Lead-free ceramics with the composition 0.91K1/2Bi1/2TiO3–0.09(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3) were prepared using a conventional solid state, mixed oxide route. The ceramics exhibited a high strain of 0.16% at 6 kV mm1, stable from room temperature to 175 C, with a variation of <10%. The materials were fabricated into multilayer structures by co-firing with Pt internal electrodes. The prototype multilayer actuator exhibited constant strains up to 300 C with a variation of 15%. The composition showed fatigue resistant behaviour in both monolithic and multilayer form after bipolar loading of 106 cycles

Acosta M.

Amir Khesro

Antonio Feteira

Dawei Wang

Derek C. Sinclair

Fayaz Hussain

Galassi C.

Ian M. Reaney

Smolenskii G. A.

Uchino K.

Lead-free ceramics with the composition 0.91K1/2Bi1/2TiO3-0.09(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3) were prepared using a conventional solid state, mixed oxide route. The ceramics exhibited a high strain of 0.16% at 6 kV mm-1, stable from room temperature to 175 °C, with a variation of <10%. The materials were fabricated into multilayer structures by co-firing with Pt internal electrodes. The prototype multilayer actuator exhibited constant strains up to 300 °C with a variation of ∼15%. The composition showed fatigue resistant behaviour in both monolithic and multilayer form after bipolar loading of 106 cycles

Khesro, A.

Wang, D.

Hussain, F.

Sinclair, D.C.

Feteira, A.

Reaney, I.M.

