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DEVELOPMENT OFHIGH ELECTRICALLY INDUCED STR AIN AND LO WHYSTERIC LOSS COMPOSITIONS FOR ACTUATORSSuryaM. upta, Z. Xu and Dwight Viehland

Department of Materials Science and Engineering, and Materials Research Laboratory,University of Illinois at Urbana Champaign,IL 6 1801ABSTRACT - Elecmcally-induced strain and polarizationstudies of La modified (1 to 10 at%) morphotropic phaseboundary Pb(M gll3N ~2/3 )03-Pb Ti03 PMN-PT) ceramicshave been performed. Charge imbalance due to La3+substitution on the A-site was compensated by : (a) B-sitevacancies, and (b) changing Mg/Nb/Ti ratio on the B-site.Tendencies of non stoichiometric ordering were found toincrease when La was compensated by B-site vacancies (La-PMN-PT) and to decrease for La compensation by changingMgMblTi ratio (PLMN-PT). Quantitative analysis usingEnergy Dispersive X-ray Spectroscopy (EDS) revealed acontinuou s decrease in Nb and corresponding increase in Ticontents for La-PMN-PT with increasing La substitution. ForPLMN-PT, no change in the B-site cation concentration wasnoticed. The spontaneous polarization and remanent strainwere found to be higher for La-PMN-PT than PLMN-PT atlow La contents (< 8at?!) and to be lower at high La contents(> 8at%). In this study, the composition 2/65/35 as found toexhibit a maximum remanent strain of 1.6 x for La-PMN-PT and 1.4 x for PLMN-PT. The effect of nonstoichiometric ordering on the d ielectri c propexties and phasestability were then investigated for the directed purpose ofdeveloping new high-performance high-strain actuators withgood micropositioning capabilities.

IntroductionA wide varieties of actuators applications inadvanced precision engineering have stimulated needfor a material with high strain and minimum hysteresisloss [l-21. Piezoelectric actuators are capable ofproducing high displacement in comparison to that inelectrostrictive actuators [3]. The main drawback ofpiezoelectric actuators is their significantly highhysteresis loss. Therefore, intensive studies have beendirected in developing electrostrictive materials. Thenewly developed materials are lead magnesium niobate(PMN) based ceramics [4-51. This material does notcontain domains, and so does not show hysteresis losswhen a field is reversed.Lead magnesium niobate, a high permittivityferroelectric material, shows large electrostrictive effectnear the temperature of its dielectric maximum. Thetransition temperature can be shifted near to roomtemperature by aIlexing with lead titanate (PbTi03).0.9 PMN-0.1 PbTi03 has been reported [6] to exhibitstrains up to - 0.1% without hysteresis at roomtemperature under an electric field of 15 KV/cm.Structural studies of PMN as a hc t io n of leadtitanate (PT) content have previously been reported byViehland et al. [7]. Viehland et al. demonstrated thepresence of polar nanodomains for PT contents less than30 at%, tweed like structure for PT contents between 30

and 35 at%, and normal micron-sized ferroelectricdomains for PT contents greater than 35 at%. Inaddition, La-modification of PMN-PT was found toresult in the evolution of polar nanodomains and relaxorcharacteristics from the tweed-like structures. Otherstudies by Hilton et al. [8] reported a reduction in the1/2[111] reflection intensity of PMN upon addition ofPT. It was then concluded that Ti modification reducesthe driving force for the ordering process.The purpose of the present study was todevelop PMN based materials with higher electricallyinduced strains and lower hysteretic losses for actuatorsapplications. Several different charge compensationmechanisms for La3+ substitution onto the A-site werestudied: (i) by B-site vacancies while maintaining theMg/Nb/Ti ratio unchanged, and (ii) by changing theMg/Nb/Ti ratio while maintaining the B-site vacancyconcentration fmed. The effect of these different La-compensation mechanisms on the electrically inducedstraia and polarization were then determined.

