Poling Field Dependence of Piezoelectric Properties and Hysteresis Loops

of Polarization versus Electric Field in Alkali Niobate Ceramics

Toshio Ogawa, Masahito Furukawa1, and Takeo Tsukada1

Department of Electrical and Electronic Engineering, Shizuoka Institute of Science and Technology,

2200-2 Toyosawa, Fukuroi, Shizuoka 437-8555, Japan1Materials and Process Development Centre, TDK Corporation, 570-2 Matsugashita Minamihatori, Narita, Chiba 286-8588, Japan

Received June 2, 2009; accepted June 16, 2009; published online September 24, 2009

The DC poling field dependence of piezoelectricity was investigated to evaluate the mechanism of domain alignment in lead-free ceramics

of the form of (1� x )(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 (x ¼ 0{0:07) by comparison with those of Pb(Zr,Ti)O3 (PZT), PbTiO3 (PT), and

BaTiO3 (BT) ceramics. Poling was conducted at 150 �C for 30min while varying the poling field (E ) between �4:0 kV/mm. By increasing xfrom 0 to 0.07, the relative dielectric constant ("r), electromechanical coupling factor in the planar mode (kp), frequency constant in the kpmode (fcp), and piezoelectric strain constant (d33) vs E plots showed domain clamping at a specific E . E was the coercive field estimated

from the DC poling field dependence. The changes in "r, kp, fcp, and d33 with E became smaller at x ¼ 0:06{0:07, because of the proximity

to the paraelectric phase. The ceramics with x ¼ 0, such as PT or BT ceramics, show similar "r, kp, fcp, and d33 vs E plots. The maximum kp(48%) and d33 (307pC/N) were obtained for x ¼ 0:05 with the lowest fcp of 2964Hz�m, as shown in the "r, kp, fcp, and d33 vs E plots for

ceramics such as tetragonal hard PZT ceramics. Since domain alignment in the ceramics was accompanied by the deformation of crystals

as a result of applying the poling field, it was clarified that the lead-free ceramics must have a high kp and high d33 to realize a low fcp, which

corresponds to a low Young’s modulus. In addition, the optimal ceramic composition was obtained in the typical domain-clamping state

from the poling field dependence. Furthermore, it was found that a higher kp was realized at a larger remnant polarization and a smaller

coercive field in a symmetrical hysteresis loop of polarization vs electric field (P–E hysteresis loop), because of the easy alignment of

ferroelectric domains by applying a poling field. The results of the poling field dependence were also supported by results of expansion

strain measurement. # 2009 The Japan Society of Applied Physics

DOI: 10.1143/JJAP.48.09KD07

1. Introduction

Material research on lead-free piezoelectric ceramics hasreceived much attention because of global environmentalconsiderations. The key practical issue is the difficulty inrealizing excellent piezoelectric properties, such as electro-mechanical coupling factors and piezoelectric strain con-stants. The planar coupling factor of a disk (kp) is closelyrelated to the degree of orientation of ferroelectric domainsin the DC poling process. We have already shown themechanism of domain alignment in Pb(Zr,Ti)O3 (PZT),1–5)

PbTiO3 (PT),6) and BaTiO3 (BT)

7) ceramics and in a relaxorsingle crystal of Pb[(Zn1=3Nb2=3)0:91Ti0:09]O3 (PZNT)8) bymeasuring the piezoelectricity vs DC poling field character-istics. In this study, the poling characteristics, especially theDC poling field dependences of dielectric and piezoelectricproperties, were investigated in lead-free ceramics andcompared with those of PZT, PT, and BT ceramics.Moreover, a direction for new research on lead-free ceramicswith higher piezoelectricity is proposed to study themechanism of domain alignment on the basis of the polingfield dependence.

The well-known lead-free ceramics focused on in thisstudy are composed of bismuth titanate,9) alkali niobate,10)

and barium titanate.11) These ceramics possess specificcharacteristics as follows: low piezoelectric strain constant(d33), low relative dielectric constant ("r), high mechanicalquality factor (Qm), and high Curie temperature (Tc) forbismuth titanate; high d33 and relatively high Tc of over250 �C for alkali niobate; high d33 and low Tc of approx-imately 130 �C for barium titanate. Since the most promisingcandidate for replacing PZT ceramics is an alkali niobateceramic owing to its large piezoelectricity for practicaluse, we selected lead-free ceramics composed of alkaliniobate to investigate the mechanism of ferroelectric domainalignment.

