/Ti ratio of 52:48 owing to the structural changes going from tetragonal to rhombohedral via an intermediary monoclinic phase;292 this D


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Figure 7. d33 vs TC of (a) lead-based and (b) lead-free materials, derived from a number of publications.4,5,16−19,35,50,59,68,97,101,102,106,120,315−339

boundaries (e.g., R−O, R−T, or O−T). Although a larger d33 can usually be realized in BaTiO3-based materials, a low TC is a fatal shortcoming for high-temperature practical applications.35−38,40−43,45,50,53,59,84 However, such materials are suitable to prepare actuators with usage temperatures below 100 ◦C. Similar to BaTiO3-based materials, the low “depoling temperature” hinders the practical applications of Bi0.5Na0.5TiO3-based materials,63−73 and a large strain comparable or even superior to PZT has been developed by Rödel and Jo et al.66−68,73−77 More interestingly, we can see from Figure 7b that KNN-based materials have wider d33 and TC distributions (e.g., d33 = 171−490 pC/N, TC = 178−475 °C) (refs 12, 17, 97, 101, 102, 104, 106−118, 166, 277, 284, and 285), which is determined by the phase boundary types and the composition itself. In particular, we achieved a breakthrough in the piezoelectricity of KNN-based ceramics by forming new R− T phase boundaries.106−118 As a result, the construction of new phase boundaries has become an efficient way to further enhance the piezoelectricity of KNN materials, narrowing the gap between lead-free and lead-based materials.

compositions but also the temperatures, as shown in Figure 5a. Such a phase boundary is obviously different from that of PZT (see Figure 5b). Regardless of the presence of MPB or PPT characteristics, the polarization of a piezomaterial can be more easily rotated among different symmetries when the compositions are located at the phase boundaries, inducing an enhancement in dielectric and piezoelectric properties. However, recent advances indicate that there is a limited improvement in the d33 values for KNN materials with O−T or R−O phase boundaries,90−105,250−252 which is also inferior to most PZT ceramics5 or even textured KNN.12 As a result, the formation of R−T boundaries that are similar to PZT may become necessary to further enhance the piezoelectric activity of KNN, because the easy rotations of polarization axes can be induced in compositions near such a phase boundary.11,35,70,76,106−117 In addition, we recently confirmed this assumption in potassium−sodium niobate piezoceramics by experimental methods,106−116 and a larger d33 value has been attained by constructing R−T phase boundaries.106−112 If the R−T phase boundary of KNN is expected to form, it is necessary to concurrently move their TR−O and TO−T values close to room temperature by choosing two or more additives, 1 0 6 − 1 1 6 , 2 5 5 − 2 6 3 such as Ta and Sb and Bi0.5(Na0.82K0.18)0.5ZrO3,106 Sb and Bi0.5(Na0.82K0.18)0.5ZrO3,107 BaZrO3 and Bi0.5Na0.5TiO3,255 Sb and BaZrO3,256 Sb and LiTaO3 and BaZrO3,257 LiSbO3 and BaZrO3,258 Sb and Bi0.5Na0.5TiO3,259 Sb and (Bi,Na,K,Li)ZrO3,260 Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3 and B0.5Na0.5TiO3,261 etc. As a result, it is possible to develop high-performance KNN-based materials by the application of new ideas. The phase boundaries of a material can be driven by its chemical composition or other factors (refs 4, 10, 35, 50, 59, 68, 97, 101, 102, 106, and 316−340) that determine the electrical properties. Figure 7a and 7b shows the d33 vs TC of lead-based (e.g., PZT, 4,5,316 Pb(Ni 1/3 Nb 2/3 )O 3 −PbTiO 3 (PNN− PbTiO3),317−321 BiScO3−PbTiO3 (BSPT),322−332 and relaxorPbTiO3333−340) and lead-free (e.g., BaTiO 3 , 35−38,40−43,45,50,53,59,84 Bi 0.5 Na 0.5 TiO 3 , 63−73 and (K,Na)NbO390−265) materials with compositions at the phase boundaries, wherein all data were derived from the references. A giant d33 value was observed in PNN-based and relaxor PT single crystals with an MPB,317−321,333−340 while PZT-based materials possess a wide d33 distribution,4,5,316 which is sensitive to the doping elements, as shown in Figure 7a. Figure 7b displays the d33 and TC values of BaTiO3-, Bi0.5Na0.5TiO3-, and K0.50Na0.50NbO3-based lead-free ceramics with different phase

3. CHARACTERIZATION TOOLS OF PHASE BOUNDARIES Although the phase boundary has some incomparable advantages, it is very difficult to know the actual phase compositions and types of phase boundaries in a material. Researchers have invested much effort in exploring the origin of ultrahigh piezoelectricity in lead-based perovskite oxides with MPBs by analyzing the phase constituents using experimental and theoretical methods.65,287,298,323,341−391 Previously, it was thought that 14 possible equivalent polarization vectors determine the ultrahigh piezoelectric activity in MPB leadbased materials.341 However, it was observed by means of highresolution X-ray powder diffraction measurements that intermediate phases may coexist in the R−T phase boundary of PZT,293,342,343,347 and an intermediate phase induces an ultrahigh piezoelectricity. In particular, the monoclinic (M) phase, as an intermediate phase of MPB, is considered to effectively enhance the piezoelectricity of PZT.342 Since an M phase was found in MPB-PZT,343 much attention has been given to the determination of its phase compositions. Some powerful tools can identify the existence of the intermediate phases and thus explain the physical origin of enhanced piezoelectricity, including high-resolution synchrotron X-ray powder diffraction,293,298,342,344 neutron diffraction,359,360 spectroscopic techniques (i.e., Raman346,347,367−369 and infrared E


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spectroscopies369,370), dielectric measurements,350 transmission electron microscopy (TEM),361−366 theoretical studies,354,355 and Rietveld studies.376 In addition, two or more methods are often used together to clarify the phase compositions near the phase boundary regions of a material.371−375 In this section, we briefly introduce the methods of characterization of the phase compositions in KNN-based materials.

phase transitions from a nonpolar to a ferroelectric state.365 Yao et al.366 reported that the ferroelectric domains and local structures of Bi0.5Na0.5TiO3−BaTiO3 single crystals could be identified by TEM, the size of polar nanoregions were refined, and the tetragonal phase volume fraction was shown to increase as the BT content increased, indicating that both polarization rotation and polarization extension lead to large electric-fieldinduced strains in materials with MPBs.366 Recently, Zuo et al.268 discussed the origin of the high piezoelectric activity in (Na0.52K0.48−x)(Nb0.92−xSb0.08)O3−xLiTaO3 ceramics by characterization of the domain evolutions using TEM, indicating that the nanodomain morphologies are responsible for their enhanced piezoelectricity. In the past, the relationships between grain size and domain of ferroelectric materials were extensively investigated using TEM in order to understand the size effect on their electrical properties.377−383 It is well known that the domain size of ferroelectric materials is closely related to the corresponding grain size, as shown here.382,383

3.1. High-Resolution Synchrotron X-ray Diffraction298,342,343,356−358

Structural information on lead-based piezoelectrics with MPBs can be obtained by high-resolution synchrotron X-ray diffraction.293,298,342,344,356−358 For example, Noheda et al.342 found that a monoclinic phase bridges tetragonal and rhombohedral regions of PbZr0.52Ti0.48O3 using high-resolution synchrotron X-ray powder diffraction measurements. In 2000, Noheda et al.345 also established the stability region of three phases (e.g., monoclinic, tetragonal, and rhombohedral phases) in PbZr1−xTixO3 (0.42 ≤ x ≤ 0.52) according to data from high-resolution synchrotron X-ray powder diffraction measurements. These results indicate that PbZr1−xTixO3 (0.46 ≤ x ≤ 0.51) ceramics possess a stable monoclinic phase when measured at 20 K, the phase composition region becomes narrower and narrower with increasing temperatures, and the role of such a monoclinic phase in the ultrahigh piezoelectricity of the materials was especially emphasized.293 In addition, this method could also be used to detect the phase compositions of other material systems.298,358 An orthorhombic phase in PbZn1−xTixO3 and Pb(Zn1/3Nb2/3)O3−PbTiO3 systems was identified by high-resolution synchrotron X-ray diffraction,298 and a monoclinic phase (Cm) of 0.65Pb(Mg1/3Nb2/3)O3− 0.35PbTiO3 was also confirmed.358

Domain Size ≈ (Grain Size)m

(1)

According to eq 1, the grain size of the samples used for TEM observation can strongly affect their domain size,382 that is, the prepared samples are in agreement with the bulk domain structures. As a result, it becomes critical to accurately characterize the domains to evaluate the underlying physical mechanisms. To attain useful information about material structure and properties, TEM becomes an indispensable tool. However, the size effects of ferroelectric materials presumably play a part in the TEM observations of domains, since the TEM samples are quite thin. Therefore, the sample preparation technique is very important for TEM analysis in order to gain real insight into a material’s structure.384 It is usually very challenging to prepare a suitable sample of polycrystalline ceramics for TEM since some additional cracks and artifacts may be introduced during the sample preparation.384 In addition to the TEM observation of domains, other tools (scanning electron microscopy,269 piezoresponse force microscopy,270,271 etc.) should also be used to further confirm the domain structure and to avoid artificial information. As a result, TEM can directly characterize domain structure and morphologies, and the size effects should be given special attention in the observation of domains.

3.2. Neutron Diffraction359,360

Neutron diffraction allows the atomic and/or magnetic structure of a material to be determined by neutron scattering, and this technique has been used to evaluate the symmetry of a material.359,360 For example, neutron diffraction methods can identify the symmetry of Pb(Zn1/3Nb2/3)O3−9%PbTiO3 with an MPB, showing that a new polarization rotation line Mc# (the orthorhombic−monoclinic line) coexists with rhombohedral and tetragonal phases.359 In addition, in situ neutron diffraction experiments show that the varied coherence lengths of polar nanoregions and internal stresses induced by domain switching are responsible for different d33 values in textured and polycrystalline 0.93Bi0.5Na0.5TiO3−0.07BaTiO3 ceramics.360

3.4. Spectroscopic Techniques

Spectroscopy is used to investigate the interactions between a material and radiated energy;342,347,367−370 thereby, the phase transitions of a material can be analyzed by spectroscopic techniques including Raman and infrared spectroscopies.367−370 3.4.1. Raman Spectroscopy. Raman spectroscopy is used to characterize vibrational, rotational, and other low-frequency modes in a material and can also identify the phase structure of a material.346,347 In 2000, Souza Filho et al. confirmed the MPB consisting of tetragonal and monoclinic phases 346 in PbZr0.52Ti0.48O3 by Raman spectroscopy with the frequency and temperature of lattice modes.347 Confocal Raman microscopy can allow high-resolution chemical mapping of 2D or 3D materials. Recently, confocal Raman microscopy (CRM) coupled with atomic force microscopy has been used to observe the domains of KNNbased materials.272,273 For example, Rubio-Marcos et al. identified the spatially resolved structure of the ferroelectric domains in (K,Na)NbO3-based ceramics by confocal Raman

3.3. Transmission Electron Microscopy268,270,378

In 1931, Max Knoll and Ernst Ruska invented the first transmission electron microscope (TEM), allowing the structural information on ferroelectric materials to be directly characterized.268,361−366 For example, both orthorhombic− orthorhombic and paraelectric−ferroelectric phase transitions in SrBi4Ti4O15 have been directly observed by TEM.363 Randall et al.364 identified the differences between domain structure and symmetry of (1−x)BiScO3−xPbTiO3 materials using TEM, clearly showing the domain structures with R and T mixed phases. In addition, the domain structure evolutions under an applied electric field were characterized in situ in (Bi1/2Na1/2)TiO3−BaTiO3−(K0.5Na0.5)NbO3 ceramics using TEM.365 This result indicates that the domain structure cannot be clearly observed when an electric field is retrieved, while a lamellar domain structure can be driven by an alternating electric field, graphically showing the involvement of electric field-induced F


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Figure 8. Possible chemical modifications of (K,Na)+ and Nb5+ sites in (K,Na)NbO3.

Although theoretical methods can further guide experimental studies, there are also limitations inherent in some methods. Most theoretical methods are only capable of modeling realistic systems,385,386 and the final results are also strongly dependent on different simulated methods.387−389 For example, simulations of the domain structures of ultrathin ferroelectric films have mainly focused on conventional perovskite ferroelectrics (e.g., BaTiO3,390 PbTiO3,391 etc.). In these studies, only a few simple factors (e.g., boundary conditions, external electrical, and mechanical loads) were considered,390,391 while treatment of complicated situations and the detailed physics of the surface and interface (i.e., octahedral rotation, magnetic ordering, other degrees of freedom) was seriously flawed. In addition, theoretical methods have so far been unable to model the true dynamics of the domain structures of ultrathin ferroelectric films.385 As a result, satisfactory simulation of broad and complicated systems will require further development of computational methods,385 and special attention should be given to the limitations of theoretical methods on the whole.

microscopy, where the characteristics of domain walls and structure−piezoelectricity correlation were clearly addressed.272 3.4.2. Infrared Spectroscopy. Infrared spectroscopy uses radiation from the infrared region of the electromagnetic spectrum, covering a range of techniques based on absorption spectroscopy. It is well known that the onset of ferroelectric states can cause a change of infrared vibrational frequencies owing to its temperature-dependent phase transitions and thus can be used to characterize their phase transitions.369,370 For example, Araujo et al.369 showed that monoclinic → tetragonal phase transitions in PbZr0.51Ti0.49O3 occur at 237 K by infrared spectroscopy, confirming this technique as a useful tool to analyze phase transitions. The monoclinic → tetragonal phase transitions in PbZr0.50Ti0.50O3 can be confirmed by infrared spectroscopy with different measurement temperatures, and the obvious differences in higher frequency ν1-(Ti−O) and ν1-(Zr− O) modes can show the structural changes in monoclinic → tetragonal phases.370 3.5. Theoretical Methods274−276,287,355,364,365,385−391

3.6. Combined Tools323,371−375

Theoretical methods rely on the creation of hypothetical models of materials. Such methods can predict the underlying physical mechanisms of enhanced electrical properties well, effectively supporting experimental results. In the past, theoretical methods have also been used to explain the origin of ultrahigh piezoelectricity of lead-based materials with an MPB, such as a first-principle study by Fu and Cohen,287 and higher-order Devonshire theory studies by Vanderbilt and Cohen.355 Fu et al. also reported that the large piezoelectricity in BaTiO3 is driven by the polarization rotation induced by an external electric field using a first-principles study.287 In addition, some theoretical methods were also used to investigate single-phase KNN materials.274−276 For example, Zhu et al. indicated a combination of negative sphericalaberration imaging using first-principles calculations and studied the atomic details across 60°/120° domain walls in (K0.46Na0.54)NbO3, providing an atomic basis for further investigation of the polarization mechanism.274 However, few attempts have been made to predict the phase boundaries in KNN materials.

