Texture Development And Electrical Properties Of PbTiO₃-based Ternary Relaxor Ferroelectric Ceramics

Ferroelectrics have been widely used in various areas as a kind of impotant multifunctional materials.Recently,lead-based ternary relaxor ferroelectric solutions,such as lead indium niobate-lead magnesium niobate-lead titanate?PIN-PMN-PT?and lead ytterbium niobate-lead magnesium niobate-lead titanate?PYN-PMN-PT?,have attracted much attention owing to high Curie temperature and large coercive field.Manufacture of large-sized piezoelectric materials with high electrical properties and high stability has become an important research topic.Texturing technology is an efficient way to solve this problem.With producing oriented grain growth in polycrystalline ceramics,electrical properties can be enhanced significantly.In this thesis,[001]-oriented PIN-PMN-PT and PYN-PMN-PT ternary relaxor ferroelectric textured ceramics with rhombohedral structure were synthsized by templated grain gowth method?TGG?.Dopants were added to further promote the sintering behavior and electrical performance of textured ceramics.Effects of texturing and dopants on the phase structure,microstructure and dielectric,piezoelectric and electromechanical properties were discussed,and the origin of high piezoelectricity in textured ceramics was also explorated.Perovskite SrTiO₃ and BaTiO₃ mirco-plates with high aspect ratios were fabricated by topochemical conversion method.The diameters and thicknesses of SrTiO₃ and BaTiO₃platelets were¹1-16?m,1.0-1.5?m and⁵-10?m,0.5-1.0?m,respectively.The phase transition and microstructure evolution during SrTiO₃ platelets topochemical conversion were explorated by Differential thermal analysis,X-ray diffraction,scanning electron microscopy and transmission electron microscopy etc.The topochemical conversion mechanism was discussed.It shows that the Aurivillius Bi₄Ti₃O₁₂ directly converted into perovskite SrTiO₃.These was no stable intermediate phase?s?appeared and crystal orientation remained during conversion processing.The topochemical conversion occurred though three steps including structure conversion,partial exfoliation,recrystallization and reshaping.The effects of template types and contents on oriented growth,phase and microstructure of PIN-PMN-PT textured ceramics during templated grain growth were studied and highly[001]-oriented PIN-PMN-PT textured ceramics were fabricated.Dielectric,ferroelectric,piezoelectric and electromechanical properties of PIN-PMN-PT textured ceramics were discussed.ST plates dissolved into the matrix and failed to yield oriented growth.PIN-PMN-PT textured ceramics with high texture degree F₀₀₁⁹3%were achieved by using of BT templates.The PIN-PMN-PT textured samples shown much higher phase transition temperatures than binary systems like PMN-PT,i.e.,T_RT¹20?,T_c²03?.Because of the highly[001]-orientation and“4R”domain configuration,the electrical properties of textured ceramics were substantially enhanced compared with randomly oriented PINPMN-PT ceramics:d₃₃=824pC/N,k_p=0.81,E_c=8.3 kV/cm and d^*₃₃=1555 pC/N.Piezoelectric coefficient of highly textured samples was twice of random samples,and electromechanical coupling coefficient also increased sharply.PYN-PMN-PT relaxor-based ferroelectric ceramics with F₀₀₁>95%were synthesized by templated grain growth using BaTiO₃ platelet templates.The phase formation,morphology evolution and texturing development were anylysized to clarify the texture mechanism during TGG in PYN-PMN-PT textured ceramics.Furthermore,the effects of templates and texturing on dielectrical,ferroelectric,piezoelectric and electromechanical properties and thermal and electric filed stability of PYN-PMN-PT ceramics were studied,and the origin of improved electrical performance in textured ceramics was discussed.It shows that lower templates loading was beneficial to electrical properties of textured ceramics with high texture degree.Particularly,PYN-PMN-PT textured ceramics with 3 vol%BT present the best properties:dielectric constant?_r=1973,coercive field E_c=8.7 kV/cm,piezoelectric coefficient d₃₃=772pC/N,electriomechanical coupling coefficient k_p=0.82 and strain S=0.38%at 50 kV/cm with low hysteresis H_s=4.5%.High Curie temperature?T_c=200 ^oC?did not change with BT addition.Existence of heterogeneous templates and the lattice mismatch of matrix led to increasing of tetragonal phase in textured ceramics,which would weak the R-T phase transition.Thus,the thermal stability of PYN-PMN-PT textured ceramics was improved,and strain under high electric field kept stable from room temperature to 140 ^oC.Both polarization and strain remained stable after 1×10⁵ cycles with applying an AC electric field,confirming the promoted fatigue resistance in[001]-textured PYN-PMN-PT ceramics.Effects of different dopants on texture evolution and electrical properties of PYN-PMN-PT textured ceramics were explorated.The origin of high performance in PYN-PMN-PT textured ceramics was also discussed.It shows that Zn and Mn doping reduced the[001]-orientation of PYN-PMN-PT textured ceramics and led to“hard”electrical properties via pinning effect on domain switch due to the presence of oxygen vacancy.For the Zn/Mn-doped ceramics,E_c increased to 10.3 kV/cm and 12.6 kV/cm,tan?decreased to⁰.81%and⁰.58%and enhanced mechanical quality factors Q_m were 110 and 133,respectively.Besides,liquid phase formed at low temperature promoted sintering and texturing evolution of PYN-PMN-PT textured ceramics.PYN-PMN-PT textured ceramics with F₀₀₁⁹9.3%was achieved at a much lower sintering temperature of 975 ^oC.The sintering temperature was 225?lower than undoped samples and the texture degree was increased.The low-temperature sintered PYN-PMN-PT textured ceramics shown extra high d₃₃¹087pC/N and k_p⁰.94.The ultrahigh piezoelectricity of Cu-doped textured ceramics was mainly attibute to intrinsic contribution.Because of enhanced[001]orientation,multiphase coexistence,increased dielectric constant and local polarization disorder result in the super high piezoelectric performance.This work can offer experimental and theoretical basis for the design,preparation technology and performance control of the new generation high-performance relaxor ferroelectric texture materials,and can provide reliable high-performance materials for new generation high-temperature piezoelectric devices applications,which are of important significance in scientific reseach area and engineering application filed.