Advances in the characterisation of domain switching in ferroelectric ceramics

In ferroelectric materials, extrinsic contributions such as domain wall vibrations, domain switching, and interphase boundary motion contribute to fatigue and aging, nonlinearity, and hysteresis in the piezoelectric response. It also affects the deformation behaviour, and influences the fracture mechanics. The most prevalent extrinsic contribution is ferroelectric/ferroelastic domain switching. Recent advances in diffraction techniques offer many opportunities for novel characterisation of domain switching and its influence on macroscopic properties. This paper presents the results of two such techniques which have been used to characterise extrinsic mechanisms in situ in a soft lead zirconate titanate (PZT) ceramic. In the first example, stroboscopic, time-resolved neutron diffraction is used to represent the ferroelectric/ferroelastic domain switching during a single cycle of a 1 kHz unipolar electric field with a magnitude of half of the coercive field. In the second example, highenergy X-ray microdiffraction is used to measure the spatial distribution of ferroelastic domain switching around a crack tip in transmission geometry under an applied stress intensity factor of KI=0.71 MPa•m. Introduction Ferroelectric ceramics are used in a wide variety of electromechanical devices including sonar, ultrasound, sensors and actuators, non-volatile memories, and micro-electromechanical systems (MEMS). In ferroelectric ceramics, changes in the ferroelectric/ferroelastic domain structures can affect the measurable bulk properties in an advantageous or disadvantageous manner. For example, domain wall motion during dynamic actuation increases the electric-field-induced strain but also contributes to nonlinearity, hysteresis, and fatigue [1]. Domain switching also contributes to fracture toughness enhancement in ferroelectric/ferroelastic ceramics, which is expressed both as an increase in the initiation toughness as well as a rising R-curve behaviour. It is therefore important to characterise such mechanisms in order to reveal their exact role in these and other cases. 442 European Powder Diffraction Conference, EPDIC 10 A schematic diagram of the process of domain switching is shown in figure 1. As shown, the application of an electric field or stress along particular directions of a bulk ceramic sample leads to a change in the volume fraction of domain types present within individual grains. Domain structures have been characterised using diffraction for decades [2]. The essence of such measurements is the relationship between the diffracted intensity of particular Bragg peaks and the volume fraction of ferroelectric/ferroelastic domains present within the sample. The simplest representation (i.e., for a sample with tetragonal symmetry) of the changes in the diffraction pattern is shown in figure 1(c).