A Route Forwards to Narrow the Performance Gap betweenPZT and Lead-Free Piezoelectric Ceramic with Low Oxygen Partial Pressure

Processed (Na0.5K0.5)NbO3

Keisuke Kobayashi,,, Yutaka Doshida, Youichi Mizuno, and Clive A. Randall

Center for Dielectric Studies, Materials Research Institute, The Pennsylvania State University,University Park, Pennsylvania 16802

Materials R&D Department, R&D Laboratory, Taiyo Yuden Company, Ltd., 5607-2 Nakamuroda, Takasaki, Gunma370-3347, Japan

Reduced atmosphere ring of (Na0.5K0.5)NbO3 (NKN) ceram-ics opens up an important opportunity for lead-free piezoelec-tric materials; one of the promising applications is base metalcored multilayer actuators. Here, we extend an investigationon the eects of reduced atmosphere ring (PO2 = 10

10 atm)on the piezoelectric properties and associated point defects inNKNLiF ceramics. To access the domain dynamics contribu-tion to the converse piezoelectric properties and consider pointdefects in the sintered ceramics, Rayleigh analysis and thermalstimulated depolarization current (TSDC) were used. Thepiezoelectric properties and Rayleigh analysis of NKNLiFceramics demonstrated that the intrinsic and extrinsic piezo-electric response is essentially independent of ring atmo-sphere. Furthermore, reduced red ceramic was found to havemuch higher resistivity than in the air-red case. From theTSDC analysis, the space charge in the reduced red ceramicwas considered to be much smaller than in the air-red cera-mic, implying that the alkali volatilization was suppressed inreduced red NKNLiF ceramic.

I. Introduction

LEAD oxide-based ceramics, such as PbZrO3PbTiO3(PZT) and Pb(Ni1/3Nb2/3)O3PbZrO3PbTiO3, are wellknown for their superior piezoelectric properties1 and areused for a wide variety of sensor and actuator applications.24

However, because of the toxicity of lead, the development ofenvironmentally friendly, lead-free materials has beendesired. There has been over 15 years of extensive eorts tond lead-free piezoelectric ceramics, and there have beenmarginal successes. There are essentially two major sub-groups based on perovskite structure, including the(Na0.5Bi0.5)TiO3 (NBT)

1,57 and (Na0.5K0.5)NbO3 (NKN)systems.810 However, the basic piezoelectric coecients inall the lead-free piezoelectric ceramics are inferior for actua-tor and sensing applications.11 Therefore, without an unfore-seen, revolutionary new lead-free piezoelectric material, oneof the possible routes forward to establish equivalent dis-placements to PZT actuators is decreasing the active layerthickness and increasing the number of electrodes in theactuator stack. As an engineering solution, we have also toconsider the manufacturing cost, especially the cost of theinner electrodes. Base metals, such as Ni and Cu, have a

great cost advantage compared with the precious metals,such as silver-palladium, which is usually used as inner elec-trode in PZT multilayer actuators.12 The ability to process alead-free piezoelectric material with low-cost base metalinner electrodes oers important advances to the design ofpiezoelectric actuators with properties close to the PZT-based compositions.NBT systems are dicult to be cored with base metal,

because Bi2O3 can readily be reduced to its metallic state andform alloys with the base metal under low oxygen partialpressure (PO2) atmosphere rings. In contrast, Kawadaet al.13 recently studied the 0.96(K0.5Na0.5)NbO30.04CaZrO30.03ZrO2 system and demonstrated it is capable ofthe coring with Ni inner electrode under low PO2 atmo-spheres. Their nding opens a new opportunity of lead-freeNKN ceramic for various applications and elevates NKN asthe most important lead-free piezoelectric material for multi-layer piezoelectric applications.An important thing to be considered with low PO2 ring

of typical perovskite ceramics is the formation of oxygenvacancies, which give rise to high electronic and ionic con-ductivity.14 Furthermore, when ring NKN ceramics underambient air atmospheres, the alkali metals, especially potas-sium, can easily volatilize at around 800C and make stoi-chiometric control dicult. Therefore, a detailed discussionon defect structures, such as oxygen and alkali vacancies inreduced red ceramic, is required for designing the basemetal cored NKN ceramic. In spite of so many extensivestudies about NKN ceramics, very few studies were reportedon the low PO2 red NKN ceramics,