It is usually difficult to identify the phase compositions of a material using only one tool; thus, multiple techniques should be used to determine the actual phase compositions.323,371−375 In the past, combined tools have been used to identify the phase structures of lead-based and lead-free materials.323,372−375 For example, both variable-temperature transmission electron microscopy and dielectric measurements were used to characterize the phase compositions of (1−x)BiScO3-xPbTiO3 single crystals in which x = 0.64.323 Schonau et al. studied the phase compositions and microstructural information on Pb(Zr1−xTix)O3 (0.40 ⩽ x ⩽ 0.475 and x = 0.55) materials using three different tools (e.g., high-resolution synchrotron Xray diffraction, transmission electron microscopy, and electron paramagnetic resonance) and found that a monoclinic phase is correlated to a miniaturization of its average domain structure.371 The phase compositions of (1−x)Pb(Ni1/3Nb2/3)O3−xPbTiO3 (x = 0.28−0.42) solid solutions were confirmed by using both high-resolution synchrotron radiation X-ray diffraction and dielectric measurement, indicating that the intermediate monoclinic phase exists between rhombohedral G


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and tetragonal phases.372 Singh et al. investigated the temperature-dependent phase structure in monoclinic Pb(Zr0.525Ti0.475)O3 powders by Rietveld analysis of highresolution synchrotron powder X-ray diffraction data in combination with its dielectric constant and planar electromechanical coupling coefficient.373 These powders, with a tetragonal phase, possess better piezoelectric activity than those with a monoclinic phase because of an anomalous softening of the elastic modulus.373 Fu et al. studied the origin of enhanced piezoelectricity in (Na,K)(Nb,Sb)O3−LiTaO3−BaZrO3 ceramics by combined tools consisting of high-resolution synchrotron X-ray data and dielectric measurements, showing that an intermediate phase between R and T leads to a larger piezoelectricity.374 Maurya et al. addressed the origin of high piezoelectricity in 0.93(Na0.5Bi0.5)TiO3−0.07BaTiO3 with an MPB by comparative analyses of high-resolution transmission electron microscopy and neutron diffraction, confirming that a pathway for polarization reorientation induced by an intermediate monoclinic phase with nanotwins results in enhanced piezoelectricity.375 As a result, multiple characteration methods can compensate for the shortcomings of a single method, thereby more accurately showing the phase compositions of a material.

In this section we mainly review the development of different types and constructions of phase boundaries in KNN materials, and some advice is provided for attaining piezoelectric activity comparable to that of PZT. In addition, the detailed discussions on how to construct phase boundaries by chemical modifications are summarized here. 4.1. Orthorhombic−Tetragonal Phase Boundaries

The O−T phase boundary has become the most familiar one since a large breakthrough was achieved in KNN-based materials.12,101 The formation of the O−T phase boundary is one of the mosty hotly contested goals of research into KNNbased materials (refs 101, 102, 132, 151−157, 159−168, 170−178, 182−197, 199−218, 220, 221, 224, and 229−247). In 2004, the original breakthrough in its d33 value was achieved by varying O−T phase coexistence to or near room temperature using Li, Ta, and Sb.12 For example, a high d33 value of ∼416 pC/N was induced in KNN-based textured ceramics due to coexistence of O and T phases at room temperature as well as the use of a new preparation technique (e.g., RTGG),12 while at the same time LiNbO3- or LiTaO3modified KNN ceramics have attained enhanced piezoelectric activity by construction of the same phase boundary using conventional solid-state methods.101,102 These original works have greatly increased worldwide research interest in lead-free piezoelectrics, and thus, scientists have devoted much energy into further promotion of the piezoelectric activity by constructing O−T phase boundaries (refs 101, 102, 132, 151−157, 159−168, 170−178, 182−197, 199−218, 220, 221, 224, and 229−247). In the past, a number of successful attempts were made to construct O−T phase boundaries in KNN-based ceramics in order to induce enhancement of its piezoelectric activity, including ion substitutions (e.g., using Li+,101,160,161,166−181 Li+/Sb5+,182−192 Li+/Ta5+,102,162,163,193−203 Li+/Sb5+ /Ta5+,12,156,204−214 etc.), solid solutions with other ABO 3 -type perovskites 215−228 or ABO 3 multicomponents,229−243 variation of the K/Na ratio,156,186,244−247 new preparation techniques,109,125,165,166,175−177,190,196,210,245 etc. In this section we focus on reviewing the methods of construction of O−T phase boundaries. 4.1.1. Li+ Substitution. Ion substitution has become a popular way to promote the piezoelectric properties of KNNbased materials by the construction of O−T phase boundaries. It is now well known that Li+, Sb5+, and Ta5+ are the leading doping elements,12,102,156,160−163,166−214 which are able to quickly shift the O−T phase boundary at or near room temperature, resulting in an enhanced d33 value owing to the facilitated polarization rotations induced by the structural instability.11,35,70,76 However, doping with Li+ is first used to improve the piezoelectric activity of KNN materials.101,166−173,394 Table 1 displays the piezoelectric activity of Li+-doped KNN materials.101,162,166−173 There are a number of advantages of introducing Li+ into KNN materials. First, adding Li+ can increase TC, decrease TO−T, and slightly affect TR−O values of KNN ceramics;174 thereby the O−T phase boundary can be formed, improving the piezoelectric activity of the material.101 Second, the doping with Li+ can also decrease the sintering temperatures and improve the density of KNN materials.101,166−173 It was shown that d33 values of 188−324 pC/N could be obtained by doping 6−8 mol % Li+ into KNN ceramics due to the formed O−T phase boundary,101,166−173 and the actual Li+ content at maximum d33 also depends on the different processing methods.101,166−173 It is also worth noting

4. TYPE AND CONSTRUCTION OF PHASE BOUNDARIES It is widely accepted that the piezoelectric activity of a material can be promoted by forming phase boundaries by chemical modification (e.g., ion substitutions, binary or ternary solid solutions, etc.).11,35,70,76,286−311 As far as KNN-based materials are concerned, three kinds of phase boundaries (i.e., O−T, R− O, and R−T) can be driven by doping with additives (refs 96−119, 131, 132, 136, 151, 152, 157, 161, 166, 169, 186−188, and 392−405), resulting in enhanced piezoelectricity. Among these methods, chemical modifications are often used to tailor the phase compositions in the phase boundary regions of KNN materials. To graphically show the chemical modifications of KNN materials, we depict the chemical modifications of (K,Na)+ and Nb5+ sites in (K,Na)NbO3 in the Periodic Table (Figure 8). The colored boxes represent different chemical modifications of (K,Na)NbO3. Usually monovalent cations (e.g., Li+101,162,166−173,392−394 and Ag+ ) are able to easily substitute the (K,Na) site, and the Nb5+ site is often replaced by pentavalent cations (e.g., Ta5+ 102,162,163,193−203 and Sb5+ 182−190) because of their identical valence and similar ionic radii. In addition, ABO3 can be constructed by combining (Mg, Ca, Sr, Ba, Pb) and (Ti, Zr, Hf, Sn) with Bi and (Y, Fe, Sc, Ga, Al, In),229−242 and this ABO3 can be used to modify the KNN materials. The KNNbased materials are also modified by doping elements (e.g., V, Mn, Co, Ni, Cu, Zn, B, La, Ce, Pr, Nd, Dy, Yb, etc.) in the form of oxides, and electrical properties can be controlled by tailoring the defect types. Recently, special ABO3 additives containing Bi3+ and Zr4+ have been found to greatly promote the piezoelectric activity of KNN-based materials by the construction of R−T phase boundaries,106−113 where Bi3+ and Zr4+ can, respectively, lower TO−T and increase TR−O and Sb5+ ions can further compact the O phase zone.106−113 By the thought of phase boundary, a large d33 of >400 pC/N was attained in KNN-based ceramics by doping of Bi0.5M0.5ZrO3 (M = Na+ and/or Li+ and/or K+ and/or Ag+),106−113 and the different d33 values originate from the variance in TR−T.277 H


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Table 1. Piezoelectric Activity of Li+-Modified KNN Materials with O−T Phase Boundaries material system K0.47Na0.47Li0.06NbO3 (K,Na,Li)NbO3 0.94K0.49Na0.51NbO3− 0.06LiNbO3 0.92Na0.535K0.48NbO3− 0.08LiNbO3 0.93K0.5Na0.5NbO3− 0.07LiNbO3 (K0.5Na0.5)0.935Li0.065NbO3 (K0.5Na0.5)0.94Li0.06NbO3 0.94K0.5Na0.5NbO3− 0.06LiNbO3 0.93K0.5Na0.5NbO3− 0.07LiNbO3

Tc (°C)

d33 (pC/ N)

kp

235 324 246

0.38 0.54 0.42

475 467

Guo101 Li and Wang166 Li167

280

0.48

475

Li and Wang168

is sensitive to not only the phase boundary and but also the compositions; thus, further investigation is necessary. 4.1.2. Li+ and Sb5+ Substitution. Many studies have involved systematic investigation of the piezoelectric properties of Li+- and Sb5+-codoped KNN materials, the general idea being to shift TO−T to (or near to) room temperature by modifying the Li+ and Sb5+ content.182−192 In fact, Li+ plays a large part in the decreased TO−T of KNN,101,162,166−173,392−394 and Sb5+ can simultaneously decrease TO−T and increase TR−O,106−110,251,392 as shown in Table 2. As a result, doping

ref

Du169

274 188 235 215

471 0.42 0.41

240

0.35

450

Table 2. Effects of Li+, Sb5+, and Ta5+ on Both TO−T and TR−O of a Pure KNN Ceramic101,251,254

Wongsaenmai170 Kakimoto171 Du172 Song173

1 mol %

Li+

Sb5+

Ta5+

TO−T (°C) TR−O (°C)

−30

−10.7 +14.8

−4.0 +2.3

with Li+ and Sb5+ can further move a material’s TO−T close to room temperature. As shown in Table 3, codoping with Li+ and

that Guo et al. achieved the first breakthrough in d33 values of Li+-modified KNN ceramics prepared by the conventional solid-state method,101 and an enhanced d33 value of ∼235 pC/ N was reached when the TO−T was tuned to near room temperature, which is one-third as much as a pure KNN ceramic.101,166−174 However, such a d33 value is still inferior to those of most PZT ceramics,4,5 and thus, other attempts must be made to further increase its d33 value.169,173,175−181 For example, Song et al. increased the d33 values of 0.93KNN− 0.07LN ceramics from ∼195 to 240 pC/N by increasing their grain sizes using two-step sintering methods (sintering at 1030 °C and subsequent annealing at 1050 °C).173 Du et al. studied the effects of both sintering temperatures and poling conditions (e.g., poling temperature, poling electric field, etc.) on the piezoelectric activity of KNN−LiNbO3 ceramics and found that two methods can further optimize their piezoelectric properties.169,175,176 Zhao et al.177 reported that high piezoelectricity (d33 ≈ 314 pC/N) can be attained in Li+-modified KNN ceramics by adding excess Na+ (∼2.9%) and choosing optimum sintering temperatures (∼1060 °C). Li and Wang et al.178 reported that a high converse piezoelectric coefficient (d33* ≈ 538 pm/V) compared to PZT could be observed in Li+modified KNN ceramics when a low applied electric field (∼1 kV/mm) was used. In addition, they also developed a new poling method to further improve the piezoelectricity by aging and repoling-induced enhancement of the piezoelectricity (d33 ≈ 324 pC/N) in such a ceramic owing to both the defect migration and the rotation of the spontaneous polarization.166 This is considered the second major breakthrough with regard to the d33 values of Li-modified KNN ceramics. The Bridgman method was used to prepare (001)-oriented 0.95KNN−0.05LN single crystals, leading to a high d33 of 405 pC/N and a large thickness electromechanical coupling factor (kt) of 61%,181 which can be regarded as the third breakthrough for this material system. It is well known that the TO−T in KNN has both advantages and disadvantages, i.e., a large d33 vs a poor temperature stability.182 Higashide et al.,160 Hollenstein et al.,161 and Zhai et al.245 investigated the temperature stability of the piezoelectric properties of Li+-modified KNN ceramics, and a poor temperature stability was demonstrated when the compositions were at the O−T phase boundary zone. However, an opposite result has been found by Li and Wang et al., and there is a slight variance in field-induced strain of Li+-doped KNN from room temperature to 175 °C.278 The authors wondered whether the temperature stability of KNN materials

