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J Mater Sci: Mater Electron (2011) 22:1393–1399 DOI 10.1007/s10854-011-0319-8

Piezoelectric, ferroelectric and mechanical properties of lead zirconate titanate/zinc oxide nanowhisker ceramics Da-Wei Wang • Mao-Sheng Cao • Jie Yuan • Quan-Liang Zhao • Hong-Bo Li • Hai-Bo Lin De-Qing Zhang

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Received: 24 October 2010 / Accepted: 3 February 2011 / Published online: 26 February 2011 Ó Springer Science+Business Media, LLC 2011

Abstract New lead zirconate titanate/zinc oxide nanowhisker (PZT/ZnOw) ceramics were fabricated by a conventional solid state processing and their structures, piezoelectric, ferroelectric and mechanical properties were studied. Both the PZT perovskite and ZnO phases can be observed from the X-ray diffraction patterns. The grain size of ceramics is reduced due to the ZnOw addition. The incorporation of ZnOw into the PZT ceramics improves the strength and toughness, while deteriorates the piezoelectric and ferroelectric properties. For the PZT/ZnOw ceramics with 1–2 wt% ZnOw, the mechanical properties become optimum, meanwhile maintain good piezoelectric and ferroelectric properties: rc = 376–484 MPa, rf = 115–121 MPa, KIC = 1.41–1.54 MPa m1/2, d33 = 442–490 pC/N, kp = 0.54–0.55, er = 3,322–3,980, Qm = 99–101, tand = 1.6%–1.7%, Pr = 21.5–26.9 lC/ cm2 and Ec = 8.1–8.6 kV/cm.

1 Introduction Lead zirconate titanate (PZT) piezoelectric ceramics exhibit excellent piezoelectric properties and are extensively used in D.-W. Wang M.-S. Cao (&) Q.-L. Zhao H.-B. Li D.-Q. Zhang School of Materials Science and Engineering, Beijing Institute of Technology, 100081 Beijing, China e-mail: [email protected] J. Yuan School of Information Engineering, Central University for Nationalities, 100081 Beijing, China e-mail: [email protected] H.-B. Lin China Astronautics Standards Institute, 100071 Beijing, China

applications of numerous electronic devices, such as actuators, sensors, capacitors, resonators and high-power transducers [1–7]. However, piezoelectric ceramics suffer from the low mechanical strength and the reliability under severe circumstances [8–11]. Several approaches to improve mechanical properties have been used to investigate incorporating polymers, metals, fibers or whiskers etc. [12–19]. Nhuapeng et al. [18] have studied the PZT-based composites incorporating polymers. Hwang et al. and Li et al. have carried out the PZT/metal ceramics with Ag and Pt particles, respectively [15, 20]. Although the mechanical properties of these PZT-based composites were reinforced, electrical properties were deteriorated greatly. Therefore, it is necessary to design the microstructure of the PZT possessing excellent mechanical properties without dramatically degrading electrical properties. In light of novel applications based on the semi-conducting and piezoelectric properties of ZnO nanostructures [21], Wang et al. have noted that the piezoelectric power generators based on ZnO nanowire arrays might be able to convert mechanical energy into electrical energy. Such devices could provide flexible power sources with potential applications in sensors, actuators and microelectromechanical systems (MEMS), etc. [22]. As a result, nanostructured ZnO is a suitable piezoelectric material for electromechanical energy conversion. ZnO nanowhiskers (ZnOw), due to the high-temperature strength and excellent chemical stability, have received much attention for industrial applications as reinforced composite materials [23–28]. Therefore, it is expected that ZnOw embedded in the PZT-based ceramics would be a good solution to improve the mechanical behavior without deteriorating electrical properties severely. In this work, PZT-based ceramics embedded with ZnOw were prepared by a normal sintering process. The effects of

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J Mater Sci: Mater Electron (2011) 22:1393–1399

ZnOw on the microstructures, piezoelectric, dielectric, ferroelectric and mechanical properties of PZT are investigated. The reinforcement mechanisms of ZnOw on the mechanical responses of PZT are discussed.

4.2 mm in thickness. The flexural strength was determined using the three-point bending method on a 3 9 4 9 36 mm bar with a span of 30 mm and a crosshead speed of 0.5 mm/min.

