Preparation and properties of (Bi1/2Na1/2)TiO3–Ba(Ti,Zr)O3 lead-free piezoelectric ceramics
Introduction
The most widely used piezoelectric materials are PbZrO3–PbTiO3-based ceramics due to their superior piezoelectric properties, but they are not environmentally friendly for their lead oxide toxicity. Recently, lead-free materials are increasingly desired, especially in consumer products such as cars, various kinds of smart systems and sound generators. Thus, it is necessary and urgent to search for lead-free piezoelectric ceramics with excellent properties, which should be comparable with those found in the lead-based ceramics.
Bismuth sodium titanate ((Bi1/2Na1/2)TiO3, BNT), discovered by Smolensky et al. in 1960 [1], is one of the most important lead-free piezoelectric materials with a perovskite crystal structure. BNT shows strong ferroelectric properties with a large remnant polarization Pr and coercive field Ec at room temperature [2], [3]. Thus, BNT is considered to be a promising candidate for lead-free piezoelectric ceramics with balanced ferroelectric properties. Many scientists have been devoted to improving the piezoelectric properties of BNT-based ceramics. Recently, some investigations have been concentrated on the search for the new morphotropic phase boundaries in the BNT-based binary and ternary systems [4], [5], [6], [7], [8], [9], [10], [11]. Takenaka et al. have reported that the (Bi1/2Na1/2)TiO3–0.06BaTiO3 ceramics [4] show a relatively low dielectric constant and high electromechanical factor. (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 system [5], reported by Sasaki et al., shows the highest piezoelectric response due to the MPB between rhombohedral BNT and tetragonal BKT.
On the other hand, early X-ray crystalline structure studies revealed that Ba(Ti1−xZrx)O3 (BZT) form a complete solid solution and a phase diagram was subsequently constructed for up to 30 mol% Zr substitution in late 1950s [12], [13]. Yu et al. reported that Ba(Ti0.95Zr0.05)O3 ceramics showed fairly satisfactory piezoelectric response, with the d33 value of 236 pC/N at room temperature [14]. However, the large temperature dependence of BZT ceramics, combined with the low Curie temperature, limits their practical application as a piezoelectric material. Considering that BNT ceramics have higher Curie temperature, we then investigated the BNT–BZT binary system as a piezoelectric material. Moreover, special emphasis was focused on searching for the MPB of this system to achieve great enhancement in the piezoelectric property. With a special emphasis on the MPB composition between BNT (rhombohedral) and BZT (tetragonal), the piezoelectric, ferroelectric and dielectric properties of BNT–BZT ceramics were studied. As a result, a high piezoelectric constant d33=147 pC/N was obtained at the composition near MPB.
Section snippets
Experimental procedure
High purity Bi2O3, Na2CO3, TiO2, BaCO3 and ZrO2 (purity >99.0%) powders were used as starting materials. Our preliminary experiments found that BNT–BZT ceramics were not well synthesized by mixing and calcinating all these above powders as used in the conventional oxide mixing process. The BNT–BZT ceramics were therefore prepared by using a modified two-step sintering approach, in which BNT and BZT source powders were calcined separately and then blended together. To prepare the BNT source
Results and discussion
Fig. 1 shows the X-ray diffraction patterns of BNT–BZT ceramics in the 2θ range of 20° to 80°. Without BZT doping, the pure BNT ceramic, labeled BNT–BZT0 in this case, shows rhombohedral phase, whose (110) peak is detected at 32.68° (2θ). For the BNT–BZT ceramics with different compositions, the separated (110) peaks corresponding to the pure BNT or BZT were not observed, implying the formation of a solid solution rather than a simply mixed compound. The (110) diffraction peak of the BZT powder
Conclusion
Ba(Ti,Zr)O3 (BZT) was incorporated into (Bi1/2Na1/2)TiO3 (BNT) to form a lead-free piezoelectric ceramic system with a morphotropic phase boundary (MPB). Highly dense BNT–BZT ceramics were prepared by a two-step sintering method at 1180 °C. Phase transition from rhombohedral to tetragonal was observed with increasing BZT fraction in the range from 3 to 12 mol%, indicating the existence of MPB in the BNT–BZT system. Ferroelectric, dielectric and piezoelectric properties of the BNT–BZT ceramics
Acknowledgments
This work was supported by natural science foundation of China (Grant No. 50325207) and Toyota as a part of the international cooperation between Tsinghua University and Toyota Motor Cooperation.
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