Properties Of Acoustic Guided Waves In Lead Magnesium Niobate-Lead Titanate Single Crystals

The discovery of relaxor-based ferroelectric lead magnesium niobate-lead titanate single crystals (PMN-PT) is considered a great achievement in ferroelectric field in recent decades because of their extremely high piezoelectric properties. This series of crystals could be used in next generation electromechanical devices, including piezoelectric ultrasonic motors, piezoelectric transducers used in structural health monitoring and medical diagnostics, active noise control systems, etc. This thesis is on the surface wave propagation in these PMN-PT single crystals with composition near the morphotropic phase boundary (MPB). A detail theoretical analysis has been performed on the Lamb waves and shear horizontal waves propagating in PMN-PT single crystals poled in different crystolographic directions with the help of PMN-PT data. Dispersion relations are calculated for waves propagating in different directions. The dependence of displacements on space and time for each mode of interest is also calculated.A detail theoretical analysis has been conducted on the dispersion relations of acoustic guided waves propagating in PMN-PT single crystal plates using the partial wave method. In order to avoid the influence of environmental conditions and to focus on the characteristics of PMN-PT single crystals, infinite plates are used under stress free condition. Some interesting phenomenons about the dispersion relationes of acoustic guided waves in PMN-PT plates are found: (1) There are a lot of crossings of the dispersion curves and strong couplings appear among the same symmetry modes, which make the dispersion relations very complicated. Many dispersion curves show spiral shape. (2) If there are no mode couplings, the phase velocities of most modes show a check mark"√"shape as a function of frequency. That is, with the increase of frequency, the phase velocities of these modes decrease rapidly from a low cut-off frequency, after passing a minimum, the velocities begin to increase slowly with frequency and eventually saturate to the sagittal shear wave velocity at high frequencies. (3) When Lamb modes and SH modes are coupled together, a new symmetry mode (Antisymmetry mode for single domain case) with low velocities appear in some cases. The limit velocity of this new mode at high frequency is the Rayleigh wave velocity.We have also studied the displacement characteristics of Lamb waves propagating in different directions. The results show that Lamb modes and SH modes are coupled together in most cases when the propagating direction is off the principal directions. Only in special directions, Lamb modes and SH modes are decoupled. The displacement pattern repeats periodically with orientation. The period isπ/4 andπ/2 respectively for [001]c and [011]c poled PMN-PT single crystals with the Lamb waves and the SH waves having the same symmetry, while the period isπ/3 for [111]c poled PMN-PT single crystals but the coupled Lamb waves and SH waves have opposite symmetries in this case.There is only one complete set of electromechanical coupling coefficients of a [111]c poled single-domain PMN-PT crystal in the literature measured under a big bias field, which may affect the nature of thte sueface waves. Therefore, a complete set of electromechanical coefficients of single-domain PMN-0.29PT crystal is measured. Based on the new data, the dispersion relations are again calculated. It was found that the bias field does not affect the dispersion curves very much, all modes behavior are the same as those calculated using non-biased data. Moreover, the aspect ratio dependence of electromechanical coupling coefficient k31 is studied for the PMN-PT single crystal system. It was found that the influence is different for crystals poled in different directions. Our more accurate results indicate that the textbook formulas for measuring the electromechanical coupling coefficients have very large error while the aspect ratio is near 1. Generally speaking, when the aspect ratio is more than 3 or less than 0.3, the percentage error will decrease rapidly so that one could use the textbook formua for such cases.Dispersion relations and displacement of acoustic guided waves in lead magnesium niobate-lead titanate single crystals have been studied using patial wave method in this thesis. Some novelty results are obtained, which could be a good foundation for PMN-PT applications in guided wave devices.