2 Experimental

3 Results and discussion

2.1 Sample preparation

Figure 1 shows the SEM images of ZnO nanowhisker synthesized by the simple combustion oxidation method without any catalysts or additives. It can be clearly seen that ZnOw is uniform and has tetra-needle-like nanostructures. The individual structure consists of needle-shaped legs with high aspect ratios. The thickness of narrow tips is some decades of nanometer and the length is about 10 lm. Figure 2 shows the XRD patterns of the ZnOw and PZT/ ZnOw ceramics with various amounts of ZnOw, respectively. A series of characteristic peaks of pure PZT and ZnO are observed. The diffraction peaks of ZnO in the ceramics gradually intensify with the increase of ZnOw content from 0 to 10 wt%, indicating that the PZT matrix is a phase compatible with ZnOw. In addition, no other peaks

The fabrication of PZT/xZnOw (x = 0, 1, 2, 5, 8, 10 wt%) ceramics were performed by a conventional solid state reaction sintering. ZnOw was synthesized by combustion oxidation of fine zinc powders (purity 99.999%) without any catalysts or additives. Detailed discussions on the synthesis of ZnOw have been reported elsewhere [29]. A commercially available PZT powder (PZT-5MN, Hongsheng Industry, Baoding, China) was used as the raw material. Appropriate amounts of the powders mixed with the as-fabricated ZnOw for the designated ceramics were ball-milled for 24 h with zirconia balls as the grinding media and alcohol as the solvent. After milling, the slurry was dried at room temperature and then the dried powders were mixed with proper polyvinyl alcohol (PVA) liquid binder addition. The mixed powders were compacted under a pressure of 200 MPa by die pressing. After binder was burned out in a furnace, the green compacts were sintered at 1,150 °C for 2 h in a closed alumina crucible that contained PbZrO3 powders to minimize lead volatilization. For electrical measurements, silver paste was printed to form electrodes on both sides of the sintered samples, which were subsequently fired at 800 °C for 10 min. Poling treatment was carried out in silicon oil at 120 °C for 20 min with an electric field of 3 kV/mm. 2.2 Characterization The bulk density was determined by the Archimedes method in distilled water. The crystalline phases of the sintered samples were examined by the X-ray diffraction (XRD) with Ni-filtered Cu Ka radiation. Morphologies of the nanowhiskers and ceramics were studied by a scanning electron microscope (SEM). The piezoelectric properties were measured by using the ZJ-3AN piezoelectric tester, and electromechanical and dielectric characteristics were calculated with a Model HP 4194 impedance analyzer. The hysteresis loops and ferroelectric properties were measured using a Radiant Precision Workstation ferroelectric tester system. For the Fracture toughness KIC, a single-edge notched beam test was used with a cross-head speed of 0.05 mm/min and a span of 20 mm on the samples with 2 9 4 9 20 mm. Compression tests were performed on a CSS-2220 testing machine using cylinders with 12 mm in diameter and

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Fig. 1 SEM micrographs of the ZnOw microstructures


Fig. 2 XRD patterns of the ZnOw and PZT/ZnOw ceramics with various amounts of ZnOw

but the PZT and ZnO are observed in the XRD spectrum, indicating that there is no significant chemical reaction occurring between PZT and ZnOw during sintering at 1,150 °C. Figure 3 shows the SEM images of the typical fracture surfaces of PZT/ZnOw ceramics with various ZnOw contents. All the sintered ceramics become dense and the grain boundaries are distinct. It is observed that the grain size of the PZT/ZnOw ceramics is much smaller than that of the PZT ceramics. The grain size is found to decrease from 15–20 lm for the PZT to about 10 lm for 2 and 5 wt%

