Toughening of bismuth-based lead-free piezoceramics

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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.