J Mater Sci: Mater Electron (2013) 24:1463–1468 DOI 10.1007/s10854-012-0953-9

Effects of ZnO nanoneedles addition on the mechanical and piezoelectric properties of hard PZT-based composites Hong-Bo Li • Yong Li • Da-Wei Wang Ran Lu • Jie Yuan • Mao-Sheng Cao

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Received: 1 July 2012 / Accepted: 20 October 2012 / Published online: 4 January 2013 Ó Springer Science+Business Media New York 2013

Abstract Hard PZT (PZT4)-based composites embedded by ZnO nanoneedles (denoted as PZT/ZnOn) were fabricated by a solid state sintering technique. The characteristic diffraction peaks of the perovskite PZT and ZnO phases were identified from the studied composites, indicating the retention of ZnOn. With increasing ZnOn content, the grain size of the composites was reduced gradually. In contrast with the pure PZT, the PZT/ZnOn composites possessed more excellent mechanical properties, while the piezoelectric properties were reduced by a certain extent. The best mechanical properties of PZT/ZnOn composites were obtained by sintering at 1,150 °C with 1.5 wt% ZnO nanoneedles addition: fracture toughness KIC * 2.04 MPa m1/2, flexural strength rf * 105.44 MPa, compressive strength rc * 543.89 MPa. The piezoelectric properties of the PZT/ZnOn composites were found to be lower than that of the pure PZT with dielectric permittivity er of 768–893, piezoelectric coefficient d33 of 240–260pC/N, mechanical quality factor Qm of 340–650 and planar electromechanical coupling kp of 0.5–0.55.

1 Introduction Hard PZT (PZT-4) ceramics exhibit excellent piezoelectric and ferroelectric properties, which have led to numerous H.-B. Li  Y. Li  D.-W. Wang  R. Lu  M.-S. Cao (&) School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China e-mail: [email protected] J. Yuan (&) School of Information Engineering, Central University for Nationalities, Beijing 100081, China e-mail: [email protected]

applications in electronic devices, such as actuators, sensors and transducers [1–7]. The high dielectric permittivity and piezoelectric properties of PZT are obtained for compositions close to the morphotropic phase boundary (MPB), which is located around the PbZrO3:PbTiO3 ratio of 1:1 [8–14]. Furthermore, PZT ceramics could be modified or doped with different additives, which make them more attractive for specific applications [15–21]. However, PZT ceramics still generally have poor mechanical properties, especially low fracture strength and toughness [22, 23], which result in low electrical reliability [24] and become a critical limitation on applications. Therefore, it is further important for the applications of PZT to increase strength and toughness. In the past years, a few studies have dealt with improving the mechanical properties of PZT by incorporating metals, oxides and polymers, or by adding particles, fibers and whiskers [25–39]. In 1985, the SiC whiskers were used to improve mechanical strength and toughness of PZT by Yamamoto et al., which were very disadvantageous to electrical properties [40]. Hwang et al. enhanced PZT by adding Ag and Pt in 1997, whereas the gradient descent of piezoelectric properties was still unavoidable [41]. Therefore, although adding reinforced phase reduces the electrical properties of PZT ceramics more or less, the tremendous increase of strength and toughness is obvious, which is the state-of-art technique to improve the mechanical properties of piezoelectric ceramics. Recently, studies revealed that the incorporation of ZnO whiskers improved the mechanical properties of soft PZT composites significantly [42–44]. However, according to the microstructure of PZT/ZnO whiskers composites, the ZnO whiskers were fractured and formed ZnO nanoneedles (denoted as ZnOn) in the sintering process. In addition, to our knowledge, limited studies have been carried out on the hard PZT-based composites

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modified by ZnO nanoneedles. Therefore, it is expected that ZnO nanoneedles embedded in the hard PZT would improve the mechanical properties without deteriorating electrical properties severely. In this work, hard PZT composites incorporated with ZnO nanoneedles were prepared by a normal sintering technique to solve the electric-mechanical coupling problem. The PZT/ZnOn composites were prepared at different process conditions (sintering temperature and time) and their sintering, microstructure, mechanical properties and piezoelectric properties were investigated in detail.