2. Experimental Procedure

The lead-free ceramics evaluated are of the form of(1� x)(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 (x ¼ 0{0:07)with a small amount of MnO,12) the compositions of whichwere chosen through research by trial and error on thechemical compositions necessary to obtain a relatively highd33 of over 200 pC/N in alkali niobate. The ceramics werefabricated by a conventional ceramic manufacturing processunder firing conditions of 1100–1200 �C for 2 h. The DCpoling temperature and poling time were fixed at 150 �C and30min, respectively, for the ceramic disk (dimensions:14mm� � 0:5mmT) with a Ag electrode, while the DCelectric field was gradually (0.2 kV/mm or 0.25 kV/mmor 0.5 kV/mm each) varied from E ¼ 0 ! þ4:0 ! 0 !�4:0 ! 0 to +4.0 kV/mm. After each poling, the dielectricand piezoelectric properties were measured at room temper-ature using an LCR meter (HP4263A) and a precisionimpedance analyzer (Agilent 4294A), respectively. Hystere-sis loops of polarization vs electric field (P–E hysteresisloops) measured at 150 �C were observed using a ferro-electric test system (Radiant RT6000HVS) by applying abipolar triangle pulse, the period of which was 400ms.Moreover, the expansion strain in the thickness (0.5mm)direction of the fully poled ceramics (poling conditions:E ¼ 4:0KV/mm at 150 �C for 30min) was measured usinga photonic sensor (MTI Instruments MTI-2000) whileapplying a unipolar triangle pulse (period: 50ms) in thepoling direction.

3. Results and Discussion

3.1 Dielectric and piezoelectric propertiesFigures 1(a)–1(d) respectively show the relationshipsamong the SrZO3 (SZ) composition (x) vs "r, kp, thefrequency constant in the kp mode ( f cp), and d33 for(1� x)(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 (x ¼ 0{0:07)

Japanese Journal of Applied Physics 48 (2009) 09KD07 REGULAR PAPER

09KD07-1 # 2009 The Japan Society of Applied Physics

with a small amount of MnO. DC poling was conducted at150 �C for 30min by applying a poling field of 4.0 kV/mm.Although "r slightly decreased at x ¼ 0:02, "r increased withx, and a maximum "r of 1931 was obtained at x ¼ 0:06.

A maximum kp of 48% was realized at x ¼ 0:04. Therelationship between f cp (half of the bulk wave velocity)and x shows a reverse tendency in the case of kp vs x.The maximum-kp composition region (0:04 5 x 5 0:06)corresponds to the minimum-f cp composition region(0:04 5 x 5 0:06). It was considered that a higher crystalorientation (domain alignment) produced by DC polingcould be achieved by softening the ceramics, which meansthat the ceramics should have a low Young’s modulus (lowf cp). A maximum d33 of 307 pC/N was obtained at x ¼ 0:05because of the difference in composition for realizing themaximum "r (x ¼ 0:06) and maximum kp (x ¼ 0:04).

3.2 P–E hysteresis loopsFigures 2(a)–2(c) show the hysteresis loops measured at150 �C under various applied fields (E), with a maximum Eof �5:0 kV/mm, at the compositions x ¼ 0, 0.05, and 0.07,respectively. The loop at x ¼ 0:05 shows a typical ferro-electric P–E loop such as the loop of BT, while the loop atx ¼ 0 was similar to the loop of PT, which is an asym-metrical (propeller shape) loop. The propeller-type loopsobserved were caused by mechanical stress generated byapplying the polar field. Since the loop at x ¼ 0:07 stillshows ferroelectricity accompanied by the existence of asmall dielectric loss, it is narrow in comparison with that atx ¼ 0:05. Therefore, at compositions of x > 0:07, theceramic crystal phase approaches the paraelectric phasewith decreasing "r and Tc.

