Papers by Author: Yuji Hiruma

Abstract: Grain-oriented Bi₄Ti_2.92Nb_0.08O₁₂ (BITN) ceramics were prepared by hot-forging (HF) method. The grain orientation factor, F, of BITN ceramic determined by the Lotgering method were approximately 0.96. The resistivity, ρ, along the c-axis of HF-BITN ceramic is higher than that of the a,b-axis of HF-BITN and OF-BITN ceramics for the temperature range from 100 to 400°C. The ρ along the direction parallel to the c-axis is found to be about 10⁹ Ω·cm at 400°C, which is about ten times larger than that of OF-BITN ceramics. From resonance and anti-resonance characteristics, electromechanical coupling factor, k₂₄, was 0.026 at room temperature.

Abstract: Grain-oriented 0.8(Bi_1/2K_1/2)TiO₃-0.2BaTiO₃ (BKT-BT20) ceramics were prepared by the Reactive Templated Grain Growth (RTGG) method. The BKT-BT20 ceramics sintered at 1070°C for 100 h. The grain-oriented BKT-BT20 exhibited relatively high orientation factor, F, of 0.87 and density ratio of 92%. A resistivity of textured BKT-BT20 was 1.29×10¹³ Ωcm. Piezoelectric strain constant, d₃₃, and the normalized strain, d₃₃^*, of the textured BKTBT20 ceramic in the direction parallel (//) to the tape stacking direction were 117 pC/N and 243 pm/V (at 80 kV/cm), respectively.

Abstract: The high-power piezoelectric characteristics of lead-free piezoelectric ceramics, based on a bismuth layer-structured ferroelectric, MnCO₃-doped (Sr_0.7Ca_0.3)₂Bi₄Ti₅O₁₈ (abbreviated as SCBT0.3 + Mn x wt%), were studied. SCBT0.3 + Mn x wt% lead-free ceramics showed an extremely high mechanical quality factor (Q_m) of more than 3000 in the (33) vibration mode under small-amplitude vibration. The high-power piezoelectric characteristics of SCBT0.3 + Mn x wt% were measured using the high-power measurement method based on frequency sweep driving under a constant voltage condition. It was found that the vibration velocity v_0-p of SCBT0.3 + Mn 0.2 wt% linearly increased up to approximately 3.0 m/s. Therefore, the Mn-doped SCBT0.3-based ceramics are a promising candidate for lead-free high-power applications.

Abstract: High-power piezoelectric characteristics of (Bi_1/2Na_1/2)TiO₃ (BNT)- and KNbO₃ (KN)-based ceramics at continuous driving were investigated. The selected compositions of BNT- and KN-based ceramics were 0.88(Bi_1/2Na_1/2)TiO₃-0.04(Bi_1/2Li_1/2)TiO₃-0.08(Bi_1/2K_1/2)TiO₃ doped with 0.6 wt% MnCO₃ [BNLKT4-8Mn0.6] and KNbO₃-MnCO₃ 0.8 wt% [KN-Mn0.8] ceramics. It was found that the vibration velocity v_0-P of BNLKT4-8Mn0.6 and KN-Mn0.8 ceramics linearly increased up to approximately 0.8 and 0.6 m/s, respectively. The temperature increases of BNLKT4-8Mn0.6 and KN-Mn0.8 ceramics were approximately 5°C under continuous driving at the vibration velocity of 0.6 m/s.

Abstract: Grain-oriented (Bi1/2K1/2)TiO3 (BKT) ceramics were prepared by the reactive templated grain growth (RTGG) method, and a hot-pressing (HP) method was modified on the basis of the RTGG method to obtain dense ceramics. The BKT ceramic sintered at 1060°C showed a single-phase perovskite structure. The grain-oriented BKT prepared by the HP method exhibited a relatively high orientation factor F of 0.82 and a high density ratio of 99%. SEM micrographs of the HP-grain-oriented BKT indicated a textured and poreless microstructure. On the basis of the density ratio of 87% for the grain-oriented BKT without HP, the HP method was considered very effective for improving the density ratio even for the RTGG sample. In addition, the resistivity of the HP-grain-oriented BKT was 1.73 x 1013 •cm.