ExperimentalThe specimens studied in this investigationwere fabricated according to the formula: (a) Pbl-x Lax

[(Mg1/3M2/3)0.65 Ti 0.3511-x/4 0 3 (La -P m- PTx/65/35) for OcxcO.1 where La3+ is compensated bycreating B-site vacancies, and (b) Pbl-xLax[(Mgl +x/3M2-&0.65Ti( 1-d4)0.35103 ( P L m - P Td65/35) for OcxcO.1 where La3+ is compensated bychanging Mg/b/Ti ratio on the B-site. The purity ofthe starting materials was 99.9%, except for La203whch was 99.99%. The specimens were prepared bythe columbite precursor method [9]. The columbiteprecursor (MgNb206) was first prepared by mixingpredetermined amounts of MgO and Nb205 inisopropanol and ball milling for 5 hours using a ZrO2p d i n g media. The slurry was b e d at 80 OC and thepowder then calcined at 1100 O C for 4 hours. Singlephase formation o f MgNb206 was confirmed by x-raydifiaction (XRD). The columbite precursor was thenmixed and ball milled with predetermined amounts ofPbO, Ti02 and La203 powders and calcined at 850OCfo r 4 hours. The calcined powders were mixed againand pressed into cylinders using polyvinyl alcohol(PVA) as a binder under a hydrostatic pressure of 25kpsi. These cylinders were fired at 1250OC for 4 hoursafter binder burnout at 450 OC for 3 hours. Firing wasperformed in a closed alumina crucible, in order to

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minimize lead oxide loss. The calcined and sinteredspecimens were analyzed by XRD (Rigaku, D-maxCuKa radiation) for second phase formation. Thesintered blocks were then cut into thin disks andpolished on different grades of emery papers to obtainparallel surfaces. The polished surfaces wereultrasonically cleaned to remove dust particles.Electroding was carried out by sputtering gold, followedby athincoating of airdried silver paint.Field induced longitudinal strainmeasurementswere made using a linear variable differentialtransformer (LVDT) t 10Hz frequency. Electrically-induced polarization studies were performed using amodified Sawyer-Tower technique.Scanning electron micrographs were takenfrom fractured and chemically etched surfaces. Sectionof sintered specimens were etched by HC1 (Conc.) fortwo minutes. The chemically etched and fracturedsurfaces were sputtered with gold and palladium. Thechemically etched surfaces were used for quantitativestandardless energy dispersive spectroscopy and thefractured surfaces were used for grain size andmorphology determinations. Both studies were doneusing a Zeiss DSM-960 scanning electron microscopy(SEW. TEM specimenswere prepared by ultrasonicallydrilling 3-mm discs which were mechanically polishedto -100 p. he center portions of these discs werethen further ground by a dimpler to - 0 pm, and argonion-milled to perforation. Specimens were coated withcarbonbefore examination. The TEM studies were doneon a Phillips EM-400 microscope operating at anaccelerating voltage of 120kV.

Resultsand DiscussionFigures l(a)-(e) illustrate room-temperaturebright field images for La-modified PMN-PTcompositions of 0/65/35, 5/65/35, 10/65/35, L-5/65/35and L-10165135, respectively. [Specimens in which

La3+ was compensated by B-site vacancies andchanging Mg/Nb/Ti ratio are represented by x/65/35 andL-x/65/35, respectively.] For the composition 0165135(Figure l(a)), ferroelectric domains were observed.These domain structures confirm the existence of long-range ferroelectric order.OnLa-substitution, long-rangeferroelectric order was broken down and polarnanodomains became the stable domain-like structure,as can be seen in Figures l(b)-(e). The average size ofthese polar nanodomains decreased with increasing La-content for compositions in both La-PMN-PT andPLMN-PT sequences. For L-5/65/35 (Figure 1 d)),ferroelectric domains (indicated by an arrow) werefound to coexist with polar nanodomains, however nosuch coexistence was observed for 5/65/35. SAED patterns are shown in Figuresl(a)-(e) for 0165135, 5/65/35, 10/65/35, L-5/65/35, andL-10/65/35, respectively. In these figures, the presenceof superlattice reflections are indicated by arrows. For

