【作者】 徐倩；

【导师】 王清远；

【作者基本信息】 四川大学 ， 固体力学， 2023， 博士

【摘要】 新能源、航空航天以及国防工业等现代工业的高速发展,急剧增加了对复杂环境下服役压电陶瓷的需求。钛酸铋(Bi₄Ti₃O₁₂,BIT)压电陶瓷因其较高的居里温度（～675℃）和优异的抗电疲劳性而备受关注。然而,较低的压电活性严重限制了该类陶瓷的应用。因此,现有的研究主要集中于BIT基压电陶瓷电学性能的优化。然而,力学性能对于压电陶瓷器件在实际使用中的可靠性、稳定性以及持续性也至关重要。因此,系统地了解BIT基压电陶瓷,尤其是微观结构对于宏观的电学行为和力学行为影响机制对其进一步的推广应用有重要的意义。本文通过在BIT的A位引入Ce离子和B位引入W/Nb离子的方式实现了电学性能优化,化学式为Bi_3.96Ce_0.04Ti_3-2xW_xNb_xO₁₂（BCTWN）。利用实验仪器表征了BCTWN陶瓷在不同烧结温度和掺杂浓度下的晶体结构、微观组织形貌和铁电畴结构演化机制。详细分析了BCTWN陶瓷结构特征与电学性能之间的内在关系,阐述了烧结温度和掺杂浓度调控BIT基陶瓷性能的物理机制。系统地研究了BCTWN陶瓷的力学行为,并且详细地分析了畴结构对BCTWN陶瓷的压痕变形行为、断裂破坏、裂纹扩展各向异性行为及铁弹变形行为的调控机制。主要的研究内容和结果如下:（1）研究了BCTWN陶瓷的晶体结构、微观组织形貌和铁电畴结构,探讨了烧结温度对BIT基陶瓷性能的调控机制。实验结果表明,BCTWN陶瓷晶格畸变主要来源于氧八面体旋转,并且较高的温度下烧结的陶瓷拥有较大的晶格畸变。此外,BCTWN陶瓷的片状晶粒呈随机分布,晶粒尺寸随着烧结温度的升高而急剧增大。晶粒表面的畴结构主要由90°和180°畴组成,在较高烧结温度下烧结的陶瓷具有较大尺寸的90°畴与较低密度的90°畴壁。烧结温度不同导致BCTWN陶瓷的微观结构出现明显地差异,从而产生不同的电学行为。（2）表征了不同掺杂含量下BCTWN陶瓷的晶体结构、微观组织形貌以及铁电畴结构的演化。测试了各掺杂含量下BCTWN陶瓷的电学性能,并深入分析了电学性能变化与微观结构演化的联系,阐明了掺杂含量调节BIT基陶瓷性能的物理机制。实验结果表明,BCTWN陶瓷的电学性能取决于结构特征,而W/Nb的掺杂含量影响着陶瓷的结构特征。随着掺杂含量的增加,BCTWN陶瓷的晶体结构对称性先降低后升高,片状晶粒尺寸先增大后急剧减小。陶瓷晶粒表面的畴结构主要由90°和细小的层状畴结构组成,且掺杂含量的变化可以有效调控该细小畴结构的密度。这些细小畴结构的尺寸位于0.05-1μm范围内,属于亚微米级别的180°层状畴结构。这类畴结构在相同电场下的翻转能力优于90°畴,因此细小层状畴结构密度的提高有利于增强陶瓷的电学性能。BCTWN-1.75陶瓷拥有最大的晶格畸变、晶粒尺寸、最低的致密度以及最多的亚微米级别的180°层状畴结构,因此其电学性能最佳(T_C=642.00℃,2P_max=30.46μC/cm²,d₃₃=38.50 pC/N,ρ=3.27×10⁶??cm)。（3）通过化学腐蚀实验发现,极化过程会使铁电畴结构由无序转变为有序。然而,温度的升高会促进陶瓷的退极化过程,导致铁电畴的取向由有序逐渐转为无序。因此,陶瓷的压电性能随着温度的升高而降低。此外,采用纳米压痕和维氏压痕技术研究了BCTWN陶瓷在不同掺杂含量和选择性退火温度下的压痕变形行为。实验结果表明,BCTWN压电陶瓷的硬度值与掺杂含量有较强的相关性,晶粒尺寸越小、密度越高的陶瓷硬度越大。对于极化试样,较高的退火温度可以恢复更多的畴壁、产生更多的氧空位、更高的弹性回复率、较低的残余应变并且产生额外的变形阻力,使试样变硬。通过提出缺陷辅助畴壁钉扎机制的原理图,进一步说明了退火策略增强的变形抗力。（4）采用单轴压缩的方法研究了BCTWN陶瓷的力学行为,系统地分析了微观结构调控陶瓷的力学性能参数以及由畴结构翻转所导致铁弹变形行为的物理机制。实验结果表明,掺杂含量和极化状态决定了BCTWN陶瓷的抗压强度和弹性模量等力学性能参数。此外,畴结构翻转导致BCTWN陶瓷的应力-应变曲线呈现出非线性,而畴结构翻转的饱和应变、饱和应力和临界应力都与掺杂含量和极化方向有较强的关联。此外,采用循环加载的方法探索了BCTWN陶瓷的铁弹变形行为,发现畴翻转的永久应变累积会达到饱和,提出了基于畴结构翻转产生的饱和应变分析陶瓷永久应变的方法。（5）采用三点弯曲技术表征了BCTWN陶瓷在不同掺杂含量下的断裂行为,同时采用单边切口梁（Single-edge notched beam,SENB）和维氏压痕法研究了陶瓷试样在不同掺杂含量和选择极化方向下的裂纹扩展行为,深入地探究了BIT基压电陶瓷的断裂和裂纹扩展机制。实验结果表明,掺杂含量对断裂行为有显著影响,晶粒尺寸越大的试样断裂强度越小。此外,晶粒尺寸越大,微观缺陷越多的试样获得的断裂韧性越低。对于极化试样,断裂韧性取决于畴结构相对于极化方向的取向,裂纹扩展呈现出各向异性。在SENB中,沿厚度方向极化的试样获得了最高的断裂韧性。在维氏压痕中,当裂纹扩展方向与极化方向平行时试样的断裂韧性最大。裂纹扩展的各向异性响应归因于铁电陶瓷的畴翻转增韧机制,断裂韧性与裂纹尖端附近的可翻转的畴结构呈现正相关。更多还原

【Abstract】 With the rapid development of modern industries such as new energy,aerospace,and national defense,there is an urgent need for piezoelectric ceramics that can be operated under extreme environments.Bismuth titanate（BIT）piezoelectric ceramics have attracted significant attention,owing to the high Curie temperature（～675℃）and excellent resistance to electrical fatigue.However,BIT ceramics have been greatly limited because of the low piezoelectric activity.Consequently,existing research mainly focuses on optimizing the electrical properties of BIT-based piezoelectric ceramics.Nevertheless,mechanical properties are equally crucial for the reliability,stability,and sustainability of piezoelectric ceramic devices in practical use.Therefore,it is of great significance to systematically understand BIT-based piezoelectric ceramics,especially elucidating the complex interplay between microstructural characteristics and their macroscopic electrical and mechanical properties for further promoting the application.The ceramic achieved optimized electrical properties by introducing Ce ions into the A site and W/Nb ions into the B site of BIT,with a chemical formula of Bi_3.96Ce_0.04Ti_3-2xW_xNb_xO₁₂（abbreviated as BCTWN）.Experimental