Table 3. Piezoelectric Activity of Li+- and Sb5+-Modified KNN Ceramics with O−T Phase Boundaries material system 0.948(K0.5Na0.5)NbO3− 0.052LiSbO3 0.95(K0.48Na0.52)NbO3−0.05LiSbO3 0.94(K0.5Na0.5)NbO3−0.06LiSbO3 0.95K0.49Na0.51NbO3−0.05LiSbO3 0.95(K0.40Na0.60)NbO3−0.05LiSbO3 0.95(K0.42Na0.58)NbO3−0.05LiSbO3 (Na0.5K0.5)1−x(LiSb)xNb1−xO3 0.95K0.5Na0.5NbO3−0.05LiSLbO3 (Na0.474K0.474Li0.052)(Nb0.948Sb0.052) O3 (Na0.5K0.5)0.945Li0.055Nb0.96Sb0.04O3

d33 (pC/ N)

kp

Tc (°C)

262 212 256 280 270 260 215 242 240

ref Zhang182

265 0.46 0.46 0.43 0.49 0.47 0.50 0.42 0.42

373 358 340 364 364 390 386

Wu183 Lin184 Li185 Wu186 Wu187 Zang188 Palei189 Zhao190 Li191

Sb5+ can enhance the d33 values (∼210−280 pC/N) of KNN materials.182−191 We can see from Table 3 that two different Li+ and Sb5+ doping methods were used to modify KNN, including LiSbO3182−190 and Li+/Sb5+.191 The O−T phase coexistence of KNN−LiSbO 3 ceramics can be effected with LiSbO 3 compositions at 5−6%, leading to d33 values of 210−280 pC/ N.182−190 For example, Zhang et al.182 reported that an enhanced d33 value of ∼265 pC/N was observed in 0.948(K0.5Na0.5)NbO3−0.052LiSbO3 ceramics due to the formation of O−T phase boundaries, showing the strong temperature dependence of d33. In the development of KNNbased materials, Sb5+ plays a large part in the construction of phase boundaries, especially for R−T.106−110 In addition, we confirmed that adding Sb5+ can decrease the TR−O and TO−T values of KNN materials to room temperature (see Figure 9),392 and thus, Sb5+ and other additives can result in a high piezoelectricity by forming R−T phase boundaries.106,107 As a result, Sb5+ plays a very important role in the construction of phase boundaries and the enhancement of piezoelectric activity of KNN-based materials. 4.1.3. Li+ and Ta5+ Substitution. For KNN materials, adding both Li+ and Ta5+ has advantages similar to those of Li+ and Sb5+,102,162,163,193−203 that is, the decreased TO−T near room temperature can lead to a high piezoelectric activity.102,162,163,193−203 However, we still find some differences I


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ries,101,102,162,163,166−173,182−203,392−394 the d33 values of the materials are usually less than 300 pC/N. As a result, a major goal is the discovery of KNN-based materials with higher d33 values. Among the range of chemical modifications, codoping with Li+, Ta5+, and Sb5+ can cause rapid decreases of TO−T in KNN materials (see Table 5),12,156,157,204−214 and finally the O−T phase boundary near room temperature can be more rapidly tuned by controlling the elemental contents. For example, Satio et al.12 reported that a high d33 value of ∼416 pC/N can be achieved in Li+, Ta5+, and Sb5+ codoped KNN textured ceramics. Although such a high d33 can be comparable to those of PZT materials,12 the expensive RTGG methods used seriously limit their practical applications. If a similar material system with a high d33 can be prepared by conventional solid-state methods, the pace of their practical applications may be further accelerated. As a result, efforts have focused on developing KNN ceramic systems containing Li+, Ta5+, and Sb5+ by conventional solid-state methods, and d33 values of >300 pC/N can often be attained by controlling the ratios of three elements (e.g., Li+, Ta5+, and Sb5+).156,157,204−210 As shown in Table 5, the composition-induced O−T phase boundary is largely responsible for the enhanced piezoelectricity of KNN-based ceramics.12,156,157,204−214 For example, excellent piezoelectric properties (d33 ≈ 413 pC/N) were observed when low Li+ (∼2%) and high Ta5+ (∼18%) contents were doped into KNN-based ceramics,204 and (K0.52Na0.45)(Nb0.88Sb0.09)O3−0.03LiTaO3 ceramics also exhibit a high d33 value of ∼400 pC/N.205 However, these results show that their d33 can be improved greatly by sacrificing the TC (≤230 °C) values of KNN,204,205 as shown in Table 5. In addition, a high Ta5+ content is still necessary to obtain a large d33 value in these studies;157,204,207,211 otherwise, a lower d33 is often encountered.212,214 As a result, codoping with Li+, Ta5+, and Sb5+12,156,204−208 frequently contributes to the enhancement of piezoelectric activity of KNN ceramics with respect to those doped by one or two of the three elements, 101,102,162,163,166−173,182−203,392−394 but high costs (Ta5+) and low TC values may be two stubborn issues impacting their practical application. 4.1.5. Addition of ABO3. Perovskite ABO3 materials are often used to modify KNN in order to induce enhancement in piezoelectric activity by favoring the formation of phase boundaries,215−228 such as Bi0.5A0.5TiO3 (A = Na+,215,216 K + ,217 Li + 218 ), BTiO 3 (B = Ba 2+, 219 Sr2+ , 219 Ca 2+, 223 (Ba0.95Sr0.05)2+,224 [(Bi0.5Na0.5)0.94Ba0.06]2+,220 BiMO3 (M = Sc3+,225 Al3+,226 Fe3+,227 Co3+ 228)), and others. Table 6 lists the piezoelectric properties of ABO3-modified KNN ceramics.215−228 Doping with ABO3 can effectively promote the

Figure 9. Phase diagram and d33 values of (K0.48Na0.52)(Nb1−xSbx)O3 ceramics.392

between the two, namely, the addition of Li+ and Ta5+ more slowly decreases its TC value with respect to those modified by Li+ and Sb5+ (see Table 2).196,197,251,254 As a result, doping with both Li+ and Ta5+ is another effective way to promote the piezoelectric activity of KNN materials without seriously sacrificing its TC,196,197,254 as shown in Table 4. Guo et al.102 first reported that KNN−LiTaO3 ceramics possess the enhanced piezoelectric properties of d33 ≈ 200 pC/N and kp ≈ 0.36 when the compositions are located at the O−T phase boundary. In addition, some references also show that a high Ta5+ content leads to a high d33 in KNN-based systems.193,195 For example, a d33 value of >250 pC/N was observed in KNNbased ceramics by doping a Ta content of ≥15%.193,195 However, production costs inevitably increase when doping excess Ta5+ into KNN materials, limiting their large-scale industrial applications. As a result, it is necessary to develop methods to further promote the piezoelectricity of KNN ceramics using a low Ta5+ content.196−198 For example, using excess Li2O as a sintering aid and the formation of O−T phase boundaries induces a high d33 of ∼250 pC/N in 0.95(Na0.5K0.5)NbO3−0.05LiTaO3,198 and a high d33 value (∼268 pC/N) in (K,Li)(Nb,Ta)O3 ceramics can also be attained by both constructing the same phase boundary and introducing excess Na+.202 Although the piezoelectric activity of KNN ceramics could be enhanced by adding Li+ and Ta5+, the addition of Ta5+ should be avoided as far as possible or be used at low content levels with an eye toward practical applications. 4.1.4. Li+, Sb5+, and Ta5+ Substitution. Although Li+, Li+ and Ta5+ or Li+ and Sb5+ can improve the piezoelectric activity of KNN materials by forming O−T phase bounda-

Table 4. Piezoelectric Activity of Li+- and Ta5+-Modified KNN Ceramics with O−T Phase Boundaries material system

d33 (pC/N)

kp

0.93(K0.5Na0.5)NbO3−0.07LiTaO3 (K0.5−x/2Na0.5−x/2Lix)(Nb1−y,Tay)O3 (Li0.04Na0.48K0.48)(Nb0.80Ta0.20)O3 [(K0.458Na0.542)0.96Li0.04](Nb0.85Ta0.15)O3 [(K0.5Na0.5)0.97Li0.03](Nb0.8Ta0.2)O3 (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 (Na0.535K0.485)0.926 Li0.074(Nb0.942Ta0.058)O3 [(Na0.535K0.480)0.942Li0.058](Nb0.94Ta0.06)O3 Li2O−0.95(Na0.5K0.5)NbO3−0.05LiTaO3 (K0.5Na0.5)0.96Li0.04(Nb0.775Ta0.225)O3

200 190 215 298 234 262 276 232 250 208

0.36 0.46 0.43 0.52 0.51 0.53 0.46 0.39 0.37 0.48 J

Tc (°C) 310 366 358 325 ∼450 462 320

ref Guo102 Hollenstein162 Wang163 Chang193 Satio194 Zhang195 Shen196 Zhao197 Kim198 Lin199 DOI: 10.1021/cr5006809 Chem. Rev. XXXX, XXX, XXX−XXX

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Table 5. Piezoelectric Activity of Li-, Ta-, and Sb-Modified KNN Ceramics with O−T Phase Boundaries material system

d33 (pC/N)

kp

(K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 (K0.38Na0.58Li0.04)(Nb0.91,Ta0.05,Sb0.04)O3 (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 (K0.52Na0.48−x)(Nb1−x−ySby)O3−xLiTaO3 (Na0.52K0.44Li0.04)Nb0.87Sb0.08Ta0.05O3 (Na0.5K0.5)0.975Li0.025Nb0.76Sb0.06Ta0.18O3 (Na0.52K0.4375)(Nb0.9075 Sb0.05)O3−0.0425LiTaO3 (Na0.52K0.48−xLix)Nb1−x−ySbxTayO3 (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375)O3 Li0.04(Na0.54K0.46)0.96Nb0.81Ta0.15Sb0.04O3 Li0.02(Na0.55K0.45)0.98(Nb0.77Ta0.18Sb0.05)O3 0.96(K0.48Na0.52)(Nb0.95Ta0.05)O3−0.04LiSbO3

416 306 345 413 400 376 352 321 308 304 293 256 250

0.61 0.48

material system 0.97(K0.5Na0.5)NbO3− 0.03(Bi0.5Na0.5)TiO3 0.975(K0.5Na0.5)NbO3− 0.025(Bi0.5Na0.5)TiO3 0.98(K0.5Na0.5)NbO3−0.02(Bi0.5K0.5) TiO3 0.98(K0.5Na0.5)NbO3− 0.02Li0.5Bi0.5TiO3 0.94(Na0.5K0.5)NbO3−0.06BaTiO3 0.95(Na0.5K0.5)NbO3−0.05SrTiO3 0.96(Na0.5K0.5)NbO3− 0.04(Bi0.5Na0.5)0.94Ba0.06TiO3 (K0.5Na0.5)NbO3−Ba(Zr0.05Ti0.95) O3−MnO2 K0.5Na0.5NbO3− CaTi0.9Zr0.1O3+MnO2 0.95(Na0.5K0.5)NbO3−0.05CaTiO3 0.97(Na0.5K0.5)NbO3− 0.03(Ba0.95Sr0.05)TiO3 0.9825K0.5Na0.5NbO3−0.0175BiScO3 0.99(Na0.5K0.5)NbO3−0.01BiAlO3 0.99K0.5Na0.5NbO3− 0.01Bi0.8La0.2FeO3 (1−x)(Na0.5K0.5)NbO3−xBiCoO3 (x = 0.01)

kp

Tc (°C)

195

0.43

375

Zuo215

256

0.48

373

Du216

251

0.49

376

Du217

172

0.37

381

Jiang218

150 195 256

0.34 0.37 0.43

279 289 395

Wang219 Wang219 Chen220

234

0.49

318

Lin221

203

0.45

342

Lin222

241 136

0.41 0.21

306

Park223 Kim224

253 202 144

0.48 0.46 0.34

351 372 370

Du226 Zuo225 Zhang227

165

0.40

370

Xiao228

ref Satio12 Wu156 Akdogan157 Gao204 Zuo205 Du206 Du207 Fu208 Ming209 Pang210 Noh211 Yoo212 Wu214

253 ∼249 ∼225 ∼230 ∼270 ∼210 315 339 271 340

0.50 0.54 0.44 0.47 0.52 0.51 0.48 0.46 0.42 0.45

348

formation of O−T phase boundaries by decreasing a material’s T O−T values, inducing enhanced piezoelectricity.215−224 Although ABO3-type additives can increase the d33 values of KNN-based mateials,215−228 the resulting values are still inferior to those of KNN ceramics containing Li+, Ta5+, and Sb5+,156,157,204−211 as shown in Tables 5 and 6. However, the low cost of ABO3 may make it a potential candidate for practical applications. 4.1.6. Addition of ABO3 Multicomponents. KNN ceramics doped with ABO3 multicomponents usually possess enhanced piezoelectric behavior (d33 = 220−305 pC/N) together with high TC values of >300 °C,229−242 as shown in Table 7. Results show that doping with most ABO 3 multicomponents can shift a material’s TO−T to room temperature, resulting in the formation of O−T phase boundaries. As indicated in Table 7, the ABO3 materials containing Bi3+ are often used to modify KNN-based ceramics, such as BiFeO3,229−232 BiScO3,233−235 BiAlO3,241 etc. Among BiMeO3 (Me = Fe, Sc, Al, etc.) additives, doping with BiFeO3 or BiScO3 greatly favors increases in d33, and d33 values of >250 pC/N are often obtained in such ternary systems by formation of O−T phase boundaries.229−231,233 The differences in increases in d33 mainly originate from the velocity of shifting TO−T. For example, Jiang et al.229 investigated KNN−LiSbO3− BiFeO3 ternary systems in order to shift the TO−T nearer to room temperature, finding that low amounts of BiFeO3 (0.4 mol %) increase the d33 values from ∼202 to 260 pC/N. As a result, BiFeO3 as an ABO3 component can effectively enhance

Table 6. Piezoelectric Properties of ABO3-Modified KNN Ceramics d33 (pC/ N)

