Crystal Structure, dielectric and ferroelectric properties of (Bi0.5Na0.5)TiO3–(Ba,Sr)TiO3 lead-free piezoelectric ceramics

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Abstract

The crystal structure, microstructure, dielectric and ferroelectric properties of (1  x)(Bi0.5Na0.5)TiO3x(Ba0.7Sr0.3)TiO3 (BNBST30-x, x = 0–0.12) lead-free piezoelectric ceramics were studied in this work. Structural variation according to (BNBST30-x) composition was carefully investigated by X-ray diffraction analyses. A rhombohedral–tetragonal morphotropic phase boundary (MPB) is found at BNBST30-8 composition which shows the highest remanent polarization value. Smallest coercive field value is observed at the tetragonal-side of MPB, i.e. BNBST30-10, instead of MPB composition. The addition of Sr2+ ions plays a role as soft dopant on the dielectric properties in the present BNBST systems. In comparison with the MPB compositions containing various Sr2+ ratios, the dielectric and piezoelectric properties (ɛ33T/e0 and d33) are effectively enhanced when Sr2+ ratio is higher. Besides, Sr2+ allows to enlarge the cell volume that is also advantageous for the improvement of dielectric and piezoelectric performances.

Introduction

Lead-based Pb(Zr,Ti)O (PZT) piezoelectric ceramics with perovskite structure are widely used as actuators, sensors and micro-electro-mechanical devices owing to their superior dielectric properties. However, toxic lead oxide may evaporate during the heating process due to its high vapor pressure and is strongly harmful for human health. Accordingly, developing new environment-friendly materials to replace PZT-based materials has become one of the most important issues. Therefore, various lead-free piezoelectric ceramics with perovskite structure were studied, such as KNbO3-, BaTiO3-, (Bi0.5K0.5)TiO3-, and (Bi0.5Na0.5)TiO3-based solid solutions [1].

(Bi0.5Na0.5)TiO3 (BNT) is well known as a complex ABO3 perovskite ceramic with a ferroelectric rhombohedral phase at room temperature, and its remanent polarization (Pr) and coercive field (Ec) of single crystal are 38 μC/cm2 and 7.3 kV/mm, respectively [2]. As temperature increases, the phase of BNT will transform from rhombohedral to tetragonal and finally into cubic phase. The transition temperatures of rhombohedral–tetragonal phase (TR–T) and tetragonal–cubic phase (TT–C) are 300 and 540 °C, respectively, which were firstly reported by Zvigzds et al. [3]. Recently, Jones and Thomas have indicated that the coexistence of rhombohedral/tetragonal and tetragonal/cubic phases occurs in the heating temperatures between 255–400 °C and 500–540 °C, respectively [4].

BNT is expected to be a candidate for replacing lead-based piezoelectric ceramics due to the presence of morphotropic phase boundary composition (MPB) for some BNT-based ceramics which generally exhibit relatively higher piezoelectric properties than single phased BNT compound. Binary material systems, such as BNT–BaTiO3, and BNT–(Bi0.5K0.5)TiO3, and ternary systems, such as BNT–BaTiO3–(Bi0.5K0.5)TiO3, BNT–BaTiO3–(Bi0.5Ag0.5)TiO3, and BNT–(Bi0.5K0.5)TiO3–(Bi0.5Li0.5)TiO3, are typical systems with MPB composition [5], [6], [7], [8], [9], [10]. To improve their piezoelectric properties further, various dopant and novel ceramic processes had been used in prepared these ceramics [11], [12], [13], [14], [15], [16], [17]. For example, highly textured BNT–BaTiO3 ceramic can be fabricated by reactive templated grain growth (RTGG) method and shows a significant improvement in piezoelectric properties [16]. The relation between MPB composition and tolerance factor in these BNT-based ceramics had been revealed in our previous study [18]. However, a decrease of depolarization temperature (Td) is always found during the attempts of modifying piezoelectric properties by doping other materials, and thus limits its application. Some researches had devoted to clarify the controlling mechanisms of the variation in Td [19], [20], [21], [22], [23], [24]. Among them, Hiruma et al. have developed a method to accurately measure Td and explain the variation in Td from the viewpoint of crystal structure [22], [23], [24]. In order to develop a material system with both high piezoelectric performance and high Td, to investigate the mechanisms that leads to a variation of Td according to the crystal structure are still important objects.

Ferroelectric (Ba1−aSra)TiO3 (BST) material is widely used in microelectronic device due to its high dielectric properties and is a good doping compound to improve the dielectric properties of pure BNT [25]. According to the tolerance factor calculation as mentioned above [18], the MPB composition of (1  x)(Bi0.5Na0.5)TiO3x(Ba1−aSra)TiO3 system is corresponding to a tolerance factor of around 0.991 in which the x value shifts from 0.06 to 0.08 as the Sr2+ content increases from a = 0–0.3. In this work, the solid solution of (1  x)(Bi0.5Na0.5)TiO3x(Ba1−aSra)TiO3 (x = 0–0.12, a = 0–0.3) was prepared. Their microstructures, crystal structures and phase transformation behaviors in correlation with dielectric properties were studied. The doping effect of Sr2+ ions in MPB composition on the dielectric performance will be also discussed.

Section snippets

Experimental

Conventional solid-state reaction was used to prepare the ceramics of (1  x)(Bi0.5Na0.5)TiO3x(Ba0.7Sr0.3)TiO3 system (labeled BNBST30-x). The powder of Bi2O3 (Alfa Aesar, 99.97%), Na2CO3 (Showa Chemical, 99.5%), BaCO3 (Alfa Aesar, 99.8%), SrCO3 (Alfa Aesar, 99.7%), and TiO2 (anatase, Alfa Aesar, 99.9%) were used as starting materials and stoichiometrically mixed in ethanol to obtain various BNBST30-x compositions with x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.12, respectively. The mixtures were

Microstructure and crystal structure analysis

Microstructure images of BNBST30 ceramics are shown in Fig. 1 confirming that the ceramics are densely sintered at 1125 °C for 2–4 h. It is found that the addition of BST30 allows to shorten the sintering duration to attain a dense sintered bulk with a similar grain size. It is worthy to mention that the sintering condition has to be carefully controlled [27] because phase decomposition due to the evaporation of Na+ and Bi3+ may occur that often results in a formation of second phase. Such phase

Conclusions

The presented work investigates the fabrication, crystal structure, dielectric properties, and the effect of Sr2+ content of (1  x)(Bi0.5Na0.5)TiO3x(Ba1−aSra)TiO3 (BNBSTa-x) ceramics. It is observed that the addition of (Ba0.7Sr0.3)TiO3 (BST30) generates a phase transition from rhombohedral to tetragonal phase and the MPB composition is found at BNBST30-8. The study confirms that the ferroelectric properties strongly depend on the structural parameters such as αR angle in rhombohedral phase and

Acknowledgement

This work was supported by the National Science Council of Taiwan under contract no. NSC 97-2221-E-006-009.

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