3)O3–PbHfO3–PbTiO3 ternary system

Available online at www.sciencedirect.com

Journal of the European Ceramic Society 33 (2013) 2491–2497

Investigation of dielectric and piezoelectric properties in Pb(Ni1/3Nb2/3)O3–PbHfO3–PbTiO3 ternary system H. Tang a,b , M.F. Zhang a,c , S.J. Zhang a,b,∗ , Y.J. Feng b , F. Li b , T.R. Shrout a a

b

Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, PR China c Centre for Composite Materials, Harbin Institute of Technology, Harbin 150080, PR China Received 21 December 2012; received in revised form 5 April 2013; accepted 8 April 2013 Available online 10 May 2013

Abstract The dielectric and piezoelectric properties were investigated in the (1 − x)Pb(Hf1−y Tiy )O3 –xPb(Ni1/3 Nb2/3 )O3 (PNN–PHT, x = 0.05–0.50, y = 0.55–0.70) ternary system. The morphotropic phase boundary (MPB) was determined by X-ray powder diffraction analysis. Isothermal map of Curie temperature (TC ) related to the compositions in the phase diagram was obtained. The optimum dielectric and piezoelectric properties were achieved in ceramics with the MPB compositions, with the maxima values being on the order of 6000 and 970pC/N, respectively. Rayleigh analysis was used to study the extrinsic contribution (domain wall motion) in PNN–PHT system, where the extrinsic contribution was found to be ∼30% for composition 0.49PNN–0.51PHT(30/70), showing a high nonlinearity. © 2013 Elsevier Ltd. All rights reserved. Keywords: Pb(Ni1/3 Nb2/3 )O3 –PbHfO3 –PbTiO3 ; Piezoelectric properties; Morphotropic phase boundary; Curie temperature; Rayleigh analysis

1. Introduction PbZrO3 –PbTiO3 (PZT) piezoelectric ceramics have been mainstay for numerous electronic devices, such as actuators, sensors and transducers due to their high dielectric and piezoelectric properties,1–3 which are obtained with compositions in the vicinity of the morphotropic phase boundary (MPB). The MPB is a nearly temperature independent phase boundary separating ferroelectric phases with rhombohedral and tetragonal structures. The coupling between the two equivalent energy states, i.e., tetragonal and rhombohedral phases gives rise to enhanced polarizability, allowing optimum domain reorientation during poling.4,5 Thus, extensive studies have been focused on the exploration of MPB compositions in binary or ternary PZT based systems. In addition, the relaxor ferroelectric lead nickel niobate [Pb(Ni1/3 Nb2/3 )O3 , PNN] has been studied,6–8 showing a broad dielectric maximum near −120 ◦ C, with values being ∗ Corresponding author at: Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 8148632639; fax: +1 8148657173. E-mail address: [email protected] (S.J. Zhang).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.010

around 4000,9 revealing a potential relaxor end member to form binary and/or ternary systems with MPB compositions, which were expected to possess enhanced dielectric/piezoelectric properties. For example, Pb(Ni1/3 Nb2/3 )O3 –Pb(Zr,Ti)O3 10–12 and Pb(Ni1/3 Nb2/3 )O3 – Pb(Zn1/3 Nb2/3 )O3 –PbZrO3 13 exhibit high dielectric permittivities and piezoelectric coefficients near MPB compositions. Analogous to PZT, the PbHfO3 –PbTiO3 (PH–PT or PHT) solid solution also exhibits MPB composition at ∼50%PT.14–16 Studies on Pb(Mg1/3 Nb2/3 )O3 –PHT based ternary compositions shown improved dielectric and piezoelectric properties, comparable to PZT based counterparts,17,18 however, no investigations on PNN–PHT ternary system was reported, which is the topic of this research. 2. Experiments (1 − x)Pb(Hf1−y Tiy )O3 –xPb(Ni1/3 Nb2/3 )O3 (x = 0.05–0.50, y = 0.55–0.70) polycrystalline ceramics were prepared using two-step precursor method.19 Raw materials of NiO (99.9%, Alfa Aesar, Ward Hill, MA), Nb2 O5 (99.9%, Alfa Aesar), HfO2 (99.9%, Alfa Aesar) and TiO2 (99.9%, Ishihara, San Francisco, CA) were used to synthesize precursors of NiNb2 O6

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Fig. 1. Compositions studied in (1 − x)Pb(Hf1−y Tiy )O3 –xPb(Ni1/3 Nb2/3 )O3 ternary system.

