Preparation And Properties Of BaTiO₃-Bi_0.5Na_0.5TiO₃ Lead-free Piezoelectric Ceramics

The piezoelectric ceramics are the important materials which are widely used for piezoelectric actuators,sensors and transducers.The most present widely used piezoelectric ceramics are lead-based materials such as lead zirconate titanate(PZT),which caused environmental pollution by volatilization of toxic PbO during high-temperature sintering. Therefore,it is necessary to develop lead-free piezoelectric ceramics to replace the lead-based ones.In recent years,several lead-free piezoelectric ceramic systems such as BaTiO₃-based oxides, Bi_0.5Na_0.5TiO₃-based oxides,bismuth layer structure oxides and potassium sodium niobate based ceramics have been studied.Among these systems,Bi_0.5Na_0.5TiO₃-BaTiO₃(abbreviated to BNT-BT)system has been found to be a promising lead-free piezoelectric material.Many researches have been focused on the systems near the MPB composition.Only few reports are involved with the compositions close to BaTiO₃(BT),and there is little information about the BT-BNT solid solution ceramics with the composition from MPB to BT-rich zone.In this work, the(1-x)BaTiO_3-xBi_0.5Na_0.5TiO₃(BT-BNT100x)(x=0～0.96)systems were synthesized by liquid cladding process,and the crystal structure,dielectric and piezoelectric properties of the solid solutions were studied.The preparation of(1-x)BaTiO₃-xBi_0.5Na_0.5TiO₃ ceramics was investigated,the properties of the ceramics with different BNT content were analyzed to seek for the best ceramics,and the variation of properties versus the BNT content for(1-x)BaTiO₃-xBi_0.5Na_0.5TiO₃ ceramics were summarized.It was found that all the ceramics form single-phase solid solutions with perovskite structure.The crystal structure and microstructure vary gradually with the increase of Bi_0.5Na_0.5TiO₃(BNT)content.The Curie temperature,T_c,shift monotonously to high temperature as BNT increased,from 118℃for BT-BNT1 up to 244℃for BT-BNT90.The ceramics with 20 to 90 mol%BNT have relatively low and stable dielectric loss,and the ceramics exhibit a dispersion phase transition characteristics with BNT content increased.The piezoelectric constant, d₃₃,enhances with the increase of BNT content through a maximum value in a composition of 90 mol%BNT and then tends to decrease.The ceramic with 90 mol%BNT shows well dielectric and piezoelectric properties,and the properties measured are as follows:T_c=244℃,ε_r=2180, tanδ=0.0557,d₃₃=125pC/N,k_p=0.164,k_t=0.237,Q_m=51,N_p=2450.In order to improve the piezoelectric properties of BT-BNT ceramics,the BT-BNT90 ceramics with(Bi,Na)nonstoichiometry and doped with Zr,K,Li and Mn were investigated. The crystal structure,dielectric and piezoelectric properties of the solid solutions were studied. The effect and the mechanism of the dopants on the ceramics were discussed.The properties of the ceramics can be improved when the addition of(Bi,Na)is excess or deficient.When the excess amount of(Bi,Na)is 0.1mol%,d₃₃of the ceramic is 142 pC/N,and d₃₃of the cermic is 144 pC/N when the deficient amount of(Bi,Na)is 0.08mol%.The properties of the ceramics can be improved effectively by an appropriate addition of Zr. The ceramic with 0.055 mol Zr shows good piezoelectric properties:d₃₃=150pC/N,k_p=0.189, k_t=0.200,Q_m=50.7,ε^T₃₃/ε₀=1246.The piezoelectric constant,d₃₃,of the ceramics is enhanced with the increase of Li content and then tends to decrease.The maximum value of d₃₃is 140pC/N when the adding amount of Li is 0.075mol.Q_m and N_p show the similar variation as d₃₃.The piezoelectric properties of the ceramics are improved slightly by codoping with Zr and Li on the base of the(Bi,Na)excess.The maximum value of d₃₃,154 pC/N,of the ceramic with 0.1 mol%excess amount of(Bi,Na)is obtained when the additions of Zr,K and Li are 5.5mol%, 6mol%and 10mol%,respectively.For the ceramics doped with Mn,the value of d₃₃,k_p and k_t have a trend of descending with the increase of Mn amount.When x≤0.3 wt%,theε_r and tanδdecrease with the Mn amount increased,and the Q_m and N_p show a opposite variation,which indicates that the ceramics have a typical characteristic of "hard" doping.However,the overfull addition of Mn will worsen the properties of the ceramics.When x≥0.3 wt%,Q_m decreases rapidly,and the permittivity decreases also,whereas,the loss tangent increases.