Properties Of Bismuth Titanate Sodium Based Textured Piezoelectric Ceramics And Multilayer Piezoelectric Actuator

Piezoelectric multilayer actuators（MLAs）possess the advantages of a wider speed range,faster response,and insensitivity to the magnetic field,and therefore have been widely used in the fields of micro-actuation and micro-manipulation.Currently,the ceramics of Pb-based solid solutions are the mainstream materials for the most commercial MLAs.However,because of increased environmental awareness,environmentally friendly and high-performance lead-free piezoelectric ceramics have been attracting more and more attention.Among the various lead-free ceramic systems,(Na_0.5Bi_0.5)Ti O₃-based（NBT-based）ceramics can possess a high electric-field induced strain response,making themselves potential to be applied in MLAs.However,the industrialization of NBT-based MLAs still has many challenges,such as higher sintering temperature,large driving electric field,and large strain hysteresis.To address these problems,this study uses grain-orientation engineering and low-temperature co-fired technology to regulate the electrical and mechanical properties of materials and devices at the multifunctional sequential levels,such as lattice,domain morphology,phase structure,grains,grain boundaries,and multilayer interfaces,and analyze the"method-structure-performance"structure-effect relationship Finally,the MLA with low-temperature sintering,low hysteresis,high strain,and high stability was achieved.The specific research contents are as follows.（1）In order to prepare high-quality NBT-based textured ceramics,the growth kinetics of lamellar seeds（templates）and the grain orientation growth mechanism were investigated.The formation mechanisms of(Na_0.5Bi_0.5)Ti O₃（NBT）and Na Nb O₃（NN）templates were first investigated,while the grain orientation growth mechanisms and properties of the textured(Na_0.5Bi_0.5)Ti O₃-25Sr Ti O₃（NBT-25ST）ceramics were analyzed using the templated grain growth（TGG）technique after the introduction of NBT templates and NN templates.When using the NBT templates to prepare textured samples,the element diffusion between the NBT template and the matrix could be found due to the similar crystal structure and elemental composition,resulting in the composition deviation of the textured samples using NBT templates.For the textured specimen using NN templates,although there was almost no elemental diffusion observed,the interfacial stress between oriented grains and NN templates could provide an enhanced relaxor behavior.Therefore,the phase structures of both textured ceramics deviated from the initial morphotropic phase boundary（MPB）region compared to the untextured samples.Ultimately,the textured NBT-25ST ceramics with NBT template（texturing degree of 81%）provided 0.28%strain at 60 kV/cm and the textured NBT-25ST ceramics with NN template weave（texturing degree of 83%）provided 0.30%strain at 60 kV/cm,which is not a significant improvement in strain performance compared to the untextured sample（0.25%at 60 kV/cm）.（2）For the high sintering temperature of NBT-based ceramics,their multilayer low-temperature co-fired technique was investigated.A synergistic sintering additive of Li₂CO₃and Cu O was utilized to explore the effect on sintering behavior and strain of(Na_1/2Bi_1/2)_0.935Ba_0.065Ti_0.975(Fe_1/2Nb_1/2)_0.025O₃（NBT-BT-0.025FN）incipient piezoceramic.The optimal firing temperature of NBT-BT-0.025FN decreased from 1160°C to 940°C due to the formation of a liquid phase,with limited degradation on the strain.When sintered at940°C,a large strain of 0.39%was achieved.Subsequently,the NBT-based MLA with an inner electrode of 70 Ag/30 Pd was prepared using this low-temperature co-firing technology.The Ag/Pd metal in the MLA had good compatibility with the NBT-based ceramic at a sintering temperature of 940°C.The strain performance of the MLA under an electric field of 70 kV/cm was 0.32%.（3）To reduce the sintering temperature of NBT-based textured ceramics and their MLAs,the changes in domain structure,phase structure,grain orientation growth,microscopic morphology,electrical properties,and mechanical properties of textured ceramics and devices were investigated after the introduction of sintering aid.Since the introduction of NN templates into NBT-based ceramics enhances the relaxation properties of the matrix,according to the above theory,for the best strain properties of the textured samples,the matrix composition needs to be modified to a nonergodic state（NBT-BT-0.022FN）,and then Li₂CO₃ and Cu O sintering aids are introduced to assist the NN template to induce grain growth.The sample yielded a very high F_00l of 95%at a low sintering temperature（1000°C）with a dense structure.Due to the promotion of the liquid phase,the NN templates gradually dissolved into the oriented grains during the sintering process,resulting in PNRs in the matrix.The highly oriented structure induced by the texturing process and the dissolution of the NN template led to a reduction of the driving electric field of the ceramic and produced a strain value of 0.36%and(9₃₃^* value of 907 pm/V at 40 kV.To simultaneously achieve a high-strain output and effective co-firing with 70Ag/30Pd internal electrodes,this work applied a grain orientation engineering approach to preparing a（001）_c-textured NBT-based((Na_1/2Bi_1/2)_0.935Ba_0.065Ti_0.978(Fe_1/2Nb_1/2)_0.022O₃)MLA by low-temperature sintering.Notably,the textured MLA had an excellent strain level of 0.46%（at60 kV/cm）which was about 2.6 times as high as that of the non-textured counterpart.Furthermore,the impedance and capacitance of the textured MLA exhibited similar characteristics to those of the non-textured MLA and exhibited excellent thermal stability and electric fatigue resistance showed.However,Young’s modulus and fracture strength are reduced.（4）An novel technique of using dual-template to texture NBT-based ceramics was developed,which is a breakthrough from the conventional TGG method The textured NBT-25ST ceramics were prepared using dual templates（NBT template and NN template）to integrate two different template-induced oriented grains into one ceramic.Benefiting from the polarization coupling effect caused by the distinct discrepancies in dielectric constants,a 56%increase in the(9₃₃^* values from 416 pm/V to 650 pm/V at 60 kV/cm（0.39%strain value）could be achieved.After that,low-temperature co-fired<001>oriented MLAs were prepared using the double-template texturing technique,and the effect of different dielectric layer thicknesses on the strain characteristics was investigated.Moreover,as the thickness of the single dielectric layer of the textured MLA gradually decreases,the residual compressive stress in the MLA increases,so the strain hysteresis characteristics also decrease.The hysteresis of the textured MLA with the monolayer thicknesses of 35μm at30 kV/cm was about 38%,which was about 29%lower than that of the non-textured sample.The large strain output of 0.324%at a low driving voltage of 175 V was simultaneously achieved in the textured MLA with monolayer thicknesses of 35μm,which is better than that of the lead-based MLA.