and Hf1−y Tiy O2 at 1000 ◦ C and 1200 ◦ C, respectively. Pb3 O4 (99.5%, Alfa Aesar), NiNb2 O6 and Hf1−y Tiy O2 powders were then batched stoichiometrically according to the nominal compositions as shown in Fig. 1. The mixed powders were calcined at 1000 ◦ C for 4 h and then vibratory milled in alcohol for 12 h, subsequently granulated and pressed into pellets with 12 mm in diameter. Following binder burnout at 550 ◦ C, the pellets were sintered in sealed crucibles at the temperature of 1250–1280 ◦ C for 2 hs. The phase purity and structure were determined using X-ray powder diffraction (XRD) on the grounded samples. The density was measured by Archimedes method. The microstructure was determined by SEM on fractured surface. For electrical measurements, the samples were polished and then electrode on the parallel surfaces using fire-on silver paste. Poling was carried out in silicon oil at 120 ◦ C for 10 min with an electric field of 30 kV/cm. Dielectric measurements were performed using a multi-frequency precision LCRF meter (HP 4184A). Curie temperatures were determined from the temperature dependent dielectric behavior, measured by the same LCR meter, connecting to a computer controlled high temperature furnace. The piezoelectric coefficients were measured using a Berlincourt d33 meter. Polarization hysteresis was determined using a modified Sawyer–Tower circuit driven by a high voltage power supply (TREK Model 610, TREK, Medina, NY). The planar electromechanical coupling factor kp was determined from resonance and antiresonance frequencies, which were measured

Fig. 3. (a) XRD patterns for xPNN–(1 − x)PHT(45/55) (the small peaks around 33◦ are due to the pyrochlore second phase); (b) corresponding expanded XRD patterns of the compositions in the range of 2θ from 42◦ to 48◦ .

using an Impedance Gain-phase analyzer (HP 4194A) according to IEEE standards on piezoelectricity.20,21 The electric-fieldinduced strain was measured using a linear variable differential transducer driven by a lock-in amplifier (Stanford Research system, Model SR830). For Rayleigh analysis, the maximum amplitude of the applied electric field was selected to be smaller than half of the coercive field.22 3. Results and discussions Scanning electron microscope (SEM) images of xPNN– (1 − x)PHT(45/55) ceramics are shown in Fig. 2. All samples exhibited intergranular fracture, with very few pores observed, revealing high densities, being consistent with the densities measured by Archimedes method, around >97% of the theoretical values. The grain size was found to be 4–8 ␮m for samples with x = 0.05, maintained the similar values with increasing PNN content, being 5–8 ␮m for x = 0.15, while decreasing to 2–4 ␮m for rhombohedral composition (x = 0.22). XRD patterns of xPNN–(1 − x)PHT(45/55) ceramics with x ranging from 0.05 to 0.25 are shown in Fig. 3(a). All the samples were found to be pure perovskite phase. It was observed that with increasing PNN content, the (2 0 0)/(0 0 2) peaks gradually merged to one peak, as shown in Fig. 3(b), indicating a phase transformation from tetragonal to rhombohedral phase,

Fig. 2. SEM images of xPNN–(1 − x)PHT(45/55) (a) x = 0.05; (b) x = 0.15; (c) x = 0.22.

H. Tang et al. / Journal of the European Ceramic Society 33 (2013) 2491–2497

Fig. 4. (a) XRD patterns for 0.20PNN–0.80PHT(45/55) as a function of temperature; (b) corresponding expanded XRD patterns in the range of 2θ from 44◦ to 46◦ .

with space group symmetries of P4mm and R3m, respectively, based on which, compositions of xPNN–(1 − x)PHT(45/55) with x = 0.18–0.22 were identified as the MPB region. Using this approach, MPB regions for other compositions of xPNN– (1 − x)PHT(43/57) and xPNN–(1 − x)PHT(30/70) were determined to be x = 0.22–0.25 and x = 0.46–0.50, respectively. Based on these XRD results, the MPB region of PNN–PHT ternary system was confirmed and given in Fig. 1. The XRD patterns for composition 0.20PNN–0.80PHT(45/55) as a function of temperature are given in Fig. 4. It was observed that this composition was in rhombohedral phase at room temperature, with only (2 0 0) peak located at 45◦ , while the (2 0 0) peak became broad and start to split as the temperature increased to 85 ◦ C, revealing the coexistence of the rhombohedral and tetragonal phases, clear (2 0 0)/(0 0 2) peaks were observed at temperature of 120 ◦ C, indicative of tetragonal phase, demonstrating that the rhombohedral to tetragonal phase transition occurs at temperature in the range of 85–120 ◦ C. In order to further explore the phase transition temperature, the temperature dependence of dielectric permittivity for xPNN–(1 − x)PHT(45/55) is measured and presented in Fig. 5.