White Rose Research Online

Temperature stable and fatigue resistant lead-free ceramics for actuatorsAmir Khesro, Dawei Wang, Fayaz Hussain, Derek C. Sinclair, Antonio Feteira, and Ian M. Reaney  Citation: Applied Physics Letters 109, 142907 (2016); doi: 10.1063/1.4964411 View online: http://dx.doi.org/10.1063/1.4964411 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/109/14?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Robust CaZrO3-modified (K, Na)NbO3-based lead-free piezoceramics: High fatigue resistance insensitive totemperature and electric field J. Appl. Phys. 118, 134102 (2015); 10.1063/1.4932144  Fatigue and failure responses of lead zirconate titanate multilayer actuator under unipolar high-field electriccycling J. Appl. Phys. 114, 024101 (2013); 10.1063/1.4813219  Fatigue response of a PZT multilayer actuator under high-field electric cycling with mechanical preload J. Appl. Phys. 105, 014112 (2009); 10.1063/1.3065097  Improvement of fatigue resistance for multilayer lead zirconate titanate (PZT)-based ceramic actuators byexternal mechanical loads J. Appl. Phys. 104, 126103 (2008); 10.1063/1.3042226  Influence of temperature on the electromechanical and fatigue behavior of piezoelectric ceramics J. Appl. Phys. 83, 5342 (1998); 10.1063/1.367362    Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.167.254.160 On: Thu, 06 Oct2016 14:53:26Temperature stable and fatigue resistant lead-free ceramics for actuatorsAmir Khesro,1,a) Dawei Wang,1 Fayaz Hussain,1 Derek C. Sinclair,1 Antonio Feteira,2and Ian M. Reaney11Functional Materials and Devices Lab, Department of Materials Science and Engineering,Sir Robert Hadfield Building, University of Sheffield, Sheffield S1 3JD, United Kingdom2Materials and Engineering Research Institute, Sheffield Hallam University, Howard Street,Sheffield S1 1WB, United Kingdom(Received 4 August 2016; accepted 24 September 2016; published online 6 October 2016)Lead-free ceramics with the composition 0.91K1/2Bi1/2TiO3–0.09(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3) were prepared using a conventional solid state, mixed oxide route. The ceramics exhibited ahigh strain of 0.16% at 6 kV mm1, stable from room temperature to 175 C, with a variation of<10%. The materials were fabricated into multilayer structures by co-firing with Pt internal electrodes.The prototype multilayer actuator exhibited constant strains up to 300 C with a variation of 15%.The composition showed fatigue resistant behaviour in both monolithic and multilayer form after bipo-lar loading of 106 cycles. VC 2016 Author(s). All article content, except where otherwise noted, islicensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4964411]A piezoelectric or electrostrictive actuator converts anelectrical signal into a precisely controlled mechanical strain.The ability to control physical displacements with high preci-sion is vital for applications such as cameras, phones, micro-scopes, fuel injectors, micro-pumps, ink cartridges, andmedical instruments.1 Many harsh environments including oilwell drilling, aerospace, automobile, and power plant enginesrequire technological devices capable of retaining functionalproperties at elevated temperatures.2,3 Pb(Zr,Ti)O3 (PZT)-based piezoelectric and Pb(Mg1/3Nb2/3)O3 (PMN)-based elec-trostrictive ceramics are most widely used in actuator devicesdue to their large electromechanical coupling coefficients.4Electromechanical strain in PMN-based ceramics is not stablein high temperature regimes due to the notable temperaturedependence of polarization above the permittivity maxi-mum.5,6 In the case of piezoelectrics, electromechanical cou-pling is restricted by the Curie or depoling temperature (Tcand Td, respectively), and functional properties are lost abovethese transitions. High temperature applications, therefore,require materials with both high Tc and Td. Long term reliabil-ity is another important aspect for real world applications.Continuous electrical cycling can cause severe degradationin strain as evident from many studies on PZT-basedpiezoelectrics.7–9Recent concern, however, is the amount of toxic leadused in both PZT- and PMN-based systems, giving rise tohealth and environment concerns over the use of these mate-rials in actuators. Lead-free piezoelectric/electrostrictivematerials are therefore required with properties at least com-parable and most preferably exceeding those containinglead, especially in terms of temperature stability and fatigueresistance.Over the last two decades, many lead-free systems havebeen investigated, among them (K,Na)NbO3 (KNN)-based,Na1/2Bi1/2TiO3 (NBT)-based, and BaTiO3 (BT)-based sys-tems are the most prominent.10 All these ceramics have theirown shortcoming inhibiting their use for high temperatureapplications. BT-based ceramics have a low Tc,11 and KNN-based ceramics suffers from temperature sensitivity of thepolymorphic phase boundaries.12,13 NBT-based ceramicshave a low Td and require high driving fields, where self-heating becomes an issue.14Less widely investigated than NBT, KNN, and BT,K1/2Bi1/2TiO3 (KBT) has also shown promise as a potential pre-cursor for the development of lead-free piezoelectrics. KBT hasthe perovskite structure with tetragonal symmetry at room tem-perature (RT) and a Tc of 380C.15,16 The piezoelectric proper-ties of KBT are moderate (d33¼ 69 pC/N and strain 0.1% at8 kV/mm) for hot pressed ceramics.15 Doping with BiFeO3(BF) improves the piezoelectric properties of KBT,17–20 andMorozov et. al.19 reported a high RT strain