Tc (°C)

ref

Table 7. Piezoelectric Properties of KNN Ceramics with ABO3 Multicomponents material system

d33 (pC/N)

kp

Tc (°C)

ref

0.996(0.95Na0.5K0.5NbO3−0.05LiSbO3)−0.004BiFeO3 0.95(0.995Na0.5K0.5NbO3−0.005BiFeO3)−0.05LiSbO3 0.996[(K0.458Na0.542)0.96Li0.04](Nb0.85Ta0.15)O3−0.004BiFeO3 0.995Li0.03(Na0.53K0.48)0.97Nb0.8Ta0.2O3−0.005BiFeO3 0.995(0.95K0.5Na0.5NbO3−0.05LiSbO3)−0.005BiScO3 0.992(0.95K0.5Na0.5NbO3−0.05LiSbO3)−0.008BiScO3 0.99(Na0.485K0.485Li0.03)NbO3−0.01BiScO3 0.998[0.95(K0.5Na0.5)NbO3−0.05LiSbO3]−0.002BiFe0.8Co0.2O3 0.96(0.99K0.5Na0.5NbO3−0.01Bi0.5Na0.5TiO3)−0.04LiSbO3 0.938Na0.5K0.5NbO3−0.002BiNiO3−0.06LiSbO3 (Li,K,Na)(Nb,Sb)O3−LiSbO3−(Ba,Ca)(Ti,Zr)O3 K0.5Na0.5NbO3−LiNbO3−BNKT 0.998(0.948 K0.5Na0.5NbO3−0.052LiSbO3)−0.002BiAlO3 0.996(K0.475Na0.475Li0.05)(Nb0.95Sb0.05)O3−0.004Bi(Ni0.5Ti0.5)O3

260 257 261 238 280 305 225 276 250 240 237 240 233 253

0.52 0.52 0.58 0.47 0.49 0.54 0.38 0.48 0.29 0.44 0.49 0.46 0.35 0.52

365 365 345 300 ∼340 315 402 340 339 360 335 469 349 ∼350

Jiang229 Jiang230 Chao231 Zhou232 Li233 Jiang234 Sun235 Zhao236 Liu237 Liu238 Wu239 Chen240 Liu241 Yang242

K


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the piezoelectric activity of KNN−LiSbO3 ceramics by forming O−T phase boundaries at or near room temperature. However, doping with BiScO3 has different effects on the piezoelectric activity of KNN-based ceramics.233,234 For example, a d33 value of ∼305 pC/N was shown in K0.5Na0.5NbO3−LiSbO3−BiScO3 ternary systems with pure tetragonal structures; the authors reasoned that the phase boundary cannot play an important role in the improved piezoelectric properties.234 However, we found that the addition of BiScO3 results in increased TR−O values and decreased TO−T values of KNN;108,109 thus, the new R−T phase boundary could be finally formed at room temperature. In addition, the dielectric constant of this material gradually increases with decreasing measurement temperatures,234 suggesting the involvement of phase boundaries at or near room temperature. As a result, further investigation into this material system is necessary for understanding the origin of its enhanced piezoelectricity.234 To further clarify this assumption, we characterized the dielectric constant of 0.992(0.95K 0.5 Na 0.5 NbO 3 −0.05LiSbO 3 )−0.008BiScO 3 ceramics at different temperatures, as shown in Figure 10. The

Table 8. Effect of K/Na Ratio on Piezoelectric Properties of KNN-Based Ceramics composition (KxNa0.96−xLi0.04)(Nb,Ta,Sb)O3 (x = 0.38) 0.95(KxNa1−x)NbO3−0.05LiSbO3 (x = 0.40) (KxNa1−x)0.95Li0.05(Nb,Ta)O3 (x = 0.42) (KxNa1−x)0.94Li0.06NbO3 (x = 0.50) (NaxKy)(Nb,Sb)−LiTaO3 (KxNa0.96−xLi0.04)(Nb,Ta)O3 (x = 0.44)

Tc (°C)

d33 (pC/ N)

kP

306

0.48

Wu156

280

0.49

Wu186

242

0.46

Wu244

220 183 291

0.43 0.33 0.54

Zhai245 Kang246 Chang247

ref

4.1.8. Other Ions. As discussed before, chemical modifications (ion substitution (Li+, Ta5+, Sb5+), ABO3, etc.) can improve the piezoelectric activity of KNN materials by constructing O−T phase boundaries. However, other elements may also affect the piezoelectricity of KNN materials,118,399−405 as shown in Table 9. We can observe from Table 9 that the piezoelectric activity of KNN materials results from ion substitutions of A and/or B.118,399−405 For example, doping with Ag+ can enhance the d33 value of KNNST ceramics by shifting TO−T to near room temperature.118,399 It was found that the addition of Bi3+ and Zr4+ induces improvement in d33 of KNN by shifting a phase transition from pseudocubic to orthorhombic phase.401 As a result, different ion substitution can result in different piezoelectric activity of KNN materials mainly due to the involvement of different phase boundaries,118,399−405 and the enhancement in their piezoelectricity of most ceramics is due to the O−T phase boundary.118,399,402 Figure 11a displays the d33 and TC values of ceramics with different chemical modifications. It was found that the piezoelectric activity is strongly dependent on the types of the chemical modifications. In terms of the piezoelectric activity, Li+-, Ta5+-, and Sb5+-modified KNN ceramics possess larger d33 values with respect to others. As far as the TC value is concerned, ceramics doped with Li+ or Li+−Ta5+ have higher TC values than those doped with other additives. To further understand the effects of doping elements on the piezoelectricity, we give some samples and plot the d33 vs TO−T of KNN ceramics with different ion substitutions,101,102,156,169,182,184,188,193,195,204,214 as shown in Figure 11b. For the same doping elements, the d33 value is very sensitive to the corresponding TO−T, and the d33 value can be greatly increased for a TO−T near room temperature. More interestingly, we found that a higher d33 value can only be attained with Li+, Ta5+, and Sb5+ codoped KNN ceramics, even if a similar TO−T is involved.182,193,204 As a result, the piezoelectric activity of KNN-based materials is strongly affected by not only the phase boundary but also the TO−T value. In addition, some other factors also play a part in a material’s piezoelectric properties, such as poling conditions,169 microstructure,204 and electronegativity.204

Figure 10. εr−T curve of 0.992(0.95K0.5Na0.5NbO3−0.05LiSbO3)− 0.008BiScO3 ceramics.

εr−T curve measured from −150 to 200 °C shows the involvement of R−T phase boundaries in this material. This suggests that enhanced d33 values of this material should be due to the R−T phase boundary and not a T phase. 4.1.7. Ratio of K/Na. In 2007, our group observed a large d33 of ∼306 pC/N in KNN-based ceramics by modifying K/Na ratios,156 and related research by other groups has further clarified this issue.245−247 In fact, the K/Na ratios of KNN materials were thought to modify PPT near room temperature,156,186,244−247 helping to enhance their piezoelectric activity. Although the K/Na ratios strongly affect the piezoelectric activity of KNN-based ceramics, the optimum K/Na ratio is very sensitive to the actual compositions of the KNN-based materials,156,186,244−247 as shown in Table 8. In the references, the d33 values of KNN-based materials could be improved by changing the K/Na ratio as their TO−T is intrinsically shifted. 156,186,244−247 Zhai,245 Kang, 246 and Chang247 et al. also promoted the piezoelectric activity of KNN-based ceramics by similar methods, but the K/Na ratio is obviously different. As a result, the piezoelectric properties of KNN-based ceramics were enhanced mainly due to the evolutions of phase structure induced by changing the K/Na ratio.

4.2. Rhombohedral−Orthorhombic Phase Boundaries

It is well known that a pure KNN undergoes a series of structural phase transitions as the measurement temperature increases (see Figure 13): TR−O(−123 °C) → TO−T(210 °C) → TC(410 °C).5,248,249 In section 4.1, much attention has been paid to discussion of the O−T phase boundary of KNN materials, which has been an effective tool for enhancing their piezoelectric activity over the past 10 years (refs 101, 102, 132, L


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Table 9. Piezoelectricity of KNN-Based Ceramics with Different Ion Substitutions composition

d33 (pC/N)

kP

Tc (°C)

ref

(K0.42Na0.52Li0.04Ag0.02)(Nb0.91Ta0.05Sb0.04)O3 0.98[(K0.4725Na0.4725Li0.055)0.98Ag0.02(Nb0.98Ta0.02)O3 (K0.5Na0.5)0.82Ag0.18NbO3 (K0.50Na0.50)0.97Bi0.01(Nb0.98Zr0.02)O3 (K0.5Na0.5)0.94Li0.06(W2/3Bi1/3)0.008Nb0.992O3 (K0.5Na0.5)0.92Li0.04Sr0.02Nb0.98Sb0.02O3 (K0.5Na0.5)(Nb0.998Cu0.005)O3 (K0.48Na0.48Ca0.02)(Nb0.85Ta0.15O3)

263 237 186 161 282 142 112 155

0.45 0.39 0.43 0.41 0.45 0.38 0.38

353 470 355 370

Wu118 Xiao399 Lei400 Wu401 Du402 Hao403 Tan404 Coondoo405

353 440

Figure 11. (a) d33 vs TC of ceramics with different chemical modifications,12,97,101,102,104,106−118,166,277−285,392−405 and d33 vs TO−T values of the ceramics.101,102,156,169,182,188,184,193,195,204,214

Figure 12. Phase diagrams of (a) (K0.5Na0.5)(Nb1−xTax)O3254 and (b) (1−y)K0.5Na0.5NbO3 −yBaZrO3397 ceramics.

Figure 13. Phase transitions of a pure KNN.

addition, it was found that doping with Ta5+ could decrease both TC and TO−T and increase TR−O of KNN,254 similar to Sb5+.251,392 However, the TR−O value of Ta-doped KNN ceramic could not reach room temperature in the investigated composition ranges (see Figure 12a),254 showing that the R−O phase boundary is difficult to induce by chemical modification. Figure 12b plots the phase diagram of (1−x)K0.50Na0.50NbO3− xBaZrO3 solid solutions. For a pure KNN ceramic, the addition of AZrO3 (A = Ba2+, Sr2+, or Ca2+) can shift its TR−O to room temperature.397,398 Although the R−O phase boundary can be formed in KNN,250−252 the d33 value of ≤230 pC/N is usually

151−157, 159−168, 170−178, 182−197, 199−218, 220, 221, 224, and 229−247). However, the R−O phase boundary of KNN is often neglected, as such a phase transition is at low temperatures (ca. −123 °C)248,249 and provides a poor d33.392 Recently, additives have been used to increase the TR−O values of KNN, including Sb5+,251,253,392 Ta5+,254 AZrO3 (A = Ba2+, Sr2+, Ca2+),397,398 BiScO3,252 etc.250,255,256,258−263 For example, the TR−O value of (K0.48Na0.52)(Nb,Sb)O3 can be tuned to near room temperature by refining the Sb content (see Figure 9); thereby an improved d33 (∼230 pC/N) was demonstrated due to the coexistence of R−O ferroelectric mixed phases.251 In M


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KNN, as collected from these references.250−254,397,398 The addition of ions or ABO3 materials (e.g., Sb5+,251,392 Ta5+,254 BaZrO3,397 CaZrO3,398 SrZrO3,398 etc.) could simultaneously increase TR−O and decrease TO−T of KNN materials. As a result, it is reasonable that the R−T phase boundary should be formed in KNN materials by controlling the composition ratios of the doped additives. According to a number of references, we developed new methods to form new phase boundaries (e.g., R−T) in KNN materials, as shown in Figure 14. Some additives were able to

poorer with respect to those with O−T phase boundaries, as shown in Table 10. Their d33 values are determined not only by Table 10. Piezoelectric Properties of KNN-Based Ceramics with TR−O material system

d33 (pC/ N)

(Na0.52K0.48)(Nb1−ySby)O3 (y = 0.09) (1−x)KNN−xCaZrO3 (x = 0.04) (1−x)(Na0.5K0.5)NbO3− xBiScO3 (x = 0.02)

kp

230

TPPT (°C)

Tc (°C)

ref

∼Tr

∼190

Zuo251

154

0.32

∼Tr

320

Liang250

210

0.45

∼Tr

340

Zuo252

their R−O phase boundary but also by their chemical modification types,250−252 and unfortunately the d33 is usually increased by sacrificing TC.250−252 Although R−O KNN materials possess a poor comprehensive performance of d33 and TC, it may help to design new phase boundaries (e.g., R− T) by simultaneously moving TO−T and TR−O close to room temperature.106−110 Detailed analysis is provided in section 4.3. 4.3. Rhombohedral−Tetragonal Phase Boundaries

As shown in sections 4.1 and 4.2, researchers have invested much energy into the construction of two kinds of phase boundaries (i.e., O−T (refs 101, 102, 132, 151−157, 159−168, 170−178, 182−197, 199−218, 220, 221, 224, and 229−247) and R−O250−254,397,398) in KNN materials because of their enhanced piezoelectric behavior, and a number of construction methods were systematically addressed. More recently, several attempts were made to shift the R−O boundary to near room temperature,250−254,397,398 and a low piezoelectric activity of d33 ≤ 230 pC/N could be induced. In addition, it was proved that two kinds of phase boundaries (i.e., R−O and O−T) in KNN belong to a polymorphic phase boundary (PPB), rather than a typical morphotropic phase boundary (MPB).17,18,182,200,205 The materials with PPBs possess a strong temperature dependence of piezoelectric properties.17,18,182,200,205 At the same time, the d33 value of R−O or O−T KNN-based ceramics is still poorer with respect to the lead-based ones. Faced with such a bottleneck, we asked ourselves how to clarify these issues concerning KNN. Previously, it was reported that the R−T phase boundary of PZT possesses a weak temperature dependence of piezoelectric properties and a large d33;4,5 thus, we wanted to know whether a similar R−T phase boundary could be induced for further improving both the piezoelectric properties and the temperature stability of KNN materials. By reviewing a number of the references, it seems feasible to design a R−T phase boundary in KNN according to the regulation of the formation of R−O and O−T phase boundaries by doping in additives (refs 101, 102, 132, 151−157, 159−168, 170−178, 182−197, 199−218, 220, 221, 224, 229−247, and 250−254). Table 11 displays the influences of different additives on the phase transition temperatures of

