Broadband Electromechanical Spectroscopy
We perform dynamic experiments to characterize the thermo-electro-mechanically-coupled response of materials with a particular focus on ferroelectric ceramics, ceramic-ceramic and metal-ceramic composites. In our lab we have developed a technique called Broadband Electromechanical Spectroscopy (BES) which provides unique insight into the viscoelastic behavior of materials resulting from microstructural mechanisms under combined mechanical, electrical, and thermal loading. Similarly to Broadband Viscoelastic Spectroscopy (BVS) and in contrast to Dynamic Mechanical Analysis (DMA), multiaxial mechanical loads are applied in a contactless fashion via electromagnetism and deformation is recorded by a laser-detector setup. Loading modes include bending, torsion, and combinations thereof with time-varying moments. Specialized clamps and grips enable us to apply large electric fields to the specimen through surface electrodes. The entire setup is enclosed in a massive metal chamber with wall cooling, so that the specimen can be examined over a wide range of temperatures. In addition, the chamber can be evacuated to pressures below 1 nanobar, while the temperature is controled by graphite resistance heaters. This eliminates air flow inside the chamber which may lead to spurious damping and has been shown to have a significant impact on the measured damping capacity. Current research investigates the thermo-electro-mechanically-coupled response of ferroelectric ceramics (including polycrystalline PZT and BaTiO3), particularly exploring the impact of microstructural domain wall motion on the macroscopically observable mechanical damping. Furthermore, through sustained testing we can investigate the fatigue behavior of materials. Overall, this experimental technique allows us to quantify microstructural mechanisms of internal friction (induced by thermal, electrical, or mechanical fields) on the macroscale in an average manner.
Figures from left to right: specimen with attached magnet/mirror driven mechanically by electromagnetic coils; setup in the open BES chamber; BES chamber closed with laser and detector and vacuum pump visible; electronics rack in our lab.
|temperature range:||room temperature - 400°C|
|pressure range:||10-6 mbar - 1 bar|
|mechanical frequency range:||1 mHz - 1 MHz|
|electrical frequency range:||1 mHz - 10 kHz|
|voltage amplitude range:||up to +/- 10 kV|
Additional equipment in our lab to control the BES apparatus:
- rotary vane pump (Pfeiffer Vacuum, Asslar, Germany)
- turbomolecular vacuum pump (Pfeiffer Vacuum, Asslar, Germany)
- pressure gauges (Active Pirani/cold cathode transmitters)
- high-voltage amplifier 10/10B-HS with 0 to +/-10 kV DC/0 to 10 mA DC (Trek, Lockport, NY)
- 5 mW 633 nm helium-neon laser (Research Electro-Optics, Boulder, CO)
- SpotOn analog position detector (Duma Optronics Ltd., Nesher, Israel)
- 2.6 kW power supply (0-50 V-DC and 0-52 A-DC, Magna Power, Flemington, NJ)
- lock-in amplifier SR830 (Stanford Research Systems, Sunnyvale, CA)
- Dual Channel Arbitrary Function Generator AFG 3022B (Tektronix, Inc., Beaverton, OR)
- Sawyer-Tower circuit
View through the Kodial window of the BES apparatus into the test chamber.
3D Digital Image Correlation
Our latest addition to the BES setup was a dual-camera system for 3D Digital Image Correlation. Two high-speed cameras (Fastec, up to 500 frames per second) record the surface deformation of specimens during electro-mechanical experiments. Specimens are covered with a speckle pattern whose motion is tracked and interpolated to obtain a 3D map of displacements and strains of the specimen surface (with a displacement resolution of 0.3 microns in-plane and 0.6 microns out-of-plane). In contrast to the original BES design, this allows for full-field displacement and strain measurements and reveals local details of those fields.
The DIC systems determines a three-dimensional map of the surface strains and displacements of electro-thermo-mechanically loaded samples; an example strain field of a PZT sample under electric acuation is shown on the right.
For composite specimen synthesis, we use methods of powder metallurgy. To this end, we have access to individual equipment of Caltech's Kavli Nanoscience Institute (KNI). In addition, our lab hosts a furnace, a manual Carver press as well as a fumehood for specimen processing.
- 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.
- D. M. Kochmann, C. S. Wojnar, J.-B. le Graverend. Broadband Electromechanical Spectrocopy, US patent 20140298918 A1 (2016).