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ZnOw, and then about 8 lm for 8 wt% ZnOw ceramics. It is suggested that the grain growth of the PZT/ZnOw ceramics is effectively limited by the incorporation of ZnOw in the PZT matrix. In addition, changing of fracture mode could also be easily observed. The fracture mode of the PZT ceramics is completely intergranular, while the fracture mode of the PZT/ZnOw ceramics changes from completely intergranular to partially intragranular. It is revealed that purposely initiated cracks in the PZT ceramics propagate mostly along intergranular and interphase boundaries, as shown in Fig. 3a. However, with addition of ZnOw, the cracks propagate partially in the intragranular mode (Fig. 3b–d) and the fracture path is significantly extended into polyphase sinters [30]. The relative densities of PZT/ZnOw ceramics are listed in Table 1. All highly dense bodies of the PZT/ZnOw ceramics are obtained at more than 96% of the theoretical density value. With the increase of ZnOw content, the relative densities of the ceramics change slightly. The mechanical properties of PZT/ZnOw in terms of compressive strength rc, flexural strength rf and fracture toughness KIC are investigated as shown in Fig. 4 and the values are listed in Table 1. It can be seen that the mechanical characteristics of the PZT/ZnOw ceramics are superior to those of the PZT ceramics. Particularly, the PZT/ZnOw ceramics with 2 wt% ZnOw possess the maximum mechanical properties with compressive strength of 484 MPa, flexural strength of 121 MPa and fracture toughness of 1.54 MPa m1/2. The strength enhancement of the PZT/ZnOw over the PZT ceramics is mainly associated with the grain size

Fig. 3 SEM micrographs of the PZT/ZnOw ceramics with a 0 wt%, b 2 wt%, c 5 wt% and d 8 wt% ZnOw

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Table 1 Mechanical properties of PZT/ZnOw ceramics rf (MPa)

1/2

ZnOw content (wt%)

Relative density (%)

rc (MPa)

0

96.5

252

84

1.12

1

96.6

376

115

1.41

2

96.4

484

121

1.54

5

96.5

292

98

1.42

8

96.8

307

92

1.36

10

96.3

420

102

1.37

KIc (MPa m )

Fig. 4 Variations of a rc, b rf and c KIC for different compositions of PZT/ZnOw ceramics

reduction due to the incorporation of ZnOw as shown in Fig. 3, which can be described by the Hall–Petch equation rf ¼ r0 þ kd 1=2 ;

ð1Þ

where r0, k and d represent the resistance for deformation in crystal, influence parameter of grain boundary and average grain size, respectively. As is seen in Fig. 3, with the addition of ZnOw, the grain size of PZT decreases obviously, which lead to the increase of strength according to Eq. 1. On the other hand, the improvement of fracture toughness is probably attributed to the contributions of the whisker bridging and pull-out [31]. Figure 5 shows the typical micrographs of ZnOw in the PZT ceramics and the corresponding schematic illustrations of the reinforcement mechanism, such as whisker pull-out (Fig. 5a) and

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bridging (Fig. 5b). During the whisker pull-out, the energy that will normally cause crack propagation is partially expended by debonding and friction as the whisker slid against adjacent microstructure features [32]. This can effectively increase the fracture toughness. In addition, according to the elastic fracture mechanics [33], the bridging mechanism of PZT/ZnOw ceramics clearly represents an intrinsically higher energy source of fracture resistance for matrix grains. Many researchers [34] have reported that the increase of fracture toughness is mostly achieved by changing the mode of crack propagation in the case of ceramics, when the increase of effective fracture energy may occur. It can be easily observed that the purposely initiated cracks propagate mostly along the intergranular boundaries of the PZT ceramics shown in Fig. 3a. However, for the PZT/ZnOw ceramics, cracks partially break through the sintered ceramic grains and propagate in the intragranular mode shown in Fig. 3b–d. As a result, the incorporation of ZnOw is effective in suppressing the crack propagation [30] and changing the fracture modes of the PZT ceramics, which leads to the improvement of fracture toughness [35]. Furthermore, as shown in Fig. 4, when the content of ZnOw is over 2 wt%, all mechanical properties of PZT/ZnOw decrease obviously, which is probably attributed to the agglomeration of ZnOw [36, 37]. The piezoelectric and dielectric properties of PZT/ZnOw ceramics are shown in Figs. 6 and 7 and the values are listed in Table 2. With the increase of ZnOw content, the piezoelectric constant d33, electromechanical coupling factor kp and relative dielectric constant er of the ceramics generally decline from 550 pC/N, 0.59 and 4,895 to 195 pC/N, 0.29 and 2,336, while the mechanical quality factor Qm and dielectric loss tand greatly increase from 71 and 2% to 112 and 6.8%, respectively. The variation of piezoelectric and dielectric properties is believed to be affected by the domain clamping caused by the significant reduction of grain size as shown in Fig. 3 [38]. As a result of the reduced grain size, the domain switching in PZT is prevented by domain clamping, leading to the insufficiency of poling and the decline of piezoelectric properties [38]. Moreover, the low-piezoelectric properties of ZnOw in PZT matrix can be another reason for the decrease of piezoelectric and dielectric constants [31]. However, the PZT/1–2 wt% ZnOw ceramics with excellent mechanical and electrical properties possess lower tand and higher Qm than those of the PZT, which is quite attractive to the application of PZT in electronic devices. Ferroelectric hysteresis behavior was studied for all the compositions of PZT/ZnOw ceramics at room temperature, which has been shown in Fig. 8. The variations of remnant polarization Pr, saturation polarization Ps, coercive field Ec and remnance ratio Pr/Ps determined from the hysteresis


