s2010135x11000409

A REVIEW ON LEAD-FREE PIEZOELECTRICCERAMICS STUDIES IN CHINA

YI-QING LU and YONG-XIANG LI*

The Key Lab of Inorganic Functional Materials and DevicesShanghai Institute of Ceramics, Chinese Academy of Sciences

1295 Dingxi Road, Shanghai 200050, P. R. China†[email protected]

Received 3 May 2011Revised 27 May 2011

There are a large number of research publications on the hot topic of environmental friendly lead-free piezoelectric materials worldwide in the last decade. The number of researchers and insti-tutions involved from China is much larger than other countries or regions. The publications byChinese researchers cover a broad spectrum on the preparations, structures, properties andapplications of lead-free piezoelectric ceramics. This has motivated us to come out with a reviewon recent advances in development of lead-free piezoelectric ceramics in China. The emphases areespecially on the preparation and electric properties of barium titanate-based materials, bismuthsodium titanate and related materials, alkaline niobate and related materials, bismuth layer-structured materials, as well as texture engineering of ceramics and some of their single crystals.Hopefully, this could give further impetus to the researchers to continue their e®orts in thispromising area and also draw the attentions from legislature, research o±ce, industrial andpublics.

Keywords: Lead-free; piezoelectric ceramic; barium titanate; bismuth sodium titanate; potassiumsodium niobate; bismuth layer-structured ferroelectrics; texture; single crystal.

1. Introduction

From the early 1960s, Prof. Zhi-Wen Yin, who wasthe founder of the functional ceramics center ofShanghai Institute of Ceramics, Chinese Academy ofSciences, China, headed a group with more than 50researchers studied BaTiO3 and PbZrTiO3 piezo-electric ceramics and the applications. Variousdevices were developed such as sensors, transducersand actuators. The technology was transferred, andhundreds of piezoelectric components and ultrasonicequipment companies were established across China.

In 1973, Yan-Yi Guo et al. studied tungsten bronzestructure barium sodium niobate (Ba2�xSrxNaNb5O15,

BNN) ceramics, and three di®erent frequencies of465kHz, 915kHz and 1.5MHz narrow band ¯lters weredeveloped and commercialized.

In 1977, Da-ren Cheng et al. studied bismuthlayer-structured ferroelectric (BLSF) Bi4MTi4O15

(M ¼ Ca, Sr, Ba, Pb) ceramics, and high tempera-ture vibration transducers used at 350�C weredeveloped, which were used in the aeronautics andastronautics industry of China.

The above nonperovskite structure piezoelectricmaterials studies and applications were mainlybecause BNN ceramics have excellent propertieswith high frequency constant (Np ¼ 3650Hz�m),

*Corresponding author.

Journal of Advanced DielectricsVol. 1, No. 3 (2011) 269�288© World Scienti¯c Publishing CompanyDOI: 10.1142/S2010135X11000409

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high mechanical quality factor (Qm ¼ 2000), lowtemperature coe±cient of resonant frequency(35� 10�6, �55�C to 85�C) and BLSF ceramicshave high d33 (>25 pC/N) and high Curie tem-peratures (TC > 650�C). Unfortunately, these earlystudies were only archived as internal technicalreports or published in Chinese journals.

From 1978, Tian-bao Wang et al. initiated theinvestigations on the perovskite structure lead-free pie-zoelectric ceramics, ð1�xÞBi0:5Na0:5TiO3�xBaTiO3

(BNT�BT), ð1�xÞBi0:5Na0:5TiO3�xBi0:5K0:5TiO3

(BNT�BKT), which were the ¯rst attempt to¯nd substitutions of PZT ceramics in China.1�3 Theyfound that good piezoelectric properties forBNT�BT system on the MPB region of x ¼ 6%, forBNT�BKT system of x ¼ 19%. The optimizedcompositions of two systems named E6C7 andBNNE�A were used for ultrasonic transducers formedical imaging, which showed great potentials.

At the 9th International Meeting on Ferroelec-tricity (August 24�29, 1997, Seoul, Korea), Prof.Ding-Quan Xiao was aware of the demands of eco-materials or environmental friendly materials, andpointed out that \ferroelectric materials shouldcomprise non-hazardous substances with a smallenvironmental load, and the manufacturing pro-cesses for these materials should also be with a smallenvironmental load," and \the research on lead-freepiezoelectric ceramics is a typical example."4 Sincethen, his group has paid great attention to theresearch of lead-free piezoelectric ceramics, especiallyconcentrated on perovskite structure lead-free pie-zoelectric ceramics. For details, a review paper can befound elsewhere.5

Starting from new millennium, global warming,energy shortage and environment protection havebeen serious concerns. The European Union (EU) in2003 included lead (Pb) in its legislature to besubstituted as a hazardous substance. There aresimilar activities and regulations in the countries allover the world. In China, the EU legislations wereadopted since July 1, 2006.

The WEEE (Waste Electrical and ElectronicEquipment) and RoHS (Restriction of the Use ofCertain Hazardous Substances in Electrical andElectronic Equipment) directives spurred theresearch for lead-free piezoceramics, which hadstarted from 2000. It can be clearly seen from Fig. 1that the research on lead-free piezoceramics world-wide re°ected by the number of publications isincreasing steadily in last decade. Publications from

East Asia are predominant. Especially, more than45% papers are fromChina (Fig. 2). There are also 148patents related to \lead-free piezoelectric ceramic"(2000�2010) registered by State Intellectual PropertyO±ce of P. R. China, which are not included in thisreview. This is because there are large groups fromresearch institutions, and a large numbers researchprograms/projects funded from The National NatureScience Foundation of China (NSFC), TheMinistry ofSciences and Technology of China (MOST) through973-projects and 863-projects, many of provincialgovernments. More importantly, there are hundredssmall- and medium-sized enterprises in Yangtze River

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Fig. 1. Publications on lead-free piezoelectric ceramic over thelast decade (2000�2010) in refereed journals. Total number ofpapers is 1755. The search and statistics are conducted from thedatabase of \ISI Web of Science," SCI-EXPANDED, CPCI-S,CCR-EXPANDED and IC, in a period of time 2000�2010,using query of TS ¼ (piezoelectric SAME ceramics) NOTTS ¼ (Pb or lead or ¯nite element or PbTiO3* or PbZrO3* orPZT* or PMN* or PT or PbO* or PMS* or PZN* or PLSZT*or PIN* or xPb*) OR TS ¼ (piezoelectric SAME ceramics)AND TS ¼ (lead SAME free or Pb SAME free) NOT TS ¼(¯nite element).

45.87%

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Fig. 2. The percentage of publications on lead-free piezo-electric ceramics divided from countries and regions.

270 Y.-Q. Lu & Y.-X. Li

and Pearl River Delta economic zones which showtheir enthusiasm and demands for lead-free piezo-electric ceramics and devices. The import and exportof electronic products from these regions occupy adecisive position and in°uence in the internationalmarkets, which must meet the regulations of di®erentnationals.

There are many universities and laboratories inChina studying in the area of lead-free piezoelectricmaterials; the top 10 institutions are listed in Fig. 3which are ranked with their publications in theyear 2000�2010. The current review is based onthese publications, and organized into compositionalfamilies including: barium titanate-based materials,bismuth sodium titanate and related materials,alkaline niobate and related materials, and bismuthlayer-structured materials. In addition, researcheson the texture engineering and single crystals oflead-free piezoelectric materials are reviewed withspecial emphasis.

2. Barium Titanate-Based Materials

Barium titanate BaTiO3 (BT) is the ¯rst practicallyused piezoelectric ceramics and the prototypepolymorphic phase transition (PPT)-based high-performance piezoelectric material. However, thepoor piezoelectric properties (d33 ¼ 190 pC/N) andlow Curie temperature (TC ¼ 120�C) of BT ceramicshave been the main obstacle for their wider com-mercial application in actuators and sensors.

