| Sound band gap materials (i.e. phononic crystals) are artificial functional composite materials with spatial periodicity of density and/or elastic stiffness coefficient. Based on the fundamental properties of waves, such as scattering and interference,“band gaps” can be created in phononic crystals, where waves at certain wavelengths cannot propagate through the structure. Thus, the sound band gap materials can be extensively applied in sound filter, wave-guide, sound isolation, and noise suppression. Compared with traditional elastic band gap materials, piezoelectric single crystal, such as PMN-PT and PZN-PT, has superior piezoelectric and electromechanical perpeties, and much stronger anisotropy and higher strain. Therefore, the band gap materials consisting of the single crystals demonstrate broader applications over the traditional elastic band gap materials. In this thesis, the propagation property of shear horizontal waves and longitudinal acoustic wave in laminated composites based on PMN-PT was investigated.The propagation characteristics of shear horizontal waves in PMN-PT piezoelectric single crystals/polymer periodic laminated composites were studied using the global matrix method. The influence of the piezoelectric effect, polarization direction and PT content on the shear horizontal wave was analyzed. The numerical results show that the piezoelectric effect has significant effect on the shear horizontal wave propagation in composite structure. Due to the piezoelectric effect, the bandgaps width was enlarged in the composite structure containing [011]c and [111]c poled PMN-0.28PT. While its influence was neglectable for the composite structure consisting of [001]c poled single crystal. The difference comes from the different elastic coefficients of the single crystals with the three polarization directions. The bandgap width was enhanced with the filling ratio of single crystal increasing. In addition, the first bandgap (FBG) width showed strong dependence upon the poling directions when the filling fraction of PMN-0.28PT was larger than0.5. The FBG width in the composite structure with single crystal poled along [111]c direction was the largest. For the single crystal with the same polarization direction, the FBG showed nearly unchanged width in the differernt PT component.Longitudinal wave propagation in one-dimensional phononic crystal containing a Fe-doped relaxor-based ferroelectric PMN-0.38PT single crystal defect layer was studied by the transfer matrix method. A narrow passband can be produced in the stopband with the inserted PMN-0.38PT layer at the thickness around its half wavelength. The passband transmission is as high as100%. Since PMN-0.38PT single crystal is able to generate strain under external electric field, the passband frequency of the phononic crystal is tunable flexibly. Our simulation results show that as the strain of the defect layer increasing, the passband shifts to the low frenquency and keeps its bandwidth unchanged. More importantly, because0.2mol%Fe-doped PMN-0.38PT defect layer demonstrates a large0.8%strain under relatively low1.2kV/mm voltage, its passband frequency can be tuned up to about7.4kHz. Also, the influence of acoustic impedance of periodic constitutive materials (layers A and B) on the passband was investigated. The passband bandwidth was shrinked as the acoustic impedance ratio of layer A and B (ZA/ZB) increasing.In the piezoelectric single crystals sound band gap materials, temperature stability of single crystal impacts strongly the device quality. To achieve high band gap stability in the composite structure, we studied the influence of the Mn doping on horizontal shear wave propagation. The results indicate that shear wave propagation depends upon the frenquency and polarization. At low frequency, the influence of Mn doping was neglectable in the composite structure containing [001]c and [011]c polarized single crystal, while the band gap width decreased by12%in [111]c direction. As frenquency increasing, Mn doping demonstrated more obvious influence on the wave propagation in all three polarization directions. Moreover, it was found that Mn doping caused more significant change for the composite structures with [011]c and [111]c polarized crystal than that consisting of [001]c poled direction. In addition, we obverved that the temperature stabilities of the piezoelectric, electromechanical coupling, and mechanical quality factor were improved with the Mn substitution (30-40°C improvement of the usage temperature), but the piezoelectric and electromechanical coupling decreased. Based on the experimental data, the simulation results show that Mn substitution enhanced the thermal stability of the shear wave transmission spectrum in piezoelectric single crystal composite. |
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