The E dependences of remnant polarization (�Pr) andcoercive field (�Ec) [see Fig. 2(a)] are illustrated inFigs. 3(a) and 3(b), respectively. The Pr vs E curves shiftupward with an increase in x from 0 to 0.05 and then shift

Fig. 1. SrZrO3 composition (mol%) dependences of (a) relative

dielectric constant ("r), (b) planar coupling factor (kp), (c) frequencyconstant (fcp), and (d) piezoelectric strain constant (d33) in

(1� x )(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 (x ¼ 0{0:07) with a small

amount of MnO.

-25-20-15-10-505

10152025

-6000 -4000 -2000 0 2000 4000 6000

P (µ

C/c

m2)

E (V/mm)

+Pr

-Pr

+Ec-Ec

-20

-15

-10

-5

0

5

10

15

20

-6000 -4000 -2000 0 2000 4000 6000

P (µ

C/c

m2)

E (V/mm)

-25-20-15-10-505

10152025

-6000 -4000 -2000 0 2000 4000 6000

P (µ

C/c

m2)

E (V/mm)

(a)

(b)

(c)

Fig. 2. P–E hysteresis loops at (a) x ¼ 0, (b) x ¼ 0:05, and (c)

x ¼ 0:07 for (1� x)(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 with a small

amount of MnO at various applied fields (E ). The maximum E is

�5:0 kV/mm. The remnant polarization (�Pr) and coercive field (�Ec)

are shown in (a).

0

5 67

4

0

2

57

(a)

(b)

Fig. 3. Applied field dependences of remnant polarization (�Pr) and

coercive field (�Ec) at compositions with x ¼ 0{0:07. The numbers

0–7 in the figures correspond to the mol% of x .

Jpn. J. Appl. Phys. 48 (2009) 09KD07 T. Ogawa et al.

09KD07-2 # 2009 The Japan Society of Applied Physics

downward with an increase from x ¼ 0:06 to 0.07. Inaddition, the degree of asymmetry in the Ec vs E curvedecreased and þEc specifically decreased with an increasefrom x ¼ 0 to 0.05. Since the mobility of ferroelectricdomains during the application of the polar field wassuccessfully evaluated by the shape of the P–E loop andthe relationships between Pr=Ec and E, it was found thatthe reason for obtaining a higher kp at x ¼ 0:05 was thesymmetrical P–E loop, larger Pr, and smaller Ec observed.

3.3 Poling field dependenceFigures 4–7 respectively show the effects of a DC polingfield (E) on "r, kp, f cp, and d33 at various SZ compositions xwhen E was varied from 0 to �4:0 kV/mm. By increasing xfrom 0 to 0.07, the relationships of "r, kp, f cp, and d33 with Eshow domain clamping at a specific E (Fig. 8). It wasconsidered that the minimum "r, kp, and d33 and themaximum f cp, owing to electrical domain clamping, denotedby "# [the arrow (") means domain alignment], occurred atcoercive fields (Ec) corresponding to a specific E, asmentioned earlier. Ec decreased with increasing x from x ¼0:02 to 0.07. The changes in the "r, kp, f cp, and d33 vs Ecurves became smaller at x ¼ 0:06{0:07, because of theproximity to the paraelectric phase, with abrupt decreases inkp and d33. The ceramics at x ¼ 0 such as BT ceramics7)

