Phase Structure And Polarization Thermal Stability Of Bismuth Sodium Titanate Based Ceramics

In the past decades,the utilization of lead-based piezoelectric ceramics has caused a lot of pollution,hence,it is of great significance to develop lead-free materials.Bismuth sodium titanate-based lead-free piezoelectric ceramics are A-site composite perovskite ferroelectric materials with R3c phase at room temperature,with high Curie temperature,stable mechanical quality factor and large electrostriction,it is a promising lead-free alternative.Nonetheless,the relatively low depolarization temperature hinders its practical application.In this thesis,bismuth sodium titanate-based lead-free piezoelectric ceramics are used as the research object to improve the piezoelectric properties of materials by doping,air quenching,composite Zn O and other modifications,and explore the corresponding influence mechanism.The main works are as follows:（1）This thesis synthesizes NBT-BFO-100x BT（x=0.02,0.04,0.05,0.06,0.07,0.08,0.09,0.10）ceramics by using solid state reaction method.Research indicates the addition of BT transforms the material to from a rhombohedral phase to a rhombohedral/tetragonal coexistence phase,exhibiting the largest piezoelectric constant at x=0.07(d₃₃₌87 p C/N).In addition,as the BT content increased,the samples undergo transition from ferroelectric to relaxor,and the depolarization temperature decreases.（2）Air quenching of NBT-BFO-100x BT ceramics show that quenching increases the lattice distortion inside the crystal,induces relaxor-ferroelectric transition,and significantly increases the depolarization temperature of ceramics,the depolarization temperature increases from 420℃to 580℃at x=0.04.Furthermore,the compositions with strong relaxor characteristics realize the simultaneous enhancement of d₃₃ and T_d.For example,the piezoelectric constant is increased to97 p C/N after quenching at x=0.10,and the depolarization temperature is increased to 380℃,demonstrating potential use in high temperature piezoelectric applications.（3）This thesis utilizes strong relaxor characteristic component NBT-BFO-10BT composited Zn O,prepares NBT-BFO-10BT:100x Zn O（x=0.01,0.03,0.05,0.07）ceramics.EDS images demonstrate Zn²⁺has two distribution forms in ceramics,some enters the lattice to form doping substitution,the others gather around the grain boundary to form Zn O enrichment regions,and the excess Zn O will also react with the matrix to form the second phase Zn Fe₂O₄.The results show that proper amount of Zn O can significantly improve the piezoelectric constant and the temperature stability of the piezoelectric constant of the sample.x=0.03,the d₃₃ at room temperature increases from 62 p C/N to 106 p C/N,and it still retains 89%of room temperature at240℃.In addition,the quenching process is still effective for the composite samples,and the depolarization temperature of the samples increases to varying degrees after air quenching.（4）Finally,aiming at the problem of large leakage conductivity of NBT-based ceramics at high temperature,prepare NBT-100x BA（x=0.02,0.06,0.07,0.08,0.10）ceramics by doping BA into NBT,followed by air quenching.Compared with NBT-BFO system,the addition of BA significantly reduces the dielectric loss of the material at high temperature,and the dielectric loss of all samples at 400℃is below 0.3,which improves its high temperature insulation.Quenching treatment stabilizes the ferroelectric order inside the samples,increases the piezoelectric constant and depolarization temperature of components with high BA content.such as d₃₃ from 33 p C/N to 84 p C/N for 7BA samples and 60℃to 140℃for T_d,widening its application temperature.