Fig. 5. The temperature xPNN–(1 − x)PHT(45/55).

dependence

of

dielectric

permittivity

for

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Fig. 6. Isothermal map of Curie temperatures with various compositions in PNN–PHT ternary system; Curie temperatures of PHT were obtained in reference 14.

It was obvious that with increasing PNN content, the Curie temperature TC decreased from 337 ◦ C to 222 ◦ C. It should be noted that permittivity anomalies existed prior to their Curie temperatures for compositions with x ranging from 0.18 to 0.25, corresponding to the rhombohedral to tetragonal phase transition temperatures, associated with the strong curved MPB. The ferroelectric phase transition was found to be 105 ◦ C, in consistent with the value determined by the temperature dependent XRD. Based on the dielectric measurements, isothermal map of Curie temperatures with various compositions in xPNN–(1 − x)PHT ternary system is given in Fig. 6. The compositions measured are marked in the graph. It is evident that the Curie temperature TC decreased with increasing PNN regardless of the various PH/PT ratio. Similar behavior of the compositional dependent phase transition temperatures was also observed in other PNN-based binary and ternary systems.10,12 Polarization hysteresis and bipolar strain for xPNN– (1 − x)PHT(45/55) with x ranging from 0.05 to 0.25 are shown in Fig. 7(a) and (b), respectively. The corresponding remnant polarization Pr was found to increase with increasing PNN content, reaching a maximum value at x = 0.22, being on the order of 34 ␮C/cm2 . Highest Pr value is expected in MPB region due to the coexistence of ferroelectric rhombohedral and tetragonal phases, with increased crystallographic orientations. Coercive field EC , on the contrary, was found to decrease monotonously with increasing PNN content, as shown in Fig. 7(a), indicating that the domain switching becomes easier, attributed to the decreased tetragonal phase. The bipolar electric-field strain curves in Fig. 7(b) exhibit butterfly shapes, which is typical for ferroelectric materials. With increasing PNN content, the negative strain (which denotes the difference between the zero strain and the lowest strain) associated with domain back-switching increased from 0.03% to 0.19%. In order to understand the relationship between composition/phase and intrinsic/extrinsic contributions, Rayleigh analysis was performed on xPNN–(1 − x)PHT ceramics. For the case of piezoelectric response under low electric field (less than

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Fig. 7. (a) Ferroelectric hysteresis loops and (b) bipolar electric-field strain curves for xPNN–(1 − x)PHT(45/55) ceramics.

half of the coercive field: sub-coercive field), the Rayleigh law can be expressed using the following formulae22 : S(E) = (dinit + αE0 )E ±

α(E02 − E2 ) 2

(1)

S(E0 ) = (dinit + αE0 )E0

(2)

d(E0 ) = (dinit + αE0 )/pC/N

(3)

where S(E) is the electric-field-induced strain, and E0 is the level of applied electric field. The coefficient dinit describes the reversible piezoelectric response, including the intrinsic (lattice) and reversible internal interface motion. The latter contribution is relatively small in ferroelectric materials.22,23 In this work, the measured coefficient dinit was considered to arise from the intrinsic contribution. The coefficient α is the irreversible motion of internal interfaces, thus αE0 represents the extrinsic contribution to the total piezoelectric response. dinit is defined as a non-unit coefficient and the unit of α is given as centimeter per kilovolt, as expressed in Eq. (3). Eq. (1) describes the Rayleigh hysteresis, where the signs “+” and “−” correspond to decreasing and increasing electric field, respectively. The total piezoelectric coefficient d33 was calculated from the peak to peak strain, Sp–p , measured for each excitation electric field (E0 ): d33 =

Sp−p 2E0

(4)