Abstract: Electrical and piezoelectric properties of CaBi2Ta2O9-based ceramics have been studied. Temperature dependence of dielectric properties indicated that the Curie temperature, TC, was 923°C. Coercive field, EC, and remanent polarization, Pr, became saturated with increasing temperature. It is expected that higher-temperature than 250°C could promote the domain wall motion during the poling treatment. From resonance and antiresonance measurements, piezoelectric properties such as the maximum phase, max, electromechanical coupling factor, k33, and piezoelectric g constant, g33, were 86.1º, 0.085 and 8.4 × 10-3 V∙m/N, at room temperature, respectively.

Abstract: (1-x)(Bi1/2Na1/2)TiO3-xBiAlO3 [BNTA100x (0 ≤ x ≤ 0.14)] ceramics were prepared by a conventional ceramic fabrication process. Single-phase perovskite structures were formed when x ≤ 0.08 for BNTA100x. The depolarization temperature, Td, at which piezoelectricity disappears, decreased with increasing x. The piezoelectric constant, d33, increased with increasing x because of the increase in free permittivity accompanied with decreasing Td. d33 and the electromechanical coupling factor, k33, of BNTA6 were 93 pC/N and 0.41, respectively. Normalized strain d33* (= Smax/Emax) at 60 kV/cm at the maximum field-induced strain was 122.2 pm/V for BNTA8. The value of d33* was higher than the calculated value of d33.

Abstract: The phase transition temperatures of divalent and trivalent ions substituted (Bi1/2Na1/2)TiO3 ceramics were investigated in detail by electrical measurement. BNT ceramics substituted by divalent (Ca2+, Sr2+ and Ba2+) and trivalent (La3+, Nd3+, Ho3+ and Yb3+) ions were prepared by conventional ceramic fabrication process. For these ceramics, the depolarization temperature, Td, the rhombohedral-tetragonal phase transition temperature, TR-T, and the temperature of the maximum dielectric constant, Tm, were determined on the basis of the temperature dependences of dielectric and piezoelectric properties. In this study, we demonstrated the relationship between phase transition temperature and ionic radius of ions substituted in the A-site of BNT ceramics.

Abstract: The piezoelectric properties of Nd and V cosubstituted bismuth titanate ceramics, (Bi4-y,Ndy)1-(x/12)(Ti3-x,Vx)O12 [BNTV-y (x=0.01, y=0.00~1.00)] , are investigated. Furthermore, the grain orientation effects of BNTV-y ceramics on their piezoelectric properties are studied by the hot forging (HF) method. The electromechanical coupling factor, k33, of ordinarily fired (OF) BNTV-y0.75 ceramic was 0.21, which is one of the highest k33 values for bismuth layer-structured ferroelectrics (BLSFs). On the other hand, the k33 of the grain oriented (HF) BNTV-y0.25 ceramic was 0.30.

Abstract: Vanadium-substituted strontium bismuth niobate, SrBi2Nb2-xVxO9 (SBNVx), ceramics were synthesized by a low-temperature ceramic fabrication process; their electric properties were characterized, and grain orientation effects on their electric properties were also investigated using a hot-forging (HF) method. Resonance characteristics of the (33) and (15) modes of grain-oriented SBNV0.1 (HF) showed relatively high electromechanical coupling factors of k33=0.272 and k15=0.151 as well as piezoelectric constants of d33=31.7 pC/N and d15=24.8 pC/N. The temperature dependences of the resonance frequency (TC-f) of the (33) and (15) modes of the SBNV0.1(HF) ceramics were -16.5 ppm/°C and –25.5 ppm/°C, respectively.