Rintaro Aoyagi

Ferroelectric and Piezoelectric Properties of (Bi1/2K1/2)TiO3 Ceramics

The dielectric, ferroelectric and piezoelectric properties of bismuth potassium titanate, (Bi1/2K1/2)TiO3 (BKT), ceramics were studied. Single-phase BKT ceramics with a high relative density of 97% were obtained by the hot pressing (HP) method. The resistivities of BKT ceramics hot-pressed at 1060 and 1080°C (hereafter abbreviated as BKT-HP1060°C and BKT-HP1080°C) were fairly high being of the order of 1013 Ωcm at room temperature (RT). The Curie temperature Tc of BKT-HP1060°C was 437°C, which is relatively higher than those of other lead-free piezoelectric materials. In this study, the ferroelectric properties of BKT ceramics were successfully obtained with fully saturated hysteresis loops. The remanent polarization Pr and coercive field Ec of BKT-HP1080°C were 22.2 µC/cm2 and 52.5 kV/cm, respectively. D–E hysteresis loops for these ceramics were observed even at 260°C. The electromechanical coupling factor k33 and piezoelectric constant d33 of BKT-HP1080°C were 0.28 and 69.8 pC/N, respectively. The second-phase transition temperature T2 of 340°C was determined from the temperature dependence of piezoelectric and dielectric measurements.

Piezoelectric properties of BaTiO3-(Bi1/2K1/2)TiO3 ferroelectric ceramics

The dielectric, ferroelectric and piezoelectric properties of barium titanate (BaTiO3)-based solid solution, (1-x)BaTiO3–x(Bi1/2K1/2)TiO3 (0≤x≤1) (BT–BKT100x), a candidate lead-free piezoelectric ceramic, were studied using a starting material of fine BaTiO3 powder. The Curie temperature, Tc, of the BT–BKT ceramics shifted to higher temperatures than those of BT with increasing the amount (x) of (Bi1/2K1/2)TiO3 and Tc was higher than 200°C at x=0.2. Electromechanical coupling factor, k33, and piezoelectric constant, d33, were 0.37 and 75.8 pC/N for BT–BKT20 + MnCO3 (0.1 mass%), and 0.40 and 100 pC/N for BT–BKT5 + MnCO3 (0.1 mass%), respectively.

(Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3-Based Lead-Free Piezoelectric Ceramics

The piezoelectric properties of lead-free x(Bi1/2Na1/2)TiO3-y(Bi1/2K1/2)TiO3-zBaTiO3 [BNBKy:z(x); y:z = 4:1 and 2:1, x+y+z=1] with tetragonal compositions were investigated, focusing on the secondary phase transition T2 from the ferroelectric to antiferroelectric phases. Although the T2 temperatures of BNBK near the MPB composition remained at about 115°C, T2 rapidly shifted to a higher temperature of 200°C or higher for the off-MPB composition [x≤0.75 for BNBK4:1(x) and x≤0.78 for BNBK2:1(x)]. The piezoelectric activities of off-MPB compositions decreased with decreasing the amount (x) of BNT. The larger piezoelectric constant d33=135 pC/N, the higher secondary phase transition temperature T2=197°C, and the coupling factor k33=0.45 with a good temperature dependence were obtained for BNBK2:1(0.78).

Dielectric Anisotropy near Morphotropic Phase Boundary in Pb(Zn1/3Nb2/3)O3–xPbTiO3 Crystals

It was proposed that large dielectric constants in Pb(Zn1/3Nb2/3)O3–xPbTiO3 (PZN–xPT) crystals are mainly due to the fluctuation of polarization along the spherical direction (perpendicular to the spontaneous polarization) near the morphotropic phase boundary (MPB), named transversal instability [M. Iwata and Y. Ishibashi: in Ferroelectric Thin Films, ed. M. Okuyama and Y. Ishibashi (Springer, 2005) Part III, p. 127]. The dielectric anisotropy in the MPB region of PZN–xPT was investigated to clarify the origin of the large dielectric constant in perovskite-type relaxors. It was found that the dielectric constant perpendicular to the spontaneous polarization indicating transversal instability markedly increases as it approaches MPB. The dielectric anisotropy and large dielectric constant near the MPB were discussed in terms of the phenomenological theory.

Piezoelectric Properties of Sodium Bismuth Tantalate Na0.5Bi2.5Ta2O9 Dense Ceramics

The piezoelectric properties of sodium bismuth tantalate Na0.5Bi2.5Ta2O9 (NBTa) dense ceramics were determined using 0.1 wt% Mn-doped samples. The ferroelectric properties of the NBTa ceramics were confirmed. The remanent polarization Pr and coercive field Ec were 15.5 µC/cm2 and 81.5 kV/cm at room temperature, respectively. Optimum poling conditions (temperature Tp, time tp and electric field Ep) were investigated for piezoelectric measurements. The independent electromechanical coupling factors of the NBTa dense ceramics were obtained as follows: kp=10.0, k31=6.1, k33=12.4 and k15=5.6%. These values are comparable to those of the reported bismuth layer-structured ferroelectrics (BLSFs) that are composed of two pseudo-perovskite layers. The physical constants of the NBTa ceramics were also determined. The piezoelectric constants were d33=10.8, d31=5.2, and d15=6.0 pC/N.

