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??_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.