10165135 and L-5/65/35, 1/2{11 1) superlatticereflections are clearly apparent. However, only weakdiffuse reflections were observed for 5/65/35 and L-10/65/35. In general, it was observed that thesuperlattice reflection intensity increased withincreasing La-content between 0 and 10 at.% forcompositions in the La-PMN-PT sequence, however theintensity decreased for compositions in the PLMN-PTsequence.SEM images of fracture surfaces for 0165135,5/65/35, 10165135, L-5/65/35, and L-10/65/35 are shownin Figures 2(a)-(e), respectively. It should be noticedthat the micrographs for 5/65/35 and 10165135 (Figures2(b) and (c)) clearly demonstrate a secondary phase, asindicated by arrows. However, no secondary phase wasobserved for L-5/65/35 and L-10/65/35 (Figures 2(d)and (e)). The secondary phase can be identified aspyrochlore on the basis of its octahedral morphology[lo]. The composition of the pyrochlore phase wasquantitatively determined by EDS on chemically etchedsections of sintered ceramics. The composition of thepyrochlore phase was determined to be

Relative to the composition of the perovskitegrains, the pyrochlore composition is deficient in Mgand Ti, but rich in Nb. Typical compositions of theperovskite grains for 10/65/35 and L-10/65/35 weredetermined to be

Pb0.98LaO.lNb0.41MgO.15Ti0.3502.98Pb0.96La0.1Nb0.46Mg0.12Ti0.3403.08,

respectively. The compositions of the perovskite grainsfor 0165135, 5/65/35 and L-5/65/35 are tabulated inTable I. It would seem apparent for La-PMN-PT thatLa3+ is compensated on the B-site positions by theoutward diffusion of Nb5+ nd the inward diffusion ofTi+4. Consequently, for the La-PMN-PT sequence, La-substitution results in Nb-rich and Nb-poor regions. TheNb-rich regions are pyrochlore, consistent with earlierinvestigation [lo], whereas the Ti-rich remainperovskite. For lower La-content PLMN-PTcompositions, no evidence for the diffusion of B-sitecations was observed. However, evidence for anoutward diffusion of Mg and a small inward diffusion ofNb were observed at higher La-substitutions (>8 at%)by changes in composition. Small degrees of inwarddiffusionof Nb should favor pyrochlore phase, althoughthis was not found to be the case for L-10/65/35.XR D was used to determine the amounts ofpyrochlore phase present in each specimen and arepresented in Table 1. The ratio of relative intensities ofthe (222) pyrochlore peak (Ipyro) and the (110)perovskite PMN peak ( I p m ) were used to determine

and

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the volume fraction of pyrochlore present. From Table1, it should be noticed that the volume fraction ofpyrochlore phase was increased from 0% to 14% for La-PMN-PT as La-content was increased from 0 to 10 at%.However, for PLMN-PT, no pyrochlore peaks weredetected as the La-content was increased from 0 to 8aPh, but a small amount of pyrochlore phase (-1 at.%)was detected for L-10/65/35. These result are consistentwith the compositional findings presented in sectionIII.1,where the development of a secondary pyrochlorephase in La-Ph4N-PT was attributed to the formation ofNb-rich and Nb-poor regions.

TABLE 1Comparisonofperovskiugrain compositionsand pyrochkmphase cooce~uation ithvariaticmofLa-subsitution a La-PMN-PT and PLMN-PTOmpoSltlOns0/65/35 pbl .07MRl. 18Nbo.44Tb.3102.975/65/35 pb l .OOh .@gO . 18%.4$%.3202.991016565

L-5/65/35 Pbo.9 6h.m g0.26% .42T %.3 102.96L- 10/65/35 1 Pbo.96h.l@gO.l2%.46Ti0.3403.08The value of ~ m a x30 KV/cm) is plottedagainst La-content for La-PMN-PT and PLMN-PT in