Figure 14. Diagram for constructing new phase boundaries in KNN ceramics.

decrease TO−T of KNN to room temperature, including LiAO3 (A = Nb 5+ , 101,172 Ta 5+ , 102,193,196 Sb 5+ 182,183,186,187 ), Bi0.5B0.5TiO3 (B = Na+,215,216 K+,217 Li+ 218), CTiO3 (C = Ba2+,219 Sr2+,219 Ca2+ 223), etc.; their TR−O could be shifted to room temperature by the addition of DZrO3397,398 (D = Ba2+, Sr2+, Ca2+), BiEO3 (E = Sc3+,226,252 Al3+,225 Fe3+,227 Co3+ 228), Ta5+,254 Sb5+,251,253,257,392 etc. We were ultimately able to induce the R−T phase boundary in KNN by doping different additives, and a series of KNN-based material systems with R− T phase boundaries has been developed.106−117,256,277,284,285 In 2013, our group first reported a high d33 value of ∼425 pC/N in 0.96(K0.5Na0.5)0.95Li0.05Nb1−xSbxO3-0.04BaZrO3 ceramics by constructing the R−T phase boundary (see Figure 15),256 matching previously reported results (∼416 pC/N) of textured KNN-based ceramics.12 In this paper, both Sb5+ and BaZrO3

Table 11. Effect of Additives on the Phase Transition Temperatures of KNN Ceramics substitution (1 mol %)

Ta5+

Sb5+

CaZrO3

BaZrO3

SrZrO3

TC (°C) TO−T (°C) TR−O (°C)

−6.5 −4.0 +2.3

−25.1 −10.7 +14.8

−28.0 −11.0 +17.0

−28.0 −7.0 +19.0

−32.0 −13.0 +19.0

Figure 15. Phase diagram of KNLNSx−BZ ceramics. (Inset) Temperature dependence of εr and the composition dependence of d33.256 N


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Table 12. Piezoelectric Properties of KNN-Based Ceramics with TR−T material system

d33 (pC/N)

kp

Tc (°C)

ref

(Na0.52K0.40)(Nb0.84Sb0.08)O3−(0.08−x)LiTaO3−xBaZrO3 (x = 0.025) KNN−BaZrO3−LiSbO3 (1−x)K0.5Na0.5Nb1−xSbxO3−xBi0.5Na0.5TiO3 0.96(K0.5Na0.5)0.95Li0.05Nb1−xSbxO3−0.04BaZrO3 (1−x)(K0.48Na0.52)(Nb0.95Sb0.05)O3−xBi0.5(Na0.7K0.2Li0.1)0.5ZrO3 (0.97−x)K0.48Na0.52NbO3−0.03Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3−xB0.5Na0.5TiO3 0.992(K0.46Na0.54)0.965Li0.035Nb1−xSbxO3−0.008BiScO3

365 344 380 425 380 285 325

0.45 0.324 0.35 0.50 0.46 0.40 0.48

∼170 ∼176 ∼267 ∼197 290 347 358

257 258 255 256 260 261 109

Figure 16. Relationship between phase boundary and d33 of (1−x)(K1−yNay)(Nb1−zSbz)O3−xBi0.5(Na1−wKw)0.5ZrO3 ceramics: (a) y = 0.48, z = 0.05, w = 0.18; (b) x = 0.04, z = 0.05, w = 0.18; (c) x = 0.04, y = 0.48, w = 0.18; (d) x = 0.04, y = 0.48, z = 0.05.106

were used to simultaneously decrease the TO−T values and increase the TR−O values of KNN materials.251,397 However, we can observe from Table 12 that the ceramics often possess low TC values of 99%) at a low processing temperature of ∼920 °C. In addition, they also prepared a high-density KNN piezoceramic by the combination of a facile hydrothermal route and SPS,447 the d33 of which was shown to reach ∼135 pC/N, which is almost twice as much as those of samples using powders prepared from the solid-state reaction. More recently, Wang et al. 448 reported that highly dense (Na 0.52 K 0.44 Li 0.04 )(Nb0.86Ta0.06Sb0.08)O3 nanoceramics can be fabricated by SPS methods using nanopowders (11−34 nm), and a piezoelectricity of d33 ≈ 296 pC/N was observed, showing that the combination of nanopowders and SPS techniques are an effective way to prepare fine-grained potassium−sodium niobate ceramics with excellent elelctrical properties. These results show that an enhanced piezoelectric activity of the SPSsintered samples could be attributed to the lower sintering temperatures and the higher densities, which is superior to those sintered by the conventional pressureless technique.449 In addition, related investigations were able to more clearly clarify the relationships between microstructure and electrical properties of KNN-based materials.127,447−452 As far as practical applications are concerned, SPS is not as widely applied in industry as the RTGG method is. 4.5.4. Nonstoichiometry. The composition fluctuations seriously affect the electrical properties of KNN-based materials due to alkali metal evaporation, which is a serious issue despite the significant breakthroughs in its synthesis and characterization.453 The deviation from stoichiometry during the synthesis is a result of the strongly hygroscopic nature of alkaline carbonates, especially potassium carbonate.454 As a result, controlling the alkali metal loss in KNN during its processing and sintering is a critical issue.453,455 It is well known that dense KNN ceramics must endure a high processing temperature of ∼1100 °C, whereby chemical composition fluctuations cannot be avoided due to the evaporation of some elements.113,453−456 In addition, chemical composition deviation may be also caused during weighing due to the deliquescence of Na2CO3/K2CO3. As a result, the loss of alkali metals negatively affects the electrical properties of KNN materials, and much attention has been focused on how to maintain the stoichiometric ratios of KNN.166,168,456−467 Table 16 shows the d33 values of published nonstoichiometric KNNbased ceramics.113,166,168,461−463 Excess alkali metals were used to suppress the chemical composition fluctuations of KNNbased ceramics,113,166,168,461−467 and enhanced piezoelectric properties were observed (see Table 16). These results show that the calcining and sintering temperatures of KNN have been strongly affected by the nonstoichiometric content.282,283,456 The sintering temperatures of KNN were decreased when the A site was excessive168 and then raised when the B site was excessive.457 For example, Rubio-Marcos et al.283 reported that excess K+ may induce a displacement of Li+ from (K0.44+xNa0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3, seriously affect-

ing their piezoelectric activity by changing the stabilization of the crystalline symmetries. More importantly, nonstoichiometric KNN-based ceramics possess much larger d33 values than stoichiometric ones with the same composition, as shown in Tables 16 and 17.101,184,212 In 2014, we attained a high Table 17. Piezoelectric Properties of Stoichiometric KNNBased Ceramics compositions (1−x)(Na0.50K0.50)NbO3−xLiNbO3 (x = 0.08) 0.94(K0.5Na0.5)NbO3−0.06LiSbO3 Li0.02(Na0.55K0.45)0.98(Nb0.77Ta0.18Sb0.05)O3 0.96(K0.46Na0.54+x)Nb0.95Sb0.05O3 −0.04Bi0.5(Na0.82K0.18)0.5ZrO3

d33 (pC/N)

kP

ref

164 212 256 436

0.35 0.46 0.42 0.44

Guo101 Lin184 Yoo212 Wu113

piezoelectric activity (e.g., d33 ≈ 496 pC/N and kp ≈ 47%) in 0.96(K0.46Na0.54+x)Nb0.95Sb0.05O3−0.04Bi0.5(Na0.82K0.18)0.5ZrO3 ceramics by doping nonstoichiometric sodium,113 and a rhombohedral−tetragonal phase boundary was constructed in ceramics with a wide Na+ nonstoichiometry.113 A higher d33 value was observed in ceramics with an excess of 0.5% Na+ with respect to those with stoichiometric ratios.113 Thus, the addition of nonstoichiometric alkali metals makes a large contribution to promoting the piezoelectric activity of KNNbased ceramics. Previously, it was shown that nonstoichiometric KNN-based ceramics with A/B < 1 possess an improved piezoelectricity due to the microstructure evolutions induced by the formation of a ferroelectric secondary phase.132,135,457,465 For example, RubioMarcos et al.132 systematically studied the effects of A/B (A = (K0.44Na0.52Li0.04), B = (Nb0.86Ta0.10Sb0.04), A/B = 0.94) on the phase structure and electrical properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 ceramics, and the electrical properties of nonstoichiometric compositions were found to improve due to good homogeneity induced by the grain growth as well as the presence of a secondary ferroelectric tungsten−bronze phase (e.g., K3LiNb6O17). As a result, the effects of nonstoichiometricity and stoichiometricity on the microstructure and electrical properties of KNN-based materials require further clarification, and further physical analysis should be conducted. 4.5.5. Two-Step Sintering. The normal sintering technique is to heat the green compact at a given rate; then the sample is kept at a higher temperature in order to reach the maximum density. However, it is difficult to achieve a material with high density, homogeneous microstructure, and small grain size by normal sintering. In 2000, a two-stage sintering (TSS) technique developed by Chen et al. was shown to resolve this issue.546 In the TSS method, the full densification without grain growth after the second-step temperature can be achieved by the competition between the driving forces of grain boundary-controlled densification and grain boundary-conS


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trolled grain growth.114,245,471−483,546 First, the green samples are heated to a certain temperature for guaranteeing a uniform pore microstructure (relative densities > 75%) and minimizing grain growth; they are then cooled and held at a lower temperature for a long dwell time, where a low sintering temperature results in decreased grain growths, and a long dwell time provides enough energy for improving its density.546 Slower kinetics are involved when second-step sintering proceeds in a frozen microstructure, but it is enough to achieve full density and suppress grain growth.114,245,471−483,546 To suppress or eliminate grain growth of electroceramics during sintering, the TSS technique has attracted much attention because it is possible to decrease the grain size and improve the density of sintered samples. For solving the narrow processing window of a ceramic, the TSS technique has also been used to widern the sintering temperatures of lead-based and lead-free piezoceramics, 114,245,471−483,546 such as BaTiO3,471−475 Bi0.5Na0.5TiO3,476−478 BiScO3−PbTiO3,479 and K0.5Na0.5NbO3.114,245,480−483 The processing procedure for this method is summarized in Figure 20: The samples are first

heated to a high temperature (T1) in order to obtain a relatively high density and then are rapidly cooled and held at a low temperature (T2). Usually, the piezoelectricity of KNNbased ceramics is very sensitive to the sintering temperatures,112,165,177,421−431,470 that is, enhanced piezoelectric activity can be only attained in a narrow sintering temperature. However, the TSS method is suitable for preparing KNN-based ceramics because the ceramics can be densitified by eradicating the pores due to the decreased loss of the alkali metals with a low melting point.114,245,480−483 For example, a pure KNN ceramic was prepared by the TSS method,482 and the resulting dense microstructure led to its enhanced piezoelectricity. In addition, the sintering temperature (T2) range can be widened significantly for pure or chemically modified KNN ceramics using TSS,245,480−483 after which their electrical properties were mostly improved or maintained. The 0.9625Na0.5K0.5NbO3− 0.0375LiTa0.4Sb0.6O3 ceramics possess good electrical properties together with a wide sintering temperature range (T2 = 1000−1075 °C) when the TSS technique was used,483 and the TSS method also widens the sintering temperature range (1090−1130 °C) of (KxNa1−x)0.94Li0.06NbO3 ceramics and improves its piezoelectric activity.245 Recently, we also realized a high d33 (348−416 pC/N) and wide sintering temperatures (800−1130 °C) in 0.955(K0.42Na0.58)(Nb0.96Sb0.04)O3−0.045(Bi0.5K0.5)0.90Zn0.10ZrO3 ceramics using the TSS technique.114 As a result, the TSS technique can improve the density and widen the sintering temperatures of KNN-based material systems, resulting in an enhanced d33 value. 4.5.6. Single Crystals. The ferroelectric domains of polycrystalline piezoceramics cannot be aligned completely along one direction during poling owing to their randomly oriented grains, easily resulting in degraded electrical properties. However, the domains of single crystals can be more easily rotated during the poling fields.120,484−508,549−551 As a result, single crystals may become an effective way to improve the piezoelectric activity of alkali niobate materials by controlling their domain engineering and crystal orientations (refs 484−493, 498, 504−509, and 547−551). Several methods have been used to prepare KNN-based single crystals, such as solid-state crystal growth,485,498,505,507,508 the flux method,486,487,490,491,493 the Bridgman method,181 top-seeded solution growth,488,489,504 and the floating zone method.492 Table 18 shows the piezoelectric properties of KNN-based single crystals grown by different methods. Among reported results, one of the most exciting is that 0.95KNN−0.05LiNbO3

Figure 20. Program schedule of conventional sintering and two-step sintering.

