Large Electric Field Induced Strains In Bismuth Sodium Titanate Lead-free Piezoelectric Ceramics

Piezoelectric material is a kind of functional materials, which are usually used to achieve a convert between mechanical energy and electric energy. So far they have been widely used in buzzer, filters, sensors, drives and kinds of underwater acoustic, ultrasonic, acoustic transducers in daily life, industrial production and military fields. Bi0.5Na0.5TiO3(BNT) has been considered as one of most promising candidates of lead-free piezoelectric materials, due to its large residual polarization, high Curie temperature and large electromechanical coupling coefficient of anisotropy. At present, the researches of BNT are mainly focused on improving piezoelectric coefficients, reducing resistivity and coercive field. Learning from the experience of PZT preparation, binary or multiple phase solid solutions would be found to exhibit excellent piezoelectric properties near the morphotropic phase boundary. The present study focused on the BNT-BT system, BNT-BKT system research is few. A huge field strain curve in BNT-BT system was found by Shan-Tao Zhang in2007, and the maximum strain could reach0.45%, Smax/Emax=d*33=560pm/V, which attracts more and more attention on this material. In the paper, BNT ceramics with different components are synthesized by traditional solid state reaction method, and the structure, dielectric, piezoelectric and ferroelectric properties are studied systematically. The main contributions of this dissertation are summarized as follows:1. The recombination of perovskite structure BKT and BNT at A site is carried out with a series of0.84BNT-0.16BKT ceramics in Morphotropic Phase Boundary. The study on crystal structure, dielectric properties and ferroelectric properties of binary solid solutions near the morphotropic phase boundary are carried out. X-Ray diffraction proves a single perovskite structure in0.84BNT-0.16BKT. From the result of temperature dielectric spectra, relative dielectric constant at high temperature does not change much after polarization, and dielectric loss shows a peak at the temperature of ferroelectric phase-relaxation phase transition and goes down around room temperature. Electric hysteresis loop test under different temperature shows that there is a phase transition at the ferroelectric phase-relaxation phase transition temperature, ferroelectric phase gradually to the relaxation phase, electric hysteresis loop gradually compressed, coercive force field (Ec) and the residual polarization (Pr) decreases, ferroelectricity gradually weakened, and field induced strain effect increase at the same time. At150℃bi-directional strain could reach1.28%, and unilateral strain1.03%. Meanwhile inverse piezoelectric constant Smax/Emax=d*33=1716pm/V, which value is higher than previous reports.2. Based on the research of piezoelectric ceramics0.84BNT-0.16BKT, we dope Nb5+at B site, a series of Bi0.5(Na0.84K0.16)0.5Ti1-xNbxO3ceramics are prepared with expect of reducing the depolarization temperature TF-R of0.84BNT-0.16BKT to improve the piezoelectric properties around room temperature. We analyze the influence of Nb substitution on0.84BNT-0.16BKT ceramic structure and performance systematically. As shown in XRD patterns, the sample is in constraint of cubic phase when x≤0.03, besides that the second phase of Bi2Ti2O7occurs when x≥0.05. When substituted with Nb, samples show lower coercive field, residual polarization and depolarization temperature. Sample for x=0.03show the most outstanding performance, the ferroelectric phase-relaxation phase transition temperature TF-R is as low as63℃lower than that of0.84BNT-0.16BKT. Though ferroelectric property fall shown in electric hysteresis loop, the variables around room temperature are improved to0.35%,Smax/Emax=d*33=500pm/V, which can be used in large displacement piezoelectric actuator.3. Recombination on BNT with Ba(Al0.5Ta0.5)O3and Ba(AIo.5Sbo.5)O3are carried out. Series of lead-free piezoelectric ceramics (1-x)BNT-xBa(Al0.5Ta0.5)O3and (1-x)BNT-xBa(Al0.5Sb0.5)O3are prepared. All the samples are all constraint cubic structure, and on second phase occurs. The corresponding Q value of0.027m4C-2, when x=0.045of (1-x:)BNT-xBa(Al0.5Ta0.5)O3is significantly higher than that of the typical lead electrostrictive material Pb(Mg1/3Nb2/3)O3(PMN). Among (1-x)BNT-xBa(Al0.5Sb0.5)O3ceramics, the sample of x=0.035shows a maximum single pole variable0.27%,,Smax/Emax=d*33=386pm/V, kp=20.1%,kt=30.4%, which indicates a stronger aeolotropism in the recombination.