Research On Surface Acoustic Waves In Periodic Piezoelectric Structures Coated With A Layer

Functional materials such as piezoelectric materials,magnetoelectric materials,and piezoelectric semiconductor materials are widely used in mobile communication,microelectromechanical systems?MEMS?and energy harvesting systems.In recent years,the propagation of acoustic waves in periodic structures?phononic crystals?composed of piezoelectric or piezomagnetic materials have become one of the researches focuses.This kind of structure can filter,guide or focus elastic waves,and can improve the performance of surface acoustic wave?SAW?devices such as filters,resonators and waveguides.Based on the theory of linear piezoelectric elasticity,the propagation of surface acoustic waves in composite structure of piezoelectric phononic crystal with a coating layer is studied by plane wave expansion method and finite element method.In addition,the effects of bias field on the surface waves in phononic crystals are discussed.The main contents of this paper are divided into the following aspects:?1?The propagation of Love wave in piezoelectric phononic crystal structures coated with a waveguide layer is investigated.The effects of the thickness of the waveguide layer,initial stress and the imperfect interface between waveguide layer and substrate on the band gap are analyzed.We found that the width of the band gap decreases with the increase of the thickness of the waveguide layer.The existence of initial tensile stress makes the band gap shift towards higher frequency while the initial compressive stress makes the band gap shift towards lower frequency.The interface imperfections have significant effects on the band structures,as the interface imperfection increase the band gap shift toward lower frequency and the width of the band gap decrease.?2?Two kinds of phononic crystal structures based on periodically poled lithium niobate are proposed and the propagation characteristics of Rayleigh waves in these two structures are studied.The first structure is composed of periodically poled lithium niobate substrate with a layer;The second kind of structure is periodically poled lithium niobate film with electrodes on elastic substrate surface.For the first type of structure,the effects of the layer on the band structure of Rayleigh waves is investigated.the propagation of Rayleigh wave in periodically poled lithium niobate coated with a layer is investigated.We focus on the effects of material,geometry parameters and electrical boundary conditions on the band gap.The results show that for the first structure,the band gap width and frequency position have small variation for electrically short or open boundary conditions.However,the characteristics of the coating material have a significant influence on the bandgap,materials with high wave velocity increase the bandgap width and frequency,while materials with lower wave velocity have the opposite effect.In addition,with the increase of the thickness of the layer,the inhibition of the band gap on the surface wave is weakened.For the second structure,the band gap can be tune by electrical boundaries,we find that the band gap is very small with electrically open condition while the band gap is obvious with electrically short condition.Hence,the band gap can be open or closed by electric boundary conditions.?3?The propagation of Rayleigh wave in piezoelectric phononic crystal coated with a piezomagnetic layer is investigated.The effects of the piezomagnetic layer on the band structure are discussed.We find that as the thickness of the layer increase the band gap shift toward higher frequency and the width of the bandgap increase for the substrate consists of BaTiO₃/PZT-5A.And for the substrate consists of periodically poled lithium niobate the band gap shifts toward lower frequency and the width of the bandgap decrease at the same time.?4?The propagation of surface acoustic waves in two dimensional phononic crystal with giant magnetostrictive material as scatterer is investigated.Based on the L-Z model,by using finite element method,we calculated two types of phononic crystal:the first type is made of Terfenol-D cylinders embedded in the substrate and the second type is made of Terfenol-D pillars attached on the surface of the substrate.Research results show that with specific magnetic field a new bandgap will open for the first type of phononic crystal.For the second type of phononic crystal,with the magnetic field increase the higher order bandgaps have significant change.?5?We study electromechanical fields in a piezoelectric semiconductor rod nonuniformly doped with impurities producing holes and electrons.The phenomenological theory of piezoelectric semiconductors consisting of the equations of piezoelectricity and the conservation of charge for holes and electrons is used,which was reduced to a one-dimensional model for thin rods.In this paper the one-dimensional theory is linearized for low electric potential or voltage.Solutions from the linearized one-dimensional equations are obtained for three specific doping profiles:piecewise linear doping with fundamentally important applications in PN junctions,sinusoidal doping,and continuously graded doping.Various electromechanical fields produced by the doping are calculated and examined.The results are fundamental to piezoelectric semiconductor devices or piezotronics.We investigate the extension of a piezoelectric semiconductor fiber with consideration of the electrical nonlinearity due to the drift current which is the product of the unknown electric field and the unknown carrier concentration.The analysis is based on a one-dimensional model.A perturbation analysis is performed.Numerical results show that when the axial load is small the linear and nonlinear solutions are essentially the same.As the axial load increases,the linear and nonlinear solutions gradually become apart.The nonlinear solution is valid for a larger range of load and establishes the range of applicability of the linear solution.It is also found that while the electromechanical fields predicted by the linear solution are either symmetric or antisymmetric about the middle of the rod,the fields described by the nonlinear solution lose these symmetry or antisymmetry.