Research On The Structures And Properties Of Bismuth Sodium Titanate Based Multi-component Lead-free Piezoelectric Ceramics

In this dissertation, sodium bismuth titanate (abbr. BNT), which is a kind of A-site compound perovskite ferroelectrics, is investigated. Aiming at the shortcoming of the BNT and according to the modified method of traditional piezoelectric ceramic, the BNT ceramics was modified systemically to improve the piezoelectric properties and achieve the practical objective. To increase the density and breakdown field strength, Li+ substitution for Na+ in BNT was researched. According to the thought that the excellent properties would be obtained in the morphotropic phase boundary, the multi-component free-lead piezoelectric ceramics which were BNT-BKT, BNT-KN and BNT-BKT-KN, were formed respectively. To further improve the piezoelectric properties, La2O3 and MnO2 were doped to BNKT18 ceramics and the performance of this material was studied.Li substitution for Na in BNT ceramics which affects on the sintering characteristic, structure and performance of the BNT material was systemically studied. The experimental results indicate: high relative density of more than 95% is obtained from proper Li substitute which may improve the sintering process. When x=0.1, the resistivity reaches the maximum value, 1.56×1012Ω.cm. The results of XRD show that there are Li2O and Bi2Ti2O7 phases which result from the instability of perovskite at x=0.3. Li substitutes don't change the structure symmetry of rhombohedra. When x=0.15, this composition expresses the optimal properties: d33=110pC/N, kp=18%, Pr=21μC/cm2. The diffuse phase and the dielectric dependence on frequency from the dielectric thermogram show that the Bi0.5(Na1-xLix)0.5TiO3 ceramics are the relaxor ferroelectrics. Above Tc, the relation of dielectric and frequency doesn't accord with the Curie-Weiss law with the increase of the temperature, which arouses from the space charge polarization induced by ions conductance.For (1-x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 system: The patterns of XRD show that the range of x=0.16~0.22 is the morphotropic phase boundary with rhombohedral and tetragonal co-existed structure. When x=0.22, the piezoelectric properties which d33 and kp are 110pC/N and 18% respectively, are the best properties at the present time. La2O3 dopant can enhance the electromechanical coupling factor and piezoelectric constant but decrease the mechanical quality factor. At the La2O3 amount of 0.5wt%, the best properties are: kp=0.32, d33=157pC/N,εT33/ε0=963, Qm=139, tanδ=0.039. Based on 0.5wt% La2O3 doped BNKT18 ceramics, MnO2 dopant can enhance the depolarization temperature but decrease mechanical and dielectric loss. At the MnO2 amount of 0.3 wt%, the best properties are: kp=0.34, d33=158pC/N,εT33/ε0=865, Qm=291, tanδ=0.02. At the same time, the mechanics that the"soft or hard"adulterant influence the depolarization temperature was analyzed.(1-x)Bi0.5Na0.5TiO3-xKNbO3 system was prepared by the tradional ceramics fabrication, in range of the researched composition, XRD results show that KNbO3 don't change the structure symmetry of rhombohedra and enhanced the structure symmetry to pseudo-cubic structure with the increase of KNbO3 content. At amount of x=0.05, the best properties are: kp=0.23, d33=125pC/N,εT33/ε0=830.For (1-x-y)BNT-xBKT-yKN ternary system, the phase diagram of the BNT-BKT-KN ternary system was put forward. Near the phase boundary of the phase diagram, the optimal piezoelectric properties are: for the composition of x=0.10 and y=0.03, kp and d33 are 38% and 138 pC/N, respectively. For the composition of x=0.16 and y=0.02, kp and d33 are 35% and 197pC/N, respectively. The d33 value reaches the maximum in the ternary system.The results of XRD, SEM and EDAX indicate: the K+ act as key function in the sintering process, the sintering temperature and the width of sintering temperature are decided by K+ content and independent of the component in the BNT based samples. Generally, the width of the sintering temperature is broad when the K+ content is little. For higher K+ content, the result is contrary. The density sample with higher K+ content can be gained by decreasing the sintering temperature and prolonging the sintering time.The results of dielectric thermogram and hysteresis loop at different temperature indicate: the BNT based ceramics are the relaxor ferroelectrics; above Td, there isn't anti-ferroelectric in the temperature region of ferroelectric and paraelectric; the reason of dielectric abnormity at low temperature is the transition from ferroelectric macro-domain to ferroelectric micro-domain.