Research On The Preparation And Doping Modification Of BNT-BT Lead-free Piezoelectric Ceramics

In this paper,lead-free piezoelectric ceramics of Bi_0.5Na_0.5TiO₃-Ba TiO₃ binary system were prepared by solid state synthesis.Firstly,Bi_0.5Na_0.5TiO₃ was used as matrix,and the amount of Ba TiO₃ was changed by adding the second component of BaTiO₃.According to the results of phase analysis of XRD,the two-phase interface of three-party and tetragonal phase in the range of XRD is found accurately.The phasestructure,microstructure,piezoelectric and dielectric properties of the components near the morphotropic phase boundary?MPB?are also studied.The best composition formula is selected as the matrix,and the next step is to study the doping modification.In the second step,the best composition near the quasi-homogenous phase boundary of Bi_0.5Na_0.5TiO₃-Ba TiO₃ binary ceramics is chosen as the matrix.In order to improve the sintering characteristics and improve the piezoelectric properties of Bi_0.5Na_0.5TiO₃ perovskite A site Na is partially replaced by the homovalent elementK and Li.The effects of MnO₂ doping on the sintering and mechanical properties of Bi_0.5Na_0.5TiO₃-BaTiO₃ binary ceramics at the same phase boundary were studied by doping MnO₂ and rare earth oxide CeO₂ respectively.The phase structure and piezoelectric properties of Bi_0.5Na_0.5TiO₃-BaTiO₃ doped with CeO₂ were studied.The main contents and results are as follows:1.Bi_0.5Na_0.5TiO₃ is rhombohedral phase and BaTiO₃ is tetragonal phase at room temperature.BaTiO₃ can solute well with Bi_0.5Na_0.5TiO₃,and when x?0.08for a single perovskite structure,show that Ba²⁺has entered the lattice and appears impurity phase at x=0.10;With the increase of BaTiO₃ content x,the crystal structure is transformed for rhombohedral phase to tetragonal phase.When x?0.05,the ceramic is a single rhombohedral phase structure,and 0.06?x?0.08 the ceramicis a morphotropic phase boundary region where rhombohedral and tetragonal phases coexist,and the ceramic transformed into tetragonal phase structure at x=0.10.The addition of BaTiO₃ can improve its performance.The piezoelectric constant d₃₃3 is increased from 80pC/N to120pC/N when the composition in the morphotropic phase boundary.In addition,the Curie temperature of ceramic samples decreased with the increase of Ba TiO₃ content.The BNBT7 samples sintered at 1100?¹180?can form the perovskite structure ceramics with the coexistence of rhombohedral phase and tetragonal phase.When the BaTiO₃ content at x=0.07 and the sintering temperature is 1160?,the ceramic samples have the best comprehensive properties:d₃₃=132pC/N,k_p=0.221,?_r=831,tan?=0.0255,?=5.79g/cm³,T_C=280?.2.On the basis of?1-x?BNT-xBT and selecting component at x=0.07near the MPBcomposition,replacingpartialNawithKandLiformthe0.93Bi_0.5(Na_0.82K_0.18-xLi_x)_0.5TiO₃-0.07BaTiO₃ ceramics.The substitution of K and Li for Na can take up the Na position of the original A site and cause the distortion of the unit cell.With the addition of Li,the impurity phaseK₂Ti₆O₁₃3 was produced when the doping amount of Li was 0.02?x?0.06,and the impurity phase disappeared when the Li content was add to at x=0.08.The long columnar grain disappeared when the Li content was low,and when the Li content reached x=0.06,the long columnar grain disappeared,and the impurity phase K₂Ti₆O₁₃3 disappeared.The ceramic sample has the best piezoelectric properties and dielectricproperties when the addition of Li content atx=0.06 and the sintering temperature is 1160?:d₃₃=145pC/N,k_p=0.180,Q_m=77,?_r=1061,tan?=0.0264.3.On the basis of 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃,we choose MnO₂ and CeO₂ as additives to improve some properties of the ceramics.As a multivalent element,a small amount of Mn doping can be helpful to the sintering process,and improve the mechanical quality factor Q_m and piezoelectric properties of ceramics.When doped with ceramic matrix,Ce ions will gather at the grain boundary,which will hinder the grain growth and refine the grain size of ceramics.When the doping amount of MnO₂and CeO₂ is a small amount,the dielectric properties can be well dissolved into the matrix and no impurity phase can be produced.The piezoelectric and dielectric properties of the ceramics can be improved when the doping amount is less,and when the doping amount exceeds a certain amount,the properties of the ceramics become worse.