SAW device applications of longitudinal leaky surface waves on lithium tetraborate

The possibility of high frequency SAW device applications of longitudinal leaky surface waves (LLSW) on lithium tetraborate (Li₂ B₄O₇; LBO) is investigated in this paper. An electrical equivalent circuit model (ECM) is extended in order to consider effects of bulk wave scattering for the LLSWs. The equivalent circuit parameters used in the extended ECM for designing the LLSW devices are directly determined from numerically calculated dispersion curves. For applications of the LLSW, high frequency SAW filters on LBO with the Euler angles (0°, 47.3°, 90°) are demonstrated. As examples of the high frequency devices, 1.5 GHz and 1.2 GHz SAW filters using the mode are designed by using the extended ECM, and fabricated by using conventional patterning processes. One is for the filter of the global positioning system (GPS), another is for the 1.2 GHz band data transmission radio system in Japan. As a result, low loss SAW filters can be obtained easily without submicron fabrication techniques by using the LLSWs on LBO. Furthermore, the frequency response calculated by the extended ECM are in a good agreement with the experiments     

Propagation of longitudinal leaky surface waves under periodic SiO (2)/Al structure on Li(2)B(4)O(7) substrate

The properties of longitudinal leaky surface waves (LLSW) under a periodic SiO(2)/Al structure on Li(2)B(4)O (7) (LBO) substrate, were investigated theoretically and experimentally, in order to improve the high propagation losses of LLSWs under a periodic Al grating with the normalized thickness over 2%. In the theoretical analysis, the previously presented method based on the boundary integral equations for the periodic metal grating structure on the substrate was extended to include the dielectric layer. In the experiments, devices with Al electrodes recessed into a SiO(2) groove on LBO were fabricated, and the propagation losses of them were estimated. As a result, it was shown that, when the surface of the structure was flattened, the propagation losses were sufficiently low and the first Bragg stopband width decreased.     

Theoretical study of surface acoustic waves in (n11) GaAs-cuts

The propagation characteristics of true and leaky or pseudo surface acoustic waves (TSAW and PSAW=LSAW) on (n11) GaAs-cuts, n=1, 2, 3 and 4, are theoretically calculated as a function of propagation direction. They include phase velocity (V), electromechanical coupling constant (K(2)), and attenuation factor (alpha) of wave propagation on a metallized surface. The results show that PSAW mode velocities are significantly higher than corresponding TSAW velocities, and for certain propagation directions the attenuation factor is extremely small (10(-5) dB/lambda). Highly coupled PSAW modes exist for propagation directions where the TSAW are very poorly coupled. For certain isolated directions, attenuation of the wave is null (alpha=0), PSAW becoming a non-leaky SAW with partial polarization. And in this case the corresponding TSAW are decoupled from the surface electric excitation. Analysis of relations between various modes (TSAW, PSAW and SSBW, surface skimming bulk wave) is made with the help of the effective surface permittivity function and the generalized slowness diagram. A coupling constant definition different from the usual 2DeltaV/V is used, its validity and application conditions are discussed.     

A method of determining acoustical physical constants for piezoelectric materials by line-focus-beam acoustic microscopy

We developed a new method of determining acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of piezoelectric materials with high accuracy. This method acquires velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface, measured by line-focus-beam (LFB) acoustic microscopy, and bulk velocities of longitudinal and shear waves, measured with planewave transducers replacing the LFB device in the same system, together with the dielectric constants and density measured independently, for a small number of specimens. For LiNbO₃ and LiTaO₃ crystals, we demonstrated that we could accurately determine the constants by choosing proper propagation directions of LSAWs and bulk waves for three principal X-, Y-, and Z-cut specimens and one rotated Y-cut specimen [(104) plate for LiNbO₃ and (012) plate for LiTaO₃]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities     

SH wave propagation in piezoelectric coupled plates

The propagation of shear horizontal (SH) wave in a piezoelectric coupled plate is investigated in this paper. Full account is taken of the piezoelectric coupling effect to the isotropic metal core in the mathematical model. One of the applications of this research is in the damage detection of the host metal structure from the wave propagation signal excited by the piezoelectric layer which is surface bonded on the surface of a metal core. This research is distinct from the previous works on SH propagation in piezoelectric structures because the piezoelectric materials were used as the core structure in the previous studies, and the potential of the studies was mainly on time-delay devices. The dispersive characteristics and the mode shapes of the transverse displacement and the electric potential of the piezoelectric layer are theoretically derived. The results from numerical simulations show that the phase velocity of the plate structure tends to the bulk shear wave velocity of the host metal core at high wave number when the shear wave velocity of host plate is larger than that of PZT bonded on it. Furthermore, there are three asymptotic solutions of wave propagation when the shear wave velocity of the host plate is smaller than that of PZT. The mode shape of the electric potential of the piezoelectric layer changes from the quadratic shape at lower wave number and with thinner piezoelectric layer to the shape with more zero nodes at higher wave number and with thicker piezoelectric layer. These findings are significant in the application of wave propagation in piezoelectric coupled structures     

