Mechanics of ferroelectric materials
The thermo-electro-mechanical coupling and the ability to retain permanent polarization makes ferroelectrics very promising active materials for applications ranging from sensor and transducer technology to data storage. In contrast to conventional piezoelectrics, ferroelectric materials can be subjected to large electro-mechanical forces hence allowing for superior electrical polarization switching and mechanical actuation capabilities. However, at large electric fields the electrical polarization and coupled mechanical response is strongly non-linear and exhibits characteristic hysteresis. This non-linear behavior still remains poorly understood.
Kinetics of ferroelectric switching
Permanent electrical polarization in a ferroelectric material originates within the single crystal unit cell. Most ferroelectric crystals have a perovskite crystal structure with a body-centered positive ion displaced with respect to the center of negative ions. This separation between the center of positive and negative ions is called "non-centrosymmetry" which causes permanent polarization in a single crystal. This non-centrosymmetry (hence ferroelectric state) is stable only below a critical temperature called the "Curie temperature". At length scales spanning nanometers up to micrometers, the material consists of a spatial distribution of different polarized states. Each of these regions is called a "ferrolectric domain" and interfaces between them "domain walls". The macroscopic polarization of a ferroelectric sample can be thought of as the volume-averaged polarization across these domains.
When large electric fields are applied to these materials, the nucleation and growth of domains are responsible for the macroscopic ferroelectric hysteresis and mechanical actuation observed in experiments. This is a classical multi-scale mechanics problem in this sense with the added complexity of multi-physical coupling (mechanical and electrical). Macroscopic measurements are made on different ferroelectric ceramics using an in-house experimental setup in our lab (Mechanics and Materials) called Broadband Electromechanical Spectroscopy. By controlling the electrical loading history we measure the polarization evolution in-situ. In combination with theoretical and computational studies within our group, we then interpret the effects of loading rates and electric field amplitudes on the kinetics of domain evolution and hence macroscopic polarization switching in ferroelectric materials.
Viscoelastic stiffness and damping in ferroelectric materials
The effect of polarization on the mechanical stiffness and damping of ferroelectric ceramics is another lesser understood problem with applications in active damping of structures. Using the BES, we probe the viscoelasticity of ferroelectric ceramics in-situ during the application of electric fields. Previous measurements have shown a dramatic increase in viscoelastic damping during polarization switching. This dynamic effect is believed to be due to the evolution of microscopic interfaces (domain walls in our case) within the material during electrical cycling. We perform controlled experiments in the laboratory to test our hypothesis. This problem has very interesting technological applications in the design of active dampers by controlling microstructural kinetics of domain walls.
Collaborators
Dr. M. Trassin , Department of Materials, ETH Zürich
Relevant Publications
A pictorial description of our ferroelectrics research