15,16 and the eects oflow PO2 ring on piezoelectric properties and defects struc-tures in this system are not understood. The objective ofthis study is to investigate the eects of low PO2 ring inNKN ceramics in terms of piezoelectric properties, defects,and reliability by using various methods, including thermalstimulated depolarization current (TSDC) and Rayleighanalysis.

II. Experimental Procedures

Ceramics with the composition of NKN were synthesized bya conventional solid-state method. The starting materialsused in this study were reagent grade K2CO3, Na2CO3, andNb2O5 (>99.5% purity; Alfa Aesar, Ward Hill, MA). Thestarting materials were dried at 200C for 24 h to removeabsorbed water. These powders were then accurately weighedin stoichiometric ratios and mixed with zirconia media andethanol by ball milling for 24 h. After calcination at 900Cfor 5 h, 5 mol% of Lithium Fluoride (LiF) powder (98.5%purity; Alfa Aesar) was added to 100 mol% of NKN powderas a sintering aid.17 The powders were subsequently granu-

D. Damjanoviccontributing editor

Manuscript No. 31047. Received February 07, 2012; approved April 11, 2012.Author to whom correspondence should be addressed. e-mail: kei-koba@jty.

yuden.co.jp

1

J. Am. Ceram. Soc., 16 (2012)

DOI: 10.1111/j.1551-2916.2012.05266.x

2012 The American Ceramic Society

Journal

lated and pressed into disk-shaped pellet of 10 mm diameter.The pellets were sintered in two dierent conditions: (1)1020C peak hold temperature for 2 h in air atmosphere and(2) 1020C for 2 h in reduced atmosphere withPO2=10

10 atm. Hereafter, we shall refer to these ceramics asair and reduced red NKNLiF, respectively. The oxygenpartial pressure of the ambient atmosphere during ring wascontrolled by N2 gas and 95 vol% N2/5 vol% H2 mixturegas. The heating and cooling rate was 5C/min. For dielectricand piezoelectric measurements, samples were polished to0.5 mm thickness, and platinum electrodes were sputtered bysputter coater (SCD 050; Bal-Tec, Balzers, Liechtenstein). Tomeasure piezoelectric properties, samples were poled at30 kV/cm and 150C for 15 min and aged 1 week prior tomeasurement. The electromechanical coupling factor (kp) andthe mechanical quality factor (Qm) were evaluated by the res-onanceantiresonance method, using an impedance analyzer(Model HP4294A; Agilent Technologies Inc., Santa Clara,CA). The piezoelectric coecient d33 was measured by Ber-lincourt d33 meter (Model ZJ-2; Institute of Acoustics Acade-mia Sinica, Beijing, China) at 20 Hz. The converse strainresponse was measured with a frequency of 1 Hz and sinewaves, using a modied Sawyer-Tower circuit and a linearvariable dierential transducer strain transducer. The micro-structure of the ceramics was observed using scanning elec-tron microscope (Model S-3500; Hitachi Corp., Tokyo,Japan).

