(K, Na)NbO<sub>3</sub>-based lead-free piezoceramics: Phase transition, sintering and property enhancement

doi:10.1007/s40145-012-0003-3

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Abstract

Most widely used piezoelectric ceramics are based on Pb(Zr,Ti)O₃ (PZT) composition which has adverse environmental and health effects due to its high lead content. Environmental and safety concerns with respect to the utilization, recycling, and disposal of lead-based piezoelectric ceramics have induced a new surge in developing lead-free piezoelectric ceramics. Among all the lead-free ceramics, (K,Na)NbO₃ (KNN) has drawn increasing attention because of its well-balanced piezoelectric properties and better environmental compatibility. On basis of the author's work, this review summarizes the progress that has been made in recent years on development of KNN-based piezoelectric ceramics, including crystallographic structure and phase transition analysis, pressurized solid-state sintering as well as liquid-phase-assisted sintering process, and poling treatment for property enhancement. All in all, KNN is a promising lead-free system, but more research is still required both from academic and industrial interests.

Key words： electrical properties ferroelectric properties piezoelectric properties perovskites niobates lead-free

Received: 16 February 2012 Published: 27 October 2015

Corresponding Authors: Jing-Feng LI

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Cite this article:

Ke WANG,Jing-Feng LI. (K, Na)NbO₃-based lead-free piezoceramics: Phase transition, sintering and property enhancement[J]. Journal of Advanced Ceramics, 2012, 1(1): 24-37.

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http://jac.tsinghuajournals.com/EN/10.1007/s40145-012-0003-3 OR http://jac.tsinghuajournals.com/EN/Y2012/V1/I1/24

67]. ? 2007, American Institute of Physics)">

The structure illustration for KNN of orthorhombic structure at room temperature (a) the perovskite type ABO₃ subcell; (b) the projection of the subcell along b axis; (c) four adjacent subcell projections in an exaggerated manner to show the geometrical relationship, while omitting Nb and O atoms. (Reprinted with permission from Ref. [67]. ? 2007, American Institute of Physics)

The X-ray diffraction patterns of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics

The X-ray diffraction patterns of N = 12 and N = 16 groups in (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics

SEM micrographs of (a) SPS and (b) normally sintered (K_0.5Na_0.5)NbO₃ ceramics

Density and piezoelectric coefficient d₃₃ of KNN, LKNN and LKNNT samples as a function of sintering temperature

Weight loss of alkaline elements in KNN-based ceramics as a function of sintering temperature

Shrinkage curve and SEM micrographs of 0.92(Na_0.535K_0.48)NbO₃-0.08LiNbO₃

Properties and phase structures of 0.92(Na_0.535K_0.48)NbO₃-0.083LiNbO₃ samples sintered at different temperatures

Piezoelectric properties and tetragonal-orthorhombic phase transition temperature (T_O-T) of (1-x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ after both the first and second poling

Patterns of (222) and (004) lattice plane series for the sample 0.92(Na_0.535K_0.48)NbO₃-0.08LiNbO₃, both before and after the first poling, as well as after the second poling, respectively. The P_s evolution inside domains is listed aside, assuming that the electric field during poling is parallel to <001> direction