In order to improve the piezoelectric propertiesand temperature stability of BT ceramics, extensive

e®orts centered on compositional investigationshave been carried out. It has been discovered thatthe substitution of Ti4þ with quadrivalence ions(such as Zr4þ and Hf4þÞ can improve the propertiesof BT ceramics. Zheng et al. found that the piezo-electric temperature stability could be greatlyenhanced by a proper combination of partially sub-stituting Ti4þ with Zr4þ and adding a small amountof CuO additive.6 CuO-modi¯ed BaTi0:9625Zr0:0375O3

ceramics possess excellent piezoelectric properties ofd33 ¼ 300 pC/N, and its kp remains larger than 0.40in a broad temperature range from �43�C to 73�Cand is almost constant between �25�C and 55�C.Tian et al. reported that the three phase transitionsin BT are pinched by substituting Ti with Hf,and the Curie temperatures decrease as the Hfcontent increases in the compounds.7 A high piezo-electric parameter (d33 ¼ 305 pC/N) is obtained forBaHf0:05Ti0:95O3 composition between the orthor-hombic and rhombohedral phases.

In 2009, Liu et al. reported a high performancelead-free piezoelectric system Ba(Zr0:2Ti0:8ÞO3�x (Ba0:7Ca0:3ÞTiO3(BZT�xBCT) which shows asurprisingly high piezoelectric coe±cient d33 of620 pC/N at the morphotropic phase boundary(MPB) composition x ¼ 50.8 The phase diagram ofthis BZT�xBCT system and a few typical dielectricpermittivity (") versus temperature (T ) curves areshown in Fig. 4. BZT�xBCT shows a phase dia-gram similar to the lead-based systems like PZT andPMN�PT, that is, with a MPB starting from atriple point of a paraelectric cubic phase (C), ferro-electric rhombohedral (R) and tetragonal (T) phase.Subsequent investigation con¯rmed that the highpiezoelectric properties could be ascribed to the lowpolarization anisotropy as well as the elastic soft-ening at MPB.9 Their work gives a new insight intohow to design lead-free piezoelectric ceramics withhigh piezoelectricity.

However, the Curie temperature of BZT�50BCTis relatively low (93�C). By increasing the TC of bothtetragonal and rhombohedral terminals of the lead-free system BZT�xBCT, the modi¯ed system Ba(Zr0:15Ti0:85ÞO3�x(Ba0:8Ca0:2ÞTiO3 (BZ0:15T�xBC0:2T)with a higher TC of 114�C at x ¼ 53 was obtained.14

The phase diagram and temperature dependence ofdielectric permittivity of BZ0:15T�xBC0:2T systemare shown in Fig. 5. It was found that both d33 and "along MPB decreased with a deviation from thecubic�tetragonal�rhombohedral triple point.

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Fig. 3. The publications from top 10 research institutions inChina on lead-free piezoelectric ceramics.

A Review on Lead-Free Piezoelectric Ceramics Studies in China 271

Other reports also indicated that optimizingCa and Zr content is an e®ective way to enhancethe piezoelectric properties of (Ba, Ca)(Ti, Zr)O3

ceramics.10�13,15,16 The properties of some compo-sitionally modi¯ed BT ceramics are summarized inTable 1.

Recently, structural engineering techniques byseveral groups have led to enhanced piezoelectricproperties in BT ceramics. The structural engin-eering techniques, which including optimization ofgrain size and orientation as well as domain engin-eering, focuses on controlling the micro and/ornanoscale structure of a piezoelectric material.17

For BT ceramics, it is believed that there is also acritical grain size, below which the lattice changes

from tetragonal to cubic and ferroelectricity is lost.Wang et al. investigated the grain size e®ect on thenanometer scale of nanocrystalline BT ceramicsobtained by spark plasma sintering (SPS) and two-step sintering.18,19 Their results revealed that fer-roelectricity could remain in BT ceramics with agrain size as small as 8 nm in diameter.19

Shao et al. synthesized BT ceramics through theconventional solid-state reaction route starting fromordinary BaCO3 and TiO2 powders.20 They foundthat the d33 decreased from 419 pC/N to 185 pC/Nwhen its average grain size increased from 3.5 to9.5�m, while its average domain width remainedapproximately constant at around 240 nm. Theysuggested that the largeness of the domain wall is an

Fig. 4. (Color online) (a) Phase diagram of pseudo-binaryferroelectric system BZT�xBCT. (b)�(d) Dielectric permittiv-ity curves for 20BCT, 50BCT and 90BCT, respectively.8

Source: Reprinted with permission. Copyright 2009 by theAmerican Physical Society.

Table 1. Properties of some compositionally modi¯ed BT ceramics.

Compositions TC (�C) "r d33 (pC/N) kp (%) kt (%) k33 (%) k31 (%) References

BaTi0:9625Zr0:0375O3 þ CuO 1546 300 49.3 48.9 65.1 30.4 6BaTi0:95Hf0:05O3 109.2 305 57 7BaðZr0:2Ti0:8ÞO3�50ðBa0:7Ca0:3ÞTiO3 93 4050 620 53 42 65 31 8(Ba0:92Ca0:08ÞðTi0:95Zr0:05ÞO3 110 365 48.5 10ðBa0:84Ca0:16ÞðTi0:9Zr0:1ÞO3 4800 328 37.6 11ðBa0:99Ca0:01ÞðTi0:98Zr0:02ÞO3 115 375 44.1 12ðBa0:96Ca0:04ÞðTi0:95Zr0:05ÞO3 2070 338 36 13

Fig. 5. (Color online) (a) Phase diagram of BZ0:15T�xBC0:2Tsystem. (b)�(f) Temperature dependence of dielectric permit-tivity for BZ0:15T�xBC0:2T (x ¼ 15, 20, 40, 53, 90).14

Source: Reprinted with permission from IOP.

272 Y.-Q. Lu & Y.-X. Li

important factor, which signi¯cantly in°uences thepiezoelectric properties. In other works, ¯ne-grainBT ceramics with enhanced piezoelectric propertieswere prepared by SPS and microwave sinteringusing hydrothermally synthesized nanoscale par-ticles. Shen et al. found that d33 of BT ceramicsprepared by SPS showed great dependence on thesize of ferroelectric domains.21 Their results showedthat nanodomain smaller than 50 nm could beachieved in the case of large grain size (>10�m) bySPS (Fig. 6), suggesting that nano-domain structureis a dominant factor to in°uence the piezoelectricproperties of BT ceramics. The physical propertiesof some structurally engineered BT ceramics aresummarized in Table 2.

3. Bismuth Sodium Titanateand Related Materials

Bismuth sodium titanate (Bi0:5Na0:5)TiO3 (BNT)with a rhombohedral perovskite structure have beenstudied for a long time as a promising alternative tolead-based piezoelectrics. BNT shows strong ferro-electric properties of a relatively large remnantpolarization, Pr ¼ 38�C/cm2, and has a relatively

high Curie temperature, TC ¼ 320�C. However,BNT-based ceramics undergo another phase tran-sition below TC that is known as depolarization tem-perature (Td), which often occurs below 200�C.Whenthe depolarization takes place at this phase transition,the piezoelectric properties of these ceramics arereduced signi¯cantly. Consequently, Td is a moreimportant parameter than TC in the view of practicaluses. Another major challenge in the performance ofpure BNT piezoelectric ceramics exists because oftheir very large coercive ¯eld and high conductivity,which leads to the poor poling treatments and thusunderdeveloped piezoelectric performance.

BT and (Bi0:5K0:5)TiO3 (BKT) are well-knownlead-free piezoelectric materials with the tetragonalsymmetry. Among the BNT-based lead-free piezo-electric systems, the binary systems of BNT�BKTand BNT�BT have obtained the most extensiveinvestigation because they have good piezoelectricperformance near the rhombohedral�tetragonalMPB. In addition, BNT-based compositionsmodi¯edwith (Bi0:5Li0:5)TiO3, Ba(Cu0:5W0:5)O3, NaNbO3

and Er2O3 have been reported.23�27

Some of the BNT-based solid solutions and theirelectrical properties are presented in Table 3.