(1) Thermal Stimulated Depolarization CurrentmeasurementsFor investigating the eects of ring atmosphere on thedefect types in the ceramics, we used the TSDC method.1821

TSDC is a powerful method for clarifying the defects types,relative concentrations, and energies of the relaxation pro-cess. For obtaining the TSDC spectrum, the ceramics wereelectrically poled under a high DC polarizing eld of 16 kV/cm and 200C for 15 min. After that, the sample was rapidlycooled down to room temperature, then the DC eld wasturned o. Finally, samples were heated from 75C to 500Cat a rate of 3C/min. The current density was recorded in thesteady state condition by picoammeter meter (ModelHP4140B; Agilent Technologies Inc.) attached to the ceram-ics under short circuit conditions. The depolarization currentis typically related to relaxation of orientated associateddefect dipoles, trap charges, and/or space charge migration.The detailed explanation of TSDC and experimental congu-rations can be found elsewhere.18,20

(2) Rayleigh AnalysisThe Rayleigh model, which was originally developed for ana-lyzing the nonlinear response of ferromagnetic materials,22

has been successfully applied to ferroelectric materials, suchas Pb(Zr, Ti)O3, BiScO3-PbTiO3, NKN, BaTiO3 in bulkceramics, and thin lms.2327 It has proven to be a very use-ful method to describe the nonreversible and reversibledomain wall motion contributions to dielectric and piezoelec-tric properties in both bulk and thin lm form. The Rayleighmodel can describe the linear dependence of piezoelectricconstant on the electric eld amplitude, and this is one of thesimplest approaches for obtaining details on nonlinear phe-nomenon and the relative contributions due to the domainmotion. The eld applied is conned to the so-calledRayleigh region, where the density and structure of thedomain walls remain unchanged with AC eld cycle, and isapproximately one-half to one-third of the coercive eld.Under these conditions, the real part of piezoelectric constantd will increase linearly with the amplitude of the appliedelectric eld (E0). This follows from the Rayleigh relation-ships describing the induced peak strain xmax and theconverse piezoelectric coecient,

xmax dinitE0 adE20 (1)

d 0 dinit adE0 (2)

where dinit and ad are the initial reversible piezoelectricresponse at zero electric eld and the piezoelectric Rayleighcoecient, respectively. The dinit may include both intrinsicand extrinsic contributions. Any extrinsic contribution con-sidered in dinit must be due to a reversible (lossless) process.The relative magnitude of ad is an excellent indicator of theextrinsic domain wall contribution to piezoelectric and dielec-tric properties of ferroelectric.22 The relative contributions ofthe domain wall motion can be sensitive to composition, fer-roelectric phase, grain size, substrate clamping in thin lms,and secondary phase transitions, such as octahedral tilting.27

The ad and dinit values can be experimentally determinedfrom the slope and intersection of the linear plot of the realpart of d33 as a function of E0, which can be obtained by theconverse piezoelectric strain-eld subswitching loops mea-sured at various E0 amplitudes.

27

III. Results and Discussions

A comparison of the microstructure of air and reduced redNKNLiF ceramics is shown in Fig. 1; dierences werefound in grain morphology and grain size. The reduced redceramics consisted of round-shaped and larger grains com-pared with the air-red ceramics. The relative geometricdensities of the air red and reduced red ceramics were alsosignicantly dierent: 91% and 95%, respectively.The basic dielectric and piezoelectric properties of the

samples are summarized in Table I. The dielectric and piezo-electric properties of reduced red sample were comparableto that of air-red sample, and our results of air-red cera-mic are in good agreement with other literature with similarcompositions.28 It should be noted that the electrical resistiv-ity of reduced red ceramic was much higher than air-redsamples. This is quite dierent from the experience in lowpartial pressure ring with BaTiO3 dielectrics; the resistivityin that case is typically greater in the air-red case.14 Thisresult implies that there are lower concentrations of pointdefects in the reduced red materials.Figure 2 shows the temperature dependence of permittivity

over the temperature range of 25500C. There were twodielectric anomalies observed over this range associated withthe ferroelectric orthorhombictetragonal phase transition ataround 130C and the ferroelectric tetragonalparaelectriccubic phase transition at higher temperature. The reducedred ceramics had higher dielectric constant at temperaturesabove 130C; this is mostly associated with the higher densityof the reduced red ceramic, as the magnitudes are consistentwith the density correction through a the logarithmic mixinglaw.29 The phase transition behavior of our air and reducedred ceramics are essentially consistent with 0.95(Na0.5K0.5)NbO30.05LiNbO3 solid-state ceramics.