Fig. 8 (a) shows the ac field dependent d33 (E0 ) for xPNN–PHT(30/70) ceramics, where x ranges from 0.42 to 0.49. The calculated d33 s at various electric fields for each composition are linear fitted. It can be observed that the piezoelectric coefficient exhibits a good linear behavior as a function of drive-field amplitude, demonstrating that the piezoelectric response can be described using the Rayleigh law. Therefore, by determination of the ac electric field dependence of the piezoelectric coefficient d33 (E0 ), the Rayleigh parameters and associate errors could be obtained by fitting Eq. (3), as given in Fig. 8(a). The values of dinit and α obtained for composition 0.49PNN–0.51PHT(30/70) were found to be on the order of 738 and 302 cm/kV, respectively, indicating that the extrinsic

contribution to piezoelectric response was up to ∼30%. From the obtained dinit and α, the S–E loop was calculated using Eq. (1). The comparison between the calculated and measured S–E loops was given in Fig. 8(b), where it can be observed that the two loops are nearly coincided, revealing that the obtained dinit and α are in good agreement with the experimental results. The minor discrepancy may be due to the measurement error or the presence of other hysteretic mechanisms in piezoelectric response.22 The Rayleigh parameters as a function of compositions are given in Fig. 9. The tetragonal (T), MPB region with coexistence of tetragonal and rhombohedral (T + R), and rhombohedral (R) phase regions are marked accordingly. It was observed that the intrinsic piezoelectric response, dinit , reached maximum values in the MPB region, being on the order of 331, 418, 738 for 0.82PHT(45/55)–0.18PNN, 0.79PHT(43/57)–0.21PNN and 0.51PHT(30/70)–0.49PNN with MPB compositions, respectively. The flattening of the Gibbs free-energy profile (Gibbs free-energy instability), induced by the coexistence of ferroelectric phases near MPB compositions, leads to the enhanced piezoelectric properties,24,25 which in Rayleigh analysis, corresponds to the reversible piezoelectric response and accounts for the improved intrinsic piezoelectric coefficient dinit .26 The Rayleigh parameter α also reached peak values in the vicinity of MPB, being on the order of 100 cm/kV, 101 cm/kV, and 302 cm/kV, respectively. Correspondingly, the extrinsic contribution αE0 increased significantly as the compositions approaching to MPB, indicating that the irreversible domain wall motion was enhanced. The ratio of extrinsic contribution to total piezoelectric response, αE0 /(αE0 + dinit ), as a function of composition is shown in Fig. 9(d). The ratio of extrinsic contribution was found to be in the range of ∼10–30%, increasing as compositions approaching to the MPB. As shown, the compositions in the rhombohedral phase generally have larger extrinsic contribution than the compositions in the tetragonal phase. It was known that ferroelastic domain wall motion can contribute to the piezoelectric response of ferroelectric ceramics, where there are 90◦ domain walls and 109◦ /71◦ domain walls existing in tetragonal and rhombohedral phases, respectively. The 90◦ domain walls are partly clamped by local internal stresses induced by the high tetragonal distortion (c/a ratio, 2–6%), whereas the 109◦ and 71◦

H. Tang et al. / Journal of the European Ceramic Society 33 (2013) 2491–2497

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Fig. 8. (a) The ac electric field dependent piezoelectric coefficient d33 for xPNN–PHT(30/70) ceramics at 1 Hz; (b) comparison between the measured and calculated strain-vs-electric field hysteresis loop.

rhombohedral domain walls are relatively free to move since the internal stresses caused by the structural distortion are significantly smaller in rhombohedral phase (<0.5%)1,27 , thus, higher extrinsic contribution is expected near rhombohedral side, as confirmed by the experimental results. It should be noted that non-180◦ ferroelastic domain wall motion is the dominant factor for extrinsic contribution observed by Rayleigh analysis, other

mechanisms, however, such as intergranular lattice strain, interphase boundary motion may also contribute to the piezoelectric response.28 The detail dielectric, piezoelectric and ferroelectric properties of PNN–PHT ceramics were summarized in Table 1 and compared to PNN–PZT30 and PMN–PT31 materials. The highest d33 was obtained in 0.49PNN–0.51PHT(30/70), being

Fig. 9. Compositional dependence of Rayleigh parameter for (a) xPNN–(1 − x)PHT45/55; (b) xPNN–(1 − x)PHT43/57; (c) xPNN–(1 − x)PHT30/70; (d) ratio of extrinsic contribution.

