Electric-field-induced strain and fatigue mechanisms in lead-free ferroelectrics

Abstract

Piezoelectric actuators are used in a broad range of industrial and consumer applications. Such devices have traditionally used the piezoelectric ceramic (Pb,Zr)TiO3 (PZT), however environmental concern has instigated the development of lead-free alternatives. Compositions based on 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) are promising in this regard, with electromechanical properties rivaling those of PZT. It has been hypothesized that the large electromechanical response of these materials arises from an electric-field-induced structural transformation spanning multiple length scales. However, there is ambiguity regarding structural symmetries comprising this transformation. This electric-field-induced structural transformation was measured using in situ neutron diffraction techniques. Development of specialized apparatus and methodologies enabled the determination of crystallographic symmetry and texture of BNT-6BT ceramics under large electric fields for a range of temperatures. Crystallographic refinement of the average and nanoscale structure in the as-sintered state depicted a disordered pseudocubic average structure containing nanostructured entities embedded in the lattice. Upon electric field application, a mixture of textured ferroelectric domains with R3c (rhombohedral) and P4mm (tetragonal) symmetries is reported. Analysis of the strain contribution from these electric-field-induced ferroelectric textures shows that the transformation is predominantly responsible for the significant ferroelectric properties of BNT-6BT at room temperature. Furthermore, the large recoverable strain observed at elevated temperatures was attributed to the volume change associated with the reversible relaxor-ferroelectric transformation upon field application. In addition to the electric-field-induced strain mechanism, the influence of the structural transformation upon the bipolar electrical fatigue of BNT-6BT was investigated. The results did not show any unexpected structural transformations, but instead highlighted a fatigue mechanism by which domains are progressively pinned and fragmented. When heated close to the ferroelectric-relaxor transition temperature, the fatigue degradation reduced significantly. This improvement of the fatigue characteristics was attributed to the unstable domain structure near the relaxor transition. This thesis describes the electric-field-induced structural transformations occurring in the lead-free piezoceramic BNT-6BT, and its contribution to the macroscopic electromechanical response. The transformation did not adversely affect the susceptibility to electric fatigue. However, fatigue was reduced considerably when in the ergodic relaxor state.