30 It has beenreported by Guo et al. that the phase transition temperaturebetween the orthorhombic and tetragonal ferroelectric phases(TO-T) and Curie temperature (TC) are linearly dependent onthe amount of x in (1 x)(Na0.5K0.5)NbO3xLiNbO3 solid-state system between 0 < x

temperature ferroelectricparaelectric phase transition is therst-order transition. The Curie constant was the same inboth of two samples, and the magnitude of these parametersagrees with earlier values obtained for alkali niobate materi-als.31 The thermal dierence DT (=To Tc) was found to bedierent between air and reduced red ceramics, which canbe explained by taking into consideration the eects of lowpermittivity grain boundary phases. Applying a diphasic ser-ies dielectric model32 to the dielectric temperature data, it isinferred that an air-red ceramic has a thicker grain bound-ary phase than a reduced red one. The grain boundarystructure and chemistry plays an important role forconduction mechanism in this system, but this needs moredetailed investigation, for example, direct observation using

transmission electron microscope and detailed impedanceanalysis, but these are beyond the scope of this study.Figure 4 shows the real part of piezoelectric coecient d33

as a function of applied electric AC eld E0, calculated bystrain-eld loops measured at various electric elds (see insetof Fig. 4). Our ceramics exhibited the linear behaviorbetween the d33 and E0, which is consistent with Rayleighbehavior (Eq. (2)). The nonlinearity of strain-eld loops wasreadily accounted for through Eq. (1). The Rayleigh parame-ter dinit and ad was estimated by Eq. (2); the results are listedin Table III. The error was estimated from a least squaresanalysis of the Rayleigh plot. There were no signicantdierences in dinit and ad between air and reduced red

Table I. Characteristic Properties of NKNLiF Ceramic Fired in Reduced and Air Atmosphere

Properties Dielectric constant Dielectric loss [%] d33 [pC/N] kP [%] Qm Resistivity [m]

Reduced red 415.6 3.3 138 36 65.9 2.70 9 109

Air red 479.4 2.5 152 38 75.8 5.49 9 108

100 200 300 400 5000

1000

2000

3000

4000

5000

6000

Reduced fired Air fired

Temperature (oC)

Die

lect

ric c

onst

ant

0.0

0.2

0.4

0.6

0.8

1.0

Die

lect

ric lo

ss

O -T C

Reduced fired 132 o 456 oCAir fired 106 o 463 oC

T TCC

Fig. 2. Dielectric constant (left axis) and dielectric loss (right axis)of NKNLiF ceramics as functions of temperature at 1 kHz. Theorthorhombictetragonal (TO-T) and tetragonalcubic (TC) phasetransition temperatures are listed in the inset.

50 m

(a)

(c)

(b)

(d)

50 m

5 m

5 m

Fig. 1. SEM micrograph of NKN-LiF ceramics red in (a), (b) reduced atmosphere and (c), (d) air atmosphere.

400 420 440 460 480 5000.0000

0.0005

0.0010

0.0015

Reduced fired Air fired

1/

Temperature (oC)

T0

TC

Fig. 3. The inverse dielectric susceptibility (1/v) as a function oftemperature at 1 kHz for reduced red ceramic (solid circle) and air-red ceramic (solid triangle). The dotted lines represent ts with theCurieWeiss law.

Piezoelectric Properties of Low Oxygen Partial Pressure Processed (Na0.5K0.5)NbO3 3

ceramics, implying that the intrinsic and extrinsic contribu-tions, which are mainly related with crystal lattice deforma-tion and the non-180o domain dynamics, respectively, werenot aected by the changes in ring atmosphere. The Ray-leigh coecients estimated from this study are consistentwith the value of Li0.04(Na0.5K0.5)0.96NbO3,

26 and 23 timessmaller than PZT and BiScO3PbTiO3 piezoelectric ceramicsmaterials at their morphotropic phase boundaries.27