Field-Induced Polarization State and Phase Transition in Pb(Zn1/3Nb2/3)O3: Dielectric Dispersions and Nonlinear Dielectric Constants

The field-induced polarization state and the phase transition to a relaxor in Pb(Zn1/3Nb2/3)O3 (PZN) were investigated. It was confirmed that the field-induced polarization state in PZN can be realized by applying an electric field at room temperature for a long time. The phase transition from the field-induced polarization state to a relaxor was observed at 114 °C during zero-field heating (ZFH) after applying an electric field even at room temperature. The critical slowing down of this transition was observed, where the dielectric dispersion was almost of the single Debye type with β=0.95. The second- and third-order nonlinear dielectric constants were measured during ZFH. The sharp peak at 114 °C with a negative value in the third-order nonlinear dielectric constant was found. The field-induced polarization state and phase transition in PZN were discussed.

Piezoelectric Properties and Depolarization Temperatures of (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3 Lead-Free Piezoelectric Ceramics

The piezoelectric properties of a solid solution based on the three components of bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT), bismuth potassium titanate, (Bi1/2K1/2)TiO3 (BKT), and barium titanate, BaTiO3 (BT), that is, x(Bi1/2Na1/2)TiO3-y(Bi1/2K1/2)TiO3-zBaTiO3 [BNBKy:z(x); x+y+z=1] are investigated. Fine piezoelectric properties in lead-free piezoelectric ceramics were obtained near the MPB composition, and the highest electromechanical coupling factors, k33 and kp, and piezoelectric constant, d33, were 0.58 and 0.36 for BNBK2:1(0.89) and 181 pC/N for BNBK2:1(0.88), respectively. In this study, we also measured the depolarization temperature, Td, from the temperature dependence of dielectric and piezoelectric properties.

Crystal Growth and Characterization of Lanthanum Substituted Bismuth Titanate Single Crystals

The thermostability of lanthanum substituted bismuth titanate, Bi4-xLaxTi3O12 (BLT), with La-content up to x=0.75 was investigated using differential thermal analysis. Subsequently, BLT and Bi4Ti3O12 (BIT) single crystals were grown by a self-flux method. The BLT crystals with two different ranges of La content, x=1.0–1.5 and 0<x<0.1, were obtained using starting materials with mole ratio BLT(x=0.25):Bi2O3=1:9. Segregation phenomena of La during crystal growth process were observed. A comparison of dielectric and ferroelectric properties in BIT and BLT crystals was performed. The La incorporation led to a significant increase in the coercive field value, Ec, of BIT single crystals.

Piezoelectric Properties of Vanadium-Substituted Strontium Bismuth Niobate

Vanadium-substituted strontium bismuth niobate, SrBi2Nb1.95V0.05O9 (SBNV), ceramics were synthesized by a low-temperature process, and the detailed piezoelectric properties of the SBNV ceramics were characterized. Dense SBNV ceramics were obtained at a sintering temperature of 950°C, which is 200°C lower than that for nonsubstituted (SBN) ceramics. With the substitution of vanadium cations for niobium the ferroelectric D–E hysteresis loops were observed at room temperature and the remanent polarization, Pr, and coercive field, Ec, obtained were 5.4 µC/cm2 and 105 kV/cm, respectively. The independent electromechanical coupling factors obtained, which were characterized by the resonance-antiresonance method, were as follows: kp=0.074, k31=0.045, k33=0.175, k15=0.106, and kt=0.140. The mechanical quality factor, Qm, of the SBNV ceramics in the (33)-mode was 5500, which was about three times that of SBN. The temperature dependences of the resonance frequency (TCF) in the length extensional vibration mode of the SBNV and SBN ceramics were 42.1 ppm/°C and 34.6 ppm/°C, respectively.

Phase Transition under Zero-Field Heating after Field Cooling in (1-x)Pb(In1/2Nb1/2)O3–xPbTiO3

Phase transitions in (1-x)Pb(In1/2Nb1/2)O3–xPbTiO3 (PIN–xPT) under two processes, i.e., zero-field heating (ZFH) after the field cooling (FC) and ZFH without FC, were investigated by dielectric constant measurement. The phase diagrams of PIN–xPT (0.2≤x≤0.45) subjected to the two processes were clarified. A new phase transition between ferroelectric phases in PIN–0.2PT on ZFH after FC was found at 113 °C. It was also revealed that the phase transition observed in PIN–0.2PT connects to the morphotropic phase boundary (MPB). We conclude that the phase transition found in PIN–0.2PT is a transition between the tetragonal and rhombohedral phases.