2 Experimental procedures ZnO nanoneedles were prepared by combustion synthesis of metal assisting, with zinc powders of 99.9 % analytical reagent. Zinc powders mixed with 5 wt% copper powders as a catalyst were pushed into the middle of tube furnace with the temperature at 980 °C for 5 min. Then the pure ZnO nanoneedles were obtained [45]. Commercially available PZT powders (PZT-4, Baoding Hongsheng Acoustics Electron Apparatus Co. Ltd., Hebei Province, China) were used as raw materials. The PZT powders and ZnOn were batched according to PZT/xZnOn (x = 0, 0.5, 1 and 1.5 wt%) and wet-milled with zirconia balls in alcohol for 24 h. The dried powders were mixed with 6 wt% polyvinyl alcohol (PVA) liquid binder and uniaxially pressed into pellets. The pellets were sintered at 1,100, 1,150 and 1,200 °C for 2 h in a closed alumina crucible, respectively. Silver paste was printed on both sides of the disk samples to form electrodes. Then the samples were fired at 800 °C for 10 min in air. Poling was carried out in silicon oil at 120 °C for 10 min in an electric field of 3–4 kV/mm. The phase and morphologies of the sintered samples were determined by X-ray diffraction analysis (XRD, Ni-filtered Cu Ka radiation, 40 kV) and scanning electron microscope (SEM, Hitachi S-4100, Tokyo, Japan). The compression tests were performed using a CSS-2220 testing machine with cylinders in diameter 12 mm and in thickness 4.2 mm. The flexural strength was determined using the three-point bending method on a 3 mm 9 4 mm 9 36 mm bar with a span of 30 mm at a cross-head speed of 0.5 mm min-1. The samples with dimensions 2 mm 9 4 mm 9 20 mm were tested using a single-edge notched beam at a cross-head speed of 0.05 mm min-1 and a span of 20 mm for the fracture toughness [27]. The piezoelectric properties were measured using the ZJ-3AN piezoelectric tester, and the electromechanical and dielectric characteristics were calculated with the Model HP 4194 impedance analyzer. In addition, the ferroelectric properties were measured using a Radiant Precision Workstation

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ferroelectric tester system. The bulk density was determined using the Archimedes method.

3 Results and discussion The XRD patterns and SEM images of ZnO nanoneedles were shown in Fig. 1.The diffraction peaks of the ZnO phase could be identified in Fig. 1a [46]. As shown in Fig. 1b, ZnO nanoneedles possessed one-dimensional structure, and the average length of the ZnOn was about 15 lm. Therefore, it was confirmed that the single-phase ZnOn were obtained, which could be used to enhance PZT. The XRD patterns of PZT/ZnOn composites with 1 wt% ZnOn sintered at different temperatures were shown in Fig. 2. Figure 3 showed the XRD patterns of PZT/ZnOn composites sintered at 1,150 °C with various ZnOn content. As be marked in the XRD patterns, it was clearly found that the diffraction peaks of the PZT perovskite phase and ZnO phase could be identified from all the sintered samples, suggesting that the ZnOn were incorporated into the PZT composites and formed a stable solid solution. Furthermore, with increasing ZnOn content, the diffraction peaks of ZnO in the composites gradually intensify, which presented that PZT and ZnOn possessed an excellent compatibility. Previous studies considered that Zn ion in ZnO could enter into the crystal lattice and occupy B sites, resulting in the phase transition from tetragonal phase to rhombohedral phase [47]. However, in this work, results

Fig. 1 a XRD patterns, and b SEM images of ZnO nanoneedles

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Fig. 2 XRD patterns of PZT/ZnOn composites with 1 wt% ZnO nanoneedles sintered at a 1,100 °C, b 1,150 °C, c 1,200 °C

Fig. 3 XRD patterns of PZT/ZnOn composites sintered at 1,150 °C with a 0 wt%, b 0.5 wt%, c 1 wt%, d 1.5 wt% ZnO nanoneedles

indicated that ZnOn were incorporated into the PZT as the second phase based on the XRD patterns of PZT/ZnOn composites, which had little effect on the structure transformation of PZT. In addition, when the sintering temperature was over 1,150 °C, the peaks of the pyrochlore phase were presented, which was attributed to the high PbO evaporation rate of PZT system at high sintering temperature [48]. Figure 4 showed the SEM images of the PZT/ZnOn composites with 1 wt% ZnO nanoneedles sintered at 1,100, 1,150 and 1,200 °C, respectively. All the sintered samples were found to be dense and of granular structure. In addition, with the same ZnOn content, the grain size of the composites was increased as the sinter temperature increasing. With 1 wt% ZnOn, it was observed that the grain size of the PZT/ZnOn composites sintered at 1,200 °C (*7 lm) was much bigger than that sintered at 1,100 °C (*3 lm).

Fig. 4 SEM images of the PZT/ZnOn composites with 1 wt% ZnO nanoneedles sintered at a 1,100 °C, b 1,150 °C, c 1,200 °C

According to the kinetic grain growth equation expressed as below [49–51].   1 1 Q logG ¼ logt þ logK0  0:434 ð1Þ n n RT where G is the average grain size at the time; n is the kinetic grain growth exponent; t is the sintering time; K0 is a constant; Q is the apparent activation energy; R is the gas constant; T is the absolute temperature. It could be explained that the increase of sintering temperature could enhance the grain growth of the PZT/ZnOn composites. SEM images of the PZT/ZnOn composites sintered at 1,150 °C with 0, 0.5, 1 and 1.5 wt% ZnOn were shown in Fig. 5. It was found that the grain size of the composites was decreased with the increase of ZnOn content. At 1,150 °C, the grain size of the PZT/ZnOn composites with

