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Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3–0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

https://doi.org/10.1016/j.jeurceramsoc.2011.04.018Get rights and content

Abstract

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K0.5Na0.5)NbO3–0.05Li(Nb0.5Sb0.5)O3 were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu2+ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu2+ ion could enter the A or B site of the perovskite unit cell and replace the Nb5+ or Li+ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d33 = 263 pC/N, kp = 0.42, Qm = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.

Introduction

Among all lead-free piezoceramics (KNa)0.5NbO3 (KNN) has drawn much attention since the breakthrough made by Saito et al.1 However, the piezoelectric coefficient, d33 of pure KNN is considerably low.2, 3 It seems undesirable that KNN ceramics only show a d33 of ∼100 pC/N by ordinary sintering,4, 5, 6 and ∼160 pC/N by cold isostatic pressing which is the highest value reported to date,7 indicating that experimental results concerning KNN still have a distance from theoretical values.8, 9

Its low d33 of 100 and 160 pC/N obtained through conventional and cold isostatic pressing, respectively, seems undesirable and far away from the theoretically value.4, 5, 6, 7, 8, 9 As far as electrical properties are concerned, KNN ceramics seem to exhibit soft characteristics (high remnant polarization Pr  15 pC/N, low coercive field Ec  1 kV/mm and large loss value).7 It appears difficult for them to be used for high-power device application. Copper oxide (CuO) has been proved to be an effective acceptor dopant to induce hardening features in KNN ceramics.10 Moreover, doping of CuO could improve the densification, decrease the sintering temperature and hence enhance the properties of KNN ceramics. Besides, as the ionic radius of Cu2+ is 0.72 Å, it may substitute the B-site ions Nb5+ (0.69 Å) as an acceptor dopant. It is hence expected that, similar to the cases for MnO2-doped Pb((Zn1/3Nb2/3)0.2(Zr0.50Ti0.5)0.8)O311 and MnO2 or Fe2O3 doped Pb(Ti, Zr)O3 ceramics,12 high Qm may be obtained in Cu-doped KNN ceramics.

Although the introduction of CuO can improve the sinterability of the KNN-based ceramics, previous research has also shown it would reduce the piezoelectricity of the ceramics to a certain degree.13, 14, 15, 16, 17 Studies have suggested that employing the Sb5+ ion could increase the ferroelectricity of the KNN-based ceramics.18 Therefore, in this work, Sb5+ ions were added in the CuO doped KNN-LN ceramics to maintain the piezoelectricity of the KNN-based ceramics. Wang et al. have reported that just Li-modified KNN lead-free piezoceramics can be sintered at a temperature as low as 950 °C, whose d33 value is up to 280 pC/N.19, 20 However, there are no details about the Qm and tan δ values of this system. The 0.95(K0.5Na0.5)NbO3–0.05Li(Nb0.5Sb0.5)O3xCuO ceramics elaborately designed by us, possess not only comparable piezoelectricity with that of Wang's,19, 20 but also better electric properties such as Qm and tan δ, indicating this system is of great potential for industrial applications. Meanwhile, the doping with CuO can obviously reduce the sintering temperature and then the evaporation of alkali metals would be effectively prohibited.

In the present study, the ceramics of CuO doped 0.95(K0.5Na0.5)NbO3–0.05Li(Nb0.5Sb0.5)O3 (abbreviated as KNN-5LNS-xCu) solid solution were prepared by conventional sintering technique. The sintering behavior, microstructure evolution, and electrical properties will be discussed in detail.

Section snippets

Experimental procedure

A conventional ceramic fabrication technique was used to prepare the KNN-5LNS-xCu (x = 0, 0.002, 0.004, 0.006, 0.008 mol) ceramics using analytical-grade metal oxides or carbonate powders: Na2CO3 (99%), K2CO3 (99.8%), Nb2O5 (99.5%), Li2CO3 (99%), Sb2O3 (99.99%), and CuO (98%). These powders are all supplied by Sinopharm Chemical Reagent Co., Ltd. After being ball milled in a nylon jar with agate balls for 24 h and dried, the stoichiometric KNN-5LNS powder was synthesized at 760 °C for 5 h. After the

Results and discussion

Fig. 1 shows X-ray diffraction patterns of KNN-5LNS-xCu sintered at 980 °C for 2 h. It was observed that all the samples were pure perovskite phase and no secondary phase was found. This suggested that KNN-5LNS based ceramics were well prepared, and the CuO completely diffused into the KNN-5LNS ceramic. Meanwhile, the doping of CuO induces the distortion of lattice, which is in the form of the slight changes in 2θ of different peaks.Table 1 presents the variations of the lattice parameters and

Conclusion

KNN-5LNS-xCu lead-free ceramics have been prepared by a conventional ceramic fabrication technique, and their microstructure, piezoelectric, and other electric properties have been investigated in detail. All the ceramics possess a perovskite structure with tetragonal symmetry. The addition of CuO could decrease the sintering temperature and improve the densification of the ceramics effectively. XRD patterns indicate that there simultaneously exist two kinds of ion substitution, A site and B

Acknowledgement

This work was supported by the National Natural Science Foundation of China.

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