Table 18. Electrical Properties of KNN-Based Single Crystals Grown by Different Methods compositions

preparation method

K0.53Na0.47NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.622Na0.378NbO3 0.95KNN−0.05LiNbO3 (K,Na)(Nb,Ta)O3 K0.53Na0.47Mn0.004Nb0.996Oy K0.14Na0.86Mn0.005Nb0.995Oy Mn-doped KNN Na0.5K0.5Nb0.995Mn0.005O3 0.5% Mn−K0.5Na0.5NbO3 K0.5Na0.5NbO3 with B2O3 flux

flux method solid-state crystal growth flux method flux method top-seeded solution growth Bridgman top-seeded solution growth flux method flux method floating zone method flux method flux method flux method

Pr (μC/cm2) 17 18

9 40 37 106 52 32 T

εr 600 1015 412 240 70 185 267

424 730

K33

d33 (pC/N)

ref

0.83

110 80 148 160 161 405 200

Kizaki484 Uršič485 Gupta486 Lin487 Tian488 Chen181 Zheng489 Inagaki490 Noguchi491 Inagaki492 Inagaki490 Lin487 Saravanan493

0.64

161 270 106


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1 * * O2 + 2Nb′Nb + V •• O → OO + 2Nb Nb 2

crystals grown by the Bridgman method possess an enhanced piezoelectricity of ∼405 pC/N,181 which is almost two times that of ceramics with the same composition.101 In addition, Lin et al. also reported a larger d33 (∼270 pC/N) in 0.5%Mnmodified K0.5Na0.5NbO3 single crystals grown by the hightemperature solution method using a K2CO3−Na2CO3 eutectic composition as flux,487 which is also superior to a pure KNN due to the involvement of high domain densities, as shown in Table 19. However, as far as piezoelectricity is concerned, most

V •• O +

compositions

TO−T (°C)

Tc (°C)

d33 (pC/N)

208 193 194 183

423 416 418 412

160 270 230 220

(9)

Nb4+ is usually found to exist in KNN single crystals,128 leading to a high leakage current density due to the 4d electrons of this ion.552 However, the valence shift of Nb4+ to Nb5+ induced by oxidation treatment can reduce its leakage current density.128 It is well known that oxygen vacancies (V•• O ), which result from the K and Na vacancies during high-temperature processing, are involved in KNN single crystals.129,181,547,548 Therefore, the leakage current density of KNN single crystals becomes higher when they are annealed under a high-pressure oxygen atmosphere.128 According to eq 9, the h• conduction (p type) of oxidized KNN single crystals is generated and then dominates the leakage behavior at room temperature. As a result, it is necessary to decrease the formation of h• in KNN single crystals in order to further reduce its leakage current.128,509 For example, the addition of Mn2+ is an effective way to decrease the leakage current of KNN single crystals.487,490−492 The increasing Mn valence (Mn2+−Mn4+) can absorb h• and then realize the objective of lowering its leakage current when Mn2+ is used to substitute the Nb site in

Table 19. Piezoelectricity of Pure and MnO2-Modified KNN Single Crystal.487 K0.5Na0.5NbO3 0.5% Mn−K0.5Na0.5NbO3 1.5% Mn−K0.5Na0.5NbO3 2.0% Mn−K0.5Na0.5NbO3

1 O2 → O*O + 2h• 2

(8)

reported results on the piezoelectric properties of KNN-based single crystals are disappointing because of the involvement of high leakage current induced by defects.128,484,487,490−492,508 It was thought that the leakage current of KNN single crystals is controlled by the electron−hole (h•) conduction;128 the formation of h• is demonstrated in eqs 8 and 9.552

Table 20. Preparation Conditions and Electrical Properties of KNN-Based Thin Films553−586 material system

substrate

preparation method

d33 (pm/V)

K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.51Na0.49NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K, NaNbO3 K,NaNbO3 K0.5Na0.5NbO3 [110] (K,Na)NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 (thick film) K0.5Na0.5NbO3 Mn2+-K0.5Na0.5NbO3 K0.5Na0.5NbO3 (thick film) K0.5Na0.5NbO3 (thick film) K0.5Na0.5NbO3 (thick film) (K,Na)NbO3−BaCu1/3Nb2/3O3 (K0.48Na0.48Li0.04)(Nb0.775Ta0.225)O3 (Na,K)NbO3−LiTaO3 0.948K0.5Na0.5NbO3−0.052LiSbO3 0.94K0.52Na0.58NbO3−0.06LiNbO3 K0.44Na0.52Li0.04Nb0.84Ta0.1Sb0.06O3 Na0.52K0.44Li0.04Nb0.84Ta0.10Sb0.06O3 K0.44Na0.52Li0.04Nb0.84Ta0.10Sb0.06O3 K0.48Na0.48Li0.04Nb0.895 Mn0.005Ta0.10O3 0.92KNN−0.06BaZrO3−0.02Bi0.5Li0.5TiO3 K0.4425Na0.52Li0.0375Nb0.8825Sb0.08Ta0.0375O3 K0.44Na0.52Li0.04Nb0.84Ta0.10Sb0.06O3 K0.44Na0.52Li0.04Nb0.84Ta0.1Sb0.06O3−Mn4+ K0.44Na0.52Li0.04Nb0.84Ta0.1Sb0.06O3−Mn K0.5Na0.5Nb0.8Ta0.2O3−Mn 0.95K0.5Na0.5NbO3−0.05CaZrO3−Mn

Pt/Ti/SiO2/Si Pt/Ti/SiO2/Si SrRuO3/SrTiO3 SrRuO3/SrTiO3 Pt/TiOx/SiO2/Si Pt/TiO2/SiO2/Si LNO/Si Nb−SrTiO3 LaNiO3/Si Pt/TiO2/SiO2/Si Pt/TiOx/SiO2/Si Pt/TiO2/SiO2/Si Pt/Ti/SiO2/Si Pt/Ti/SiO2/Si Pt/Ti/SiO2/Si Pt/TiO2/Al2O3 platinum foils Pt/Ti/SiO2/Si Pt/sapphire Pt/Ti/SiO2/Si Nb−SrTiO3 Pt/TiO2/Al2O3 Pt/Ti/SiO2/Si SrRuO3/SrTiO3 Pt/MgO SrRuO3/SrTiO3 Pt/TiO2/SiO2/Si Pt/(001)MgO Pt/Ti/SiO2/Si SrRuO3/SrTiO3 SrRuO3/SrTiO3 SrRuO3/SrTiO3 Pt/TiOx/SiO2/Si Pt/TiOx/SiO2/Si

chemical solution deposition chemical solution deposition hydrothermal method rf magnetron sputtering chemical solution deposition sol−gel method rf magnetron sputtering sol−gel method rf magnetron sputtering sol−gel method chemical solution process rf-magnetron sputtering chemical solution deposition chemical solution deposition reactive template method screen printing electrophoretic deposition aerosol deposition method aerosol deposition method pulsed laser deposition hydrothermal method aerosol deposition sol−gel method pulsed laser deposition pulsed laser deposition pulsed laser deposition chemical solution deposition pulsed laser deposition sol−gel pulsed laser deposition pulsed laser deposition pulsed laser deposition chemical solution deposition chemical solution deposition

74.0 40

U

45.1 58 46 64.5 56 35 124* 82.5 40 110* 49 25 50 192 53

61

58 32

Pr (μC/cm2)

14 12.05 7 3.54 11.5 17.3 12 8 7 21.1 12 9.1 3.05

8.1 18.03 11.3 15.5 9.7 26.3 7.5 16.1 24.5 9.5 4 15 10 (Ps) 7

ref 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586


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KNN,128,495,497,509 leading to a high remanent polarization (Pr ≈ 40 μC/cm2). 4.5.7. Thin Films. Since the increase in interest in the piezoelectric activity of KNN-based ceramics, efforts have been made to prepare corresponding thin films due to their potential applications for microdevices such as microelectromechanical systems.552−590 However, two main issues still hinder the fabrication of high-quality KNN-based films: (i) composition deviations from stoichiometry and (ii) the loss of alkali oxides during preparation.553,573,589,590 Table 20 lists the preparation methods and the corresponding electrical properties of KNNbased materials in the form of thin films or thick films.553−586 One can observe from Table 20 that the ferroelectric and piezoelectric properties of KNN-based materials (e.g., thin film and thick film) can be promoted to varying degrees by some methods, such as the use of stabilizing agents,553 new preparation techniques (hydrothermal method),573 excess alkali metals,557,562 different substrates,552−586 processing conditions,555,567,574 buffer layers,556,559,561,577,582 dopants or ion substitutions,571−575,577,579−585 orientation,559−561,567,582 annealing atmosphere,564 etc. For a pure KNN thin film,553−564 some factors strongly affect their ferroelectric (Pr = 7.0−21.1 pm/V) and piezoelectric properties (d33* = 40−74 pm/V). For example, Kim et al. have shown that d33* values of 40 pm/V for KNN thin films can be attained by doping excess K and Na,554 while the piezoelectric activity can be further promoted up to d33* = 56−74 pm/V by introducing stabilizing agents (diethanolamine and ethylenediaminetetraacetic acid)553 or polyvinylpyrrolidone565 because of the decrease of crystallization temperature and the suppression of the volatility of alkali ions.587,588 A comparable piezoelectric constant (d33* = 64.5 pm/V) of KNN thin films annealed under a Na2O atmosphere can be also observed by different preparation techniques (rf magnetron sputtering).564 It was found that different substrates also affect the ferroelectric properties of KNN thin films,568,570 and the Pr values of the films deposited on Nb-doped SrTiO3560 are much higher with respect to those deposited on Pt/TiO2/SiO2/Si substrates,558 due to the involvement of the (110) orientation.560 In addition, the effects of the orientations on ferroelectric and piezoelectric properties of a pure KNN thin film were also systematically studied,559−561,567,582 and [110]-oriented KNN thin films were found to possess higher Pr values of 17.3 μC/cm2 because the [110] orientation is its spontaneous polarization direction, while a large d33 of 50.5 pm/V can be observed in the [001]oriented materials560 due to the involvement of reversal of 180° domains. Also, KNN-based thick films were also prepared, and their electrical properties were found to be comparable to the corresponding bulk materials.568,571 For example, it is of great interest to note that a large d33 of 110 pm/V can be shown in Ba(Cu1/3Nb2/3)O3-modified KNN thick films owing to the presence of large grain sizes and abnormal grain growth,571 and moreover, the KNN thick films also possess a comparable d33 to the corresponding bulk materials due to their dense microstructure.568 Recently, Goh et al. reported a piezoelectric constant of 25 pm/V in a 480 nm KNN thick film grown at a very “low” temperature of 130 °C using a new sustainable hydrothermal method, which results from the decrease of the loss of alkali metals.573 Although the ferroelectric and piezoelectric properties of KNN-based films can be improved by these methods,553−564 their electrical properties are still poor when compared to leadbased or the corresponding bulk materials. Unlike KNN-based

bulk materials, it is difficult to form phase boundaries in the corresponding material system.575−582 In the past, researchers have attempted to construct phase boundaries in KNN-based thin films using the phase boundary compositions of the corresponding bulk materials,575−582 but the results are disappointing due to a number of factors (e.g., composition deviations, lattice mismatch between thin film and substrate, etc.). As a result, there are considerable challenges for the development of high-performance KNN-based thin films. When KNN-based films are prepared, two important factors (composition deviations and the loss of alkali metals) should be first considered, and other factors should also be addressed, such as preparation parameters, buffer layers, dopants, or substitutions, etc. More importantly, much more attention should be given to the design and construction of phase boundaries as well as the relationships between strain and electrical properties if subsequent breakthroughs are desired. 4.5.8. Poling Conditions. The optimized poling conditions are a very important tool to improve the piezoelectric properties of lead-based or lead-free ceramics,166,169,284,510−522 including poling temperatures, poling electric fields, dwell times, etc. Recently, much attention has been given to the effects of the poling conditions on the electrical properties of lead-free ceramics, such as BaTiO3,45,512,513 Bi1/2Na1/2TiO3,513−519 and K1/2Na1/2NbO3.166,169,265,284,520−522 For KNN ceramics, the poling conditions (especially poling temperatures) become very important due to the intrinsic characteristics of the O−T phase boundary with a PPT.166,169,265,520−522 As far as KNN-based ceramics are concerned, one usually chooses poling temperatures near its phase transition temperature.169,265 For example, Du et al.169 and Wu et al.265 improved the d33 values of KNN-based ceramics by optimizing the poling temperatures (Tp), that is, the Tp is located at the phase transition temperatures. According to such a regulation, the d33 value can be increased greatly for KNN-based ceramics with O−T phase boundaries.520 Morozov et al.521 also studied the influence of the poling temperature on the piezoelectricity of Li-, Ta-, and Mnmodified (K,Na)NbO3 ceramics with O−T phase boundaries, showing that the low-signal dielectric and piezoelectric properties in orthorhombic phase can be enhanced by poling in tetragonal phase. Zhao et al.522 investigated the relationships between 90° domain reorientations and poling fields of (K,Na,Li)NbO3 ceramics by X-ray diffraction analysis, indicating that the 90° domain reorientation can be effected by applying poling fields (5 kV/mm), finally leading to enhanced piezoelectricity. Recently, Li and Wang et al.166 promoted the enhanced piezoelectricity of KNN-based ceramics from 190 to 324 pC/N by the aging and repoling method and proposed that both defect migration and rotation of spontaneous polarization are responsible for such a phenomenon. Recently, we investigated the effects of poling conditions on the piezoelectricity of (1−x)(K 0 . 4 2 Na 0 . 5 8 )(Nb 0 . 9 6 Sb 0 . 0 4 )O 3 − x(Bi0.5K0.5)0.90Zn0.10ZrO3 ceramics with R−T phase boundaries, whereby a high d33 can be attained when Tp is located at its TR−T. A very low poling electric field (>0.8 kV/mm) was enough to warrant full polarization due to the involvement of more polarization states.284 As a result, the optimization of poling conditions becomes very important for further enhancement of the piezoelectric activity of KNN-based ceramics and should be given further attention. 4.5.9. Sintering Aids and Sintering Atmosphere. Sintering aids can improve the electrical properties of KNN V


