Piezoelectric MaterialsChris PetorakUniversity of Central FloridaAdvising Professors: Keith Bowman and Jacob Jones |
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Introduction
A piezoelectric material forms a perovskite crystal structure below its Curie temperature. This perovskite structure allows for randomly oriented dipoles to exist in the material. When these dipoles are aligned through poling an overall dipole moment of the material can be generated. In other words, if an external stress is applied in the poled direction of the material, a voltage difference between top and bottom surfaces result. Similarly, if a voltage is applied to the electroded surface, a strain in the direction of that applied field results.
Piezoelectric ceramics are used commercially for many applications. Some applications include: to detect mechanical vibrations in microphones and hydrophones and to produce vibrations for use in actuators, to control frequency, and to generate acoustic and ultrasonic vibrations used in ultrasound technology. (1) Some general piezoelectric materials are Barium Titanate(BaTiO3), Lead Zirconate Titanate (PZT)(Pb(Zr,Ti)O3), Lead Zirconate(PbZrO3) and Lead Titanate(PbTiO3).
The properties of two bulk PZTs produced by Keramos Inc. were previously characterized by four Purdue undergraduate students. Properties such as the piezoelectric constant, elastic constant, acoustic impedance, Vickers hardness, coefficient of thermal expansion and dielectric constant were all determined. The fatigue effects on these two PZTs are investigated in this project.
Project Objectives
- Research and Design of the setup and the parameters for the cyclic loading of the PZT.
- Inspect the decreasing piezoelectric (d33 value used) and dielectric (k) constants when the material is subject to cyclic loading
- Obtain results of fatigue on two PZTs under different applied electric fields.
Experimental Approach
PZT ferroelectric properties are highly dependent upon the percent of domains that can be aligned in the poling field direction. Electrical Loading was chosen over compressive loading because the setup was more economical and accessible. The research already performed in this area gave some general outlines to be used. A high electric field needed to be obtained in order to stress the domain walls movement as close to saturation as possible. A low frequency is desirable to allow the domain walls time to overcome the 'difficult' domain wall movement. The difficult domain wall movement is responsible for the greatest amount of fatigue to the PZT. An electric field of 1.4kV/mm was chosen for cycling. A final frequency of 60Hz was chosen after the desired 20Hz could not be coupled with the approximate 3000V necessary to obtain the 1.4kV/mm electric field. The final setup, shown in figure 1, consists of a tube transformer, a variable autotransformer, the wire attachments to sample and the silicon oil bath. The tube transformer produces the high voltage necessary and the variable autotransformer controls the magnitude of the output voltage. The AC current from the wall outlet supplied the 60Hz Frequency.
Research findings
Neither material could be cycled under the 1.4kV/mm electric field without mechanically failing. The degradation of ferroelectric properties until final failure reported by earlier research was not consistent with the two PZT materials cycled at 1.4kV/mm field. Trouble shooting of the testing setup became the main focus during the remaining time span. The field was reduced to 1.2kV/mm and one of the materials, K181, showed results similar to those published. The other material, K182, continued to crack early on in the cycling. Due to time constraints, testing of K182 was abandoned allowing further investigation of testing parameters and the K181 material. The failure mode of cracking, shown in figure 2, was determined to be a result of current flow through the material. This was theoretically determined to be geometry dependent, though further trials are necessary.
Figure 1. Tube transformer, a variable autotransformer, the wire attachments to sample and the silicon oil bath.
Figure 2. Sample of K182 after failure. Black marks are where current passed through the sample.