Toughening of bismuth-based lead-free piezoceramics

多层致动器通常承受高达150摄氏度的温度，并且容易受到内电极边缘发出的裂纹扩展的影响。因此，用无铅压电陶瓷取代含铅压电陶瓷需要了解显著的机械性能，如依赖于温度的裂纹扩展。在新材料中，铋基压电陶瓷的场致铁电有序显示出了超过锆钛酸铅的应变。这些材料展示了具有遍历和非遍历弛豫以及铁电相的相图。最近发现弛豫铁电体之间的转变依赖于电场，如已知的，例如PMN-PT。BNT-BT(钛酸铋钠-钛酸钡)材料也已被证明具有依赖于机械应力的相变。因此，在这一建议中，将研究不同掺杂含量的BNT-BT化合物在预期的温度范围内的裂纹扩展，以访问遍历和非遍历驰豫以及铁电材料。重点将放在材料上，在裂纹尖端加工区，从弛豫铁电体到铁电材料的转变已经可逆或不可逆地完成，伴随着铁弹性增韧，而主体保持不变。通过测定应力-应变曲线，包括体积随温度变化的曲线，将进一步量化这种转变。在单轴应力和裂纹扩展条件下的示例性原位中子和同步辐射衍射将被用来使增韧效应和随后的R曲线行为合理化。

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.