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Fig. 5 Typical micrographs of the ZnOw in the PZT matrix and the corresponding schematic illustration of the reinforcement mechanism: a whisker pull-out and b whisker bridging

Fig. 7 Variations of a er and b tand for different compositions of PZT/ZnOw ceramics Table 2 Piezoelectric and dielectric properties of PZT/ZnOw ceramics ZnOw content (wt%)

d33

Qm

kP

er

tand

(pC/N)

Fig. 6 Variations of a d33, b kp and c Qm for different compositions of PZT/ZnOw ceramics

loops are shown in Fig. 9 and the values are listed in Table 3. It is observed that as the content of ZnOw increases, Pr decreases from 33.1 to 13.1 lC/cm2 and the Ec increases from 7.4 to 9.3 kV/cm for the PZT/ZnOw ceramics. Domain switching becomes harder due to the incorporation of ZnOw and reduction of grain size,

0

550

71

0.59

4,895

2.0

1

490

99

0.55

3,980

1.6

2

442

101

0.54

3,322

1.7

5

314

118

0.42

2,921

2.0

8

227

114

0.33

2,458

3.7

10

195

112

0.29

2,336

6.8

resulting in lower polarization as evidenced by the decrease in Pr and Ps. Pr/Ps is found to decrease with the increase of ZnOw content, which indicates that squareness of the P–E loop decreases [39].

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Fig. 8 Room temperature ferroelectric hysteresis loops for different compositions of PZT/ ZnOw ceramics

Table 3 Ferroelectric properties of PZT/ZnOw ceramics

Fig. 9 Variations of a Pr, Ec and b Ps, Pr/Ps for different compositions of PZT/ZnOw ceramics

4 Conclusions PZT/ZnOw piezoelectric ceramics have been fabricated successfully by the solid state method sintered at 1,150 °C for 2 h. XRD results show that the characteristic diffraction

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ZnOw content (wt%)

Pr (lC/cm2)

Ec (kV/cm)

PS (lC/cm2)

Pr/Ps

0

33.1

7.4

38.5

0.86

1

26.9

8.1

34.7

0.78

2

21.5

8.6

28.4

0.76

5

17.6

8.7

23.6

0.75

8

15.6

9.4

21.1

0.74

10

13.1

9.3

18.3

0.72

peaks of the PZT and ZnO can be observed in the PZT/ZnOw ceramics. SEM images exhibit the decline of grain size and the change of fracture mode due to the incorporation of ZnOw. Compared to the PZT ceramics, mechanical properties of the PZT/ZnOw ceramics are obviously improved. The enhancement of the strength is mainly due to the grain size reduction, and improvement of KIC is attributed to the ZnOw put-out and bridging. On the other hand, with the increase of ZnOw content, the d33, kp, er and Pr substantially decrease, while Qm, tand and Ec increase greatly. The new ZnOw reinforcing PZT ceramics with excellent mechanical properties and tunable electrical properties are promising candidates for further applications. Acknowledgments This research was supported by the National Natural Science Foundation of China under Grant Nos. 50742007, 50872159 and 50972014, the National High Technology Research and Development Program of China under Grant No. 2007AA03Z103, the National Defense Fund of China under Grant No. 401050301 and the Key Laboratory Foundation of Sonar Technology of China under Grant No. 9140C24KF0901.


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