show "r and f cp vs E characteristics, whereas those at x ¼ 0

such as PT ceramics6) show kp vs E characteristics. In

addition, asymmetrical shapes in the "r, kp, f cp, and d33 vsþE curves, and in the "r, kp, f cp, and d33 vs �E curves wereobserved for ceramics with x ¼ 0 and 0.02 because theceramics showed nonuniform mobility of domain alignmentupon the application of positive and negative poling fields.However, the ceramics with x ¼ 0:05 exhibited symmetricalshapes in their "r, kp, and f cp vs E curves as did tetragonalPZT hard ceramics.1,3–5) Moreover, the maximum kp (48%)and maximum d33 (307 pC/N) were obtained at the lowestf cp of 2964Hz�m at x ¼ 0:05. Since the domain alignment inthe ceramics was accompanied by the deformation of thecrystal under a DC poling field, it was clarified that a high kpand high d33 in the lead-free ceramics are necessary torealize low f cp (low Young’s modulus) in ceramic compo-sitions, as shown in Fig. 6 (x ¼ 0:05 at E ¼ �4:0 kV/mm).Moreover, the optimal ceramic composition shows thatthe typical domain clamping in the "r, kp, f cp, and d33 vsE curves occurred simultaneously at E ¼ þ0:50 and�0:75 kV/mm (x ¼ 0:05 in Fig. 8), which correspond tothe strict threshold (Ec) of domain switching. The differencebetween E ¼ þ0:50 and �0:75 kV/mm was thought to bethe effect of domain alignment caused by the DC poling ofas-fired (virgin) ceramics at E ¼ 0 ! E ¼ þ4:0 kV/mm.

3.4 Expansion strain characteristicsFigure 9(a) shows the dependence of the expansion strainmeasured at room temperature on the electric field (E)

x=0 x=0.02 x=0.04

x=0.05 x=0.06 x=0.07

0

500

1000

1500

2000

2500

-6000-4000-2000 0 2000 4000 6000

ε r

E (V/mm)

0

500

1000

1500

2000

2500

-6000-4000-2000 0 2000 4000 6000

ε rE (V/mm)

0

500

1000

1500

2000

2500

-6000-4000-2000 0 2000 4000 6000

ε r

E (V/mm)

0

500

1000

1500

2000

2500

-6000-4000-2000 0 2000 4000 6000

ε r

E (V/mm)

0 -6000-4000-2000 0 2000 4000 6000

ε r

E (V/mm)

0

500

1000

1500

2000

2500

500

1000

1500

2000

2500

-6000-4000-2000 0 2000 4000 6000

ε rE (V/mm)

Fig. 4. DC poling field dependence of "r at compositions with

x ¼ 0{0:07 for (1� x )(Na,K,Li,Ba)(Nb0:9Ta0:1)O3–xSrZrO3 with a

small amount of MnO.

x=0 x=0.02 x=0.04

x=0.05 x=0.06 x=0.07

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

k p(%

)

E (V/mm)

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

k p(%

)

E (V/mm)

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

kp

(%)

E (V/mm)

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

kp

(%)

E (V/mm)

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

kp

(%)

E (V/mm)

0

10

20

30

40

50

-6000-4000-2000 0 2000 4000 6000

kp

(%)

E (V/mm)

Fig. 5. DC poling field dependence of kp at compositions with

x ¼ 0{0:07.

x=0 x=0.04x=0.02

x=0.05 x=0.06 x=0.07

2900

3000

3100

3200

-6000-4000-2000 0 2000 4000 6000

fcp

(Hz

· m)

fcp

(Hz

· m)

fcp

(Hz

· m)

E (V/mm)

2900

3000

3100

3200

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

2900

3000

3100

3200

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

2900

3000

3100

3200

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

2900

3000

3100

3200

-6000-4000-2000 0 4000 6000

E (V/mm)

2900

3000

3100

3200

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

fcp

(Hz

· m)

fcp

(Hz

· m)

fcp

(Hz

· m)

2000

Fig. 6. DC poling field dependence of fcp at compositions with

x ¼ 0{0:07.

x=0 x=0.02 x=0.04

x=0.05 x=0.06 x=0.07

050

100150200250300350

-6000-4000-2000 0 2000 4000 6000

d 33

(pC

/N)

E (V/mm)

050

100150200250300350

-6000-4000-2000 0 2000 4000 6000E (V/mm)

0 50

100 150 200 250 300 350

-6000-4000 -2000 0 2000 4000 6000E (V/mm)

050

100150200250300350

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

050

100150200250300350

-6000-4000-2000 0 2000 4000 6000

E (V/mm)

050

100150200250300350

-6000 -4000 -2000 0 2000 4000 6000

d 33

(pC

/N)

E (V/mm)

d 33

(pC

/N)

d 33

(pC

/N)

d 33

(pC

/N)

d 33

(pC

/N)

Fig. 7. DC poling field dependence of d33 at compositions with

x ¼ 0{0:07.