characterization methods were used to investigate the crystal structure,microstructure morphology,and ferroelectric domain structure evolution mechanism of BCTWN ceramics at different sintering temperatures and doping contents.The intrinsic relationship between structural features and electrical properties of BCTWN ceramics were analyzed in details,and also elaborates the physical mechanism on sintering temperature and doping contents in regulating performance of BIT-based ceramics.Furthermore,the mechanical behavior of BCTWN ceramics was systematically studied.The effects of domain structure on the indentation deformation behavior,fracture failure,crack propagation anisotropy behavior,as well as ferroelastic deformation behavior of BCTWN ceramics were systematically investigated.The main research contents and results are as follows:（1）The crystal structure,microstructure and ferroelectric domain structure of the BCTWN ceramics were studied,and the regulation mechanism of sintering temperature on the properties of BIT-based ceramics was explored.The results show that the octahedral rotation of BCTWN ceramic primarily stems from the lattice distortion of the pseudo-perovskite layer,and the ceramics sintered at higher temperature generates more distorted crystal structure.The plate-like grains of BCTWN ceramics are randomly distributed,and the grain size increases sharply with the increase of sintering temperature.The domain structures on the grain surface are mainly composed of 90°and 180°domains,and the higher sintering temperature induces larger domain size and fewer domain wall density.The grain features and defect dipole distribution are also significantly affected by sintering temperature,thus yielding the distinct electrical behaviors.（2）This research characterized the crystal structure,microstructure,and ferroelectric domain structure evolution of BCTWN ceramics with different doping contents.The effects of doping contents on crystal structure,microstructure and ferroelectric domain structure as well as resulting electrical behaviors of BCTWN were analyzed in details.This work elaborated the physical mechanism on doping contents tailoring the properties of BIT-based ceramics.The results show that the electrical properties of BCTWN ceramics are closely associated with the structural features primarily influenced by the W/Nb doping contents.With the increase of doping contents,the symmetry of the crystal structure of BCTWN ceramics first decreases and then increases,whereas the grain size presents an approximately opposite trend.The domain structure is mainly composed of 90°and small layered structures,along with the density of these small domain structures can be effectively tailored by the doping contents.The size of these small domains lies in0.05-1μm,belonging to the submicron range of 180°layered domain structures.In particular,the ability of these domain structures to reverse under the same electric field is superior to that of 90°domains,thus increasing the density of small layered domain structures is beneficial to enhance the electrical properties of the ceramics.In this research,BCTWN-1.75 ceramic exhibits the largest lattice distortion,grain size,lowest density,and the most submicron 180°layered domain structures,which contribute to its excellent electrical properties(T_C=642.00°C,2P_max=30.46μC/cm²,d₃₃=38.50 p C/N,ρ=3.27×10⁶??cm).