performance of250 pm/V. However, the strain showed an increase by around48% when the temperature was increased from RT to 100 C.BF has a high Tc of 830C,21 which is very attractive forhigh temperature applications, but BF is usually very conduc-tive, and high fields cannot be applied to access the electrome-chanical properties.22 The high conductivity of BF has beenattributed to the presence of oxygen vacancies formed due tothe reduction of Fe3þ to Fe2þ during processing.23 Recently,Khesro et al.24 showed that electrically homogenous ceramicswith activation energy >1 eV could be obtained in the series(0.95-x)BiFeO3-xLaFeO3-0.05La2/3TiO3 and that these com-positions were either ferroelectric with R3c symmetry or anti-ferroelectric with Pbam symmetry depending on the value ofx. Further research by the present authors showed similarstructural trends as a function of x for Nd containing ana-logues of this solid solution, and it was concluded that (1-x-y)BiFeO3-xREFeO3-yRE2/3TiO3 (RE¼ rare earth) compositionsmight provide a low dielectric loss end-member in solid solu-tion with KBT for the fabrication of lead-free actuatormaterials.a)E-mail: amir.khesro@sheffield.ac.uk0003-6951/2016/109(14)/142907/5 VC Author(s) 2016.109, 142907-1APPLIED PHYSICS LETTERS 109, 142907 (2016) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.167.254.160 On: Thu, 06 Oct2016 14:53:26Based on this concept, we comprehensively studied thesolid solution (1-x)K1/2Bi1/2TiO3-x(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3). The composition 0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3 was preferred as an end member with KBTbecause of its antiferroelectric Pbam symmetry similar toPbZrO3. Optimum properties (0.16% strain at 6 kV mm1)were obtained at x¼ 0.09, hereafter abbreviated as KBNFT9.The induced strain was temperature stable up to 175 C with avariation of strain (DS)< 10% and fatigue resistant up to 106cycles at 5 kV mm1. It should be noted that higher temperaturestrain measurements could not be performed on monolithicceramics due to safety restrictions on the measuring equipment.In addition to temperature-stable, fatigue-resistant monolithicceramics, multilayer actuators (MLA) were fabricated using Ptinternal electrodes, which yielded 2 lm displacement at6 kV mm1, stable up to 300 C (DS 15%). The MLA alsoexhibited fatigue resistant behaviour up to 106 cycles at5 kV mm1 bipolar field.KBNFT9 ceramics were prepared by a conventionalsolid state oxide route, and MLAs were fabricated using thetape casting technique. The details of both processes can befound elsewhere.24,25 However, it is useful to note thatceramics both in monolithic and multilayer form were sin-tered at 1090 C for 2 h.For X-ray powder diffraction (XRD) measurements, sin-tered pellets were crushed and then annealed at 500 C over-night to reduce the effect of mechanical strain producedduring grinding. XRD was performed using a Siemens D5000diffractometer with CuKa. Optical micrographs were recordedusing a Polyvar Met optical microscope. Ceramic microstruc-tures were studied using an FEI Inspect F, Scanning ElectronMicroscope (SEM). For electrical measurements, the pelletswere ground and polished to a thickness of 0.7 mm and thenelectroded on both sides using gold paste. Multilayers weresilver painted to form termination electrodes. The LCR meas-urements were carried out at fixed frequencies of 1, 10, 100,and 250 kHz from RT to 550 C using a precision Agilent4184A LCR meter. Polarization (P) and bipolar strain (S) vs.electric field (E) were measured using an aix-ACCT (PES)ferroelectric test unit.A RT XRD pattern of KBNFT9 annealed crushed pelletsis shown in Figure 1(a). All peaks are attributed to a singleperovskite structured phase, similar to the pseudocubic struc-ture reported in KBT-BF systems.17–20 Figure 1(b) is a sec-ondary electron SEM image of KBNFT9 showing a densemicrostructure of cuboid grains with an average size of<1 lm.Relative permittivity (er) and tan d plotted against temper-ature for KBNFT9 are presented in Figure 1(c). er is stronglyfrequency dependent below the peak maximum in permittivity(Tm). The overall behaviour is similar to that of relaxor ferro-electrics, but it is important to note that tan d decreases to<2% at higher temperatures, suggesting that the relativelyhigh losses at RT arise primarily from the dielectric relaxationrather than long range conduction. Pure KBT has been shownto possess weak relaxor behaviour attributed to compositionaldisorder of Kþ and Bi3þ cations on the A-site.26,27 The forma-tion of a solution with 0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3 has significantly increased peak broadening and fre-quency dispersion indicating an enhancement in relaxation.RT strain-field (S-E) and polarization-field (P-E) loopsfor KBNFT9 are shown in Figure 1(d). P-E loop was not sat-urated at 6 kV mm1, with a low remanent polarization (Pr)of 9 lC/cm2. The slim P-E loop and low Pr is consistentwith the maximum broad permittivity (Figure 1(c)) and indic-ative of dominant relaxor-like behaviour. Moreover, the bipo-lar S-E loop shows a very low 0.01% of negative strainsuggesting the absence of long range ferroelectric order.KBNFT9 exhibits a high positive RT strain of 0.16% at6 kV mm1, which appears predominantly electrostrictive.The electrostrictive coefficient (Q33) is 0.023 m4 C2, similarto PMN-based ceramics,28 and was calculated usingFIG. 