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Table 21. Electrical Properties of KNN-Based Ceramics with Doping Oxides material system

density or relative density

KNN−Bi2O3 KNLN−CuO KNN−CuO KNLNTS−Fe2O3 KNN−Fe2O3 KNN−La2O3 KNN−(La−Fe2O3) KNN−ZrO2 KNN−ZnO KNN−SnO2 KNN−CdO KNN−CuNb2O6 KNN−ZnO−SnO2 KNN−CuO−SnO2 KNNS−K4CuNb8O23−La2O3 KNN−MnO2−K5.4Cu1.3Ta10O29 KNN−K5.4Cu1.3Ta10O29−CuO

99.3% 4.33 g/cm3 98.9% 97.5% 93.3% 96.7% 97% 98% 95.3% 4.47 g/cm3 4.48 g/cm3 97.8% 4.376 g/cm3 4.65 g/cm3 97%

ε/εo 1287 237 593 815 723 905 652 627 493 503 710 300 285

d33 (pC/N)

kp

140 109 96 257 136 92 145 100 117 108 107 92.5 124 120 160 90 94

0.46 0.36 0.389 0.53 0.41 0.34 0.43

tan δ

Qm

ref

0.005

410 1023 415

147 530 531 543 544 544 544 591 592 592 592 526 596 597 593 594 595

0.042 0.179 0.039 0.040 0.033 0.045 0.040

0.44 0.39 0.42 0.40 0.398 0.38 0.38 0.40 0.38

412 396 392 418 405 406 1933

0.010 0.013

1040 283 1900 3053

0.003 0.0018

between the extrinsic factors and the piezoelectric properties of KNN-based materials. 4.6.1. Density vs Electrical Properties. It is well known that densification of pure KNN ceramics is difficult due to their poor sinterability and high volatility;3,121,127,137,249 thus, poor d33 values are often observed by conventional solid-state methods.137,249 As a result, it is very important to understand the relationships between density and piezoelectric activity of KNN ceramics except for the phase boundaries,125 the compositions,125 or other factors. Here we gave an example and discussed the influence of the density on the d33 in a pure KNN ceramic using different preparation methods, as shown in Table 22. It is clear that a pure KNN ceramic sintered in air has

materials by the improvement in the densification and microstructure.130,140,143,147,523−545 In recent decades, many aids have been used to sinter KNN ceramics (refs 130, 140, 143, 147, 523−546, 591, 601, and 602), including K4CuNb8O23 (KCN), 130 K 5.4 CuTa 10 O 29 (KCT), 140,143,524 K 1.94 Zn 1.06 Ta 5.19 O 15 (KZT), 525 CuNb 2 O 6 , 526 CuO, 145,527−531,602 Mn3O4,532 MnO2,533,534 MoO3,535 ZnO,536−539 La2O3,540 Bi2O3,147,541,542 Fe2O3,543,544 V2O5,545 and ZrO2.591 Sintering aids have two main advantages for KNN materials: (i) improving the density, such as KZT, KCN, etc.; and (ii) enhancing electrical properties (e.g., d33, kp, Qm, etc.) due to their “softening” or “hardening” characteristics, such as CuO, KCN, KCT, etc. In addition, the electrical properties of KNN materials could be improved by doping with two sintering aids,593−597 such as K4CuNb8O23 and La2O3,593 MnO2 and K5.4Cu1.3Ta10O29,594 K5.4Cu1.3Ta10O29 and CuO,595 ZnO and SnO2,596 CuO and SnO2,597 etc. Table 21 shows the density and electrical properties of KNN ceramics with doping oxides.147,526,530,531,543,544,591−597 In particular, the ceramic matrixes used mainly involve a pure KNN147,526,531,544,546,547,594−597 not those doped with other components.530,543 As shown in Table 21, a higher d33 is attained in the ceramics because of the improved density induced by the addition of oxides,147,530,542,591,593,596,597 and a higher Qm was also shown.147,526,531,542,543,546,547,593−597 In addition, the atmosphere during sintering is an important factor to determine the electrical properties of KNN materials by changing the microstructure (e.g., grain morphology598 and density599−601). Among sintering atmospheres, an oxygen atmosphere during sintering usually has a large effect on the electrical properties of KNN ceramics due to microstructure evolution.598−601

Table 22. Relationship between Density and Piezoelectricity of a Pure KNN Ceramic Obtained by Different Groups material system K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3 K0.5Na0.5NbO3

preparation method air sintered air sintered air sintered spark plasma sintering hot pressed

ρ

relative density (%)

d33 (pC/ N)

4.30 4.26 4.25 4.47

95.3 94.4 94.24 >99.0

110 80 80 148

4.46

98.89

160

kp

ref

0.39 0.36 0.39

121 137 249 127

0.45

249

a lower density than one sintered by SPS or hot press methods;121,127,137,249 thus, higher d33 values are shown by SPS or hot-pressed samples due to the improved density.127,249 In the past decade, some methods (e.g., sintering temperature, 1 1 2 , 1 6 5 , 1 7 7 , 4 2 0 − 4 3 2 , 4 7 0 rapid sintering technique,127,447,450,451 the use of sintering aids (refs 130, 140, 143, 147, 523−545, and 602), and sintering atmosphere598−601) have been used to improve the density of KNN ceramics. Among these methods, the main goal is to suppress the loss of alkali metals112,127,165,177,421−431,447,450,451,470 or the formation of liquid phases130,140,143,147,523−545 during the sintering process. The piezoelectric activity of KNN materials can often be enhanced by increasing their density.127,147,249,530,542,591 However, it was also found that the piezoelectric activity of KNN-based materials was not only

4.6. Extrinsic Factors

In the last section, we illustrated the influences of intrinsic factors (i.e., crystalline structure, lattice parameters evolution, phase boundaries, etc.) and some special preparation techniques on the piezoelectric properties of KNN-based materials. In addition, extrinsic factors (i.e., density, microstructure evolution, domain structure, etc.) also play a crucial role in the development of technological applications. As a result, we briefly provide an overview of the relationships W


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Figure 21. SEM images of KNN-based ceramics with R−T phase boundaries reported by us.110,112,115,114,117,285

dependent on the density but also very sensitive to fluctuations of composition (e.g., the degree of volatilization) when the compositions are situated at the phase boundary regions.125 As a result, other factors should be also considered except for the density. 4.6.2. Grain Morphology vs Electrical Properties. It is usually accepted that the microstructure (e.g., grain morphology) of KNN-based materials can be modulated by a sintering aid,130,140,143,147,523−545 the use of new preparation techniques,127,249,447,450,451 various sintering atmospheres,598 and others. Many researchers have improved the sinterability of KNN-based materials using different sintering aids145,413,528,533,534,537,539,604 (e.g., CuO,145,413,528 NiO,603 ZnO,537,539 MnO,604 and MnO2533), and the existence of Asite vacancies can offset the formation of the hygroscopic secondary phases because these chemical elements substitute the B site in ABO3.124 In addition, sintering aids are often used to densify KNN ceramics by introducing a liquid phase 130,140,145,143,147,523−545 or a “transient” liquid phase,124,132,537 and the grain morphologies can be also controlled by changing the concentration of sintering aids.124,132,537 In particular, bimodal grain size distributions were often observed in KNN-based ceramics doped by oxides.132,539 It was proposed that the general grain growths observed in oxide-doped samples should be attributed to enhanced atomic mobility,132,539 dramatic grain growth can be induced by the difference in surface free energies between large grains and small grains,3 and then bimodal grain sizes can be facilitated and accelerated due to the presence of a liquid phase. However, piezoelectric activity will usually be degraded by the addition of sintering aids because a material’s phase transition temperature deviates to room temperature.145,537,413,603,604 In addition, the piezoelectric activity of KNN-based ceramics is closely related to their average grain sizes.132,605 Also, the grain growth behavior of a pure KNN ceramic can be controlled by different sintering atmospheres because of the critical driving force for 2D nucleation-controlled grain growth caused by their different edge free energies.598 As a result, the grain morphologies of KNN-based ceramics can be modified by different methods.

It was previously reported that abnormal grain growth (AGG) was usually present in KNN-based ceramics, leading to degradation of their piezoelectric properties.105,114,173,269,284,285,606 Recently, bimodal grain size distribution has also been observed in KNN-based ceramics with R− T phase coexistence108−118,285 (see Figure 21). However, a large d33 of >400 pC/N can be attained, which seems to be at odds with the reported results.105,173 For KNN-based ceramics with R−T phase boundaries, the phase boundary should play a critical role in the piezoelectric properties of KNN-based ceramics; thus, the negative effect of AGG can be almost neglected or decreased. As a result, the piezoelectric activity of KNN-based materials was more sensitive to the phase boundary with respect to the negative effect of AGG when the compositions were situated at the R−T phase boundary regions.108−118,285 Preparation of R−T KNN-based ceramics with a unimodal grain size distribution is necessary in order to further promote their piezoelectric activity. Some methods may decrease the extent of AGG in KNN materials, such as the particle size (nanopowder) of calcined powders, milling strategies, etc.607 Recently, it was shown that grain morphologies can be controlled by developing KNN-based low-dimensional materials using different preparation techniques,271,608−613 strongly affecting their electrical properties.271,610−613 For example, Li et al. reported that one-dimensional KNN nanorods fabricated by a molten-salt method possess a high piezoelectric constant (d33* ≈ 230 pm/V),612 the K/Na ratios of one-dimensional KNN nanorods seriously affect their piezoelectricity, and moreover the phase boundary is also obviously different from that of KNN bulk.271 In addition, higher d33* values of 180 pm/ V were also shown in vertically aligned lead-free (K0.6Na0.4)NbO3 nanorod arrays via the facile hydrothermal method.613 Thus, control of morphology is an efficient way to promote the piezoelectric properties of KNN-based materials.271,610−613 4.6.3. Domain Structure vs Electrical Properties. Domain structure can determine the piezoelectric properties of a ferroelectric material,446,268,269,406,614−622 that is, the electrical properties of a ferroelectric material can be modulated by changing the domain structure as well as domain−wall X


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Figure 22. (a) Relationship between phase boundary type, piezoelectric constant, and Curie temperature,12,90−201 and (b) comparison of the piezoelectric properties of PZT and KNN.

the disappearance of the polar anisotropy by means of transmission electron microscopy, which also confirms the different domain structure between PPT and MPB,268 and the R−T phase boundary is also confirmed by the tweed-like domain morphology.619 In addition, the relationships between domain patterns and time-aging stability of d33 were illuminated in (K0.5Na0.5)1−xLixNbO3 ceramics, and the involved 180° domains cause the degradation and time-aging instability of piezoelectric properties.620 It was also found that domain size seriously affects the electrical properties of KNN-based materials. For example, the decreased domain sizes can reduce the coercive field of Li0.02(K0.45Na0.55)0.98NbO3 ceramics,621 and both curved domain stripes and larger average domain width were found to improve the piezoelectric properties of (K0.5Na0.5)0.965Li0.035(Nb0.8Ta0.2)O3 ceramics with O−T phase coexistence.622 As a result, the evolutions of the piezoelectric properties can be described and explained by characterizing their domain structure, and correspondingly the piezoelectricity can be promoted by varying the domain types and sizes. In this section, the electrical properties of KNN-based materials could be further improved by microstructure variation using tools besides the phase boundary. Among those tools, their electrical properties can best be improved by controlling the microstructure evolution (e.g., density, grain size and morphology, domain structure, etc.). In addition, multiple tools are always used together to modify the KNN-based materials. As a result, more consideration should be given to these supplementary tools for preparing KNN materials.

rotations. As a result, it is necessary to clarify the ferroelectric domain structure of KNN-based materials with the phase boundary compositions in order to further understand its nanoscale properties and improve its piezoelectricity. The domain structure of KNN-based materials can be well identified by a number of measurement techniques (see section 3). For example, the domain patterns concerning 60° and 90° domain walls were observed in pure or Mn-doped KNN single crystals by polarizing optical microscopy, and the domain width was found to be a few tens of micrometers.614 The unpoled KNN ceramics possess only 90° and 180° domain walls,269 while the domain structure changes when Li is doped to KNN, that is, the domains (e.g., 60°, 90°, and 180°) and S-type domain walls were simultaneously observed in a grain.615 In addition, some researchers have investigated the domain structure of KNN-based materials with the phase boundary compositions.406,616 For example, the phase transition temperatures (e.g., TR−O, TO−T, and TC) of KNN single crystals were graphically confirmed by the changes of the domain structures under different temperatures.406 The coexisting tetragonal and orthorhombic domains were vividly observed in one grain of K0.44Na0.52Li0.04Nb0.86Ta0.1Sb0.04O3 by piezoresponse force microscopy, providing a basis for the understanding of the PPT of KNN-based materials.616 As a result, the domain structure can allow the characterization of the phase structure of KNN-based materials. There is a close relationship between domain structure and phase structure of KNN-based materials, and the phase structure can determine its piezoelectric activity.268,617−622 As a result, the piezoelectric properties of KNN-based materials can be tailored by varying their domain structure. Recently, the relationships between domain structure and piezoelectricity of KNN-based materials have been studied by considering the changes in domain structure. Zhang et al.617 investigated the evolution of domain structure in hot-pressed KNN ceramics before and after poling, and the irreversible domain wall motions were found to strongly contribute to the piezoelectric coefficient of the hot-pressed KNN ceramics. Transmission electron microscopy was used to in situ identify the evolutions of the domain morphologies and phase structures under driving electric fields (E) of 0.948(K0.5Na0.5)NbO3−0.052LiSbO3, and it was found that the enhanced piezoelectric properties result from a tilted monoclinic phase from O−T for E > 14 kV/cm.618 For O−T (Na,K,Li)(Nb,Ta,Sb)O3 ceramics, the nanodomains (e.g., 20−50 nm width) generated enhanced piezoelectric properties due to the reduced domain wall energy induced by

5. RELATIONSHIP BETWEEN PHASE BOUNDARIES AND PIEZOELECTRICITY Generally speaking, the piezoelectric activity of KNN-based ceramics can be enhanced in different degrees by constructing phase boundaries and modifying the microstructure of a material, while its piezoelectric activity is strongly sensitive to the phase boundary types. In this section, we further clarify the relationships between phase boundary and piezoelectric activity of KNN-based materials, addressing the weakness and the corresponding solution methods. Tables 1−12 display the influences of the phase boundary types on the piezoelectric activity of KNN-based materials. There is a progressive development from O−T, to R−O, to R−T in chronological order, and the trend of d33 is R−O < O−T ≤ R−T according to the piezoelectric activity, as shown in Tables 1−12. At the same time, it is much more difficult and complicated to construct R− T phase boundaries with respect to R−O and O−T, as the R− Y


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Figure 23. (a) Articles concerned with lead-free piezoelectric materials in refereed journals published between 2010 and 2014 obtained from ISI Web of Science using the keywords “lead-free piezoelectric” and “strain”. (b) Relative amounts of several kinds of lead-free piezoelectric materials in strain investigations.