The study of BNT-based lead-free piezoelectricceramics in China began in 1978. Wang et al. studiedthe piezoelectric and ferroelectric properties ofxBNT�(1�x)BKT (0:73 � x � 1:0) and yBNT�(1�yÞBT (0:81 � y � 0:99) ceramic systems.1�3 Itwas found that the MPB between tetragonal andrhombohedral phases of the systems were located atthe compositions of x ¼ 0:81 and y ¼ 0:94, respect-ively. The Na-rich compositions near the boundariesof BNT-based systems with higher thickness couplingfactor kt, lower planar coupling factor kp, higher fre-quency constants Np and lower dielectric constants"T33 were obtained, as shown in Fig. 7. Ultrasonictransducers based on these materials were developed.

The development of BNT-based ceramics in thelast 10 years are summarized in the following threesections.

Fig. 6. TEM image for poled BT ceramics prepared by SPSusing hydrothermally synthesized 100 nm BaTiO3 powders.21

Source: Reprinted with permission from the Ceramic Society ofJapan.

Table 2. Physical properties of structurally engineered BT ceramics derived from di®erent preparation methods.

BaTiO3 � (g/cm3) "r tan� (%) d33 (pC/N) kp (%) References

Normal sintering (for comparison) 5.85 3088 3.3 193 27.3 21Spark plasma sintering 5.94 4301 2.9 416 41.5 21Normal sintering (¯ne grain) 5.79 3181 1.4 419 45.3 20Microwave sintering 1570 1.12 374 22


3.1. BNT�BKT system

Recent work by Yang et al. demonstrated the for-mation of an MPB in the BNT�BKT solid solution,in the range of 0.16 to 0.2 BKT. The d33 of 144 pC/N, kp of 0.29, and dielectric permittivity of 893 wereachieved for MPB composition.28

Zhang et al. investigated the in°uence of sinteringtemperature on the piezoelectric, dielectric and fer-roelectric properties of 0.5mol% Bi2O3-compensatedBNT�BKT near the composition of MPB.58 Itwas demonstrated that both sintered density andelectrical properties were sensitive to sinteringtemperature; particularly, the piezoelectric proper-ties deteriorated when the ceramics were sinteredabove the optimum temperature. The highest d33of 192 pC/N was obtained in the ceramics of

0.78BNT�0.22BKT sintered at 1150�C. On theother hand, the MPB of 1.0mol% Bi-compensatedBNT�BKT system exists in the range of 20�24mol%BKT.29 This MPB range slightly shifts to the BKTside as compared to previous results, probably due tothe comprehensive e®ects of Bi compensation. The1.0mol% Bi-compensated 0.77BNT�0.23BKT cer-amics showed the highest piezoelectric constant d33 of207pC/N.

In order to further enhance the properties, BNT�BKT compositions have been modi¯ed with CeO2,NaNbO3, KNbO3 (KN), BiFeO3 and BiCrO3.

30�33,59

Fan et al. investigated the MPB phase diagram ofBNT�BKT�KN ternary system (Fig. 8).31 Theaddition of KNmay improve the crystal symmetry ofthe ternary system and make the rhombohedral and

Table 3. Properties of some BNT-based solid solution systems.

Compositions "r tan � (%) d33 (pC/N) kp (%) kt (%) TC (�C) Td (�C) References

0.85BNT�0.15(Bi0:5Li0:5ÞTiO3 110 18 230.995BNT�0.005Ba(Cu0:5W0:5ÞO3 328 1.5 80 18.1 308 240.98BNT�0.02NaNO3 624 5.9 88 17.9 25

0.82BNT�0.18BKT 893 3.7 144 29 280.77BNT�0.23BKT þ 1 mol% Bi2O3 1261 5.1 207 29.8 330 290.88BNT�0.12BKT þ 0.2 wt% CeO2 132 27.8 300.82BNT�0.16BKT�0.02KN 1437 4.0 215 35 310.79BNT�0.18BKT�0.03BiFeO3 170 36.6 320.805BNT�0.18BKT�0.015BiCrO3 168 330.7BNT�0.2BKT�0.1(Bi0:5Li0:5ÞTiO3 1190 4.47 231 36.8 50.5 340.7BNT�0.2BKT�0.1(Bi0:5Li0:5ÞTiO3 þ Naþ 171 30.0 334 136 350.9(0.79BNT�0.21BKT)�0.1(Bi0:5Ag0:5ÞTiO3 1110 5.2 160 30 38 36

0.94BNT�0.06BT 826 2.5 155 36.7 288 105 370.94BNT�0.06BT þ 0.4 wt% CeO2 652 1.2 152 34 380.92BNT�0.08BT þ 0.3 wt% MnO 1596 0.8 153 36.4 390.93BNT�0.07BT þ 0.8 wt% CoO 0.8 137 23 400.94BNT�0.06BT þ 0.5 mol% Co2O3 1200 2.3 139 27 46 410.985(0.94BNT�0.06BT)�0.0075La2O5 1520 4.2 133 29 40 270 420.93BNT�0.07BT þ 0.16 wt% In2O3 1046 3.6 205 50.3 430.92BNT�0.08BT þ Nb2O5 1230 3.9 149 250 70 440.995(0.94BNT�0.06BT)�0.005Ta2O5 1861 5.84 171 33 273 89 450.865BNT�0.06BT�0.075(Bi0:5Li0:5ÞTiO3 208 36.8 260 460.94[0.94BNT�0.06(Bi0:5Ag0:5ÞTiO3]�0.06BT 168 31 51.6 265 100 470.92BNT�0.06BT�0.02(Y0:5Na0:5ÞTiO3 159 28.8 267 95 480.91BNT�0.09Ba(Ti0:942Zr0:058ÞO3 881 2.6 147 244 490.92BNT�0.08Ba(Ti0:95Hf0:05ÞO3 135 259 500.95BNT�0.02BT�0.03NaNO3 110 51 51

0.90BNT�0.05BKT�0.05BT 700 2 148 34 49.2 �125 520.895BNT�0.07BKT�0.035BT 150 29.8 530.85BNT�0.12BKT�0.03BT 149 28.2 540.85BNT�0.12BKT�0.03BT þ Co2O3 �143 25.2 157 55(Bi0:98Na0:755K0:15Li0:075Þ0:5Ba0:02TiO3 1040 3.4 205 29 210 56(Bi0:95Na0:75K0:15Li0:05Þ0:5Ba0:05TiO3 1510 5.5 194 17 37 57

274 Y.-Q. Lu & Y.-X. Li

tetragonal MPB move toward lower BKT content.Near the MPB, the optimum properties are d33of 215 pC/N and kp of 35% for BNT�BKT�KN82/16/2 ceramics, and d33 of 138 pC/N and kp of38% for BNT�BKT�KN 87/10/3 ceramics. Zhouet al. investigated the e®ect of BiFeO3 addition on

microstructure and electrical properties of (0.82�x)BNT�0.18BKT�xBiFeO3 ceramics.32 The speci-mens with x � 0:05 maintained a rhombohedral�tetragonal phase co-existence and changed into arhombohedral phase when x > 0:05. The piezo-electric constant d33 and the electromechanicalcoupling factor kp showed an obvious improvementby the addition of small amount of BiFeO3, whichshowed optimum values of d33 ¼ 170 pC/N and kp ¼0:366 at x ¼ 0:03. The mechanisms of intrinsicand extrinsic contributions to the dielectric andpiezoelectric responses have been proposed. In the(1�x�y)BNT�xBKT�yBiCrO3 system, it wasfound that the MPB with rhombohedral and tetra-gonal co-existence lies in the range of x ¼ 0:18�0.21and y ¼ 0�0:02.33 The optimum values of d33, kpwere 168pC/N, 0.32 at x ¼ 0:18, y ¼ 0:015 andx ¼ 0:18, y ¼ 0:01, respectively.

