Toughening of bismuth-based lead-free piezoceramics

多层致动器通常经受高达150 ℃的温度，并且容易受到从内部电极边缘发出的裂纹扩展的影响。因此，用无铅压电陶瓷代替含铅压电陶瓷需要了解显著的机械性能，如随温度变化的裂纹扩展。在这些新材料中，铋基压电陶瓷已被证明表现出超过基于场致铁电顺序的锆钛酸铅的应变。这些材料表现出各态历经和非各态历经弛豫以及铁电相的相图。弛豫体和铁电体之间的转变最近已被确定为取决于电场，如已知的，例如对于PMN-PT。BNT-BT BT（钛酸铋钠-钛酸钡）材料也已被证明表现出依赖于机械应力的相变。因此，在该提议中，将研究具有不同掺杂剂含量的BNT-BT组合物在预期温度范围内的裂纹扩展，以获得遍历和非遍历弛豫以及铁电体。重点将放在材料的弛豫铁电体的过渡已经完成可逆或不可逆的裂纹尖端过程区与随之而来的铁弹增韧与散装剩余未转化。这种转变将通过确定应力-应变曲线进一步量化，包括作为温度函数的体积变化。在单轴应力和裂纹扩展条件下的示例性原位中子和同步加速器衍射将用于合理化增韧效应和随后的R曲线行为。

Multilayer actuators are often subjected to temperatures up to 150 oC and are susceptible to crack propagation emanating from internal electrode edges. Replacement of lead-containing piezoceramics through lead-free piezoceramics therefore requires knowledge of salient mechanical properties like temperature-dependent crack propagation. Amongst the new materials, bismuth-based piezoceramics have been shown to exhibit strains in excess of lead zirconate titanate based on field-induced ferroelectric order. These materials exhibit phase diagrams with ergodic and nonergodic relaxor as well as ferroelectric phases. The transitions between relaxor and ferroelectric have recently been identified to depend on electric field, as already known, for example for PMN-PT. BNT-BT BT (bismuth sodium titanate-barium titanate) materials have also been proven to exhibit phase transitions dependent on mechanical stress. Therefore, in this proposal, the crack propagation is to be studied for BNT-BT compositions with different dopant content in the prospective temperature regime to access both ergodic and nonergodic relaxors as well as ferroelectrics. The focus will be on materials where the transition from relaxor to ferroelectric has been completed either reversibly or irreversibly in a crack-tip process zone with attendant ferroelastic toughening with the bulk remaining untransformed. This transition will further be quantified by determining stress-strain curves including volume change as function of temperature. Exemplary in-situ neutron and synchrotron diffraction under uniaxial stress and under crack propagation conditions will be used to rationalize the toughening effect and the ensuing R-curve behavior.