[1]	Jaffe B, Cook WR, Jaffe H. Piezoelectric Ceramics. New York: Academic Press, 1971.
[2]	Eu-Directive 2002/96/Ec: Waste electrical and electronic equipment (WEEE). Off J Eur Union 2003, 46: 24-38.
[3]	Eu-Directive 2002/95/Ec: Restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS). Off J Eur Union 2003, 46: 19-23.
[4]	Saito Y, Takao H, Tani T, et al. Lead-free piezoceramics. Nature 2004, 432: 84-87.
[5]	Guo YP, Kakimoto K, Ohsato H. Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3-LiNbO3 ceramics. Appl Phys Lett 2004, 85: 4121-4123.
[6]	Zhao P, Zhang BP, Li JF. High piezoelectric d33 coefficient in Li-modified lead-free (Na,K)NbO3 ceramics sintered at optimal temperature. Appl Phys Lett 2007, 90: 242909.
[7]	Hollenstein E, Davis M, Damjanovic D, et al. Piezoelectric properties of Li- and Ta-modified (K0.5Na0.5)NbO3 ceramics. Appl Phys Lett 2005, 87: 182905.
[8]	Matsubara M, Yamaguchi T, Kikuta K, et al. Effect of Li substitution on the piezoelectric properties of potassium sodium niobate ceramics. Jpn J Appl Phys 2005, 44: 6136-6142.
[9]	Du HL, Tang FS, Liu DJ, et al. The microstructure and ferroelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics. Mater Sci Eng B-Solid State Mater Adv Technol 2007, 136: 165-169.
[10]	Klein N, Hollenstein E, Damjanovic D, et al. A study of the phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric measurements, and raman spectroscopy. J Appl Phys 2007, 102: 014112.
[11]	Song HC, Cho KH, Park HY, et al. Microstructure and piezoelectric properties of (1-x)(Na0.5K0.5)NbO3-LiNbO3 ceramics. J Am Ceram Soc 2007, 90: 1812-1816.
[12]	Wang K, Li J F, Liu N. Piezoelectric properties of low-temperature sintered Li-modified (Na, K)NbO3 lead-free ceramics. Appl Phys Lett 2008, 93: 092904.
[13]	Guo YP, Kakimoto K, Ohsato H. (Na0.5K0.5)NbO3-TaO3 lead-free piezoelectric ceramics. Mater Lett 2005, 59: 241-244.
[14]	Zhao P, Zhang BP, Li JF. Enhancing piezoelectric d33 coefficient in Li/Ta-codoped lead-free (Na,K)NbO3 ceramics by compensating Na and K at a fixed ratio. Appl Phys Lett 2007, 91: 172901.
[15]	Dai YJ, Zhang XW, Zhou GY. Phase transitional behavior in K0.5Na0.5NbO3-LiTaO3 ceramics. Appl Phys Lett 2007, 90: 262903.
[16]	Kim MS, Jeong SJ, Song JS. Microstructures and piezoelectric properties in the Li2O-excess 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 cramics. J Am Ceram Soc 2007, 90: 3338-3340.
[17]	Lin D, Kwok KW, Chan HLW. Microstructure, phase transition, and electrical properties of (K0.5Na0.5)1-xLix(Nb1-yTay)O3 lead-free piezoelectric ceramics. J Appl Phys 2007, 102: 034102.
[18]	Chang YF, Yang ZP, Ma DF, et al. Phase transitional behavior, microstructure, and electrical properties in Ta-modified [(K0.458Na0.542)0.96Li0.04]NbO3 lead-free piezoelectric ceramics. J Appl Phys 2008, 104: 024109.
[19]	Wu L, Zhang JL, Shao SF, et al. Phase coexistence and high piezoelectric properties in (K0.40Na0.60)0.96Li0.04Nb0.80Ta0.20O3 ceramics. J Phys D: Appl Phys 2008, 41: 035402.