Recently, some studies have been conducted tomodify the BNT�BKT system by incorporatingother A-site dopants, such as Liþ and Agþ, to fur-ther enhance their ferroelectric and piezoelectricproperties. Bi0:5(Na1�x�yKxLiyÞ0:5TiO3 lead-freepiezoelectric ceramics have been investigated by Linet al.34,60 The partial substitution of Naþ by Kþ andLiþ e®ectively decreases the coercive ¯eld Ec butsimultaneously maintain the very strong ferroelec-tricity, which results in an obvious improvement onthe piezoelectric properties. The optimized piezo-electric properties d33 of 231 pC/N, kp of 41.0%, andkt of 50.5% were obtained. The Td of the compo-sition with x ¼ 0:15 and y ¼ 0:075 was reported tobe approximately 195�C. Another study by Zhanget al. focused on Bi0:5(Na0:7K0:2Li0:1Þ0:5TiO3 com-position ceramics by adding Na2CO3 as sinteringaid.35 The excess of Na2CO3 could e®ectively reducesintering temperature to 1020�C without deterio-ration of the piezoelectric properties. The Bi0:5(Na0:7K0:2Li0:1Þ0:5TiO3 ceramics with an excess of5mol% Na2CO3 exhibited the optimum properties(d33 ¼ 171 pC/N, kp ¼ 30:0%, Pr ¼ 31:9�C/cm2Þ.

3.2. BNT�BT system

It has been reported that the BNT�BT system has arhombohedral�tetragonal MPB at x ¼ 0:06.44

Compared with pure BNT, the BNT�BT compo-sitions near the MPB provide substantiallyimproved poling and piezoelectric properties. In°u-ences of nonstoichiometry and doping on the struc-tures and piezoelectric properties of BNT�BT

Fig. 7. Piezoelectic properties and dielectric constants ofBNT�BKT ceramics as a function of the amount of BNT.1

Fig. 8. MPB phase diagram of BNT�BKT�KN ternarysystem.31

Source: Reprinted with permission. Copyright 2007, AmericanInstitute of Physics.


compositions near the MPB have been studiedextensively.44,61

Various oxides, such as CeO2, SnO2, MnO, CuO,CoO, Co2O3, In2O3, La2O3, Nb2O5 and Ta2O5 havebeen employed as additives for the BNT�BTceramics.38�45,62�64 Researchers have investigatedthe e®ects of small amount of dopants on theproperties of BNT�BT ceramics, and found thatintroducing these dopants can improve the piezo-electric and dielectric properties to some extent.Wang et al. reported that the addition of CeO2 at acertain amount improved the piezoelectric anddielectric properties of 0.94BNT�0.06BT ceramicssigni¯cantly, showing the double e®ects in increas-ing the coupling factor and decreasing the dissipa-tion factor.38 During sintering CeO2 di®used intothe lattice to form a solid solution, but did notchange co-existence of tetragonal and rhombohedralphases. Zhu et al. observed that the sinteringproperties of 0.92BNT�0.08BT ceramics could beimproved by adding a small amount of MnO.39 Theoptimal electrical properties were acquired at thesolution limit of MnO at 0.30wt%. The suitablesubstitution of Mn ion into the B-site induces thelattice distortion of BNT�BT perovskite. Zhouet al. synthesized In2O3-doped 0.93BNT�0.07BTceramics by direct reaction sintering.43 It was foundthat the direct reaction sintering promotes growingof ceramic grains while doping of In2O3 contributesto inhibit and homogenize the grain growth. Opti-mum properties with kt ¼ 0:503, d33 ¼ 205 pC/N,"T33="0 ¼ 1046, and tan � ¼ 0:036 were achieved insamples with 0.16wt % In2O3. Zuo et al. investi-gated tantalum-doped 0.94BNT�0.06BT lead-freepiezoelectric ceramics.45 Due to the occupation oftantalum on B-site as a donor, the material prop-erties thus get softer electrically. The piezoelectricconstants are slightly improved; however, thedesirable working temperature for a piezoelectricceramic is reduced. With increasing tantalum con-tent, the antiferroelectric phase zone gets broader.

The partial substitutions of analogous ions forA-site or B-site ions of BNT�BT, such as Naþsubstituted by Liþ or Agþ, Bi3þ substituted by Y3þ,Ba2þ substituted by Sr2þ, Ti4þ substituted by Zr4þor Hf 4þ, have been investigated in order to obtain asolid solution with enhanced piezoelectric proper-ties.46�50,65,66 In the ð1�x�yÞBNT�xBT�yBLTsystem (BLT stands for Bi0:5Li0:5TiO3Þ, the ceramicsclose to the MPB (x ¼ 0:06, y ¼ 0:075) exhibit theoptimum piezoelectricity but the lowest Td.

46 In the

(1�xÞBNT�xBa(Ti0:942Zr0:058ÞO3 system, phasetransition from rhombohedral to tetragonal wasobserved with increasing x in the range from 3 to12mol%, indicating the existence of MPB in thissystem.49 The maximum value of piezoelectric con-stant d33 of 147 pC/N was obtained for the ceramicswith the composition x ¼ 0:09 near the MPB region.

3.3. BNT�BKT�BT system

The ternary BNT�BKT�BT system has also beenwidely investigated due to relatively high Curietemperatures and large piezoelectric properties nearthe rhombohedral�tetragonal MPB.

Wang et al. investigated the e®ects of amount ofBKT on the electrical properties and crystal struc-ture of (0:95�x)BNT�xBKT�0.05BT ceramics.52

During sintering, the incorporated BKT di®usesinto the BNT�BT lattice to form a solid solution,but changes the crystal structure from rhombohedralto tetragonal symmetry at higher BKT amounts.The incorporation of 5mol% BKT enhanced thepiezoelectric properties signi¯cantly. The propertiesof 0.90BNT�0.05BKT�0.05BT composition aregood enough to replace PZT in certain applicationssuch as ultrasonic wire-bonding, transducers andaccelerometers.67,68

Li et al. investigated BNT�BKT�BT system bykeeping the BKT and BT ratio constant.53,54,69 TheMPB between rhombohedral and tetragonal existsin the range of 0:024 � x � 0:030 for ð1�5xÞNBT�4xKBT�xBT system and 0:025 � y � 0:035 for ð1�3yÞ NBT�2yKBT�yBT system at room temperature.The compositions near the MPB have relativelyhigh piezoelectric due to the coexistence of rhom-bohedral and tetragonal phases. Figure 9 depictsthe MPB phase diagram of the ternary BNT�BKT�BT system based on their XRD results andother references.69

Another report described the Co2O3-addedBNT�BKT�BT system.55 The dependence of thevalence state of Co cations on the Co2O3 additionimpacts on the microstructure, depolarization tem-perature and electrical properties of the ceramicssigni¯cantly. The Td is elevated to 157

�C for 0.5mol%Co2O3 addition; and the Qm reaches a maximum of626 at 1.5mol% Co2O3 addition.

Further enhanced performance was found inLi-modi¯ed BNT�BKT�BT system. Lin et al.reported that the multicomponent lead-free piezo-electric ceramics [Bi1�z(Na1�x�y�zKxLiyÞ]0:5BazTiO3

276 Y.-Q. Lu & Y.-X. Li

exhibited good piezoelectric performances and strongferroelectricity.56 The maximum d33 of 205 pC/Nwas obtained for x=y=z ¼ 0:15/0.075/0.02, the highdepolarization temperature of this composition wasfound to be 210�C.

4. Alkaline Niobate and RelatedMaterials

The alkaline niobates (K1�xNaxÞNbO3 (KNN)-based ceramics are considered as one of the mostpromising candidates for lead-free piezoceramics.

KNN is a solid solution of ferroelectric KNbO3 andantiferroelectric NaNbO3.