Jpn. J. Appl. Phys. 48 (2009) 09KD07 T. Ogawa et al.

09KD07-3 # 2009 The Japan Society of Applied Physics

applied in the same direction as the DC poling field.Although the induced expansion strain increased from x ¼0:02 to 0.05, the slope of strain vs E curve, whichcorresponds to the piezoelectric d33 constant, increases fromx ¼ 0:02 to 0.04 and 0.06 with increasing piezoelectricconstants such as kp and d33 measured from the impedanceresponse. From the hysteresis curve of strain vs E, the�strains calculated from the ratio of the maximum hystere-

sis in the strain curve to the maximum strain at themaximum E are shown in Fig. 9(b). The �strain at x ¼ 0:02was almost 1.5 times larger than those at x ¼ 0:04 and0.05, because the generation of a large �strain means lessmobility of ferroelectric domains under mechanical stress.These results are supported by the shapes of P–E hysteresisloops [Figs. 2(a) and 2(b)] and the relationships of Pr andEc with E [Figs. 3(a) and 3(b)].

-1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50

ε r

kp

fcp

kt

fct

d33

Ferroelectric

PropertiesSZ

(mol%)

0

Min. Min.

2

Poling field (kV/mm)

Min. Min. Min. Min.

Min. Min.Min. Min.

4

5

6

7

Max. Max. Min. Min. Min. Min.

Max. Max.

Min. Min. Min. Min. Max. Max. Min. Min.

Min. Min. Min. Min.

Min. Min.

Min. Min. Max. Max. Min. Min.

Min. Min. Min. Min.

Min. Min. Min. Min. Min. Min.

Min. Min. Min. Min.

Max. Max. Min. Min. Min. Min.

Min. Min. Min. Min. Min. Min.

Min. Min.Min. Min.

Min. Min. Max. Max.

Domainclampingε r

kp

fcp

kt

fct

d33

ε r

kp

fcp

kt

fct

d33ε r

kp

fcp

kt

fct

d33

ε r

kp

fcp

kt

fct

d33

ε r

kp

fcp

kt

fct

d33

Fig. 8. Relationships between DC poling field (E ) required to obtain minimum "r, kp, kt�, fc t

�, and d33 and maximum fcp, and SrZrO3 (SZ)

compositions with x ¼ 0{0:07. Domain clamping appears at the coercive field (Ec) required to realize minimum "r, kp, kt�, fc t

�, and d33 and

maximum fcp. A typical domain-clamping state is observed simultaneously at a specific E for compositions with x ¼ 0 and 0.05. � kt is the

electromechanical coupling factor in the thickness mode of the disk, and fc t is the frequency constant in the kt mode.

(a) (b)

Fig. 9. Applied field dependences of (a) expansion strain and (b) � strain at compositions x ¼ 0:02, 0.04, and 0.05.

Jpn. J. Appl. Phys. 48 (2009) 09KD07 T. Ogawa et al.

09KD07-4 # 2009 The Japan Society of Applied Physics

4. Conclusions

The DC poling field dependences of dielectric and piezo-electric properties were investigated in lead-free ceramicsand compared with those in PZT, PT, and BT ceramics.Domain clamping was observed in the lead-free ceramics aswell as in the PZT, PT, and BT ceramics. A higherelectromechanical coupling factor was obtained in theceramics with a low frequency constant, which correspondsto a low Young’s modulus. Furthermore, the ceramics with ahigh coupling factor showed a typical domain-clampingstate because of their easy deformation in a DC polingfield. The P–E hysteresis and expansion strain charac-teristics also showed the same results as the poling fielddependence.

Acknowledgements

This work was partially supported by a Grant-in-Aid forScientific Research C (No. 21560340) from the Ministry of

Education, Culture, Sports, Science and Technology and aResearch Foundation Grant 2009 jointly sponsored byAcademia and Industry of Fukuroi City.

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09KD07-5 # 2009 The Japan Society of Applied Physics