Figure 5 shows the TSDC spectra. The origin of TSDCpeaks found in Fig. 5 was identied by the polarization elddependence of TSDC peaks, as discussed by Liu et al.18,19;these peaks are noted in Fig. 5. There was a large spacecharge peak indicated in air-red ceramics, while in reducedred ceramics, only a small defect dipole peak was observed.The space charge behavior is associated with migration ofoxygen vacancies,19 and so these data imply that despite r-ing in low PO2, there is a lower concentration of oxygenvacancies in this case. That, in turn, implies that the low PO2suppresses the volatilization of the alkali oxides.Further evidence for improvement in the defect concentra-

tions can be inferred through the current density. The currentdensity against the applied eld (Fig. 6) indicated that thematerials had Ohmic-like conductivity behavior in the mea-surement range for both the air and reduced red materials.

This is also dierent from the reduced ring cases of basemetal dielectrics, such as BaTiO3, where the conductionmechanism is Schottky below the paraelectric transition, andspace charge limited current above the Curie temperature.33

From the physical characterization in this study, theimportant nding was that piezoelectric and dielectric prop-erties in NKNLiF ceramic are not aected by the reducedring. An additional remark that should be made here is thatthe resistivity and densication were signicantly improvedin reduced ring. These observations indicate that NKNmaterial is highly compatible with base metal coring. Below,we discuss a rationale for the superior performance ofreduced red NKN ceramic from these preliminary observa-tions.First, volatilization of alkali metal is an important aspect

of the process. The sintering of NKN has long been knownto have sintering problems associated with the volatility ofalkali metals, Na and K, through their associated suboxides.The volatilization process of alkali metal can be expressed asfollowing equation:

2MM OO$M2O" 2V0M VO (3)

where the M indicates alkali metal. Similar to PZT, therecould be a superoxidation process on cooling in air ring,changing the compensation from ionic to an electronic p-typeconduction, which possibly occurs in the grain boundaryregions:

2V0M VO 1=2O2$2V0M OO 2h (4)

Considering that reduced red ceramic did not show sig-nicant space charge peak in the TSDC spectrum (Fig. 5),

0 2 4 6 8 10100

120

140

160

180

200

220 Reduced fired Air fired

Piez

oele

ctric

coe

fficie

nt d

33 (p

m/V

)

Applied AC field E0 (kV/cm)

-6 -4 -2 0 2 4 6

-0.010

-0.005

0.000

0.005

0.010

Str

ain

(%)

Field (kV/cm)

Reduced fired Air fired

Fig. 4. Piezoelectric coecient d33 as a function of applied ACeld. The dashed lines represent linear ts with the Rayleighequations (Eq. (2)). Inset: The strain-eld loops of reduced and air-red ceramics measured at 6 kV/cm and 1 Hz.

Table III. Rayleigh Coecients for NKNLiF Ceramics.The Error Estimated From the Least Squares Analysis is

Listed Below each Coecient. For Comparison, the Data ofLi0.04(N0.5K0.5)0.96NbO3,

26 PZT with MPB Structure,27 and0.36BiScO30.64PbTiO3 (BSPT) with MPB Structure

27 arealso Shown

Properties ad 1016 [m2/V2] dinit 10

12 [m/V]

Reduced red NKNLiF 1.25 (0.06) 98 (2.1)Air-red NKNLiF 1.14 (0.03) 104 (3.7)Li0.04(N0.5K0.5)0.96NbO3 1.14 (0.06) 97.1 (3.0)PZT MPB 2.43 (0.03) 421 (3.0)BSPT MPB 3.34 (0.14) 490 (7.0)

100 200 300 400 5000

-50

-100

-150

-200

TSD

C (n

A)

Temperature (oC)

Reduced fired Air fired

Pyroelectric peak (Tetra. Cubic)

Defect dipole

Space charge

Space charge

Pyroelectric peak (Ortho. Tetra.)

