Microstructural Kinetics of Ferroelectric Ceramics
Ferroelectric ceramics are commonly exploited for their piezoelectric effect which is utilized in sensors and actuators. While such materials are usually used within their linear, reversible piezoelectric regime, we explore their performance under significantly larger electric fields. One of our goals in the study of the viscoelastic properties of ferroelectric ceramics is to create materials with both high stiffness and high damping and to create electro-active solids and structures. A myriad of engineering structures (e.g. aircraft and spacecraft) are simultaneously subjected to large mechanical loads and vibrations. Incorporating materials with high stiffness and high damping into such structures could dramatically reduce weight and complexity since additional damping mechanisms would not be needed. However, materials with both high stiffness and high damping are hard to find in nature due to contrary mechanisms on the microscale, which are responsible for stiffness and damping. Typical ferroelectric ceramics exhibit high stiffness. We seek to increase their damping capacity by utilizing their electromechanically-coupled response. That is, we apply electric fields during mechanical excitation to induce microstructural changes by domain switching. The increase in domain wall motion on the microscale leads to dissipation during cyclic mechanical loading on the macroscale. In addition to dissipation, domain switching can lead to large permanent strains. Many applications of ferroelectrics in active structures only utilize the linear piezoelectric behavior, which require constant powering to maintain a deformed configuration. As an alternative, we explore active structures with set-and-hold functionality based on controlled domain switching. To understand the dynamic behavior of ferroelectrics during domain switching, we perform experiments using Broadband Electromechanical Spectroscopy (BES), which was developed in our lab, to measure the dynamic stiffness and damping of ferroelectrics under combined mechanical, electrical, and thermal loads. In addition, we develop material models that help us understand the experimental data and predict the response of new materials and structures with optimal performance.
Figure: (a) potential energy landscape of ferroelectric perovskites above and below the Curie temperature, showing unpolarized and polarized atomic configurations, respectively; (b) application of an electric field causes domain switching which gives rise to a hysteresis in the macroscopic polarization.
Using BES, we measure the dynamic stiffness and damping of ferroelectrics under wide ranges of mechanical excitation frequencies and electric field cycling frequencies. Near the material's coercive field, the mobility of domain walls on the microscale is increased, which leads to enhanced energy dissipation (large increases in the loss tangent) and a decrease in the Young modulus on the macroscale. The damping increase occurs over wide ranges of mechanical frequencies, which makes the domain switching phenomenon a promising methods for creating broadband high damping materials. In addition, the maximum damping can be tuned by varying the electric field loading rate. By methods of powder metallurgy, we also manufactures and tests ferroelectrics with different microstructures (i.e. grain and domain structures) as well as different compositions to understand their effect on the macroscopic response of the material. With this understanding, we can further tailor the viscoelastic response to optimize the material's performance. Our latest addition to the lab is a 3D-DIC system to obtain full-field displacement and strain information from samples undergoing electro-thermo-mechanical testing.
Figure: (a) macroscopic electric displacement, (b) relative Young modulus, and (c) loss tangent vs. electric field for various frequencies of the applied cyclic electric field.
Figure: (a) macroscopic electric displacement, (b) relative Young modulus, and (c) loss tangent vs. electric field for various mechanical excitation frequencies.
In order to better understand our experimental observations and to predict the material response outside the tested regimes, we develop material models that describe the viscoelastic response of ferroelectrics. Due to the various phenomena present at different length scales (from ferroelectric domain walls on the nanoscale to grain structures on the mesoscale of polycrystals) we employ both effective continuum mechanics and micromechanical models using phase field methods. The latter allow for the simulation of domain pattern evolution under applied mechanical and electric fields.
Figure: distribution of microstructural domain patterns in a 3D polycrystal of barium titanate (shown are the shear stress distribution, which highlights domain walls, and the distribution of the polarization), obtained from a phase field simulation.
We also perform multiscale modeling of ferroelectrics via homogenization and relaxation methods to bridge the scales. Using our in-house finite element code, we apply those models to macroscopic structures. We inform and validate our models based on experimental data which can be generated in the lab on demand.
Figure: The macroscopic behavior of ferroelectric materials arises from microstructural mechanisms at different length scales.
We use the newly-gained understanding of the underlying physics of domain switching in ferroelectrics and how they affect the macroscopic mechanical response to design and test new technologies for active materials and structures. This includes materials with controllable stiffness and damping as well as a new class of set-and-hold actuators. By using the permanent strains caused by domain switching, we apply electric fields to deform the actuator permanently without the need of continuous power supply. Such devices are extremely useful in, e.g., deployable space structures for antenna, solar panels, and reconfigurable telescopes - we explore the latter in collaboration with scientists and engineers at JPL Our studies in particular focus on discovering optimal electrical loading histories to obtain desired permanent strains as well as the stability of the configurations to cyclic electrical and mechanical loading where fatigue effects become apparent.
- C. S. Wojnar, J.-B. le Graverend, D. M. Kochmann. Broadband control of the viscoelasticity of ferroelectrics by electric fields, Appl. Phys. Lett. 105, 162912 (2014).
- J.-B. le Graverend, C. S. Wojnar, D. M. Kochmann. Broadband Electromechanical Spectroscopy: characterizing the dynamic mechanical response of viscoelastic materials under temperature and electric-field control in a vacuum environment, J. Mater. Sci. 50 (2015), 3656-3685.