70 Both end members ofthe solid solution are orthorhombic at room tem-perature. Pure KNN ceramics sintered by the con-ventional solid-state reaction in air exhibit a highCurie temperature (TC = 420�C), a good ferroelec-tricity (Pr ¼ 33�C/cm2Þ, while poor piezoelectricity(d33 ¼ 80 pC/N) and bulk density.71 The obstaclesof the processing for KNN-based ceramics is largelydue to the instability of the KNN phase and vola-tility of the alkali oxides sintering at high tempera-tures, which make it di±cult to obtain high density.

Numerous studies have been carried out in orderto improve the piezoelectric properties of KNN-based ceramics, especially following Saito et al. 2004Nature paper.72 The Chinese researchers account fora large proportion of this area. Some typical physicalproperties for KNN-based ceramics published byChinese researchers are summarized in Table 4.

4.1. The optimizations of the processingfor KNN-based ceramics

To suppress the high volatilization of alkali metaloxides at a high temperature during sintering, Zhenet al. designed a double crucible method, as shown inFig. 10, to calcinate and sinter the ceramic samplesin atmospheric powder in sealed crucibles.73 It wasfound that the density increased greatly within anarrow temperature range. On the other hand, alkali

Fig. 9. MPB phase diagram of BNT�BKT�BT ternarysystem.69

Source: Reprinted with kind permission from SpringerScienceþBusiness Media.

Table 4. Dielectric and piezoelectric properties of KNN-based ceramics.

Compositions "r d33 (pC/N) kp (%) kt (%) TC (�C) TO�T (�C) References

K0:48Na0:52NbO3 � 600 160 47 � 410 � 204 97K0:5Na0:5NbO3 (SPS) 606 148 38.9 395 75

K0:5Na0:5NbO3 þ 1mol% CuO 82 39 47 421 201 98K0:5Na0:5NbO3 þ 0.75mol% K5:4Cu1:3Ta10O29 360 90 41 46 383 206 99K0:5Na0:5NbO3 þ 1wt% K4CuNb8O23 100 40 80

0.92Na0:535K0:48NbO3�0.08LiNbO3 280 48.3 475 � 50 890.92Na0:535K0:48NbO3�0.08LiNbO3 (domain engineering) 324 86(K0:55Na0:45Þ0:965Li0:035Nb0:80Ta0:20O3 1290 262 53 325 40 1000.948K0:5Na0:5NbO3�0.052LiSbO3 1372 286 51 385 35 101K0:38Na0:58Li0:04Nb0:91Ta0:05Sb0:04O3 1327 306 48 49 337 � 40 1020.91K0:5Na0:5NbO3�0.09AgSbO3 þ 0.75mol% MnO2 1400 216 44 43 215 114 103K0:42Na0:52Li0:04Ag0:02Nb0:91Ta0:05Sb0:04O3 1478 263 45.3 353 104(K0:47Na0:47Li0:06Þ0:985Bi0:005NbO3 1020 185 43 45 455 �10 93

0.98Na0:475K0:475Li0:05NbO3�0.02Bi0:48Na0:48Ba0:04TiO3 702 328 48 415 141 1050.97Na0:5K0:5NbO3�0.03Bi0:5Na0:5TiO3 195 43 375 1060.9825K0:5Na0:5NbO3�0.0175BiScO3 � 1000 253 48 49 351 100 1070.94K0:5Na0:5NbO3�0.06Ba(Zr0:05Ti0:95ÞO3 1191 234 49 48 318 � 30 1080.94K0:5Na0:5NbO3�0.06BaTiO3 þ 1mol% CuO 230 193 43 40 314 � 30 109


elements were added in excessively to compensatetheir losses at high temperature. Enhancedpiezoelectric properties are obtained for the0.058LiNbO3�0.942Na0:535K0:480NbO3 ceramics byexcessively adding alkali metal oxide (TC ¼ 490�C,d33 ¼ 314 pC/N, kp ¼ 41:2%).74

The SPS method is suitable for the improvementof the density of KNN-based ceramics because of itsadvantages of a rapid sintering process and a shortsoaking time.75�78 Post-annealing in air is alwaysrequired to reduce conductivity due to oxygende¯ciency. Li et al. used SPS method to prepareNa0:5K0:5NbO3 ceramics, achieved > 99% relativedensity and nearly twice the d33 value of conventionallysintered KNN ceramics (d33 ¼ 148 pC/N).75

The incorporation of liquid phase sintering aids,like K4CuNb8O23, K5:4Cu1:3Ta10O29 and CuO, arealso used to reduce sintering temperature and pro-mote the densi¯cation for KNN-based ceramics,giving a larger relative density > 96%.79�81 Itshould be noted that copper-compound sinteringaids make KNN-based ceramics become \hard," andthe mechanical quality factor Qm increased largely.Besides, after the addition of copper-compoundsintering aids, both TC and TO�T decrease with theP�E loop becoming constricted.

Moreover, ZnO, CdO, Sc2O3 and SnO2 canimprove the sintering behaviors, while the additionof oxides, like CeO2, Y2O3 andWO3, severely inhibitsintering.82 MnO2 lowers "r and tan �, and improvesQm and kp.

83,84 The addition of Bi2O3 increases themelting point of the system and raises the sinteringtemperature of K0:5Na0:5NbO3 ceramics as well.85

Recently, Wang et al. proposed a generalapproach, domain engineering, toward piezoelectricresponse enhancement in KNN-based piezoelectricceramics.86 A high d33 of 324 pC/N was obtained forthe 0.92(Na0:535K0:48ÞNbO3�0.08LiNbO3 ceramics,

with a second poling treatment after the ¯rst polingand room temperature aging for two months(Fig. 11). A mechanism corresponding to details ofspontaneous polarization change in the domain levelwas proposed, concerning the combined e®ect of themigration of oxygen vacancies, and interactionbetween defect dipoles and spontaneous polarizationinside the domains. This work is a successfulexample of domain engineering applied to lead-freepiezoceramics.

In addition, Du et al. reported a new approach tofurther improve piezoelectric properties of KNN-based ceramics.87 They pointed out that the opti-mum poling temperatures of KNN-based ceramicsshould be chosen near the PPT temperatures.

Fig. 10. Schematic illustration showing how to use doublecrucibles for the normal sintering of KNN-based ceramics.73

Source: Reprinted with permission from John Wiley and Sons.

(a)

(b)

Fig. 11. (a) The piezoelectric coe±cient d33 as well as the TO�T

of (1�x)(Na0:535K0:48ÞNbO3�xLiNbO3 ceramics as a function ofLiNbO3 content, after both the ¯rst and second poling;(b) Comparison of the P�E hysteresis loop for the sample withx ¼ 0:080 after the ¯rst and second poling.86

Source: Copyright Wiley-VCH Verlag GmbH & Co. KGaA.Reproduced with permission.

278 Y.-Q. Lu & Y.-X. Li

Piezoelectric constant d33 of 0.93K0:5Na0:5NbO3�0.07LiNbO3 ceramics increases from 210 to 274 pC/N by selecting optimum poling temperature.

4.2. E®ects of doping on KNN-basedceramics

A lot of research works have been explored to enhancethe piezoelectric properties of KNN-based ceramics bycomposition design. All of these studies obtainedenhanced properties mainly by shifting the orthor-hombic to tetragonal PPT temperature TO�T toslightly above or below room temperature. It isbelieved that the enhancement of the piezoelectricproperties in PPT-based ceramics results from a com-bination of a \softening" the crystal lattice andincreased alignment of ferroelectric domains followingpoling due to the coexistence of two phases.17 However,the piezoelectric properties exhibit poor temperaturestability in most modi¯ed KNN-based ceramics.