[20]	Zuo RZ, Ye C, Fang XS, et al. Processing and piezoelectric poperties of (Na0.5K0.5)0.96Li0.04(Ta0.1Nb0.9)1-xCuxO3-3x/2 lead-free ceramics. J Am Ceram Soc 2008, 91: 914-917.
[21]	Zang GZ, Wang JF, Chen HC, et al. Perovskite (Na0.5K0.5)1-x(LiSb)xNb1-xO3 lead-free piezoceramics. Appl Phys Lett 2006, 88: 212908.
[22]	Zhang SJ, Xia R, Shrout TR, et al. Piezoelectric properties in provskite 0.948(K0.5Na0.5)NbO3-.052LiSbO3 lead-free ceramics. J Appl Phys 2006, 100: 104108.
[23]	Wu JG, Xiao DQ, Wang YY, et al. Effects of K content on the dielectric, piezoelectric, and ferroelectric properties of 0.95(KxNa1-x)NbO3-.05LiSbO3 lead-free ceramics. J Appl Phys 2008, 103: 024102.
[24]	Park HY, Ahn CW, Song HC, et al. Microstructure and piezoelectric poperties of 0.95(Na0.5K0.5)NbO3-0.05BaTiO3 ceramics. Appl Phys Lett 2006, 89: 062906.
[25]	Lin D, Kwok KW, Chan HLW. Structure, dielectric, and piezoelectric properties of CuO-doped K0.5Na0.5NbO3-BaTiO3 lead-free ceramics. J Appl Phys 2007, 102: 074113.
[26]	Park HY, Cho KH, Paik DS, et al. Microstructure and piezoelectric properties of lead-free (1-x)(Na0.5K0.5)NbO3-xCaTiO3 ceramics. J Appl Phys 2007, 102: 124101.
[27]	Zhang SJ, Xia R, Shrout TR. Modified (K0.5Na0.5)NbO3 based lead-free piezoelectrics with broad temperature usage range. Appl Phys Lett 2007, 91: 132913.
[28]	Chang RC, Chu SY, Lin YF, et al. The effects of sintering temperature on the properties of (Na0.5K0.5)NbO3-CaTiO3 based lead-free ceramics. Sensor Actuat A-Phys 2007, 138: 355-360.
[29]	Chang RC, Chu SY, Lin YF, et al. An investigation of (Na0.5K0.5)NbO3-CaTiO3 based lead-free ceramics and surface acoustic wave devices. J Eur Ceram Soc 2007, 27: 4453-4460.
[30]	Wu JG, Xiao DQ, Wang YY, et al. Improved temperature stability of CaTiO3-modified [(K0.5Na0.5)0.96]Li0.04](Nb0.91Sb0.05Ta0.04)O3 lead-free piezoelectric ceramics. J Appl Phys 2008, 104: 024102.
[31]	Zuo RZ, Fang XS, Ye C. Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3-(Bi0.5Na0.5)TiO3 ceramics. Appl Phys Lett 2007, 90: 092904.
[32]	Zuo RZ, Fang XS, Ye C, et al. Phase transitional behavior and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-(Bi0.5K0.5)TiO3 ceramics. J Am Ceram Soc 2007, 90: 2424-2428.
[33]	Zuo RZ, Ye C. Structures and piezoelectric properties of (NaKLi)1-x(BiNaBa)xNb1-xTixO3 lead-free ceramics. Appl Phys Lett 2007, 91: 062916.
[34]	Du HL, Zhou WC, Luo F, et al. Structure and electrical properties' investigation of (K0.5Na0.5)NbO3-(Bi0.5Na0.5)TiO3 lead-free piezo electric ceramics. J Phys D: Appl Phys 2008, 41: 085416.
[35]	Du HL, Zhou WC, Luo F, et al. Polymorphic phase transition dependence of piezoelectric properties in (K0.5Na0.5)NbO3-(Bi0.5K0.5)TiO3 lead-free ceramics. J Phys D: Appl Phys 2008, 41: 115413.
[36]	Chang YF, Yang ZP, Hou YT, et al. Effects of Li content on the phase structure and electrical properties of lead-free (K0.46-x/2Na0.54-x/2Lix)-(Nb0.76Ta0.20Sb0.04)O3 ceramics. Appl Phys Lett 2007, 90: 232905.
