力电耦合作用下铁电单晶裂纹扩展机理研究

Understanding the fracture behavior of ferroelectrics during electro-mechanical loading is the key issue for designing and lifetime assessment of ferroelectric devices. During the past decades, the fracture behavior of ferroelectrics has been intensively investigated and discussed by measuring the fracture toughness and crack growth rate of ferroelectric ceramics under external loading. However, the actual physical process of crack propagation in ferroelectrics remains unclear, due to lack of in-situ experimental results concerning the crack propagation process and domain switching behavior near the crack tip. In addition, some contradictions still exist between theoretical models and experimental results. Therefore, we propose to investigate the fracture growth process in ferroelectric crystal in-situ by building a new experimental platform in this project, aiming to reveal the interaction mechanism between the crack propagation and the domain switching near the crack tip. Taking into account that domain switching plays an important role during crack propagation, the domain switching behavior of ferroelectric crystals under electro-mechanical loading will also be investigated by observing the domain switching process and measuring the macro response (polarization and strain) simultaneously. At last, based on the experimental results, a fracture model which takes the incompatible crack tip domain switching into consideration will be developed. The facts that influence the fracture behavior of ferroelectric crystal will be analyzed quantitatively in detail.

理解铁电材料力电耦合作用下的断裂破坏机理是对铁电器件优化设计和可靠性评估的关键。过去学者已通过测量铁电陶瓷的宏观断裂韧性和裂纹扩展速率探讨过铁电材料的断裂破坏过程，但因缺乏裂纹扩展及裂尖畴变行为的详细实验数据，到目前为止，铁电材料断裂破坏的实际物理力学过程仍不清晰，并且，理论模型与实验结果还存在诸多矛盾。为此，本项目拟搭建原位加载实验平台，通过同步观测铁电单晶在外载作用下的裂尖畴变和裂纹扩展行为，进一步了解裂纹扩展与裂尖畴变之间的相互作用机制，揭示铁电材料的断裂机理。考虑到裂尖畴变在裂纹扩展中起关键作用，我们也将研究铁电单晶在力电耦合作用下的电畴演化和宏观响应特性，从不同层面理解约束对铁电材料电畴翻转行为的影响。最后，我们将集成实验结果，细化铁电材料电畴演化行为的描述办法，建立考虑裂纹尖端非协调畴变效应的铁电单晶断裂模型，对影响铁电材料断裂破坏的关键因素进行定量分析。

理解力电耦合作用下铁电材料的断裂破坏机理是对铁电器件优化设计和可靠性评估的关键，其中，涉及到的关键科学问题包括电场作用下裂纹面的电边界条件的表征；约束条件下铁电材料电畴翻转问题；力电耦合作用下裂纹尖端畴变行为与裂纹扩展的作用机制；材料初始状态对裂纹扩展行为的影响等，国内外研究者对以上问题的研究都非常重视。在本项目的支持上，我们利用钛酸钡单晶，通过原位试验，对以上部分问题进行了表征：（1）测试了带有自然裂纹的钛酸钡单晶在电场作用下的极化响应，并观测了钛酸钡单晶靠近裂纹面的畴变行为，测试结果表明，对新裂纹，铁电材料的电边界条件为导通裂纹，随着加载周期的增加，裂纹面逐渐有颗粒物脱落，裂纹边界条件也有导通裂纹向半导通裂纹发展；（2）根据力电耦合下钛酸钡单晶的电畴翻转行为，建立了增量型电畴翻转准则，该准则可以良好的预测力电耦合作用下钛酸钡单晶的电畴翻转行为；（3）设计了一套紧凑的原位力加载试验装置，力通过压电致动器加载，该装置可以放在显微镜下面，利用该装置，测试了电场对裂纹扩展行为的影响，发现在电场作用下，裂纹会沿45o方向发生偏折；（4）研究了初始电畴结构对钛酸钡单晶电致裂纹扩展行为的影响，初始畴结构为a-a畴的试件，裂纹扩展前裂纹尖端有明显的畴变行为，而对a-c畴结构试件，裂纹扩展前没有观察到裂纹尖端发生畴变，裂纹扩展速度也要远大于具有a-a畴结构的试件。另外，在该项目的支持下，我们还研究了PZT陶瓷的高温下的本构行为和疲劳断裂行为。