4.2.1. E®ects of A-site ions substitution onKNN-based ceramics

The researches by A-site doping of KNN-basedceramics are mainly focused on the addition of Liþ,Agþ, Bi3þ, and alkaline-earth AE2þ (AE¼Mg, Ca,Sr, Ba) ions, in which Liþ modi¯ed KNN ceramicsare widely studied.74,88�94 Generally speaking, Liþdoping decreases the TO�T to room temperature atabout 6mol% A-site substitution and increases TC

while also improving densi¯cation and achieving rela-tive high piezoelectric properties (d33 > 200 pC/N).

There are some reports about Agþ substitutionfor A-site of the (K0:5Na0:5Þ1�xAgxNbO3 cer-amics.90,91 The results from Xu et al. show that afterthe addition of AgNbO3, both TC and TO�T decreaseslightly, and the electrical properties of the ceramicswith x ¼ 0:10 are found to be optimum: d33 ¼135 pC/N, kp ¼ 0:43, kt ¼ 0:46, TC ¼ 394�C.90

However, for the Agþ and Liþ co-modi¯edK0:5Na0:5NbO3 ceramics, Agþ substitution largelyincreases the Curie temperature (TC > 476�C) whiledecreasing TO�T, and exhibits good piezoelectricproperties (d33 ¼ 176�220pC/N).95,96

Du et al. found that Liþ and Bi3þ co-modi¯edKNN ceramics can possess not only high Curietemperature (TC ¼ 455�C) and piezoelectric prop-erties (d33 ¼ 185 pC/N), but also as low as possibleTO�T (�10�C), giving good temperature stability ofpiezoelectric properties.93

4.2.2. E®ects of B-site ions substitution onKNN-based ceramics

Tantalum and antimony have been widely usedas dopants in the B-site ions substitution on theKNN-based ceramics.110�114 Generally speaking,both tantalum and antimony doping hinder abnor-mal grain growth and decrease both TC and TO�T

transition temperature, while increasing the rhom-bohedral to orthorhombic PPT temperature TR�O. Itshould be noted that for the (Na0:52K0:48)(Nb1�ySby)O3 (NKNSy) ceramics, TR�O can be tuned to nearroom temperature at y � 0:09 by the substitution ofSb5þ, leading to optimum piezoelectric properties ofd33 ¼ 230 pC/N, as shown in Fig. 12.114 The exist-ence of successive PPT above room temperaturemakes the samples exhibit di®erent thermal stabilitycharacteristics from previously reported KNN-basedceramics.

4.2.3. E®ects of both A-site and B-site ionssubstitution on KNN-based ceramics

Currently, A-site and B-site ions co-modi¯cation isthe most investigated approach to improve thepiezoelectric properties of KNN-based Ceramics.The B-site ions Nb5þ can be substituted by Ta5þor/and Sb5þ ions, while the A-site is doped by Liþor/and Agþ.101,104,115�121 On the other hand, thereare lots of researches chosen alkaline earth ionsAE2þfor A-site while Ti4þ for B-site substitutions.122�125

Recently, there are some reports that used bothtrivalent ions for A-site and B-site dopants, suchas Bi3þ for A-site, while Fe3þ, Sc3þ, Al3þ forB-site.107,126�128

Fig. 12. (Color online) Phase transition temperatures chan-ging as a function of the Sb content y for NKNSy ceramics.114

Source: Reprinted with permission from John Wiley and Sons.


LiTaO3-doped KNN ceramics exhibits a coex-istence of orthorhombic and tetragonal phases at4�5mol% of LiTaO3 accompanied by enhancementof piezoelectric properties; the piezoelectric coe±-cient d33 have been reported in the range of200�265 pC/N.129 The introduction of LiSbO3 intothe KNN solid solution decreases slightly TC, butgreatly shifts TO�T to room temperature, reaching amaximum (d33 > 260 pC/N) at approximately 5mol% of LiSbO3 content.101;121 Wu et al. studied theLiþ, Agþ and Ta5þ co-modi¯ed KNN ceramics,achieved enhanced electrical properties d33 ¼263 pC/N and good aging characteristics.104 Linet al. prepared \softened" KNN ceramics by theaddition of AgSbO3, and obtained excellent piezo-electric properties (d33 ¼ 130�216 pC/N).103

AETiO3 doping of KNN produces relaxor beha-vior. Moreover, the orthorhombic to tetragonalphase transition temperature in KNN can be e®ec-tively lowered by addition of CaTiO3.

125,130Wu et al.found that CaTiO3-modi¯ed [(K0:5Na0:5Þ0:94Li0:06]�(Nb0:94Sb0:06ÞO3 ceramics possessed good electricalproperties (d33 ¼ 252 pC/N and kp ¼ 49%) andimproved temperature stability for the PPT belowroom temperature.125 Chang et al. observed thatadding MgTiO3 and BaTiO3 to KNN reduced den-sity and deteriorated electrical properties, whileadding CaTiO3 and SrTiO3 can promote densi¯ca-tion and optimize electrical properties.131

Bismuth containing compounds, such asK0:5Na0:5NbO3�BiMeO3 (Me3þ ¼ Fe3þ, Sc3þ, Al3þÞ,K0:5Na0:5NbO3�BNT solid solutions are other types ofKNN-based lead-free piezoceramics.105�107,127,132�134

The ð1� xÞK0:5Na0:5NbO3�xBiScO3 ceramics at x ¼0:0175 exhibit excellent electrical properties d33 ¼253 pC/N.107

4.2.4. E®ect of tungsten bronzecompounds doping onKNN-based ceramics

Though many solid solutions of KNN with otherferroelectrics or nonferroelectrics have been studied,there is little work on establishing phase diagramsof KNN with tungsten bronze compounds (TBC),like potassium lithium niobate (KLN) withstu®ed tungsten bronze structure. It was reportedthat KLN phase began to appear at x ¼ 0:08 inð1� xÞK0:5Na0:5NbO3�xLiNbO3 solid solution, andthe piezoelectric properties decreased markedly.135

However, the conclusion may be suitable for

Liþ-doped KNN ceramics, but not for pure KNNceramics.

Zeng et al. and Wang et al. studied the e®ects ofKLN on the phase structure, dielectric and piezo-electric properties of KNN ceramics.136,137 Theresults showed that the piezoelectric properties ofKNN ceramics modi¯ed by small amount of KLNwere improved. The addition of KLN markedlyincreased TC, but greatly shifted TO�T down to nearroom temperature. Small amount of KLN decreasedthe amount of defects, thus the remnant polariz-ation increased and the coercive ¯eld decreasedmarkedly. A coexistence of the orthorhombic andtetragonal phaseswas identi¯edat approximately 0:12 �x � 0:18 for the ð1� xÞ(K0:48Na0:52ÞNbO3�(x/5.15)K2:9Li1:95Nb5:15O15:3 ceramics (KNN�KLN100x).137

The ceramics with x ¼ 0:16 exhibited excellent pie-zoelectric properties: d33 ¼ 235 pC/N, kp ¼ 41:6%,TC ¼ 473�C, TO�T ¼ 53�C, "r ¼ 728, tan � ¼ 0:02.Moreover, the KNN�KLN100x ceramics also havean excellent thermal stability in the severe aging test upclose to their Curie temperatures, as shown in Fig. 13.

5. Bismuth Layer-Structured Materials

The bismuth layer-structured ferroelectrics (BLSFs)with general formula Bi2An�1BnO3nþ3 consist of n-perovskite layers (An�1BnO3nþ1Þ2� sandwichedbetween bismuth oxygen sheets (Bi2O2Þ2þ. A is amono-, di- or tri-valent ions or a mixture of them,B is a combination of cations well suited to form anoctahedron, and n can be 1, 2, 3, 4 or the mixture

Fig. 13. (Color online) (a) Thermal annealing temperaturedependence of d33 and tan � determined at 10 kHz of theKNN�KLN100x ceramics with x ¼ 0:16; (b) the variation of TC

and TO�T as a function of x.137

280 Y.-Q. Lu & Y.-X. Li

of the adjacent them. So far, there are total¯ve types, which are Bi3TiNO9-based, Bi4Ti3O12-based, MBi2N2O9-based, MBi4Ti4O15-based, andthe intergrowth bismuth layer-structured ferro-electrics (iBLSF), where M is Sr2þ, Ca2þ, Ba2þ,(Bi0:5Na0:5Þ2þ, (Bi0:5K0:5Þ2þ, respectively, N is Nb5þ,Ta5þ, respectively.

