Growth and characterization of Na_0.5Bi_0.5TiO₃–BaTiO₃ lead-free piezoelectric crystal by the TSSG method

Abstract

In this paper, polycrystalline material of 0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃ (abbreviated as NBBT94/6) was synthesized by solid-state reaction techniques. DTA and TG analysis indicate the proper temperature for solid-state reaction is 1200 °C. A single crystal with dimensions of 25 mm × 10 mm was successfully grown by using the top-seeded solution growth (TSSG) method. X-ray fluorescence analysis revealed that the composition of the as-grown crystal is NBBT98/2. X-ray powder diffraction results show that the as-grown NBBT98/2 crystal possesses the perovskite structure and belongs to the rhombohedral system. The unit-cell constants of the as-grown NBBT98/2 crystal are a = b = c = 3.8862 Å and α = β = γ = 89.2°. At room temperature, the dielectric constant of <0 0 1> oriented NBBT98/2 crystal is 770 at 10 kHz and it decreases to 430 after poling under the E-field of 7 kV/mm. Maximum d₃₃ values of 60, 65 and 30 pC/N were obtained for <0 0 1>, <1 1 0> and <1 1 1> oriented NBBT98/2 crystal, respectively.

Introduction

In recent years, growing attention has been given the research and improving piezoelectric properties of lead-free piezoelectric materials, which are viewed as possible substitutes for lead-based piezoelectric materials from the viewpoint of environmental protection [1], [2], [3]. Sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, abbreviated NBT) is a strong ferroelectric material with a high Curie temperature of T_c = 320 °C, a remanent polarization of P_r = 38 μC/cm², and a coercive field of E_c = 73 kV/cm at room temperature [4], [5]. In view of these good ferroelectric properties, NBT is considered to be a promising candidate for a lead-free piezoelectric material. However, the large coercive field and relatively large conductivity make pure NBT is hard to be poled and its piezoelectric properties are not desirable. Therefore NBT-based solid solutions were studied to improve piezoelectric properties [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Among these NBT-based solid solutions, (1 − x)(Na_0.5Bi_0.5)TiO₃–xBaTiO₃ (abbreviated as NBBT) system is the most attractive due to their excellent piezoelectric properties and was studied by several researchers [6], [7], [8], [9], [10], [11], [12], [13]. There also exists rhombohedral–tetragonal morphotropic phase boundary (MPB) near x = 0.06–0.07 in NBBT system and the piezoelectric properties can be enhanced near the MPB composition as it was previously reported in Pb-based perovskites, such as PMN-PT and PZN-PT [6], [20]. It was reported that NBBT94/6 ceramics presented a relatively low dielectric constant of ɛ₃₃ = 580, a high piezoelectric constant and electromechanical coupling coefficient of d₃₃ = 125 pC/N and k₃₃ = 0.55, and appear to be advantageous for high-frequency ultrasonic uses or piezoelectric actuator applications [6]. Improved electromechanical actuation can be achieved in single crystals oriented to optimize piezoelectric coefficients. Chiang has reported that NBBT94.5/5.5 single crystals on the (0 0 1) cut exhibit d₃₃ value up to 450 pC/N, which close to some PZT ceramics [7]. But the investigations focused on NBBT solid solution single crystals are still scarce and not systematic, and the reported piezoelectric results are quite different [7], [8], [13]. There is still a lack of fundamental understanding of the structure–property relations and mechanism for high piezoelectric responses near the MPB compositions in lead-free materials, about which much research and development need to be done to optimize the piezoelectric properties in lead-free materials.

Single crystals provide the opportunity to conveniently investigate the physical properties as a function of crystallographic orientations and find the optimum crystallographic cuts for practical applications. Up to now, many growth methods have been tried to obtain NBT-based crystals, including Czochralski method [5], [21], [22], [23], [24], flux method [7], [8], [22], [23], and Bridgman method [8], [13]. However, it is not easy to fabricate high-quality NBT-based single crystals by Czochralski method because the high volatility of the bismuth and potassium components at melting temperature results in crystal growth deviating from stoichiometry and combined with many defects [22]. NBT-based single crystals have been also grown by the flux slow-cooling method [7], [8], [22], [23], but these obtained crystals were too small (most of them were mm-level) to systematically characterize their piezoelectric properties as a function of crystallographic orientations. In this paper, NBT-BT single crystal with dimensions of 25 mm × 10 mm has been successfully grown by the top-seeded-solution growth (TSSG) method. The dielectric and piezoelectric properties for as-grown crystal were measured along <0 0 1>, <1 1 0> and <1 1 1> crystallographic directions.