识别薄板中兰姆波和体波的阶梯板方法

斜探头在某些频率下激励出的兰姆波,其群速度与体波的传播速度相近,所以通过判断传播速度不易区分出兰姆波和体波。通过数值模拟和实验,分别研究了激励频率为2 MHz的纵波和S0模态兰姆波在阶梯板上的反射特性,发现:在阶梯板上入射S0模态兰姆波时,有反射回波;而入射纵波时,无反射回波。基于这种反射特性的差别,提出了一种利用阶梯板区别薄板中兰姆波和体波的方法,该方法可用于确认探头的激励特性。     

Suppression of the leaky SAW attenuation with heavy mechanical loading

We discuss effects on the propagation of surface acoustic waves (SAW) due to heavy mass loading on Y-cut lithium niobate and lithium tantalate substrates. An abrupt reduction in the leaky-SAW (LSAW) attenuation is observed in the measured admittance of a long resonator test structure on 64 degrees -YX-cut lithium niobate for aluminum electrodes of thickness h/lambda(0) beyond 9-10%. This experimental fact is explained theoretically as the slowing down of the leaky wave below the velocity of the slow shear surface-skimming bulk wave (SSBW), such that energy dissipation into bulk-wave emission becomes inhibited. An infinite transducer structure is modeled using the periodic Green's function and the boundary-element method (BEM); the computed theoretical properties well explain for the experimental findings. The model is further employed to quantify the leaky surface-wave attenuation characteristics as functions of the crystal-cut angle and the thickness of the electrodes. The resonance and antiresonance frequencies and the corresponding Q values are investigated to facilitate the selection of crystal cuts and electrode thicknesses. The transformation of the leaky SAW into a SAW-type nonleaky wave is also predicted to occur for gold electrodes, with considerably thinner finger structures.     

Propagation of longitudinal leaky surface waves under periodic metal grating structure on lithium tetraborate

The dispersion properties of longitudinal leaky surface waves propagating under the periodic Al strip grating on lithium tetraborate (Li(2)B(4)O(7); LBO) are described theoretically and experimentally for applications of the mode to high frequency SAW devices. A theoretical method developed here is based on Floquet's theorem using space harmonics as an orthogonal function set and real boundary integral equations derived from the method of weighted residuals for a period of each region, i.e., substrate, metal, and free space. The boundary integral equations are solved by using the Galerkin procedure. The periodic strip gratings with both single-electrodes and double-electrodes are investigated, considering the convergency of the numerical computation for the number of the space harmonics. As a result, the propagation loss for shorted gratings was found to be relatively low in the thickness range of the Al strip below about 1% for the single-electrodes and 2% for the double-electrodes, although it greatly increases for a thickness over 2% for the single-electrodes and 3% for the double-electrodes.     

Acoustic waves in bounded anisotropic media: theorems, estimations, and computations

This paper discusses some basic achievements in theoretical studies on acoustic wave propagation along boundaries in anisotropic solids. In particular, the following issues are reviewed: existence theorems for subsonic surface and interface waves, leaky surface acoustic waves (SAW) and their relation to "supersonic" SAWs and fast exceptional bulk waves, the resonance reflection of bulk waves in the vicinity of leaky wave branches. General conclusions are illustrated by numerical examples.     

Acoustic loss mechanisms in leaky SAW resonators on lithiumtantalate

Discusses acoustic losses in synchronous leaky surface acoustic wave (LSAW) resonators on rotated Y-cut lithium tantalate (LiTaO₃ ) substrates. Laser probe measurements and theoretical models are employed to identify and characterize the radiation of leaky waves into the busbars of the resonator and the excitation of bulk acoustic waves. Escaping LSAWs lead to a significant increase in the conductance, typically occurring in the vicinity of the resonance and in the stopband, but they do not explain the experimentally observed deterioration of the electrical response at the antiresonance. At frequencies above the stopband, the generation of fast shear bulk acoustic waves is the dominant loss mechanism     

An alternative approach of acousto-ultrasonic technique formonitoring material anisotropy of fiber-reinforced composites