BLSFs are characterized by their low dielectricconstant, low dielectric dissipation factor, highCurie temperature and large anisotropy in theelectromechanical coupling factor. Therefore, theBLSF ceramics are considered as superior candi-dates for high temperature piezoelectric appli-cations. Especially for some demanding applicationswith work temperature above 600�C, while BLSFceramics show good stability of d33 at elevatedtemperature.138�141

However, owing to its two-dimensional orien-tation restriction on the permissible rotations of thespontaneous polarization and low resistivity, thepiezoelectric activities in BLSF ceramics are ratherlow. Therefore, a lot of works have been done on thedoping e®ects in order to improve of physicalproperties and high temperature stability of BLSFceramics. Doping in A-site, B-site and A/B-sitenormally takes place in the perovskite layers(An�1BnO3nþ1Þ2�. It is seldom that doping takesplace in the (Bi2O2Þ2þ layer structure. Rareelements like La3þ, Nd3þ, Sm3þ for A-site doping,

Nb5þ, V5þ, W6þ for B-site doping and their com-bination for co-doping are mainly investigated.Some of these doping e®ects on physic properties ofBLSF ceramics are listed in Table 5.

From Table 5, it can be seen that A-site doping ismore e®ective for the d33 or Pr enhancement thanthat of B-site doping. Li et al. reported that the solidsolubility of A-site has crucial in°uence than that ofB-site.154 B-site doping can signi¯cantly increaseresistivity, leading to enhanced d33 by enablingpoling at high temperature and high electric ¯eld,particularly for Bi4Ti3O12 ceramics. Zeng et al.reported that both V5þ and W6þ doping increasethe conductivity of CaBi4Ti4O15, which indicatethat CaBi4Ti4O15 ceramics show an n-type con-ductivity.149 It is di®erent from the conductivitymechanism of Bi4Ti3O12. Wang et al. reported thatB-site cobalt modi¯cation signi¯cantly enhanced thepiezoelectric activity of Na0:5Bi4:5Ti4O15 piezo-electric ceramics.139 Thermal annealing studiesdemonstrated that high temperature piezoelectricapplications are possible up to 500�C in this cobalt-modi¯ed BLSF ceramics.

Yu et al. investigated the three-dimensionaldomain structure and the variation of local elasticityon Nb-doped Bi4Ti3O12 ceramics by ScanningProbe Microscopy in a piezoresponse mode and lowfrequency acoustic mode, respectively.155 In acousticmode, the 90� domain structures contribute strongly

Table 5. Doping e®ects on BLSF piezoelectric ceramics.

Compositions TC (�C) "r tan � (%) d33 (pC/N) Pr (�C/cm2) References

CaBi2NbTiO9 936 97 1.5 5 139Ca0:9(K, Ce)0:05Bi2NbTiO9 868 118 0.18 16 139

Bi4Ti3O12 672 176 0.95 4 141Bi4Ti3O12 þ 4%mol Nb2O5 630 224 0.23 18 141Bi4Ti2:98V0:02O12:01 674 170 20 6 142Bi4Ti2:975W0:025O12:025 þ 0.2wt% Cr2O3 658 178 2 22 143Bi3:84Nd0:16Ti2:98V0:02O12:01 648 1.8 13.5 144Bi3:25La0:75Ti2:92Mn0:08O12 16.6 145Bi3:5Nd0:5Ti3O12 19 146

Na0:5Bi4:5Ti4O15 668 153 0.35 18 147Na0:5Bi4:5Ti4O15 þ 0.5wt% CeO2 655 146 0.14 28 147Na0:5Bi4:5Ti4O15 þ 0.3wt% CoO 663 152 0.1 30 148

CaBi4Ti4O15 789 143 0.19 7 5.3 149CaBi4Ti3:975W0:025O15 789 158 0.125 10 3.2 149Ca1�xSrxBi4Ti4O15 þ 1.5mol% MnO2 677 122 0.34 21 11.6 150CaBi4Ti3:95V0:05O15 791 14 5.96 151CaBi3:75Nd0:25Ti4O15 756 159 0.082 12 9.6 152

Bi7Ti4NbO21 841 150 13 10 153Bi6:25La0:75Ti4NbO21 700 180 17 12 153


to the image contrast because of the ferroelasticbehavior (Fig. 14). However, the 180� domain pat-terns have little e®ect on it and the featuresobserved do not correlate with the local elasticity.

Yi et al. investigated the ferroelectric and piezo-electric properties of the La-doped intergrowthAurivillius phase ceramics Bi5�xLaxTiNbWO15 andBi7�xLaxTi4NbO21 (x ¼ 0:00�1.75).153,156�159 Itwas found that the La doping is in favor of thedomain switching. Enhanced ferro-/piezoelectricproperties, such as 2Pr of 24.4mC/cm2 and d33 of16.6 pC/N, were obtained in the Bi7�xLaxTi4NbO21

ceramics. The high temperature electrical behaviorindicates that the Bi5�xLaxTiNbWO15 ceramics aresemiconducting whereas the La-doped Bi7Ti4NbO21

ceramics are worthy to be further studied towardhigh temperature piezoelectric applications.

Besides the doping e®ects, processing for ¯nepowders, such as co-precipitate, sol�gel, led toenhanced densi¯cation or lowered dielectric loss ofBLSF ceramics are also reported.160,161

6. Application of Texture Techniques inLead-Free Piezoelectric Ceramics

Texture control of polycrystalline ceramics is a con-venient and e®ective approach to improve the pie-zoelectric properties by tailoring the microstructureof ceramics without drastically changing the com-position of the materials. Many methods have beenused to prepare lead-free textured anisotropic cer-amics, for example, hot processing, templated graingrowth (TGG), screen-printing and other techniques.

Hot processing is a traditional grain orientationtextured technique, mainly including hot-forging,hot-pressing and hot-extrusion. Using hot proces-sing, dislocation movements take place in the crystal

interior and grain boundary slides under externalforce at a high temperature, thus ceramic grainorientation arrangement is achieved. The hot pro-cessing technique has some disadvantages (e.g.,complexity of the process), so that it has not beenwidely studied. Hao et al. prepared SrBi4Ti4O15

ceramics with a grain orientation degree of 85.7%via hot-forging technique.162 Zhang et al. preparedgrain-orientated ferroelectric Bi3NbTiO9 ceramicswith remarkable anisotropy and doubled the valueof d33 by hot-pressing technique.163

Reactive templated grain growth (RTGG) is aprocessing method in which reactive template par-ticles are mixed with complementary reactants andaligned, and the product is formed in-situ duringheat-treatment, preserving the orientation of thetemplates. RTGG technique includes two key steps,preparation of the anisotropic template grain (gen-erally plate-like or needle-like) and the templatealignment process such as tape casting or extrusion.As a versatile and e®ective processing technique forthe fabrication of textured ceramics, TGG/RTGGmethod has been widely applied to texture per-ovskite-structure, tungsten�bronze-structure andBLSF ceramics with enhanced piezoelectric proper-ties. Some of the textured lead-free piezoceramics andtheir templates by TGG/RTGG are listed in Table 6.

Screen printing, which has numerous advantagesover tape casting including the realization of poly-morphism and streamline production, is a highvolume technique employed for fabricating thick-¯lm electronic materials. But screen-printingtechnique has remained relatively neglected forprocessing textured ceramics. The use of screen-printing multilayer grain growth of texturing cer-amics was proposed by Zeng et al. for texturedCaBi4Ti4O15 BLSF with a grain orientation of 96%

(a) (b) (c)

Fig. 14. In-situ obtained images of Nb-doped Bi4Ti3O12 ceramics in piezoresponse mode; (a) Topography; (b) vertical piezoresponseimage and (c) interal piezoresponse image.155

Source: Reprinted from Ref. 155. Copyright 2005, with permission from Elsevier.

