Studying the mechanisms of piezoelectric materials requires instrumentation that can provide short real-time measurements. Wombat, the high-intensity powder diffractometer at the OPAL reactor, is capable of performing high-speed stroboscopic measurements down to the sub-millisecond time scale. This capability has been used to measure the real-time response of piezoelectric materials to cyclic electric fields.
Piezoelectric materials
Piezoelectrics are materials whose electrical and mechanical properties are deeply intertwined. Putting an electric field across a piezoelectric causes it to change shape, which makes it useful as an actuator material. Conversely, the materials are also used as mechanical sensors: putting the material under strain causes it to generate anelectric field. They have a wide range of uses in society - from pacemakers to hydrophones.
Ideally, in order to study the piezoelectric response in realistic conditions, the material structure should be measured in real time as it is subjected to a cyclic electrical load. What is required is an instrument that can measure rapid, real-time structural change in materials. The OPAL reactor has such an instrument: the Wombat high-speed neutron diffractometer.
Wombat’s speed comes from a combination of the high neutron flux from the OPAL reactor and guide system and the rapid data-acquisition capabilities of the instrument [1].
Despite their ubiquity, there is a lot still to be learned about piezoelectrics and the precise nature of the electromechanical coupling. The overarching scientific question is: what are thedetails of the structural changes happening to the material as an electric field is applied, and how does that relate to performance? Understanding this dynamic behaviour is important for developing applications and for understanding the subtleties in the physics of their behaviour.
The sample under investigation
Measurements were performed on a particular commercial PZT material called EC-65 (ITT Corporation). Commercial piezoelectrics such as PZTs are polycrystalline materials; on a microscopic scale they consist of a myriad of tiny crystals or domains, oriented in different directions.
When an electric field is placed across the material, the individual grains are “stretched” by the field. This strain is the piezoelectric effect that is the basis of the material’s usefulness.
The grains are stretched by different amounts depending on their orientations. As a result, this causes a build-up of internal strain between the grains within the material and atoms will “switch” between different domains to relieve that strain (Figure 1). However, as time goes on and the material is cycled, the domains become more fixed. This loss of mobility is a key cause of fatigue damage in these materials.
The experiment
Wombat measures the piezoelectric response using a stroboscopic technique. The instrument can measure an individual event down to the microsecond level, but even on Wombat, for a rapidly oscillating system there may not be enough neutrons measured within one cycle to obtain meaningful information.
By synchronising the detector to the electrical cycling equipment, we can measure over multiple cycles and combine them to obtain statistically useful results (Figure 2). This process is rapid, and a full measurement can usually be performed in minutes.
In a Wombat experiment, a beam of neutrons is directed at the sample, and some of the beam is diffracted at a number of different angles which relate to the spacing between different atomic planes in the materials.
If a piezoelectric material is deformed by an electric field, the change in position of the atoms can cause shifts in the diffraction peaks or changes in their intensity.
Different peaks are sensitive to the strain and domain switching components by different amounts. Wombat’s large 120o area detector, measuring multiple diffraction peaks, can observe both of these characteristics simultaneously.
We performed two types of measurements on the samples: firstly, the response of the sample to a static electric field; then time-resolved stroboscopic measurements over a range of frequencies and electric fields.
Results and future work
Our measurements, performed with electric fields cycling over a range of field strengths and frequencies between 1 and 500 Hz, one of which is shown in Figure 3. This measurement was performed at 100 Hz for a field of +/-0.4 kV/ mm.
An important detail here is that the field is sub-coercive, less than the 0.5kV/mm threshold required to induce irreversible changes in the sample. This use of real-time neutron diffraction to measure sub-coercive domain switching was pioneered at ANSTO [2].
There is a strong dependence of domain switching with field strength, however there is no variation in the domain switching with frequency. This was unexpected by the investigators because their property measurements indicate both field-amplitude and frequency dependence.
Further experiments are underway to determine the cause of this discrepancy. In particular, the diffraction measurement show that the domain switching is fast; there is no evidence of relaxation behaviour down to the ~10μs resolution of Wombat.
This work was the first post-commissioning user experiment using the stroboscopic technique to be performed on Wombat and is also the first to be published [3].
It is the first step in an ongoing programme which will continue to correlate the structure response (measured with Wombat) with other property measurements such as permittivity. In the future, the work will be expanded to look explicitly at fatiguing effects and also at characterising new classes of leadfree free piezoelectric materials.
Authors
Jacob L. Jones1, Abhijit, A1 and Andrew Studer2
1University of Florida, USA and 2ANSTO
References
- Studer A. J, Hagen M. E and Noakes T. J, “Wombat: The High Intensity Powder Diffractometer at the OPAL Reactor”, Physica B 385-386 (2006) 1013-1015.
- J. L. Jones, M. Hoffman, J. E. Daniels, and A. J. Studer, “Direct measurement of the domain switching contribution to the dynamic piezoelectric response in ferroelectric ceramics”, Appl. Phys. Lett. 89 (2006) 092901.
- Pramanick A., Prewitt A. D, Cottrell M. A., Lee W., Studer A. J., An K., Hubbard C. R., and Jones J. L., “In situ neutron diffraction studies of a commercial, soft lead zirconate