| There is a tremendous surge in research toward lead-free material owing to health concerns and environmental issues,although lead-based solid-solutions has been widely used due to their excellent electric properties.The growing interest in lead-free ferroelectric materials has leaded to immense alternatives in the preparation of high-performance lead-free materials.BTO-based lead-free materials with a perovskite structured(ABO3)have potential applications,due to their tailored electrical properties by suitable preparation technique and incorporating appropriate dopants in their lattice.Especially,(1-x)BZT-xBCT materials are comparable to those of lead-based materials due to their outstanding piezoelectric properties,which has attracted much attention in recent years.In this work,Zr/Fe doped BaTiO3(BTO)thin films and Fe/Nb doped(1-x)BZT-xBCT thin films were prepared by sol-gel process,respectively.The effects of the compositon,doping concentration on the structure,phase transition behavior and electric properties of the films were investigated.The relations between structure and electrical properties were discussed intensively.The details are as follows.(1)The BTO thin films and(1-x)BZT-xBCT thin fims were prepared by sol-gel process.It is very essential to abtain high quality films by optimizing heat treatment process.The effect of heat treatment temperature,time and the thickness of films on the structures and electric properties were studied,respectively.(2)The effect of Zr content and Fe content on the structures,phase transition behavior,dielectric,ferroelectric and piezoelectric properties of BaTi1-xZrxO3(BZT)(0.0≤x≤0.35)films and BaTi1-xFexO3(BTFO)(0.0≤x≤0.02)films were studied,respectively.With an increase of Zr content,the reduced phase transition temperature(Tm)and the enhanced diffused phase transition behaviors were observed.A relaxor-like behaviors of the films were presented when x=0.3.The effect of low Fe content on the structural,phase transition behavior,and electric properties of the BTFO thin films were studied.With an increase of Fe content,the Tm was reduced.A saddle-backed anomaly were observed at x=0.01,which was attribute to the coexistence of two different ordering structures in the films at room temperature.At low Fe contents(x<0.01),Fe ions and oxygen vacancies lead to the formation of(FeTi-Vo)defect complexes which can lessen the mobility of the oxygen vacancies that was conducive to enhancing the dielectric,ferroelectric and piezoelectric properties.With a further increase of Fe content(0.012<x<0.02),the concentration of oxygen vacancies exceeded the critcal value,and the lattice distortions of BTO resulted in the decreased electric properties.The optimal electric properties were obtained at a composition of x=0.01,which corresponded to the dielectric constant(εr)=934,the dielectric loss(tanδ)=0.053,the remanent polarization(Pr)=13.8μC/cm2,the piezoelectric constant(d33*)=300pm/V.BaTi0.99Fe0.01O3 films were annealed in O2,air and N2,respectively.With the increase of oxygen vacancy concentration,the pinning effect on the domain walls was increased in oxygen vacancie,and oxygen vacancie were easy to be trapped at the grain boundary and the electrode film interface.Therefore,it leaded to a larger leakage current and low ferroelectric properties.(3)The effect of compositions on the structures,phase transition behavior,dielectric,ferroelectric,and piezoelectric properties of(1-x)BZT-xBCT(0.1≤x≤0.5)films had been performed.With the increase of x,the Tm of films was increased from 30℃ to 60℃,and the diffused phase transition of the films were enhanced.Single rhombohedral phases existed in the compositional range of 0.1≤x≤0.2,while the rhombohedral and tetragonal phase coexist in the compositional range of 0.3<x<0.5.The optimal electric properties were obtained at the composition of x=0.3,which corresponded to εr=1340,tanδ=0.05,Pr=12μC/cm2,d33*=280pm/V.The observed enhanced electric properties were mainly attributed to the composition close to critical triple point of(1-x)BZT-xBCT system in the phase diagram and the coexistence of rhombohedral and tetragonal phases at room temperature.The coexisting phases lead to a low energy barrie,thus facilitates the polarization rotation.(4)The electrical properties of 0.5BZT-0.5BCT thin films had been modified by introducing Fe ions into lattices.It revealed that the orthorhombic phases existed in a wide temperature range in Fe-doped 0.5BZT-0.5 BCT thin films.With an increase of Fe content,the phase transition temperature from cubic to tetragonal structure(TT-C)of films was reduced from 60℃ to 15℃,and the diffused phase transition of films was enhanced.The phase structure of films evolves from two ferroelectric phase coexistence(x≤0.002)to three ferroelectric phase coexistence(0.005<x<0.02)with the increase of Fe content,which revealed that the phase structures of 0.5BZT-0.5BCT-xFe films were dependent on the Fe concentrations.The defect mechanism of Fe-doped 0.5BZT-0.5BCT was as the same as Fe-doped BTO films.The optimal electric properties were obtained at a composition of x=0.01,which corresponded to εr=1290,tanδ=0.04,Pr=14μC/cm2,and d33*=496pm/V.The significantly enhance of the piezoelectric properties was resulted from the coexistence of tetragonal,orthorhombic and rhombohedral phases,which caused the polarization anisotropy to vanish.The polarization anisotropy vanished which leaded to a very low energy barrie,thus facilitated the polarization rotation.(5)Nb doped 0.5BZT-0.5BCT-xNb(0.0<x<0.025)films were investigated.The coexistence of tetragonal and rhombohedral phases was abtained in all films at room temperature.With the increase of Nb content,the Tm was reduced,and the diffused phase transition of the films were enhanced,and the electric properties was Small Nb content lead to enhanced dielectric,ferroelectric and piezoelectric properties because of the compensation of electron and the two phases coexistence in films.Thus the electric properties of films were decreased when 0.005<x<0.025.The optimal electric properties were obtained at a composition of x=0.002,which corresponded to εr=3556,tanδ=0.062,and d33*=448pm/V. |