[37]	Chang YF, Yang ZP, Wei LL. Microstructure, density, and dielectric properties of lead-free (K0.44Na0.52Li0.04) (Nb0.96-xTaxSb0.04)O3 piezoelectric ceramics. J Am Ceram Soc 2007, 90: 1656-1658.
[38]	Hagh NM, Jadidian B, Safari A. Property-processing relationship in lead-free (K,Na,Li)NbO3 solid solution system. J Electroceram 2007, 18: 339-346.
[39]	Lin DM, Kwok KW, Chan HLW. Phase structures and electrical properties of K0.5Na0.5(Nb0.925Ta0.075)O3-LiSbO3 lead-free piezoelectric ceramics. J Phys D: Appl Phys 2007, 40: 6060-6065.
[40]	Lin DM, Kwok KW, Chan HLW. Effects of BaO on the structure and electrical properties of 0.95K0.5Na0.5(Nb0.94Sb0.06)O3-0.05LiTaO3 lead-free ceramics. J Phys D: Appl Phys 2007, 40: 6778-6783.
[41]	Qi P, Wang JF, Ming BQ, et al. Phase transition and high piezoactivity of Sb doped (Na0.53K0.435Li0.035) Nb0.94Ta0.06O3 lead-free ceramics. Chinese Phys Lett 2007, 24: 3535-3538.
[42]	Rubio-Marcos F, Ochoa P, Fernandez JF. Sintering and properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 ceramics. J Eur Ceram Soc 2007, 27: 4125-4129.
[43]	Wu JG, Wang YY, Xiao DQ, et al. Effects of Ag content on the phase structure and piezoelectric properties of (K0.44-xNa0.52Li0.04Agx)-(Nb0.91Ta0.05Sb0.04)O3 lead-free ceramics. Appl Phys Lett 2007, 91: 132914.
[44]	Wu JG, Xiao DQ, Wang YY, et al. Effects of K/Na ratio on the phase structure and electrical properties of (KxNa0.96-xLi0.04)(Nb0.91Ta0.05Sb0.04)O3 lead-free ceramics. Appl Phys Lett 2007, 91: 252907.
[45]	Yang ZP, Chang YF, Wei LL. Phase transitional behavior and electrical properties of lead-free (K0.44Na0.52Li0.04)(Nb0.96-xTaxSb0.04)O3 piezoelectric ceramics. Appl Phys Lett 2007, 90: 042911.
[46]	Akdogan EK, Kerman K, Abazari M, et al. Origin of high piezoelectric activity in ferroelectric (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 ceramics. Appl Phys Lett 2008, 92: 112908.
[47]	Juan D, Wang JF, Zang GZ, et al. (Na0.52K0.44Li0.04)Nb0.9-xSbxTa0.1O3 lead-free piezoelectric ceramics with high performance and high Curie temperature. Chinese Phys Lett 2008, 25: 1446-1448.
[48]	Lin DM, Kwok KW, Lam KH, et al. Phase structure and electrical properties of K0.5Na0.5(Nb0.94Sb0.06)O3-LiTaO3 lead-free piezoelectric ceramics. J Phys D: Appl Phys 2008, 41: 052002.
[49]	Wu JG, Peng T, Wang YY, et al. Phase structure and electrical properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 lead-free piezoelectric ceramics. J Am Ceram Soc 2008, 91: 319-321.
[50]	Zhao P, Zhang BP, Tu R, et al. High piezoelectric d33 coefficient in Li/Ta/Sb-codoped lead-free (Na,K)NbO3 ceramics sintered at optimal temperature. J Am Ceram Soc 2008, 91: 3078-3081.
[51]	Zuo RZ, Xu ZK, Li LT. Dielectric and piezoelectric properties of Fe2O3-doped (Na0.5K0.5)0.96Li0.04Nb0.86-Ta0.1Sb0.04O3 lead-free ceramics. J Phys Chem Solids 2008, 69: 1728-1732.
[52]	Wang K, Li JF. Domain engineering of lead-free Li-modified (K,Na)NbO3 polycrystals with highly enhanced piezoelectricity. Adv Funct Mater 2010, 20: 1924-1929.
[53]	Wang K, Li JF, Zhou JJ. High normalized strain obtained in Li-modified (K,Na)NbO3 lead-free piezoceramics. Appl Phys Express 2011, 4: 061501.
