Piezoelectric Properties of (Bi0.5Na0.5)TiO3-BaTiO3-SrTiO3 Ceramics - Lead-free Piezoelectric Ceramics Developed at AIST Chubu Keiji KUSUMOTO Ceramics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan 1. Introduction Ever since the discovery of piezoelectric effect, piezoelectric materials have been rapidly developed and widely used for ultrasonic transducers, sensors and actuators. At present, the most widely-used piezoelectric materials are Pb(Zr,Ti)O3 (PZT)-based ceramics because of their superior piezoelectric properties. However, because of lead oxide toxicity, it is desired that lead-free materials be used as piezoelectric ceramics from the viewpoint of environmental protection. Consequently, it is necessary to develop lead-free piezoelectric materials that have excellent properties such as the PZT-based ceramics. In this lecture, I introduce representative candidates for lead-free piezoelectric ceramic materials and piezoelectric properties of (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 ceramics developed at AIST. 2. Representative lead-free piezoelectric ceramic materials At present, tungsten·bronze-type, bismuth layer-structured-type, and perovskite-type ferroelectrics are known as candidates for lead-free piezoelectric ceramic materials. However, piezoelectric activities of conventionally sintered tungsten·bronz-type ferroelectrics (e.g., Ba 2 NaNb 5 O15 ) and bismuth layer-structured ferroelectrics (e.g., Bi 4Ti 3O 12 ) are not so large because of its large anisotropy in piezoelectric properties. Thus, the grain orientation technique such as hot-forging process and RTGG (Reactive Templated Grain Growth) process have recently been attempted in order to solve this problem. On the other hand, the perovskite-type ferroelectrics (e.g., BaTiO3, KNbO 3, and (Bi 0.5 Na0.5)TiO 3 ) are hopeful candidates for lead-free piezoelectric ceramics because its anisotropy in piezoelectric properties are small compared with other ferroelectrics. BaTiO3 is a first piezoelectric ceramics developed in 1947 and is known to show relatively good piezoelectric properties. However, BaTiO3-based ceramics are not suited for useful lead-free piezoelectric ceramic materials because their Curie temperatures (Tc=~130°C) are too low for piezoelectric applications. It is known that KNbO3 and related compounds show a high Curie temperature (Tc=~435°C) and good piezoelectric properties. However, dense KNbO3 based ceramics are difficult to obtain by conventional method because of the vaporization of potassium oxide during sintering. In the perovskite-type ferroelectrics, ((Bi 0.5Na0.5)TiO3)-based ceramics are thought to be the strong candidate for lead-free piezoelectric ceramic materials because (Bi 0.5Na0.5)TiO 3 is a strong ferroelectric with a relatively high Curie temperature (Tc=350°C). 3. (Bi 0.5 Na0.5 )TiO3 -BaTiO3 -SrTiO 3 lead-free piezoelectric ceramics developed at AIST Chubu In recent years, our group have been attempted to develop useful lead-free piezoelectric ceramic materials in order to replace the PZT-based ceramics for environmental protection. Recently, we have developed novel lead-free piezoelectric ceramic materials, (Bi 0.5 Na0.5)TiO 3 -BaTiO 3 -SrTiO3 system, via the modification of (Bi 0.5Na0.5)TiO3-BaTiO3 solid solutions with SrTiO3 . As mentioned above, (Bi 0.5Na0.5)TiO3)-based ceramics are expected to be the candidate for lead-free piezoelectric ceramic materials in the perovskite-type ferroelectrics because (Bi 0.5 Na0.5)TiO 3 is a strong ferroelectric with a relatively high Curie temperature. In the (Bi 0.5 Na0.5)TiO 3 -based solid solution, (Bi 0.5Na0.5)TiO3-BaTiO3 ceramic is considered to be a strong candidate for useful lead-free piezoelectric ceramic material because the solid solution shows good piezoelectric properties. Thus, we selected the (Bi 0.5Na0.5)TiO 3 based solid solution for investigation of lead-free piezoelectric ceramics and have attempted to improve the piezoelectric properties of the ceramics. The processing method of specimens is described below. Conventional ceramic fabrication technique was used to prepare the (Bi 0.5Na0.5)TiO 3 BaTiO3 -SrTiO3 ceramics. The starting powders were reagent-grade Bi 2O3, Na 2 CO3, BaCO 3 , SrCO 3, and TiO2 . These oxides were mixed in ethanol with zirconium balls by ball-milling for 1h and then calcined at 900°C for 3h in air. The crystal phase of the calcined powders was identified using an X-ray diffractometer. The calcined powders were ground and pressed into pellets of 17 mm in diameter and 1.5 mm in thickness. The green pellets were sintered on a Pt foil at 1200°C for 3h in air. The sintered pellets were polished and electroded with a silver paste. The temperature dependence of dielectric constant, εs, and dissipation factor, tanδ, of the samples were measured at 1, 10, and 100 kHz using an impedance analyzer. The pellets were poled in a silicone oil bath at a temperature of 60°C with a DC field of 4kV/mm for 30min. Piezoelectric properties such as a planar coupling coefficient (Kp) of the samples were determined using a resonance method and Onoe's formula. A piezoelectric charge coefficient (d33 ) of the sample was measured using a piezo d 33-meter at frequency of 110 Hz. The electric-field-induced strains were estimated by a laser-type displacement meter. Figure 1 shows the planar coupling coefficient (Kp) of (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 ceramics. It is can be seen from the figure that the compositions with a relatively large planar coupling coefficient (Kp<0.3) are observed in the (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 system. Figure 2 shows the piezoelectric charge coefficient (d33 ) of (Bi 0.5Na0.5)TiO 3 -BaTiO3 SrTiO3 ceramics. Relatively large piezoelectric charge coefficient (d33 <130 pC/N) are observed in a certain range of the composition. Figure 3 shows a typical electric-fieldinduced strain curve of (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 ceramics. As seen in the figure, although the strain of (Bi 0.5Na 0.5)TiO3 -BaTiO3-SrTiO3 ceramics is very small compared with that of Pb-based piezoelectric ceramics, the (Bi 0.5 Na0.5 )TiO3-BaTiO3-SrTiO3 ceramics shows the electric-field-induced strain curve with a relatively low hysteresis. In the case of (Bi 0.5 Na0.5)TiO 3 -BaTiO 3 ceramics, it has been reported that the piezoelectric properties of the ceramics are improved by adding small amounts of additives such as Y2O3, CeO 2, and MnO 2. Thus, in the future, we will attempt to improve the piezoelectric properties for (Bi 0.5 Na0.5)TiO 3 -BaTiO 3 -SrTiO3 ceramics by adding additives. SrTiO3 20 SrTiO3 (mol%) 10 (Bi0.5Na0.5)TiO3 Paraelectric BaTiO3 0.3 〜 0.25 - 0.3 0.2 - 0.25 Kp=0.15 - 0.2 (Bi0.5Na0.5)TiO3 10 BaTiO3 (mol%) 20 Fig.1. Planar coupling coefficient (Kp) in (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 ceramics. SrTiO3 20 SrTiO3 (mol%) 10 Paraelectric (Bi0.5Na0.5)TiO3 BaTiO3 100〜 50-100 d33= 〜50 pC/N (Bi0.5Na0.5)TiO3 10 BaTiO3 (mol%) 20 Fig.2. Piezoelectric charge coefficient (d33 ) in (Bi 0.5Na0.5)TiO3-BaTiO3-SrTiO3 ceramics. Induced-strain (%) 0.01 0.008 0.006 0.004 0.002 0 0 200 400 600 800 1000 Electric field ( V/mm ) Fig.3. 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