High-throughput Synthesis And Electrical Property Studies Of BNT-BT-KNN Based Lead-free Piezoelectric Ceramics

Ternary(Bi_0.5Na_0.5)TiO₃-BaTiO₃-(K_0.5Na_0.5)NbO₃（abbreviated as BNT-BT-KNN）is a promising lead-free piezoelectric material system that featured with abundant morphotropic phase boundaries（MPBs）,excellent piezoelectric properties and high curie temperature（T_c）.However,its composition-structure-property relationships especially the distribution of MPBs are still the subject of heightened concern.Meanwhile,as typical BNT-based lead-free piezoceramics,the materials in this system also face strong demands on low electric-field application.The solution of these problems needs the support of huge numbers of experiments and data,hence the efficiency becomes the critical bottleneck.In view of these,a high-throughput technology was built and applied in this dissertation to accelerate the studies of BNT-BT-KNN ternary system.We mapped composition,structure and properties of BNT-BT-KNN system.The preferred composition was screened and further modified by elements doping to lower its driving electric-field.The main contents and achievements of this dissertation are summarized as follows:（1）The high-throughput preparation technology of lead-free piezoelectric ceramics and its feasibility research.A fast and parallel high-throughput preparation method was established based on automatic powder weighing,high-throughput multi-channel ball milling and rubber mold batch isostatic press.The ceramics library composed of 16 components of 87BNT-6BT-7KNN ceramics was selected to verify the homogeneity and feasibility of the established high-throughput preparation method.The results showed that all samples have high densification,uniform composition,consistent microscopic morphology,and the same pseudo-cubic structure.Meanwhile,the coefficient variation（CV）of the average dielectric constant（ε_r）,piezoelectric constant(d₃₃),saturation polarization（P_s）,residual polarization（P_r）and inverse piezoelectric constant(S_max/E_max)were 1.7%,4.14%,0.94%,8.47%,and 6.12%,respectively.The smaller CV values indicates the established high-throughput parallel preparation method is feasible for the study of lead-free piezoelectric ceramics.（2）The high-throughput preparation and composition-structure-property studies of ternary（1-x-y）BNT-xBT-yKNN（0≤x≤0.12,0≤y≤0.12）system.The comprehensive analysis on the mapping phase,morphology,dielectric,ferroelectric,piezoelectric and electric-field-induced strain revealed the phase distributions of rhombohedral ferroelectric（I）,relaxor ferroelectric（II）and tetragonal ferroelectric（III）in（1-x-y）BNT-xBT-yKNN system.The excellent piezoelectric and inverse piezoelectric properties were obtained near the phase boundaries but show different distribution characteristics.The optimal piezoelectric coefficient d₃₃(d₃₃=181 p C/N)tends to I-III phase boundary while the excellent inverse piezoelectric coefficient d₃₃^*(d₃₃^*=528pm/V)is located at I-II phase boundary.The electrical properties and domain structures studies indicated that the excellent inverse piezoelectric property for the composition near I-II phase boundary was originated from the reversible transition between relaxor phase and ferroelectric phase,while the good piezoelectric property for the composition near I-III phase boundary was contributed to the electric-field-induced metastable rhombohedral or tetragonal phases.Based on above results,we successfully draw the ternary property diagram of the BNT-BT-KNN system.（3）Doping modification and property research of（0.93-x）BNT-0.06BT-0.01KNN-x ST（x=0-0.06）lead-free piezoelectric ceramics.The results showed that the incorporation of Sr TiO₃（ST）into BNT-BT-KNN matrix could destroy the long-range ferroelectric order and make the samples possess the relaxor feature with decreased P_m,P_r,E_c but significant increased electric-field-induced strain at room temperature.Benefiting from the reversible relaxor-ferroelectric phase transition,the sample with x=0.02 showed the maximum unipolar strain of～0.30%(d₃₃^*～600 pm/V)at room temperature under an electric field of 5 k V/mm.Furthermore,the results also showed that ST can enhance the temperature stability of electric-induced strain.For the sample with x=0.06,the fluctuation of the normalized strain in the temperature from 25℃to120℃was lower than 15%,reducing from 400 pm/V to～337 pm/V slowly.