Propagation Properties Of Elastic Waves In The Layered Composite Media Based On Lead Magnesium Niobate-Lead Titanat Single Crystals

The piezoelectric properties of the relaxor ferroelectric lead magnesium niobate-lead titanate single crystals (PMN-PT) are far superior to those of the traditional piezoelectric ceramics. This may result in a wide prospect in the field of medical ultrasonic, underwater sonar, microwave communication and new intelligent material systems and structures. Different from traditional piezoelectric ceramics, the macroscopic symmetry property of the PMN-PT single crystals are closely associated with its polarization direction. Moreover, the propagation of elastic waves in this structure will appear some special physical and mechanical phenomena. Thus, several basic problems for waves propagation in layered PMN-PT single crystals composites are investigated in this dissertation. The main contents are as follows.(1) Reflection and transmission of SH waves at the interface between PMN-PT and elastic media are studied. Considering the different cutting angles of PMN-PT single crystals, the variations of the reflection and transmission coefficients and energy coefficients with the incident angle are discussed. The results show that the electrical boundary conditions and the cutting angles of PMN-PT single crystals have a significant influence on the reflected coefficients, the transmission coefficients and the waves energy coefficients of the EA waves and the SH waves.(2) Reflection and transmission of quasi-longitudinal (QP) and quasi-transverse (QSV) waves at the interface between PMN-PT single crystals and elastic media are analyzed. Considering the different polarization directions of PMN-PT single crystals, the variations of the reflection coefficients, transmission coefficients and energy conservation coefficients with the incident angles are discussed. The results show that the QSV waves will split at the interface for quasi-transverse incident from PMN-PT single crystals when the single crystals are polarized along the cubic reference directions of [001]c. Moreover, the different polarization directions of PMN-PT single crystals will produce different reflected and transmission waves fields.(3) The propagation of SH waves in the PMN-PT/elastic substrate structures is investigated. The effects of boundary conditions and the cutting angles of PMN-PT layer on dispersion properties and electromechanical coupling coefficients are discussed. The results show that the phase velocities of SH waves approach to the limiting speed of the PMN-PT layer with the increasing normalized wave number under the electrically open condition. For the electrically shorted condition, the phase velocities of the first mode tend towards the speed of the B-G waves of the PMN-PT layer, while the phase velocities of the higher modes approach to the limiting speed of the PMN-PT layer. In addition, the effects of the thickness ratio on the dispersion characteristics of SH waves are very obvious when the wave number is small, and the influence disappears gradually with the increasing wave number. The material properties of the elastic substrate determine the initial velocity of in the composite structure, and the cutting angles of PMN-PT single crystals determines the convergence velocity of the SH waves.(4) The propagation of plane waves in the PMN-PT/elastic layered structures is investigated, where the layered structures include the layered half-space and the bi-material. The effects of the electrically boundary conditions, the polarization direction of PMN-PT single crystals, the thickness ratio and the material properties of elastic media the propagation of plane waves are explored. For the layered half-space, the material properties of the elastic media determine the initial velocity of the Rayleigh-type surface waves, and the polarization directions of the PMN-PT single crystals determine the asymptotic velocity. For the bi-material plate, the smaller the thickness ratio of the PMN-PT layer to the elastic layer is, the larger the phase velocity of the Lamb waves is. Moreover, it is found that there exists a high speed pseudo surface waves when Lamb waves propagates in the bi-material plate. Meanwhile, the larger the velocity of the bulk shear waves in the elastic layer is, the greater the velocity of the pseudo surface waves and the corresponding cut-off frequency are.(5) The propagation and localization of elastic waves in the PMN-PT/elastic periodically layered structures are studied. The effects of the polarization directions and the cutting angles of the PMN-PT on the localization factor, the reflection and transmission coefficients of displacement and energy are analyzed. The results indicate that the band gap characteristics of SH waves are closely associated with the incident angle, the polarization directions and the cutting angles of the PMN-PT single crystals. The widest band gap is obtained when the PMN-PT single crystals are polarized along the direction of [011]c. However, the result is opposite when the PMN-PT single crystals are polarized along the direction of [001]c. The larger the cutting angle of the PMN-PT is, the larger the localization factor is. The characteristics of the band structures are consistently described by the localization factors and the frequency response of the transmitted waves.