The effect of nano-sized BNBT on microstructure and dielectric/piezoelectric properties
Introduction
Perovskite piezoelectric ceramics containing lead oxide such as Pb(Ti, Zr)O3 (PZT), are now widely used in piezoelectric devices because of their excellent piezoelectric properties [1]. However, recently emerged environmental issues, such as the restriction of WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restriction of Hazardous Substances), will prohibit us from using lead oxide in the near future due to their toxicity. In an approach to acclimate ourselves to recent ecological consciousness trends [2], [3], a lead-free piezoelectric material, bismuth sodium barium titanate, was considered as an environment-friendly alternative for a PZT system.
Since bismuth sodium titanate, Bi0.5Na0.5TiO3 (abbreviated as BNT), was discovered by Smolenskii and Agranovskaya in 1960 [4], BNT is considered to be an excellent candidate as a key material of lead-free piezoelectric ceramics because BNT is strongly ferroelectric. Besides, pure BNT shows a characteristic of diffuse phase transition (DPT). The BNT exhibits a large remnant polarization, Pr = 38 μC/cm2, a high Curie temperature (Tc = 320 °C). The dielectric properties display also an interesting anomaly wherein a low temperature phase transition at 200 °C marks the transition from ferroelectric to antiferroelectric [5]. However, because of its high coercive field, Ec = 73 kV/cm, and relatively large conductivity, the pure BNT is difficult to be poled and cannot be a good piezoelectric material. These problems were then improved by forming solid solutions with BaTiO3(BT), Bi0.5K0.5TiO3, KNbO3, NaNbO3, (SraPbbCac)TiO3, BiFeO3, BiScO3, etc. [6], [7], [8], [9], [10], [11], [12], [13]. Among these systems, bismuth sodium barium titanate (Bi0.5Na0.5)1−xBaxTiO3 (BNBT) has been widely investigated by many researchers [7], [14], [15]. Morphotropic phase boundary (MPB) of this solid solution system is near x = 0.06, where the materials show enhanced piezoelectric and dielectric performances. Takenaka et al. [7] have reported that the composition of (Bi0.5Na0.5)0.94Ba0.06TiO3, which is near the MPB, has relatively good piezoelectric properties of kp = 0.25, k31 = 0.19, d33 = 125 pC/N. However, this composition has a higher coercive electric field and medium piezoelectric constant. These properties were afterward promoted by a few research groups [16], [17], [18]. It was believed that substitution at A site or B site may induce soft or hard properties in a piezoelectric material by forming cation or oxygen vacancies, respectively. Several kinds of cations such as La3+, Nb5+, Co3+, Mn2+, etc. were tested to further modify BNT-based piezoelectric ceramics [19], [20], [21], [22]. Many researchers have investigated to improve the properties of BNBT ceramics either by modifying fabrication technique or by dopants.
The purpose of this present study is to control the size and morphology of starting powder and to pre-synthesize BaTiO3 in order to increase the reaction activity. Another objective is to demonstrate the effect of nano-sized BNBT particles on the microstructure and the piezoelectric/dielectric properties.
Section snippets
Experimental procedure
Perovskite (Bi0.5Na0.5)0.94Ba0.06TiO3 (abbreviated as BNBT6) ceramics were fabricated by the two different processes. One is the process I, which contains the pre-milling process and the pre-synthesizing process of the starting powder and the high energy milling process of calcined powder [sometimes abbreviated as nano-sized BNBT6] and the other is the process II, which contains the conventional mixing and milling process [sometimes abbreviated as conventional BNBT6]. The detailed fabrication
Effects of pre-milled starting powders and pre-synthesized BaTiO3 on the reaction temperature and the crystal structure
Fig. 2 shows the SEM micrograph of as-received and pre-milled Bi2O3, Na2CO3, and BaCO3, exclusive of the TiO2 powder having fine and sphere type particle morphology (average particle size: 100 nm). It has been reported that fine particles of TiO2 act as a catalyst for the decomposition of carbonate [24], and the anatase-phased TiO2 has a higher activity and is more efficient in lowering reaction temperature than the rutile one due to its low density (density of anatase phase TiO2 = 3.90 g/cm3,
Conclusions
The effects of manufacturing process on the dielectric/piezoelectric properties and the microstructure in the BNBT6 ceramic were systematically investigated. The effects of the pre-milled starting powders and pre-synthesized BaTiO3 on the reaction temperature and the crystal structure were examined by using BNBT6 in the MPB region. The results of thermo-gravimetric/differential thermal analysis (TG/DTA) revealed that the pre-milling process of starting materials and the use of anatase phase TiO2
Acknowledgements
This research was supported by the Program for the Training of Graduate Students in Regional Strategic Industries and Regional Innovation Center (RIC) Program which was conducted by the Ministry of Commerce, Industry and Energy of the Korean Government.
References (35)
- et al.
Effects of compositions and Nb-doping on microstructure and piezoelectric properties of PMS-PZ-PT system
Mater. Sci. Eng. B
(2003) - et al.
Additive effects on electrical properties of (Bi1/2Na1/2)TiO3 ferroelectric ceramics
J. Eur. Ceram. Soc.
(2001) - et al.
Tantalum doped 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 piezoelectric ceramics
J. Eur. Ceram. Soc.
(2008) - et al.
Electrical properties of Na1/2Bi1/2TiO3–BaTiO3 ceramics
J. Eur. Ceram. Soc.
(2002) - et al.
Some effects of different additives on dielectric and piezoelectric properties of (Bi1/2Na1/2)TiO3–BaTiO3 morphotropic-phase-boundary composition
Mater. Lett.
(2004) - et al.
Piezoelectric properties of Mn-doped (Na0.5Bi0.5)0.92Ba0.08TiO3 ceramics
Mater. Lett.
(2005) - et al.
Lowering the synthesis temperature of high-purity BaTiO3 powders by modifications in the processing conditions
Thermochim. Acta
(2003) Agency for Technology and Standards, Industry and Energy of the Korean Government
- et al.
Dielectric polarization and losses of some complex compounds
Sov. Phys. Tech. Phys. (Engl. Transl.)
(1958) - et al.
Ferroelectric and antiferroelectric properties of (Na0.5Bi0.5)TiO3–SrTiO3 solid solution ceramics
Ferroelectrics
(1974)