6.1. Piezoelectric Energy Harvesting

T phase boundary can be only formed by adding two or more additives.106−113,256,277,284,285 In addition, the interrelationships among different phase boundary types were also established, and the methods to construct the R−O and O−T phase boundaries provide a solid practical and theoretical basis for designing the corresponding R−T, that is, the R−T phase boundary can be induced by combining two kinds of boundaries (R−O and O−T) using the optimum additives. Figure 22a shows the piezoelectric constant, phase boundary types, and Curie temperature of KNN-based ceramics, as collected from a number of studies (refs 12, 17, 97, 101, 102, 104, 106−118, 166, 201, and 266−285). The types of phase boundaries strongly affect the d33 value of KNN materials: R− O ceramics possess a poorer d33 with respect to those with R− T and O−T, the O−T ceramics have a wide d33 distribution from 200 to 350 pC/N, and a large d33 of >330 pC/N is usually induced in R−T ceramics. In addition, it was also found that the piezoelectric activity of R−T KNN-based ceramics can be comparable to part-PZT ceramics. However, during the development of KNN-based materials a general trend has emerged whereby the d33 value is enhanced by sacrificing a material’s TC, and the ceramics with a large d33 (>330 pC/N) always have a low TC ( 200 °C, the d33 of KNN-based ceramics has been comparable or even superior to PZT-based ceramics. As a result, we believe that KNN-based lead-free piezoceramics can replace PZT in some applications.

So-called “energy harvesting” devices have attracted much attention because energy can be generated from renewable sources (e.g. temperature, vibration, air flow, etc.), with the hope of solving the energy crisis if these devices can be developed and made practical.623,624 Among these energy harvesting devices, piezoelectric energy harvesting devices can convert mechanical energy into electrical energy; moreover, their energy density is usually superior to other energy devices.625−628 Since piezoelectric nanogenerators fabricated by piezoelectric zinc oxide nanowire arrays were first reported for the direct energy conversion from mechanical motion to electrical power,624 much attention has been focused on investigations into piezoelectric energy harvesting devices. In the past, most piezoelectric energy harvesting devices were fabricated using PZT-based materials due to their promising electrical properties.625,626,628,629 Recently, energy harvesting devices have been fabricated using KNN-based materials,630−632 motivated by environmental concerns over lead-containing PZT-based materials.625,626,628,629 For example, energy harvesting devices based on (K0.5Na0.5)0.97(Nb0.96Sb0.04)O3 ceramics doped with 0.2 wt % CeO2 were fabricated, possessing optimum output voltage (24.6 mVrms) and power (0.839 μW) under an external vibration acceleration of 0.7 g.630 More recently, a large-area nanocomposite generator device was fabricated using alkaline niobate-based particles and copper nanorod fillers, providing a maximum output of up to 140 V and 8 μA (∼0.5 mW).632 6.2. Piezoelectric Actuator

It is well known that actuator applications require electromechanical coupling providing high strain with high force, and this requirement can be realized by piezoelectric materials that allow direct conversion between electrical and mechanical energy.17,120 Currently, the piezoactuator markets were dominated by PZT materials in the form of the ceramics or thin films,633 and lead-free piezoelectric materials concerning strain are also experiencing gradual growth due to environmental regulations,12,66−68,93,162,178,634 as shown in Figure 23a. Recently, strain investigations have triggered strong interest in the field of lead-free piezoelectrics,12,66−68,93,162,178,634 since a high unipolar strain value of ∼0.45% was first reported in BNTbased ceramics under a high driving electric field of ∼8 kV/ mm.68,634 It is thought that, for the first time, the electrical properties (strain) of lead-free piezoelectrics have become

6. APPLICATIONS OF KNN-BASED MATERIALS The excellent piezoelectric properties of KNN-based materials result in some practical applications (e.g., piezoelectric energy harvesting, piezoelectric actuators, etc.), similar to PZT ceramics. In this section, we pay special attention to their applications in the field of piezoelectric energy harvesting and piezoelectric actuators because this review mainly concerns the relationships between phase boundary types and piezoelectricity. In addition, other practical applications are also briefly mentioned.17,120 Z


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Figure 24. Unipolar strain in 0.965K0.45Na0.55Nb0.98Sb0.02O3−0.035Bi0.5Na0.5Zr0.85Hf0.15O3 ceramics, measured at 10 Hz and room temperature.

superior or comparable to those of lead-based examples.68,634 We can see from Figure 23b that most research has focused on strain investigations with BNT-based materials because of its high strain values. However, issues related to their transfer into practical applications need to be resolved despite a high strain in the mentioned ceramics,635 that is, a high driving electric field is often required to warrant a large strain,68,634 and the strain hysteresis exhibits a strong nonlinearity.68,634 Recent research results show that KNN-based ceramics can effectively address two of these issues,93,162,178,265,270,278,285,636 where a slimmer strain hysteresis and a relatively high unipolar strain value under a low driving electric field can be attained.93,162,178,265,270,278,285,636 In addition, a high unipolar strain was also observed in KNN-based materials.93,178,270,278 In our group, 0.965K0.45Na0.55Nb0.98Sb0.02O3− 0.035Bi0.5Na0.5Zr0.85Hf0.15O3 ceramic systems have been recently developed (see Figure 24), and a high unipolar strain and a slimmer strain hysteresis compared with the BNT-based materials68,634,635 can be simultaneously observed, due to the involvement of phase boundaries, as shown in Figure 24. There are few reports of such a high unipolar strain value in KNNbased ceramics, which is two times as high as those reported for (Li, Ta, Sb)-doped KNN textured ceramics.12 Recently, Li et al.636 reported that the unipolar strain of CaZrO3 and MnO2 comodified (K,Na,Li)(Nb,Ta)O3 ceramics shows fatigue-free (only small degradation within 3%) during 107 cycles under a field amplitude of 2 kV/mm, which results from the “softening” effect. Rubio-Marcos et al.93 developed a high strain value with Smax as high as 0.17% at 3 kV mm−1 in the (K0.44Na0.52Li0.04) (Nb0.86Ta0.10Sb0.04)O3 microfiber ceramics. These results indicate that KNN-based materials are the most promising candidates in the field of lead-free piezoelectric actuators.265,270,278,285,636 In the past, transducers,148,637,638 buzzers,159,639 surface acoustic wave filters,640 and other devices have been developed using KNN-based materials. In addition, the performance of some devices is comparable to those fabricated by PZT-based materials. For a detailed introduction of applications, the reader is referred to recent works reported by Rödel and Jo.17,120

7. OUTLOOK AND FUTURE WORK We systematically reviewed the construction of phase boundaries as well as the related physical mechanisms in KNN-based materials by comprehensively analyzing the recent advances in the field as well as the achieved results. Although the piezoelectric activity of KNN-based materials can be promoted by constructing different phase boundaries as well as other supplementary tools, some issues still need to be addressed before large-scale application can be realized. According to our review of the recent advances in KNNbased materials, a number of recurring problems haunt researchers in the field, e.g., how to design new phase boundaries, identify the phase compositions, investigate the interrelationships between different phase boundary types, phase compositions, and piezoelectric properties, etc. In order to clearly solve these issues, future work will most likely focus on the following challenges. 7.1. Designing New Phase Boundaries

Although the R−T phase boundary of KNN-based materials can more significantly improve the piezoelectric activity with respect to other phase boundary types, its piezoelectric activity continues to catch up to those of PZT ceramics in terms of meeting the requirements of different applications. As a result, it is necessary to further increase the piezoelectric activity of such materials by developing new phase boundaries similar to those of PZT. 7.2. Analyzing Phase Compositions

Phase composition identification is critical to establishing the relationships between phase boundaries and piezoelectric properties, providing us with a guide to refining the phase boundaries and further promoting a material’s piezoelectricity. In addition, the relationships between domains and phase boundaries should be established; then the piezoelectric activity can be modified by varying the domain structure. 7.3. Physical Origin of High Piezoelectricity

It is very important to understand the physical origin of high piezoelectricity in order to investigate the relationships between the phase boundaries, phase compositions, and piezoelectric properties of KNN materials. The different phase boundaries can be constructed by doping additives, then the phase AA


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compositions concerning phase boundaries can be optimized by refining the types and concentration of the additives, and finally the piezoelectric activity can be enhanced by modifying the phase compositions.

Jiagang Wu received his B.S. degree from Sichuan University in 2003 and Ph.D. degree from Sichuan University in 2008 and worked as a Singapore Millennium Postdoctoral Fellow (SMF-PDF) at the National University of Singapore from 2008 to 2010 (2 years). He has been an Associate Professor at the Department of Materials Science, Sichuan University, since 2011. His main research interest is composition design and property modification of ferroelectric/ piezoelectric/multiferroic materials.

7.4. Intrinsic Characteristics of Phase Boundaries

The intrinsic differences between different phase boundary types (e.g., O−T, R−O, R−T, etc.) and the intrinsic characteristics of MPB and PPT were studied by investigating the temperature-dependent domain structure. 7.5. Investigating the Stability of Piezoelectricity

The greatest challenge for KNN-based materials is how to improve the stability of piezoelectricity. In the past, there was conflict between piezoelectricity and its stability, that is, if a large d33 induced by PPT phase boundaries shows strong temperature dependence. As a result, phase boundaries with MPB characteristics may be an effective way to solve this issue. It is still an open question as to whether KNN-based materials possess phase boundaries with MPB characteristics. 7.6. Other Issues

Potassium−sodium niobate lead-free piezoceramics will be a promising lead-free materials for actuator applications. It was wondered whether a large strain can be induced by phase transitions. As a result, it may be very interesting to investigate the relationships between phase transitions and strains of potassium−sodium niobate materials.

Dingquan Xiao received his B.S. degree from Sichuan University in 1968 and M.S. degree from Sichuan University in 1980, studied at the Queen Mary University of London from 1980 to 1982, and worked a senior visiting scholar at the University of Pennsylvania in 1990 and the University of Houston in 1998. He has worked at Sichuan University since 1983. Currently, he is a Professor in the College of Materials Science and Engineering of Sichuan University. His main research interests include ferroelectric/piezoelectric ceramics, dielectric materials, ferroelectric thin films, ecomaterials, and electrochemical techniques. He is a member of the Chinese Materials Research Society and the Chinese Physics Society.

8. CONCLUSIONS High-performance lead-free piezoceramics have become an international research frontier in the fields of high technology and new materials. In this review, we systematically reviewed the developments of phase boundaries as well as the piezoelectric properties of KNN-based materials, and some suggestions for the future development of KNN materials were also addressed. In the past decade, piezoelectric properties have been greatly increased by constructing phase boundaries, and piezoelectricity comparable to those of PZT-based materials has been achieved. As a result, we believe that KNN-based lead-free ceramics will gain wide practical application in the near future. AUTHOR INFORMATION Corresponding Author

*E-mail:[email protected] and [email protected]. Notes

The authors declare no competing financial interest. Biographies Jianguo Zhu received his M.S. degree from Sichuan University in 1987 and Ph.D. degree in materials science from Sichuan University in 1998. He has worked at Sichuan University since 1987. Currently, he is Professor and Dean of the College of Materials Science and Engineering of Sichuan University. His research activities are ferroelectric/piezoelectric ceramics, dielectric materials, and ferroelectric thin films. He is a member of the Chinese Materials Research Society and the Sichuan Silicate Society.

ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Science Foundation of China (NSFC Nos. 51102173, 51272164, 51332003, and 51472169), the Fundamental Research Funds for the Central Universities (2012SCU04A01), introduction of AB


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talent starting funds of Sichuan University (2082204144033), and the College of Materials Science and Engineering of Sichuan University. We are also thankful to a number of researchers who provided help during our research: Xiaojie Lou and Xiangjian Wang (Xi’an Jiaotong University), Hui Wang (Sichuan University), Ruishi Xie (Southwest University of Science and Technology) and Hao Tian (Harbin Institute of Technology). Finally, we would like to sincerely thank the reviewers for valuable suggestions.

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AK


Diaschisis: past, present, future.

Marchantia: Past, Present and Future.

Death: past, present, and future.

Integrons: past, present, and future.

Landfilling, past, present and future.

Macrophages: past, present and future.

Ribavirin: Past, present and future.

Psychosomatics: past, present and future.

Tonography: past, present, and future.

ALPPS: past, present and future.

Heparin: Past, Present, and Future.

Endocytosis: Past, present, and future.

Neurofibromatosis: past, present, and future.

Hysteroscopy: Past, Present and Future.

Behavior genetics: past, present, future.

Past, present and future of hirudin.

Surgery of hydrocephalus: past, present and future.

Total laryngectomy - past, present, future.

Interstitial brachytherapy: past-present-future.

Retinopathy of prematurity: Past, present and future.

Diet and atherosclerosis: past, present and future.

Vaccines and pregnancy: past, present, and future.

Postburn Hypermetabolism: Past, Present, and Future.

Hairy cell leukemia: Past, present and future.