[54]	Zuo RZ, Fu J. Rhombohedral-tetragonal phase coexistence and piezoelectric properties of (NaK)(NbSb)O3-LiTaO3-BaZrO3 lead-free ceramics. J Am Ceram Soc 2011, 94: 1467-1470.
[55]	Shrout TR, Zhang SJ. Lead-free piezoelectric ceramics: alternatives for PZT? J Electroceram 2007, 19: 111-124.
[56]	Zhang SJ, Xia R, Shrout TR. Lead-free piezoelectric ceramics vs. PZT? J Electroceram 2007, 19: 251-257.
[57]	Takenaka T, Nagata H, Hiruma Y, et al. Lead-free piezoelectric ceramics based on perovskite structures. J Electroceram 2007, 19: 259-265.
[58]	R?del J, Jo W, Seifert KTP, et al. Perspective on the development of lead-free piezoceramics. J Am Ceram Soc 2009, 92: 1153-1177.
[59]	Panda PK. Review: Environmental friendly lead-free piezoelectric materials. J Mater Sci 2009, 44: 5049-5062.
[60]	Safari A, Abazari M. Lead-free piezoelectric ceramics and thin films. IEEE Trans Ultrason Ferroelectr Freq Control 2010, 57: 2165-2176.
[61]	Tennery VJ, Hang KW. Thermal and X-ray diffraction studies of NaNbO3-NbO3 system. J Appl Phys 1968, 39: 4749-4753.
[62]	Ahtee M, Hewat AW. Structural phase-transitions in sodium-potassium niobate solid-solutions by neutron powder diffraction. Acta Crystallogr A 1978, 34: 309-317.
[63]	Egerton L, Dillon DM. Piezoelectric and dielectric properties of ceramics in the system potassium sodium niobate. J Am Ceram Soc 1959, 42: 438-442.
[64]	Dai YJ, Zhang XW, Chen KP. Morphotropic phase boundary and electrical properties of K1-xNaxNbO3 lead-free ceramics. Appl Phys Lett 2009, 94: 042905.
[65]	Zhou JJ, Li JF, Wang K, et al. Phase structure and electrical properties of (Li,Ta)-doped (K,Na)NbO3 lead-free piezoceramics in the vicinity of Na/K = 50/50. J Mater Sci 2011, 46: 5111-5116.
[66]	Trodahl HJ, Klein N, Damjanovic D, et al. Raman spectroscopy of (K,Na)NbO3 and (K,Na)1-xLixNbO3. Appl Phys Lett 2008, 93: 3.
[67]	Wang K, Li JF. Analysis of crystallographic evolution in (Na,K)NbO3-based lead-free piezoceramics by X-ray diffraction. Appl Phys Lett 2007, 91: 262902.
[68]	Ahtee M, Glazer AM. Lattice-parameters and tilted octahedra in sodium-potassium niobate solid-solutions. Acta Crystallogr A 1976, 32: 434-446.
[69]	Shirane G, Newnham R, Pepinsky R. Dielectric properties and phase transitions of NaNbO3 and (Na,K)NbO3. Physical Review 1954, 96: 581-588.
[70]	Hewat AW. Cubic-tetragonal-orthorhombic-rhombohedral ferroelectric transitions in perovskite potassium niobate - neutron powder profile refine ment of structures. J Phys C: Solid State Phys 1973, 6: 2559-2572.
[71]	Kakimoto K, Akao K, Guo YP, et al. Raman scattering study of piezoelectric (Na0.5K0.5)NbO3-LiNbO3 ceramics. Jpn J Appl Phys 2005, 44: 7064-7067.
[72]	Kano J, Sasanuma K, Tsukada S, et al. Piezoelectric (Na0.5K0.5)NbO3-SrTiO3 ceramics in the tetragonal-orthorhombic phase boundary studied by Raman spectroscopy. Ferroelectrics 2007, 347: 55-59.
[73]	Zhou JJ, Li JF, Zhang XW. Orthorhombic to tetragonal phase transition due to stress release in (Li,Ta)-doped (K,Na)NbO3 lead-free piezoceramics. J Eur Ceram Soc 2012, 32: 267-270.
