Study On Low Temperature Sintering And Properties Of BF-BT Based High Temperature Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are widely used in the fields of filters,transformers,ignition devices,electro-acoustic devices,precision measurement,and underwater acoustic energy exchange because of the piezoelectric property of converting mechanical energy into electrical energy.In recent years,with the continuous development of science and industry,many special fields,such as petroleum exploration,aerospace,automobile engine and so on,need piezoelectric materials that can still work at high temperature.Although the traditional Pb（Zr,Ti）O₃（PZT）based piezoelectric ceramics have good piezoelectric activity,it is difficult to be applied in the field of high temperature due to the low depolarization temperature.Pb element is poisonous and volatile at high temperature,which brings many adverse effects to human and natural environment.In addition,low temperature sintering can realize the use of cheap base metals such as（Ag）as the inner electrode of multilayer co-fired piezoelectric ceramics（LTCC）,which significantly reduces the production cost.Therefore,developing new high temperature lead-free piezoelectric ceramics and reducing sintering temperature is a research direction with important practical engineering significance.（1-x）BiFeO₃-xBaTiO₃lead-free piezoelectric ceramics have attracted much attention because of their good high temperature stability and piezoelectric properties.BF Curie temperature is 870℃,BT Curie temperature is 120℃,increasing the content of BF can increase the Curie temperature of BF-BT ceramics.The effects of adding different content of Ba(Cu_0.5Wu_0.5)O₃ and B₂O₃ on sintering process,crystal structure,microstructure,electrical properties,Curie temperature and depolarization temperature of ceramics were studied.The essential relationship between composition ratio,phase structure,process and properties was mastered.The theoretical and experimental basis for obtaining high temperature lead-free piezoelectric ceramics was provided.The main studies are as follows:（1）The（1-x）BiFeO₃-xBaTiO₃-0.35mol%MnO₂-0.1mol%Li₂CO₃（x=0.19,0.20,0.21,0.22,0.23,0.24）high temperature lead-free ceramics were prepared.The formula is abbreviated as（1-x）BF-xBT（x=0.19,0.20,0.21,0.22,0.23,0.24）.When x=0.21,Curie temperature（Tc）and depolarization temperature（T_d）reach the maximum value:Tc=615℃,T_d=590℃,and the minimum value of dielectric loss is 0.007.The results show that 0.79BT-0.21BT ceramics has a great prospect in the field of high temperature with high temperature stability.（2）Taking 0.79BiFeO₃-0.21BaTiO₃-0.35mol%MnO₂-0.1mol%Li₂CO₃ as the basic formula,adding different content of sintering assistant Ba(Cu_0.5W_0.5)O₃.The formula is abbreviated as 0.79BF-0.21BT-xBCW（x=0,0.25mol%,0.5mol%,0.75mol%,1mol%,1.25mol%）.It was found that the addition of BCW could reduce the sintering temperature of ceramics,and the optimal sintering temperature was reduced from 950℃to 930℃.By analyzing the element valence states,it is found that the contents of ferric ion and oxygen vacancy at x=1mol%are both lower than those at x=0,indicating that there are fewer crystal defects at x=1mol%.BCW can improve the electrical properties of the ceramics,and the piezoelectric and ferroelectric properties reach the maximum value at x=1mol%:d₃₃=86p C/N,Pr=14.4μC/cm².In addition,the ceramics have excellent high temperature stability at x=1mol%:Tc=604℃,T_d=590℃.（3）0.79BiFeO₃-0.21BaTiO₃-0.35mol%MnO2-0.1mol%Li₂CO₃-xB₂O₃ ceramics were prepared experimentally.The formula is abbreviated as 0.79BF-0.21BT-xBO（x=0.25mol%,0.5mol%,0.75mol%,1mol%,1.25mol%,1.5mol%）.The experimental results show that BO acts as a low-temperature sintering agent and the sintering temperature is reduced to900℃.XRD analysis shows that when x<0.75mol%,the sample is rhombohedral phase structure.When x=1.5mol%,the ceramic sample has a pseudo-cubic structure.By observing the microstructure of the section,with the increase of BO content,the grain size becomes larger,the ceramic density increases,and existence of liquid phase.The maximum values of ferroelectric properties,piezoelectric properties,Curie temperature and depolarization temperature were obtained at x=0.75mol%:d₃₃=71p C/N,k_p=0.195,P_r=8μC/cm²,T_C=651℃,T_d=595℃.BF-BT high temperature lead-free piezoelectric ceramics were prepared by the solid-phase synthesis method with simple process and low cost.The system has high temperature stability,low dielectric loss,dense ceramic sintering and low sintering temperature,which has great application potential in the field of LTCC and high temperature devices.