282 Y.-Q. Lu & Y.-X. Li

in 2005.164,165 Later, a screen-printing RTGG(sp-RTGG) has been used successfully for the per-ovskite-structure 0.94BNT�0.06BT ceramics with agrain orientation of 92%.166,167

In recent years, strong magnetic ¯eld has beenused on the study of textured piezoelectric ceramics.Zhao et al. prepared 0.94BNT�0.06BT texturedceramics using the pulsed strong magnetic ¯eld andtemplate grain growth.182 The gel-casting techniquehas been introduced to prepare green bodies. Apulsed strong magnetic ¯eld was applied to induceinternal grain orientation in the materials during thesolidi¯cation of the green bodies. The orientationdegree of the obtained textured ceramics reaches76%, and the d33 is 123 pC/N.

7. Lead-Free Piezoelectric SingleCrystals

Lead-free single crystals with optimal crystal-lographic orientation and domain engineering mayhave better piezoelectric properties when comparedwith its polycrystalline counterpart. While compared

with lead-free piezoelectric ceramics, the researcheson lead-free single crystals are carried out insu±-ciently until now.

Recently, high piezoelectric and ferroelectricproperties have been found in Mn-doped BNT�BTsingle crystals, which were grown by a top-seededsolution method.183 The electrical resistivity,dielectric constant, ferroelectric and piezoelectricproperties were all found to be notably enhanced byMn-doping. Tetragonal and rhombohedral phaseswere found to coexist in the as-grown condition. Aninduced phase stability change from rhombohedralto tetragonal phases occurred under an electric-¯eldapplied along the h001i direction.184

In the KNN-based system, 0.95(K0:5Na0:5ÞNbO3�0.05LiNbO3 crystal was grown by the Bridgmanmethod and the piezoelectric coe±cient d33 was foundto be on the order of 204�405 pC/N.185 Lin et al.prepared MnO2-doped (K0:5Na0:5ÞNbO3 single crys-tals by high-temperature solution method usingK2CO3�Na2CO3 eutectic composition as °ux.186

The Mn�KNN crystals were found to exhibit higherd33 and "r when compared with pure KNN crystal.

Table 6. Some of the textured lead-free piezoceramics and the templates used by TGG/RTGG.

Templates Textured composition Orientation degree (%) d33 (pC/N) References

plate-like SrBi2Nb2O9 SrBi2Nb2O9 68 168plate-like Bi4Ti3O12 0.8BNT�0.2BKT 90 195 169

0.84BNT�0.16BKT 70 134 1700.94BNT�0.06BT 95 241 171Bi4Ti3O12 92 172, 173

plate-like BNT 0.94BNT�0.06BT 87 299 1740.7BNT�0.2BKT�0.1(Bi0:5Li0:5ÞTiO3 60 300 175

plate-like Bi2:5Na3:5Nb5O18 0.92BNT�0.08BT 69 176BNT�BT 58 98 177

plate-like NaNbO3 K0:476Na0:524NbO3þCuO 99 146 178(K0:5Na0:5ÞNb0:97Sb0:03O3 97 218 179

acicular Ba2NaNb5O15 Ba2NaNb5O15 80 180acicular Sr0:39Ba0:48K0:32Nb2O6 Sr0:4Ba0:6Nb2O6 86 181

Table 7. Properties of some of lead-free single crystals.

Single crystals "r d33 (pC/N) kt (%) TC (�C) References

Mn: BNT 120 1870.7BNT�0.3BKT 160 49 365 188BNT�BT 1230 283 50 189Mn: BNT�BT 486 55.6 183K0:5Na0:5NbO3 240 160 45 393 190Mn: K0:5Na0:5NbO3 730 270 416 1860.95(K0:5Na0:5ÞNbO3�0.05LiNbO3 405 61 426 185


The smaller domain size (high domain density) inMn�KNN crystals gave rise to the enhanced dielec-tric and piezoelectric properties.

Some of the lead-free single crystals and theirelectrical properties are summarized in Table 7.

8. Summary

The current status of the research activities andprogresses from China in the last decade werereviewed in this paper, including the compositions,dopants, sintering aids, processing, texturing andsingle crystal growths of the BT-based, BNT-based,KNN-based and BLSF-based lead-free materials.(1) BZT�BCT system has excellent piezoelectricresponse, but the low Curie temperature and coer-cive ¯eld Ec make it suitable for sensors and detec-tors; (2) BNT�BKT�BT system shows largepiezoelectric constants, high curie temperature, highelectromechanical coupling factors, which is suitablefor high-power applications such as actuators andtransducers. (3) The KNN family has the advan-tages of low density, high coupling coe±cient kt,lower acoustical impedance, and higher mechanicalstrength that make it suitable for high frequencytransducers. But, the processing and sintering ofKNN-based materials are still problematic, such asvolatilization of K2O and Na2O, moisture sensitivityand density of the ceramics, more importantly thecost of Nb2O5 and Ta2O5 raw materials. (4) Some ofthe BLSF-based materials, especially the iBLSF,show a good stability and enhanced properties atelevated temperatures (> 600�C). It can be used asceramics accelerometers, sensors, resonators and¯lters with high mechanical quality factor and lowtemperature coe±cient of resonances frequency.

Till date, there is no one composition system oflead-free material that can replace PZT-based sys-tem, it is inevitable that speci¯c features of indi-vidual lead-free material can be used to the requiredpiezoelectric properties for each device. Future workon the research and development of lead-free pie-zoelectric materials seem to be focused on (1) com-positional design, modeling and computation of newmaterials, the nature of piezoelectric activity withMPB and PPT, domain and domain wall engineer-ing, control of grain size and grain boundary are thefundamentals; (2) high quality single crystals andtheir microstructure characterization (synchrotronand neutron di®raction, spherical aberration cor-rected TEM/STEM), phase transition, domain

structure and the structure-property relations arethe key for a better understanding and a path forsearching new advanced lead-free materials; (3)textured grain orientation ceramics prepared by useof TGG and RTGG (sp-RTGG) methods that canbe mass produced with high e±ciency; (4) thin ¯lms,think ¯lms and multilayered structure devices oflead-free ceramics is an important trend; (5) themethods and standard of piezoelectric measurement,the classi¯cations of lead-free materials should betaken into account from now on; (6) more impor-tantly, international cooperation and team-workwith multidiscipline are needed for the deep under-standing of the piezoelectrics of lead-free materials,industrial applications, as well as the legislatures.

Acknowledgments

The authors are thankful to many of our colleaguesand students, Prof. Qing-rui Yin, Prof. Tian-baoWang,Mr. You-liangWang,Mr. PanWang,Ms. Ya-liLi, Ms. Chang-wei Shao, Mr. Fa-qiang Zhang, Prof.Dong Wang, Dr. Qun-bao Yang, Dr. Xue-zheng Jing,Dr. Jiang-tao Zeng, Dr. Zhi-guo Yi, Dr. Ying Wang,Dr. Hong-zhang Song, Dr. Meng-jia Wu, Dr. Zheng-faLi, Mr. Yu Zhao, Ms. Yi-lin Wang and Mr. Wen-junWu, for their invaluable contributions in variousaspects of the related research programs. The ¯nancialsupports from The National Nature Science Foun-dation of China (NSFC, Nos. 50072039, 20151003,50572113, 50932007), The Ministry of Sciences andTechnology of China (MOST) through 973-projects(Nos. 2002CB613307, 2009CB623305) and 863-Projects (Nos. 2001AA325070, 2006AA03Z430), TheScience and Technology Commission of ShanghaiMunicipality (Nos. 05JC14079, 08JC1420500,10XD1404700), and Shanghai Institute of Ceramics(No. SCX200409), are gratefully acknowledged.

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