[74]	Liu W, Ren X. Large piezoelectric effect in Pb-free ceramics. Phys Rev Lett 2009, 103: 257602.
[75]	Xue D, Zhou Y, Bao H, et al. Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics. Appl Phys Lett 2011, 99: 122901.
[76]	Jaeger RE, Egerton L. Hot pressing of potassium-sodium niobates. J Am Ceram Soc 1962, 45: 209-213.
[77]	Haertling GH. Properties of hot-pressed ferroelectric alkali niobate ceramics. J Am Ceram Soc 1967, 50: 329-330.
[78]	Li JF, Wang K, Zhang BP, et al. Ferroelectric and piezoelectric properties of fine-grained Na0.5K0.5NbO3 lead-free piezoelectric ceramics prepared by spark plasma sintering. J Am Ceram Soc 2006, 89: 706-709.
[79]	Zhang BP, Li JF, Wang K, et al. Compositional dependence of piezoelectric properties in NaxK1-xNbO3 lerad-free ceramics prepared by spark plasma sintering. J Am Ceram Soc 2006, 89: 1605-1609.
[80]	Wang K, Zhang BP, Li JF, et al. Lead-free Na0.5K0.5NbO3 piezoelectric ceramics fabricated by spark plasma sintering: Annealing effect on electrical properties. J Electroceram 2008, 21: 251-254.
[81]	Shen ZY, Li JF, Wang K, Electrical and mechanical properties of fine-grained Li/Ta-modified (Na,K)NbO3-based piezoceramics prepared by spark plasma sintering. J Am Ceram Soc 2010, 93: 1378-1383.
[82]	Liu N, Wang K, Li JF, et al. Hydrothermal synthesis and spark plasma sintering of (K,Na)NbO3 lead-free piezoceramics. J Am Ceram Soc 2009, 92: 1884-1887.
[83]	Zhen Y, Li JF, Wang K, et al. Spark plasma sintering of Li/Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics: Post-annealing temperature effect on phase structure, electrical properties and grain growth behavior. Mater Sci Eng B-Adv Funct Solid-State Mater 2011, 176: 1110-1114.
[84]	Arlt G, Hennings D, Dewith G. Dielectric-properties of fine-grained barium-titanate ceramics. J Appl Phys 1985, 58: 1619-1625.
[85]	Zhen YH, Li JF. Normal sintering of (K,Na)NbO3-based ceramics: influence of sintering temperature on densification, microstructure, and electrical properties. J Am Ceram Soc 2006, 89: 3669-3675.
[86]	Li JF, Zhen YH, Zhang BP, et al. Normal sintering of (K,Na)NbO3-based lead-free piezoelectric ceramics. Ceram Int 2008, 34: 783-786.
[87]	Shen ZY, Zhen YH, Wang K, et al. Influence of sintering temperature on grain growth and phase structure of compositionally optimized high-performance Li/Ta-modified (Na,K)NbO3 ceramics. J Am Ceram Soc 2009, 92: 1748-1752.
[88]	Shen ZY, Wang K, Li JF. Combined effects of Li content and sintering temperature on polymorphic phase boundary and electrical properties of Li/Ta Co-doped (Na, K)NbO3 lead-free piezoceramics. Appl Phys A 2009, 97: 911-917.
[89]	Zhen YH, Li JF. Abnormal grain growth and new core-shell structure in (K,Na)NbO3-based lead-free piezoelectric ceramics. J Am Ceram Soc 2007, 90: 3496-3502.
[90]	Wang K, Li JF. Low-temperature sintering of Li-modified (K,Na)NbO3 lead-free ceramics: Sintering behavior, microstructure, and electrical properties. J Am Ceram Soc 2010, 93: 1101-1107.
