Study On Synthesis And Electrical Properties Of Bi_0.5Na_0.5TiO₃Lead-free Piezoelectric Materials

Sodium bismuth titanate (Bi0.5Na0.5TiO3, BNT), which has strong ferroelectrics, high piezoelectric coefficient and low dielectric constant, is considered to be one of the most excellent lead-free ferroelectric materials. However, pure BNT is difficult to be poled in DC electrical fields due to its high coercive field (Ec=7.3kV/mm) and relatively large conductivity, which limits its applications in the ferroelectric and electromechanical devices. Thus, this paper is focused on the research of preparative technique of BNT powders, modification by solid solution and ion doping, texture of BNT grains to improve the properties of BNT ceramics. The effect of reaction parameters on the phase formation and morphology during solution combustion synthesis were investigated and its mechanism of action was also revealed; The effect of parameters including sintering temperature and time on the microstructure of BNT ceramics were investigated and the grain growth mechanism in the process of sintering was discussed; The effect of BaTiO3(BT) and Eu doping concentration on the phase structure, microstructure and electrical properties of BNT ceramics were investigated and the relationship between microstructure and properties was revealed; The effect of reaction parameters on the phase formation and morphology evolution during hydrothermal synthesis were investigated and the formation mechanisms of BNT particles with different morphologies and high (110)-oriented BNT thin films were proposed. The main results of this study are listed as following:(1) Pure BNT and (1-x)BNT-xBT (BNBT) powders was successfully synthesized by a single-step solution combustion technology using urea as fuel. The effect of reaction parameters including fuel to oxidizers ratio and ignition temperature on the phase formation and morphology were investigated. Its mechanism of action was also revealed.(2) BNT powders synthesized by solution combustion method have high sintering activity and it can be well densified at1140℃for2h. The properties are as follows: Rd=95.7%, d33=65pC/N,Kp=0.20. According to the theory of phenomenological kinetic grain growth, the grain growth mechanism is controlled by volume diffusion or secondary interface and the activation energy Q≈(75±4) kJ/mol.(3) BT has diffused into the BNT lattice and formed the BNBT solid solution. A MPB between the rhombohedral and tetragonal phases exists when0.06≤x≤0.08. At this time, BNBT ceramic possesses excellent electrical properties. When x=0.06, Pr=35.96pC/N, Ec=3.22kV/mm.(4) Eu-doped BNBT6lead-free piezoelectric ceramics have been synthesized. The piezoelectric constant d33and remnant polarization Pr of BNBT6ceramics are improved and the coercive field Ec is reduced by appropriate Eu doping. When Eu doping is0.25at.%, BNBT6ceramic possesses excellent electrical properties, i.e. d33=149pC/N, Pr=40.27μC/cm2, Ec=2.95kV/mm. The effect mechanism of Eu doping on the phase structure and electrical properties of BNBT6ceramics is revealed.(5) BNT particles with different morphologies were synthesized by a hydrothermal synthesis method using a layered titanate H1.07Ti1.73O4·H2O as a Ti precursor. The effect of NaOH concentration on the phase formation and morphology evolution were investigated and its mechanism of action was revealed. When NaOH concentration is8mol/L, Na+and Bi3+ions can exchange with H3O+in the interlay space of plate-like titanates during hydrothermal treating. Then the plate-like titanates are transformed into plate-like BNT particles by an in-situ reaction. When NaOH concentration is10mol/L, the plate-like titanates are transformed into plate-like BNT particles firstly by an in-situ reaction. With the reaction progressing, the plate-like BNT particles split into nanowires whose width ranges from30to150nm while length is about several micrometers to tens of micrometers. When NaOH concentration is increased to14mol/L, the solubility of Ti precursor increases. Cubic particles are formed by dissolution-crystallization mechanism.(6) High (110)-oriented BNT film thins have been synthesized by a hydrothermal synthesis method using titanate2D-nanosheets as a Ti precursor. This thin film is composed of densly packed cubic cystal grain which has a smooth surface and size of about150nm. The orientation degree of (110) crystal facet exceeds99%. The maximum effective constant d33≈50pm/V. The in-situ growth mechanism of oriented BNT thin films is revealed.