Fig. 5. Thermal stimulated depolarization current (TSDC) spectrumsof reduced and air-red ceramic.

Table II. CurieWeiss temperature (TO), Curie Temperature(TC), and Curie Constant (C) estimated from Fig. 3

Properties TO [oC] TC [

oC] DT (=TC TO) [oC] C [oK]

Reducedred

417.5 455.6 38.1 2.27 9 105

Air red 405.0 463.3 58.3 2.28 9 105

0.1 1 10

1E-10

1E-9

1E-8

1E-7 Air fired Reduced fired

Cur

rent

den

sity

(A/c

m )2

Electric field (kV/cm)

Fig. 6. Current density as a function of applied DC electric eld.The dashed lines are linear t results of the experimental data.

4 Journal of the American Ceramic SocietyKobayashi et al.

the volatility of alkali metals is considered to be suppressedin reduced red ceramic. Combination of Eqs. (3) and (4)gives

2MM 1=2O2gas$M2Ogas 2V0M 2h (5)

Therefore, under reduced ring condition, because of thelower oxygen partial pressure minimizes the volatility of thealkali suboxide, the formation of intrinsic alkali vacanciescan be suppressed. This is consistent with the ndings ofShigemi and Wada, who showed the formation energy ofalkali and oxygen vacancies calculated by using a plane wavepseudo-potential method.34,35 According to those calcula-tions, the formation energies of K and Na vacanciesincreased as conditions changed from oxidizing to reducing,whereas that of oxygen vacancy decreased. The results implythat oxygen vacancy is preferred, and the formation of alkalivacancies can be suppressed in reduced atmospheres. This isalso consistent with the observed morphology of the NKNgrain (Fig. 1) because the point defect, such as oxygenvacancy, also causes an increase in the conguration entropy,which in turn impacts the morphology of the crystal, speci-cally crating the rounding of the ceramic grain.36,37 Fisheret al.15 discussed the relationship between concentration ofoxygen vacancies and grain morphology based on the two-dimensional nucleation-controlled theory and reported thathigher concentration of oxygen vacancies can lead to therounding shape of NKN grain.Second, the LiF added as a sintering aid should also be

discussed. Considering the ion radii of Li (1.25 for 12-foldcoordination, 0.76 sixfold coordination) is close to that ofboth A-site atoms (K: 1.64 and Na: 1.39 ) and B-siteatom (Nb: 0.68 ) in NKN,38 Li can substitute into both Aand B-site of NKN lattice. These reactions are expressed byfollowing equations:

2NaNa Li2O!Na2O 2LiNa(A-site substitution) (6)

2NbNb Li2O 4OO!Nb2O5 4VO 2Li0000Nb (B-site substitution) (7)

When the Li+ substitutes into B-site of NKN lattice, itworks as an acceptor and forms the defect dipole2VO Li0000Nb. This is considered one possible origin of thedefect dipole peak observed in TSDC spectrum, as shown inFig. 5. Considering that this defect dipole peak was notfound in air-red ceramic, B-site substitution of Li is consid-ered to occur only in reduced ceramics. This is consistentwith the hypothesis that there is a suppression of the alkalisuboxide volatility in the low PO2 atmosphere. Although thisis highly speculative at this time, there is supporting evidenceby the shift of phase transition temperature, as shown inFig. 2.Taking these two mechanisms into consideration, air-red

ceramic has p-type conduction, because alkali vacancies V0M(M = Na, K) preferably formed in air ring condition. Thisalkali volatilization also results in the weakly associateddefects dipole as 2V0Na VO , which easily dissociates andresults in the space charge, which limits the dielectric proper-ties and reliability in the ceramics. On the other hand, inreduced red ceramic, as volatilization of alkali metals wassuppressed, the concentration of alkali vacancies wasmuch smaller, and consequently p-type conduction can besuppressed.Reduced red ceramics usually have an n-type conduction

because the oxygen vacancies form intrinsically by low- PO2atmosphere:

OO!1=2O2" VO 2e0 (8)However, as Li substituted into B-site can work as

an acceptor, this n-type conduction is also suppressed. In

addition, in this system, F-ion possibly substitutes into theoxygen vacancies and partially compensates the oxygenvacancies:

FF VO 2e0!FO e0 (9)

n 2VO FO (10)These mechanisms suggest that the balance of n-type and

p-type conduction in NKNLiF can be controlled by partialoxygen pressure. Figure 7 shows the conductivity of NKNLiF ceramics red in various PO2. Considering the discussionabove, the crossover through the change from air to low PO2ring is caused by the change of conduction mechanism fromp-type (higher PO2) to n-type (lower PO2). In the PO2 rangebetween 1.0 9 104 and 1.0 9 1010 atm, conductivity wasnot changed signicantly because of the ionic compensationof the donor, which minimizes the conductivity in this sys-tem.As shown in Fig. 4, the nonlinear piezoelectric response,

which is mainly related with irreversible non-180o domainmotion, was independent of the sintering atmosphere. As iswell known, the electrically charged defects, such as oxygenvacancies, particularly, are thought to cause domain wallpinning, and suppress the nonlinear piezoelectric response inpiezoelectric ceramics.39,40 However, in this system, the oxy-gen and alkali metal vacancies are considered to be formedmainly in the grain boundary regions, and the piezoelectricproperties are mostly related with the center grain region.The clamping eect of the oxygen vacancies should occur inthe grain boundary region and aect the piezoelectricresponse; however, this contribution is relatively small.According to the discussion above, the reduced ring in

NKNLiF ceramic is considered to have various advantages:higher resistivity, higher ceramic density, and comparablepiezoelectric properties. These features are very important forpractical application of NKN-based system, because theyallow base metal coring of NKN ceramics and should opena new opportunity for NKN-based lead-free piezoelectricmaterials. The piezoelectric and dielectric properties of Ni-cored multilayer NKN ceramic are currently under studyand will be reported in a future article.Future research directions need to consider sintering in

low PO2 conditions, and detailed compositional design underthese conditions, for both piezoelectric and reliability perfor-mance. Cations, such as Sn2+, Sb4+, Ca2+, Zr4+, La3+, areall of interest to this system under these processing condi-tions.

IV. Conclusions

In this study, the eects of ring atmosphere on the piezo-electric properties in NKNLiF ceramics were investigated.

10-13 10-11 10-9 10-7 10-5 10-3 10-1 10110-13

10-12

10-11

10-10

Oxygen partial pressure (atm)

Con

duct

ivity

((m

) )-1

Fig. 7. Conductivity of the NKNLiF ceramics red at variouspartial oxygen pressures (PO2). The dashed lines are guide to the eye.

Piezoelectric Properties of Low Oxygen Partial Pressure Processed (Na0.5K0.5)NbO3 5

The primary intrinsic or extrinsic piezoelectric and dielectricproperties of NKNLiF ceramics were not aected by ringatmosphere. Furthermore, reduced red ceramics had muchhigher resistivity than air-red ceramics, which is explainedby taking into consideration that reduced ring can suppressthe volatility of alkali metals, and B-site substituted Li worksas an acceptor. These results are important from practicalviewpoint, because they open various new opportunities foralkali niobate-based lead-free piezoelectric materials as low-cost multilayer piezoelectrics, which can be manufactured toengineer equivalent actuator and sensing performance to thePZT.

Acknowledgments

The authors gratefully acknowledge technical assistance from Je Long, thelate Paul Moses, Steven Perini, and Amanda Baker at The Pennsylvania StateUniversity. Keisuke Kobayashi also wishes to thank Taiyo Yuden, fromwhom he received a fellowship to study as a visiting scientist at The PennState University. Clive Randall also wishes to thank the National ScienceFoundation for support of this program, as part of the Center for DielectricStudies under Grant No. 0628817. Thanks also go to the anonymous reviewswho raised some points that, in answering, hopefully improved the article andits impact.

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