[91]	Kingery WD. Densification during sintering in the presence of a liquid phase. I. theory. J Appl Phys 1959, 30: 301-306.
[92]	Yoo J, Lee K, Chung K, et al. Piezoelectric and dielectric properties of (LiNaK)(NbTaSb)O3 ceramics with variation in poling temperature. Jpn J Appl Phys 2006, 45: 7444-7448.
[93]	Du HL, Tang FS, Luo F, et al. Effect of poling condition on piezoelectric properties of (K0.5Na0.5) NbO3-LiNbO3 lead-free piezoelectric ceramics. Mater Sci Eng B-Solid State Mater Adv Technol 2007, 137: 175-179.
[94]	Du HL, Zhou WC, Luo F, et al. An approach to further improve piezoelectric properties of (K0.5Na0.5)NbO3-based lead-free ceramics. Appl Phys Lett 2007, 91: 202907.
[95]	Ogawa T, Furukawa M, Tsukada T. Poling field dependence of piezoelectric properties and hysteresis loops of polarization versus electric field in alkali niobate ceramics. Jpn J Appl Phys 2009, 48.
[96]	Rubio-Marcos F, Romero JJ, Ochoa DA, et al. Effects of poling process on KNN-modified piezoceramic properties. J Am Ceram Soc 2010, 93: 318-321.
[97]	Morozov MI, Kungl H, Hoffmann MJ. Effects of poling over the orthorhombic-tetragonal phase transition temperature in compositionally homogeneous (K,Na)NbO3-based ceramics. Appl Phys Lett 2011, 98.
[98]	Zuo RZ, Fu J, Lv DY. Phase transformation and tunable piezoelectric properties of lead-free (Na0.52K0.48-xlix)(Nb1-x-ySbyTax)O3 system. J Am Ceram Soc 2009, 92: 283-285.
[99]	Gao Y, Zhang J, Qing Y, et al. Remarkably strong piezoelectricity of lead-free (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 ceramic. J Am Ceram Soc 2011, 94: 2968-2973.
[100]	Safari A, Akdogan EK. Piezoelectric and Acoustic Materials for Transducer Applications. New York: Springer, 2008.
[101]	Zhen YH, Li JF, Wang K. Fabrication and electrical properties of fine-scale 1-3 piezoceramic/ epoxy composites using (K,Na)NbO3-based lead-free ceramics. Ferroelectrics 2007, 358: 1043-1050.
[102]	Shen ZY, Xu Y, Li JF. Fabrication and electromechanical properties of microscale 1-3-type piezoelectric composites using (Na,K)NbO3-based Pb-free piezoceramics. J Appl Phys 2009, 105.
[103]	Shen ZY, Li JF, Chen R, et al. Microscale 1-3-type (Na,K)NbO3-based Pb-free piezocomposites for high-frequency ultrasonic transducer applications. J Am Ceram Soc 2011, 94: 1346-1349.
[104]	Shung KK, Cannata JM, Zhou QF. Piezoelectric materials for high frequency medical imaging applications: a review. J Electroceram 2007, 19: 139-145.
[105]	Guo M, Lam KH, Lin DM, et al. A rosen-type piezoelectric transformer employing lead-free K0.5Na0.5NbO3 ceramics. J Mater Sci 2008, 43: 709-714.
[106]	Hagh NM, Jadidian B, Ashbahian E, et al. Lead-free piezoelectric ceramic transducer in the donordoped K1/2Na1/2NbO3 solid solution system. IEEE Trans Ultrason Ferroelectr Freq Control 2008, 55: 214-224.
[107]	Kwok KW, Hon SF, Lin D. Lead-free self-focused piezoelectric transducers for viscous liquid ejection. Sensor Actuat a-Phys 2011, 168: 168-171.