（3）Chemical etching reveals that the polarization is a process that the disordered domains switch to the reoriented ones by under external electric field,while depolarization of ferroelectrics exhibits an approximately opposite trend by annealing,thus resulting in a decrease in the piezoelectric properties with increasing annealing temperature.The indentation deformation behavior of BCTWN ceramics at different doping contents and selective annealing temperatures was investigated using nanoindentation and Vickers indentation techniques.The results shows that the hardness of BCTWN ceramics is strongly correlated with the doping content,wherein the ceramics with smaller grain size and higher density exhibit bigger hardness.For the poled samples,a higher annealing temperature could restore more domain walls and generate more oxygen vacancies in the sample,along with higher elastic recovery rates and lower residual strain,shedding extra deformation resistance to make sample harder.Moreover,a schematic of defect assistant domain wall pinning mechanism is proposed to further rationalize the enhanced deformation resistance by annealing strategy.（4）The compression deformation behaviors of BCTWN ceramics were investigated using uniaxial compression.Simultaneously,the physical mechanism of ferroelastic deformation caused by domain structure switching was systematically analyzed.The results show that the mechanical performance parameters of BCTWN ceramics,such as compressive strength and elastic modulus,are significantly affected by doping contents and polarization state.The stress-strain curve of BCTWN ceramics exhibits nonlinearity,resulting from the domain structure switching.The mechanical parameters with respect to domain structure switching,such as saturated strain,saturated stress and critical stress,are strongly correlated with the doping contents and poling directions.Additionally,the accumulation of permanent strain resulting from ferroelastic domain switching during cyclic loading would reach saturation.Therefore,a computational model for the permanent strain of ceramics was proposed,based on an analysis of the saturated strain generated by domain structure switching.（5）The fracture behavior of BCTWN ceramics at different doping contents was characterized using three-point bending techniques,simultaneously the crack propagation behaviors of samples at different doping contents and selective poling directions were investigated using the single edge notch beam（SENB）and Vickers indentation methods.The results shows that the fracture behavior is significantly affected by the doping contents,and the samples with larger grain size tend to attain a smaller fracture strength.In addition,the samples with larger grain size and more microscopic defects obtained the lowest fracture toughness.For the poled samples,the value of fracture toughness depends on the orientation of ferroelectric domains with respect to the poling direction,and the crack propagation shows strongly anisotropy.the samples poled along the thickness direction toughen the most.In Vickers indentation,the largest fracture toughness is obtained when the direction of crack propagation is parallel to the polarization direction,shedding extra crack propagation resistance to make sample toughen the most.The crack propagation anisotropy response is attributed to the ferroelastic domain switching toughening mechanism in ferroelectric ceramics,and the fracture toughness is proportional to the switchable domain configurations near the crack tip.更多还原