Figure 3. In this study, the composition 0/65/35 wasfound to exhibit a maximum electrically-inducedstrainof 3x10-3. In this figure, it can be noticed that La-substitution decreases ~ ~ ( 3 0V/cm) more rapidly forPLMN-PT than La-PMN-PT. The values of themaximum strain under a drive of 3OKVlcm canbe seen to be increased by minor additions of La (upto 2at%), in addition the degree of hysteresis was found toremain nearly constant for La-PMN-PT. Uponincreasing the La content to 5 at%, a significantreduction in the degree of hysteresis was observed. Onfurther increment of the La content, the value of emaxwas dramatically reduced.Clearly, in terms of electromechanicalperformance, specimens prepared by B-site vacancycompensation are superior. These specimens havehigher induced strainsand lower dielectric constants.

ConclusionsNon-stoichiometric ordering in PMN-PTcompositions close to the MPB his been investigatedby TEM, as a h c t i o n of La-content using variouscompensation mechanisms. Compensation by B-sitevacancies was found to result in secondary pyrochlorephase formation, enhanced B-site cation ordering, and adecrease in Nb-content and an increase in Ti-content ofthe perovskite phase. Electrically induced strain andpolarization measurements demonstrated that non-stoichiometric ordering leads to reduction in andthe spontaneous polarization. A small amount of La-substitution( 5at%) lead to thedevelopment of Nb-rich and Nb-poor regions andreduces Em= for La-PMN-PT. It was also shown that

compensation by B-site vacancies resulted in betterelectromechanicalperformance characteristics, suitablefor high-power transducers.AcknowledgementsThis work was supported by the Office ofNaval Research(Om)nder contract No. N00014-95-

1-0805 and by Naval Undersea Warfare Center contractNo. N66604-95-C-1536. The use of the facilities in theCenter for Microanalysis in Materials ResearchLaboratory at the University of Illinois at Urbana-Champaign s gratefully acknowledged.References1. K. Uchino,Y. suchiya, S. Nomura, T. Sato, H. Ishikawa and 0.Ikeda. "Deformable Mirror Using the PMN Elecwstrictor ". Auul.r rOatics 20.3077 (1981).2. A. K&, "A Piezoelectric Ulnasonic Motor", Jpn. J. Appl.PhyS.. 24, Suppl. 24-2,739 (1985).3. K Uchino, UPiezoelectric and Electrostrictiv e Actuators", Proc.IEEE ,61 0 (1981).4. S. Nomura and K. Uchino, "Electrostrictive Effect inPb(Mglfim/3)03-Type Materials ," Ferroelectrics , 41, 117

(1982).5. S.Nomura and K. Uchino, "Recent Application of PMN-BasedElectrostrictors", Ferroelectrics, 50 ,52 3 (1 983).6. L. E. Cross, S. . Jang, R. E. Newnham, S. Nomura and K. Uchino,"Large Electrostrictive Effects in Relaxor Ferroelectrics",Ferroelectrics, 23, 187 (1980).7. D. Viehland, M.-C. Kim, Z. Xu, and J.-F. Li, "Long-timemetastable tweedlike precursors and paraelectric clusters inferroelectrics containing strong quenched randomness", Appl.Phys. Lett. 67,247 1 (1995).8. A. D.Hilton, D. J. Barber, C. A. Randall and T. R. Shrout, "Onshort range ordering in the perovskite Lead Magnesium Niobate",J. Mat. Sci., 25,34 616 6 (1990).9. S. L. S w a m and T. R. Shrout, "Fabrication of perovskite LeadMagnesium Niobate", Mat. Res. Bull., 17, 1245-50 (1982).10. J. Chen and M. P. Harmer, "Microstructure and dielectricproperties of Lead Magnesium Niobate diphasic mixtures", J. Am.C m oc.. 73,68-73 (1990).

- a-PMN-PT -3 -X 2 -

1 -

0 ' I ' I " ' ' ' " ' '-2 0 2 4 6 8 1 0 1 2La (atyo)

Figure 3Comparison of electrically induced strain (at 30KV/cm) for various La-PMN-PT and PLMN-PT,x/65/35 compositions with La-contents between 0and 10 at%.

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