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Table 1 Piezoelectric, dielectric and ferroelectric properties of (1 − x)Pb(Hf1−y Tiy )O3 –xPNN ternary ceramics. d33 (pC/N)

kp (%)

K

Loss (%)

TC (◦ C)

Pr (␮C/cm2 )

Ec (kV/cm)

x=

5 10 15 18 19 20 22 25

160 280 340 500 470 440 380 350

34.8 43.3 49.3 60.5 61.1 60.9 60.3 59.9

1050 1400 1830 2140 1970 1870 1250 1180

1.0 1.2 1.8 1.7 1.6 1.8 1.7 1.8

337 310 280 260 256 250 238 222

10 16 24 29 28 31 34 36

12 12 11 8.0 7.5 7.5 7.3 7.2

x=

15 19 20 21 22 23 25

250 350 360 420 480 550 600

39.3 47.1 48.6 57.0 57.4 64.9 65.8

1280 1950 2000 2100 2600 2650 2200

1.4 1.5 1.5 1.6 1.8 1.8 1.7

292 266 257 252 246 240 229

19 21 23 27 28 32 34

14 10 10 9.8 9.0 8.8 7.6

x=

30 40 42 44 45 46 47 48 49 50

300 450 540 650 730 800 870 940 970 930

47.1 51.3 55.3 57.7 59.7 61.0 62.9 65.0 65.2 64.5

1900 2700 3600 4500 4950 5500 5830 5240 6000 5930

1.4 1.3 1.7 1.9 2.0 1.8 2.3 2.4 2.5 2.4

208 184 168 155 151 145 138 120 110 100

25 26 26 26 26 27 27 27 27 27

13 9.0 6.0 6.7 6.5 5.6 5.1 3.8 4.2 4.3

802 669

68.0 –

5362 4936

2.9 2.7

105 143

25 25

3.9 5.0

Samples

y = 0.55

y = 0.57

y = 0.7

0.55PNN–0.45PZT30 0.7PMN–0.3PT31

970 pC/N, with a planar electromechanical coupling factor of 60.5% and dielectric permittivity of 6000. The dielectric and piezoelectric properties follow the general trend as observed for PZT based polycrystalline ceramics, where the compositions possessing higher Curie temperature usually exhibit lower dielectric and piezoelectric properties.29 Compared to PNN–PZT polycrystalline ceramics with similar compositions,30 PNN–PHT were found to possess similar Curie temperature, but higher piezoelectric properties (970 pC/N vs 800 pC/N), similar phenomena were also observed in PMN–PHT system, showing advantage over its PMN–PZT counterpart, due to the existence of hafnium instead of zirconium.17,18 In addition, PNN–PHT system was found to possess either higher Curie temperature with comparable piezoelectric properties or higher piezoelectric properties with comparable Curie temperature, when compared to PMN–PT ceramics, due to the fact that PNN relaxor end member shows higher dielectric permittivity and PH end member exhibits higher Curie temperature when compared to PMN. 4. Conclusion (1 − x)Pb(Hf1−y Tiy )O3 –xPb(Ni1/3 Nb2/3 )O3 (x = 0.05–0.50, y = 0.55–0.70) ternary system was investigated in this work. The MPB of the ternary system was determined by X-ray powder diffraction. The Curie temperatures were found to decrease with increasing PNN content, while rhombohedral to tetragonal phase

transition temperatures increased, indicating the existence of a curved MPB region. It was observed that the intrinsic piezoelectric response reached maximum values in the MPB region due to the flattening of the free energy profile. The ratio of extrinsic contribution was found to be in the range of ∼10–30%, increasing as composition approaching the MPB region. The optimized dielectric and piezoelectric properties were achieved in ceramics with the MPB compositions, with the maxima values being on the order of 6000 and 970 pC/N, respectively. Acknowledgment The authors Hua Tang and Mingfu Zhang want to acknowledge the supports from the China Scholarship Council. References 1. Jaffe B, Cook WR, Jaffe H. Piezoelectric ceramics. London/New York: Academic Press; 1971. 2. Haertling GH. Ferroelectric ceramics: history and technology. J Am Ceram Soc 1999;82:797–818. 3. Wang DW, Cao MS, Yuan J, Zhao QL, Li HB, Zhang DQ, Agathopoulos S. Enhanced piezoelectric and ferroelectric properties of Nb2 O5 modified lead zirconate titanate-based composites. J Am Ceram Soc 2011;94: 647–60. 4. Zhang SJ, Xia R, Lebrun L, Anderson D, Shrout TR. Piezoelectric materials for high power, high temperature applications. Mater Lett 2005;59: 3471–5.

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