Research On Preparation And Performance Of BT-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are widely used in many fields,and become one of the indispensable device materials in people’s life.At present,the most widely used piezoelectric ceramics with a huge amount in the market are represented by PZT-based lead-containing piezoelectric ceramics.The lead content is over 60 wt%,which result in a grievous lead pollution during production,use,disposal or future recycling process,and has a significant toxic effect on the environment and human health.With the development of society and the progress of science and technology,people have higher and higher requirements for the quality of life,so there is an urgent need for lead-free piezoelectric ceramics to replace lead-containing piezoelectric ceramics.As one of the earliest piezoelectric ceramics material discovered,barium titanate（BaTiO₃,referred to as BT）has been used for more than80 years,but there is still a gap from broad application due to its weak piezoelectric properties and low Curie temperature.Therefore,to improve the piezoelectric properties and increase Curie temperature of BT-based lead-free piezoelectric ceramics is still a taugh and must work to fulfill.In this work,we start from exploring the preparation conditions of pure BT ceramics,and then optimize the process details,introduce new synthesis techniques and modulate the phase structure by doping for improving the comprehensive performance of BT-based ceramics.The main works are summarized as:（1）Pure BT ceramic is prepared through normal solid phase synthesis.A post oxidation treatment is applied on BT ceramic thereafer.By comparing,we found that after the post oxidation treatment,the crystal structure keeps the same,but the density increases,the microstructural defects such as microcracks and voids are reduced,and the dielectric and piezoelectric properties are greatly improved.The optimum sintering temperature of the sample is about 1350℃.After the oxidation treatment,the average grain size of BT piezoelectric ceramic reaches 36.39μm,the relative density reaches 99.0%,the Curie temperature T_c is about 126℃,the piezoelectric constant d₃₃ is 271 pC/N,the relative permittivity ε_r is above 3800 at room temperature,and the maximum relative permittivity ε_{r max} is more than 23800.（2）For high temperature applications,126℃ Curie temperature of BT is not good enough.Considering the high piezoelectric constant（>300 p C/N）of BT with Ca and Sn doping,we combine the BT piezoelectric ceramic with high piezoelectric constant（～271 p C/N）and low Curie temperature（～125℃）with BF ceramic which hold higher Curie temperature（～830℃）and lower piezoelectric constant（<150 p C/N）and obtaine 0.70BiFeO₃-0.30BaTiO₃-0.03 mol%Sn-x mol%Ca ceramics.By adjusting the amount of Ca from 0-1 mol%,0.70BiFeO₃-0.30BaTiO₃-0.03 mol%Sn-0.05 mol%Ca exhibits a high relative density of 96.4%,a average grain size of 2.22μm,and a d₃₃of 197 p C/N,and the Curie temperature is above 450℃,which is330℃higher than that of the pure BT.（3）In the final part,we introduce hydrothermal synthesis method intended to further improve the performance of 0.70BF-0.30BT piezoelectric ceramics.The results show that the crystal structure of 0.70BF-0.30BT ceramics keep the same,but the morphology and electrical properties such as dielectricity and piezoelectricity change significantly.When the hydrothermal synthesis temperature is 200℃,the0.70BF-0.30BT samples exhibit high densities,small leakage currents and large relative dielectric constant,with d₃₃=201 p C/N,T_c=530℃,meanwhile,the average grain size increases about 0.06μm,the relative density increases about 1.70%compare to conventional solid phase method.