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abstracts
Piezoelectric ceramics exhibit the remarkable property to couple elastic strain and polarization under the influence of an applied electric field. Among the various types of ferroelectric devices only actuators rely on high electric fields to generate high strains and forces. Prominent examples for actuators are multilayer stack actuators used for nanopositioning or in modern combustion engines for automobiles to control injection cycles. Despite extensive studies and elaborated measurement techniques, the correlation between macroscopic strain and structural response is still not fully understood. Most of the relevant systems found up to now are compositions close to phase boundaries linking highly correlated phases. This results in major challenges for structural analyses due to overlapping reflections. Apart from the well-known field induced structural responses such as domain switching and the piezoelectric effect we recently identified field induced phase transitions in different systems as an additional poling mechanism [1,2]. In this contribution we present a structural analysis of in situ powder diffraction data that is capable of resolving all three involved poling mechanisms within only one experiment. The key to obtain enough information for such an analysis is a combination of high resolution and a broad Q-range, together with different sample orientations. This can be done with either X-ray or Neutron powder diffraction. The results not only separately reveal the contributions of each poling mechanism to the macroscopic strain, but also different behaviours of the individual phases. Several studies on lead containing as well as lead free systems showed significant changes while crossing the phase boundaries. Additionally, the calculation of the elastic strain perfectly matches the macroscopic observations, confirming the accuracy of the applied models.
abstracts
By analogy with ferromagnetism and the hysteresis of the magnetic moment with a magnetic field, materials that exhibit a macroscopic spontaneous polarization Ps, which can be reversed under electric field E were defined as ferroelectrics. Ps, the directional order parameter can give rise to different polar structural phase transitions and finally disappear as a function of temperature T and/or hydrostatic pressure P in a transformation from a non-centrosymmetric to a centrosymmetric space group. The physical properties of ferroelectric materials are the basis of many technological applications based on their hysteretic properties (Ps / E in ferroelectric random access memories) or based on their coupled properties (η (mechanical strain)/ E in piezoelectric applications). In order to understand the origin and the mechanisms associated with the ferroelectric properties, "in-situ" structural studies as a function of E, T and P have to be performed. In addition ferroelectric materials exhibit based on their directional properties (Ps) a particular domain configuration which makes the structural understanding of these compounds much more complex. Different scales should be taken into account: from the atomic scale (individual polar displacements) to the macroscopic scale (macroscopic piezoelectric effect) and finally the mesoscopic scale in between, which is governed by the domain wall motion. High piezoelectric/ferroelectric properties in lead perovskite materials (PZT, PMN, PZN) are structurally linked to strong disorder which can be characterized by the presence of diffuse scattering in diffraction experiments and by nanosized domains. Here we will present "in-situ" characterization in lead perovskite materials as a function of the applied electric field based on X-ray and neutron diffraction and EXAFS techniques. A brief overview of the challenges to solve in future studies as a function of pressure and temperature will also be discussed.