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Fig. 5 SEM images of the PZT/ZnOn composites sintered at 1,150 °C with a 0 wt%, b 0.5 wt%, c 1 wt%, d 1.5 wt% ZnO nanoneedles

1.5 wt% ZnOn (*2 lm) was much smaller than that with 0 wt% ZnOn (*7 lm), suggesting that the grain growth of the PZT matrix could be effectively limited by the incorporation of ZnOn. In the process of grain growth, the ZnOn as the second phase were the obstacles to prevent the growth of grains [52]. Therefore, the grain size of the composites was relevant to the ZnOn contents. The dependence of KIC (fracture toughness), rf (flexural strength) and rc (compressive strength) with the ZnOn additional contents for the PZT/ZnOn composites was shown in Fig. 6. It was seen that the addition of ZnOn improved the mechanical properties of PZT obviously. With the increase of the ZnOn contents, the overall variation trend of the mechanical strength was gradually enhancive. And the variation of the strength was small when the ZnOn additional content was below 1.0 wt%. Particularly, when ZnOn contents increased to 1.5 wt%, the fracture toughness KIC * 2.13 MPa m1/2, flexural strength rf * 101.57 MPa and compressive strength rc * 545.29 MPa

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for the PZT/ZnOn composites were about 24, 43 and 41 % higher than that of the pure PZT, respectively. Compared with the previous reports of PZT based composites enhanced with ZnO whiskers [42–44], both ZnO whiskers and ZnO nanoneedles were found to be good reinforcement materials, which could be used to effectively improve the mechanical properties of PZT. The improvement of PZT/ZnOn composites was mainly attributed to the excellent mechanical properties of ZnOn [53–56]. Furthermore, under the applied stress condition, a crack was arrested upon reaching the needle at the ceramic–needle interface and then extended along the interface [57, 58]. It could be noted that a crack deflected from the resistance of the tip or root of a needle, when the crack reached the needle and then continued in the composites, leaving the needle intact in the process. The needle also could act as a barrier to prevent rupture and bridge the crack to prevent slipping of the interface. Under further loading, some needles would be pulled out from the PZT

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1467 Table 1 Piezoelectric properties of the PZT/ZnOn composites sintered at 1,150 °C ZnOn content (wt%)

q (g/cm3)

d33 (pC/N)

kp

Qm

er

tan d (%)

0

7.53

320

0.53

610

1,056

0.3

0.5

7.60

260

0.55

650

900

1.1

1

7.40

250

0.54

560

880

1.1

1.5

7.82

240

0.50

340

770

1.2

q density, d33 piezoelectric coefficient, kp planar electromechanical coupling, Qm mechanical quality factor, er dielectric permittivity, tan d dielectric loss

The piezoelectric properties of PZT and PZT/ZnOn composites sintered at 1,150 °C with different contents of ZnOn were listed in Table 1. With the increase of the ZnOn content, the piezoelectric constant d33, electromechanical coupling factor kp, mechanical quality factor Qm and relative dielectric constant er of the composites generally decreased to some extent, while the kp and Qm possessed the highest value with 0.5 wt% ZnOn. It was believed that the variation of electrical properties was affected by the domain clamping, which was the result of the reduction of grain size. On the other hand, the essential low piezoelectricity of ZnOn could also lead to the decrease of the piezoelectric and dielectric properties with adding the ZnOn into the PZT-based composites.

4 Conclusions

Fig. 6 Dependence of a KIC (fracture toughness), b rf (flexural strength) and c rc (compressive strength) with the incorporating contents of ZnO nanoneedles for the PZT/ZnOn composites

grain and another would be fractured. These mechanisms of reinforcement described above acted at the same time in the PZT/ZnOn composites. In addition, it was found that the density of the PZTbased composites was improved by the incorporation of ZnOn, and the maximum q of 7.82 g/cm3 was achieved at 1,150 °C with 1.5 wt% ZnOn. It was reported that adding ZnO could produce liquid phase in the sintering process, which enhanced the sintering rate, resulting in the increase of the density [59]. The increase of density also improved the mechanical properties of composites.

In conclusion, the PZT/ZnOn composites were prepared at different sintering temperatures from 1,100 to 1,200 °C with various ZnOn contents from 0 to 1.5 wt%. It was found that the increase of sintering temperature enhanced the grain growth and densification in effect. The optimum mechanical properties were achieved for the composites with 1.5 wt% ZnOn sintered at 1,150 °C for 2 h with KIC, rf and rc 24, 43 and 41 % higher than that of the pure PZT. However, the electrical properties of er, d33, Qm and kp were lower than that of the pure PZT. The new PZT-based composites incorporated with ZnOn possessing enhanced mechanical properties and good piezoelectric properties could be further used in various practical applications. Acknowledgments This research was supported by the National Natural Science Foundation of China under Grant Nos. 50742007 and 50872159, the National High Technology Research and Development Program of China under Grant No. 2007AA03Z103, the National Defense Fund under Grant No. 401050301 and the Key Laboratory Foundation of Sonar Technology of China under Grant No. 9140C24KF0901.

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