One of the (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions, Ba-modified bismuth sodium titanate, (Bi1/2Na1/2)1-x Ba x TiO3 (BNBT), is studied for its dielectric and piezoelectric properties as a new group of lead-free piezoelectric ceramics. A rhombohedral (Fα)-tetragonal (Fβ) morphotropic phase boundary (MPB) is shown to exist at x=0.06∼0.07 by X-ray data, and dielectric and piezoelectric properties. BNBT ceramics with the MPB composition are superior as piezoelectric ceramics in high-frequency ultrasonic applications or as piezoelectric actuator materials because of a lower free permittivity, ε33 T/ε0, and a high electromechanical coupling factor, k t or k 33, along with high mechanical strength.

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The effect of grain orientation on the dielectric properties of hot-forged ferroelectric Bi4Ti3O12 ceramics has been investigated and the properties are compared with ordinarily fired ones. The degree of grain orientation f calculated from X-ray diffraction patterns is found to be as large as 0.95. The temperature dependences of the dielectric constant εs and loss tangent tan δ are measured with a Q-meter at 12 MHz in the perpendicular [⊥] and parallel [//] directions to the forging axis, and considerable anisotropies, such as εs[⊥]/εs[//]=5 at the Curie temperature of about 680°C, are seen. The remanent polarization Pr is calculated for both cases of perfect (f=1) and random (f=0) grain orientations. Observed Pr values from hysteresis loops are smaller than calculated ones, the ratio of the former to the latter being about 82% for the hot-forged ceramics.

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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.