Stoichiometric and nonstoichiometric ͑Bi 0.5 Na 0.5 ͒TiO 3 ͑BNT͒ 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 T d , rhombohedral-tetragonal phase transition temperature T R-T , and the temperature T m 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 T d , between T d and T R-T , and between T R-T and T m from the effects of annealing on the field-induced strain, x-ray diffraction patterns, and dielectric constant of poled specimens.

Articles you may be interested inElectric-field-temperature phase diagram of the ferroelectric relaxor system (1−x)Bi1/2Na1/2TiO3−xBaTiO3 doped with manganese Lead-free piezoelectric ceramics based on (0.97−x)K0.48Na0.52NbO3-0.03Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3-xB0.5Na0.5TiO3 ternary systemIn this study, we demonstrated the relationship between the phase diagrams and the electrical properties of ͑Bi 1/2 Na 1/2 ͒TiO 3 ͑BNT͒-based solid solutions. In this study, ͑1−x͒ ϫ͑Bi 1/2 Na 1/2 ͒TiO 3 -xNaNbO 3 and ͑1−x͒͑Bi 1/2 Na 1/2 ͒TiO 3 -xKNbO 3 ͑abbreviated to BNT-NN100x and BNT-KN100x͒ ceramics were prepared by a conventional ceramic fabrication process, and ͑1−x͒͑Bi 1/2 Na 1/2 ͒TiO 3 -x͑Bi 1/2 K 1/2 ͒TiO 3 ͑abbreviated to BNKT100x͒ ceramic was prepared for comparison. We revealed the phase transition temperatures, such as the depolarization temperature T d , rhombohedral-tetragonal phase transition temperature T R-T , and the temperature T m 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 were markedly enhanced when T R-T shifted to a lower temperature, and a large piezoelectric constant d 33 of 168 pC/N was obtained at the morphotropic phase boundary ͑MPB͒ between the ferroelectric rhombohedral and ferroelectric tetragonal phases for BNKT100x. Although the piezoelectric properties almost disappeared when T d shifted to room temperature, the field-induced strain S and the normalized strain d 33 ‫ء‬ ͑=S max / E max ͒ abruptly increased to 0.22% and 259 pm/V, respectively, for BNT-NN100x. In particular, a very large reversible strain of S = 0.40 with d 33 ‫ء‬ = 498 pm/ V was obtained at the MPB between the ferroelectric rhombohedral and ferroelectric relaxor with pseudocubic ͑tetragonal͒ phases for BNT-KN100x. This very large reversible strain was clarified to be due to non-180°͑71°and 109°͒ domain switching of the field-induced ferroelectric rhombohedral phase.

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)] (1≦x≦2), displays the high Qm value (=13500) in (p)-mode at x=1.25. For resonator applications, (Sr1-xCax)2Bi4Ti5O18 (SCBT) (0≦x≦0.5) ceramics are suitable.

( 1 - x ) ( Bi 0.5 Na 0.5 ) Ti O 3 – x Sr Ti O 3 (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.

The piezoelectric properties of a solid solution of the binary system, x(Bi 1=2 Na 1=2 )TiOx ¼ 0:50 { 0:98] were investigated, focusing on depolarization temperature, T d . 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, k 33 , and piezoelectric constant, d 33 , were 0.56 for BNKT84 and 157 pC/N for BNKT80, respectively. However, the T d of BNKT80 was low (174 C). The T d of the MPB composition was low, and the T d 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, d 33 (126 pC/N), dynamic d 33 (214 pm/V), and high depolarization temperature, T d (206 C). In this study, we determined depolarization temperature, T d , from the temperature dependence of dielectric and piezoelectric properties.