1. (a) XRD pattern of KBNFT9 atRT; (b) the ceramic microstructure ofKBNFT9; (c) permittivity and tan d ver-sus temperature for KBNFT9 ceramics;(d) P-E and S-E loops for KBNFT9bulk ceramics at RT and at a frequencyof 1 Hz.142907-2 Khesro et al. Appl. Phys. Lett. 109, 142907 (2016) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.167.254.160 On: Thu, 06 Oct2016 14:53:26S3 ¼ Q33P23; (1)where S3 and P3 are the strain and polarization in the direc-tion of poling, respectively.The maximum polarization (Pmax) and Pr as a functionof temperature for bulk KBNFT9 ceramics is shown inFigure 2(a). Pr decreases with the increasing temperaturefrom 9.46 lC cm2 at RT to 2.62 lC cm2 at 175 C result-ing in slimmer loops as shown in Figure 2(b), but Pmaxexhibits negligible temperature dependence. From Equation(1), electrostrictive strain is directly proportional to thesquare of polarization. A temperature stable Pmax, as shownin Figure 2(a), should therefore lead to temperature stableelectromechanical strain. S-E loops at different temperaturesfor bulk KBNFT9 ceramics are shown in Figure 2(d). Thestrain exhibited by KBNFT9 is broadly insensitive(DS 10%) to temperature between RT and 175 C, asshown in Figure 2(c).Owing to its promising properties in thick bulk samples,KBNFT9 was co-fired with platinum internal electrodes toFIG. 2. (a) Maximum and remanentpolarization versus temperature forKBNFT9 bulk ceramics; (b) P-E loopsfor KBNFT9 bulk ceramics at differenttemperatures; (c) normalised bipolarstrain versus temperature for KBNFT9bulk ceramics; and (d) S-E loops forKBNFT9 bulk ceramics at differenttemperatures.FIG. 3. (a) Optical image of aKBNFT9 MLA cross section; (b) P-Eand S-E loops for KBNFT9 MLA atRT and at a frequency of 1 Hz; (c) dis-placement versus temperature forKBNFT9 MLA; and (d) S-E loops forKBNFT9 MLA at 22 and 300 C.TABLE I. The temperature stability of KBNFT9 vs. leading lead basedmaterials.Material Temperature range (C) Variation in strain (%) References0.9PMN-0.1PT RT-70 >40 6PZT-5H RT-80 40 30PIC 151 RT-170 20 31PZT 4 RT-160 15 29KBNFT9 RT-300 15 This work142907-3 Khesro et al. Appl. Phys. Lett. 109, 142907 (2016) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.167.254.160 On: Thu, 06 Oct2016 14:53:26form MLAs consisting of 10 active layers, as shown in Figure3(a). The thickness of each sintered layer was 83 lm, and theactive area of each electrode was 31.36 mm2. The S-E andP-E loops of MLA at RT are shown in Figure 3(b). The MLAyielded a strain of2 lm at 498 V (6 kV mm1) demonstratingthat KBNFT9 can be processed into a MLA without significantloss of performance at RT. More importantly, the MLAshowed a broadly temperature insensitive strain up to 300 C,as shown in Figure 3(c). A comparison of strain at RT and300 C is shown in Figure 3(d). KBNFT9 requires a higherfield of 6 kV mm1 for achieving a strain of 0.16% as com-pared to PZT-based materials, e.g., PZT4 requires a field of 2kV mm1 to achieve a strain of 0.14%.29 However, the tem-perature stability in KBNFT9 is by far better than commerciallead-based ceramics. A comparison of KBNFT9 with commer-cially used lead-based materials is summarized in Table I. Thevalues in Table I are taken from figures in the respectivereferences.To study the fatigue behaviour, KBNFT9 bulk ceramicswere exposed to a bipolar loading of 5 kV mm1 (3Ec) for106 cycles at a frequency of 10 Hz. Fatigue resistant behav-iour is observed as shown in Figures 4(a) and 4(b) with thebipolar strain decreasing by <6% and Pmax by <5% after106 bipolar cycles. Similar to bulk samples, the prototypeMLA also showed fatigue resistant properties, and only<9% of polarization and strain was lost after cycling 106times at 5 kV mm1 of bipolar field (5 Hz), as shown inFigures 4(c) and 4(d).In conclusion, lead-free KBNFT9 has been shown as amonolithic ceramic and as an MLA to not only exhibit tem-perature stable and fatigue resistant properties superior to allother comparable lead-free materials but also to commercialPZT formulations.A.K. and F.H. acknowledge Abdul Wali KhanUniversity Mardan and NED University of Engineering andTechnology, respectively, for Ph.D. studentships. All authorsacknowledge the financial support from the Sustainabilityand Substitution of Functional Materials and DevicesEPSRC grant (EP/L017563/1).1W. Jo, R. Dittmer, M. Acosta, J. Zang, C. Groh, E. Sapper, K. Wang, andJ. R€odel, J. Electroceram. 29, 71 (2012).2T. Stevenson, D. G. Martin, P. I. Cowin, A. Blumfield, A. J. Bell, T. P.Comyn, and P. M. Weaver, J. Mater. Sci. Mater. 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(a) Polarization and strain forbulk KBNFT9 ceramics as a functionof switching cycle; (b) S-E loops forKBNFT9 bulk ceramics at 1st and mil-lionth cycle at 10 Hz; (c) polarizationand strain for a KBNFT9 MLA as afunction of switching cycle; and (d)comparison of S-E loops for KBNFT9MLA at 1st and millionth cycle at5 Hz.142907-4 Khesro et al. Appl. Phys. Lett. 109, 142907 (2016) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.167.254.160 On: Thu, 06 Oct2016 14:53:2622K. Kalantari, I. Sterianou, S. Karimi, M. C. Ferrarelli, S. Miao, D. C.Sinclair, and I. M. Reaney, Adv. Funct. Mater. 21, 3737 (2011).23X. Qi, J. Dho, R. Tomov, M. G. Blamire, and J. L. MacManus-Driscoll,Appl. Phys. Lett. 86, 62903 (2005).24A. Khesro, R. Boston, I. Sterianou, D. C. Sinclair, and I. M. Reaney,J. Appl. Phys. 119, 054101 (2016).25D. J. Cumming, T. Sebastian, I. Sterianou, J. R€odel, and I. M. Reaney,Mater. 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Temperature Stable and Fatigue Resistant lead-free ceramics for actuators

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