Yuji Hiruma

Electrical Properties and Depolarization Temperature of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 Lead-free Piezoelectric Ceramics

The piezoelectric properties of a solid solution of the binary system, x(Bi1/2Na1/2)TiO3–(1-x)(Bi1/2K1/2)TiO3 [BNKT100x; x=0.50–0.98] were investigated, focusing on depolarization temperature, Td. Fine piezoelectric properties in lead-free piezoelectric ceramics were obtained near the morphotropic phase boundary (MPB) composition between the rhombohedral and tetragonal structures, and the highest electromechanical coupling factor, k33, and piezoelectric constant, d33, were 0.56 for BNKT84 and 157 pC/N for BNKT80, respectively. However, the Td of BNKT80 was low (174 °C). The Td of the MPB composition was low, and the Td near the MPB composition was sharply decreased. It is thought that BNKT70 is a candidate composition for lead-free actuator applications owing to its relatively large piezoelectric constant, d33 (126 pC/N), dynamic d33 (214 pm/V), and high depolarization temperature, Td (206 °C). In this study, we determined depolarization temperature, Td, from the temperature dependence of dielectric and piezoelectric properties.

Current Developments and Prospective of Lead-Free Piezoelectric Ceramics

The dielectric, ferroelectric and piezoelectric properties of perovskite ferroelectric and bismuth layered-structured ferroelectric (BLSF) ceramics are described being superior candidates for lead-free piezoelectric materials to reduce environmental damage. Perovskite-type ceramics seem to be suitable for actuator and high-power applications that require a large piezoelectric constant, d33, and a high Curie temperature, Tc, or a depolarization temperature, Td (>200 °C). For BaTiO3-based solid solutions, (1-x)BaTiO3–x(Bi0.5K0.5)TiO3 (BT–BKT100x) ceramics, Tc increases with increasing amount of x. The BT–BKT20 + MnCO3 (0.1 wt %) ceramic shows a high Tc greater than 200 °C and an electromechanical coupling factor of k33 =0.35. In the case of a(Bi1/2Na1/2)TiO3–b(Bi1/2K1/2)TiO3–cBaTiO3 [BNBK (100a/100b/100c)] solid solution ceramics, d33 is 191 pC/N for BNBK (85.2/2.8/12). KNbO3 (KN)-based ceramics are also a candidate for lead-free piezoelectrics. In Mn-doped KN ceramics, a higher k33 of 0.507 is obtained for KN + MnCO3 (0.1 wt %). On the other hand, BLSF ceramics seem to be excellent candidates as piezoelectric sensors for high temperatures and ceramic resonators with a high mechanical quality factor, Qm, and a low temperature coefficient of resonance frequency, TC-f. The k33 value of the donor (Nb)-doped and grain-oriented (HF) Bi4Ti3-xNbxO12 (BITN-x) ceramic is 0.39 for x=0.08 and is able to keep the same stable value up to 350 °C. Nd(0.01) and V(0.75) co-doped Bi4Ti3O12 ceramics, BNTV(0.01, 0.75), show a relatively low TC-f. Bi3TiTaO9 (BTT)-based solid solution, Srx-1Bi4-xTi2-xTaxO9 [SBTT2(x)] (1x2), displays the high Qm value (=13500) in (p)-mode at x=1.25. For resonator applications, (Sr1-xCax)2Bi4Ti5O18 (SCBT) (0x0.5) ceramics are suitable.

Phase diagrams and electrical properties of (Bi1/2Na1/2)TiO3-based solid solutions

In this study, we demonstrated the relationship between the phase diagrams and the electrical properties of (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. In this study, (1−x)(Bi1/2Na1/2)TiO3–xNaNbO3 and (1−x)(Bi1/2Na1/2)TiO3–xKNbO3 (abbreviated to BNT-NN100x and BNT-KN100x) ceramics were prepared by a conventional ceramic fabrication process, and (1−x)(Bi1/2Na1/2)TiO3–x(Bi1/2K1/2)TiO3 (abbreviated to BNKT100x) ceramic was prepared for comparison. We revealed the phase transition temperatures, such as the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant, from the temperature dependence of dielectric properties using poled and unpoled specimens. As a result, it was shown that the BNT-based solid solutions form three types of phase diagrams. In addition, we clarified the relationship between the phase diagrams and the electrical properties of BNT-NN100x, BNT-KN100x, and BNKT100x. The piezoelectric properties we...

Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics

Stoichiometric and nonstoichiometric (Bi0.5Na0.5)TiO3 (BNT) ceramics were prepared by a conventional ceramic fabrication process. This study revealed that the high conductivity of BNT ceramics is associated with Bi vaporization during sintering. An x-ray study revealed that a tetragonal phase exists in the temperature range between 330 and 480 °C in BNT ceramic as well as BNT single crystals. In addition, the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant were determined to be 187, approximately 300, and 325 °C, respectively, from the temperature dependences of dielectric properties using unpoled and poled specimens. The piezoelectric properties of all vibration modes and the temperature dependences of the piezoelectric properties were measured using fully poled BNT ceramics. It was also revealed that BNT ceramics exhibit three thermal depoling processes at Td, between Td and TR-T, and between TR-T and Tm ...

Large electrostrain near the phase transition temperature of (Bi0.5Na0.5)TiO3–SrTiO3 ferroelectric ceramics

(1-x)(Bi0.5Na0.5)TiO3–xSrTiO3 (abbreviated as BNST100x) was prepared by a conventional ceramic fabrication process. The depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant were determined from the temperature dependence of the dielectric and piezoelectric properties. It is revealed that BNST100x forms a morphotropic phase boundary of rhombohedral ferroelectric and pseudocubic (tetragonal) paraelectric at x=0.26–0.28 for BNST100x, and a very large strain and normalized strain d33* of 0.29% and 488pm∕V, respectively, were obtained at x=0.28. In addition, it was clarified that the intermediate phase between TR-T (⩾Td) and Tm shows relaxor behavior.

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

The phase transition temperatures of x(Bi1/2Na1/2)TiO3–y(Bi1/2K1/2)TiO3–zBaTiO3) [x+y+z=1, y:z=2:1] [abbreviate to BNBK2:1(x)] ceramics were investigated using electrical measurements. We discussed the determination of the depolarization temperature, Td, and defined the Td for (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. We also determined the rhombohedral–tetragonal phase transition temperatures, TR–T, for BNBK2:1(x), and verified them using dielectric and piezoelectric measurements. It was demonstrated that TR–T corresponded with Td at x=0.94. The existence of an intermediate phase with ferroelectric and antiferroelectric properties at temperatures higher than the Td around the morphotropic phase boundary (MPB) was also revealed.

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.

Phase transition temperature and electrical properties of (Bi1∕2Na1∕2)TiO3–(Bi1∕2A1∕2)TiO3 (A=Li and K) lead-free ferroelectric ceramics

(1−x)(Bi1∕2Na1∕2)TiO3–x(Bi1∕2Li1∕2)TiO3 and (1−x)(Bi1∕2Na1∕2)TiO3–x(Bi1∕2K1∕2)TiO3 (abbreviated as BNLT100x and BNKT100x, respectively) ceramics were prepared by the conventional ceramic fabrication process. In this study, the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature of the maximum dielectric constant Tm were determined from the temperature dependences of the dielectric and piezoelectric properties. The results showed that Td of BNLT100x and BNKT100x increased to 199 and 209°C, respectively, at the rhombohedral composition. In addition, we revealed that Td is related to the magnitude of the rhombohedrality 90-α and the tetragonality c∕a. We studied the piezoelectric properties in detail, and the relationship between Td and piezoelectric properties was clarified. Moreover, we discussed the ferroelectricity of the middle phases of Td−TR-T and TR-T−Tm.

Piezoelectric properties in (K 0.5 Bi 0.5 )TiO 3 -(Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 lead-free ceramics

Lead-free piezoelectric ceramics with compositions around the morphotropic phase boundary (MPB) x(Na_0.5Bi_0.5)TiO _3-y(K_0.5Bi_0.5)TiO₃-zBaTiO ₃ [x + y + z = 1; y:z = 2:1] were synthesized using conventional, solid-state processing. Dielectric maximum temperatures of 280degC and 262degC were found for tetragonal 0.79(Na_0.5Bi_0.5)TiO₃-0.14(K_0.5Bi_0.5)TiO₃-0.07BaTiO

Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 )TiO 3 -and (Bi 1/2 K 1/2 )TiO 3 -Based Bismuth Perovskite Lead-Free Ferroelectric Ceramics

d3 (BNBK79) and MPB composition 0.88(Na_0.5Bi_0.5)TiO₃-0.08(K _0.5Bi_0.5)TiO₃-0.04BaTiO