An alternative acousto-ultrasonic (AU) technique has been developed for nondestructive evaluation (NDE) of fiber-reinforced composites. The technique measures the time of flight (TOF) of AU waves, instead of the stress wave factor, by two low-frequency (0.5 MHz) transducers and relates TOF to material properties and fiber orientation. As the transducer separation increases, the measured time-domain AU signals clearly separate into two groups, since the excitation is under the first critical frequency, which correspond to the first two fundamental modes of the Lamb waves. One is an antisymmetric mode with slower propagation velocity and is highly dispersive, while the other is a symmetric mode with faster propagation velocity, which is very close to that of the longitudinal bulk wave, and is nearly nondispersive. The phase velocity in the composites can be accurately determined from the slopes of the TOF curves, and depends strongly on the azimuthal angle, frequency, and plate thickness. If the wave propagates away from the fiber direction, a slower but more attenuated wave is observed. Phase-velocity curves in azimuthal angles were obtained for E-glass/polyester, S-2-glass/epoxy, and Kevlar 49 composites. The theoretical solutions, for the longitudinal bulk wave and Lamb wave, are obtained by solving an eigenproblem once the material mechanical properties are defined. Good agreement is obtained between the measurements and the theoretical calculations     

Ray representation of longitudinal lateral waves in acoustic microscopy

For object materials having a large enough Rayleigh velocity, the V(z) (where V is the output voltage and z is the defocus distance) variation is mainly due to interference between the fields of the geometrically reflected wave and the leaky Rayleigh wave. However, for materials, such as organic compounds, having a low Rayleigh velocity, the leaky Rayleigh wave is not excited. For this case, the lateral wave resulting from propagation along the surface of the longitudinal wave plays a significant role in determining the V(z) dependence. The effect of the lateral wave contribution on V(z) is studied. Ray optics is to derive an expression giving the influence of the longitudinal lateral wave. Good agreement is found between the theory and measurements for z not near zero. Because of the ease with which the longitudinal wave velocity can be obtained from V(z), one can conveniently determine the elastic constant c(11 ) of isotropic materials using the acoustic microscope.     

On the application of Biot's theory to acoustic wave propagation in snow

A fluid-saturated, elastic, porous media model is used to describe acoustic wave propagation in snow. This model predicts the existence of two dilatational waves and a shear wave. In homogeneous, isotropic snow the two dilatational waves are uncoupled from one another but involve coupled motion between the interstitial air and ice skeleton. Dilatational waves of the first kind and shear waves are slightly dispersive and attenuated with distance. Dilatational waves of the second kind are strongly dispersive and highly attenuated. The model also predicts that the wave impedance for snow is close to that of air and that snow strongly absorbs acoustic wave energy.Available experimental phase velocity, impedance and attenuation data support the calculated results. Phase velocity measurements indicate three identifiable categories: fast dilatational waves (phase velocity ? 500 m/s), slow dilatational waves (phase velocity < 500 m/s) and shear waves. Wave impedance and attenuation measurements illustrate the low impedance, highly absorbing characteristics of snow. Additional impedance, attenuation and phase velocity data are required to further test and improve the model.     

Plate mode propagation losses in solidly mounted resonators

This paper investigates the acoustic losses of propagating eigenmodes through the acoustic mirror of a solidly mounted resonator (SMR) to clarify how resonator properties are influenced by reflection coefficients for the thickness shear (TS) wave as well as that for the thickness extensional (TE) wave. To this end, we analyze the effective acoustic admittance for several test structures with different mirror properties. Leaky modes are distinguished from plate-like modes and the propagation losses are quantified by calculating mode quality factors. The dependence of the propagation properties of leaky eigenmodes is compared with the mirror properties in terms of bulk wave transmission coefficients obtained by the one-dimensional Mason?s model. It is shown that the TE-like main mode couples with TS-like spurious modes, which then influence the leaky loss of the main mode as well. The coupling strength is strongly frequency-dependent and drastically changes with the mirror design. This result explains previous experimental results reported on SMR design.     

Propagation of transverse bulk and surface acoustic waves in LiNbO (3) variable time-delay devices

Experimental measurements are reported on voltage-controlled acoustic time-delay lines operating at 1 GHz in the nearly pure shear-horizontal (S-H) mode in 38 degrees rotated Y-cut LiNbO(3). The high-acoustic velocity (4800 m/s) in conjunction with the large electroacoustic effect exhibited by this orientation allows high-frequency operation and optimum time-delay tuning sensitivity with a planar, single surface, device geometry. The authors demonstrate fractional time delay of 0.3x10(-6) V(-1 ) for surface electrodes that produce an in-plane E-field. However, the simultaneous excitation and propagation of both a leaky surface-acoustic wave (LSAW) and surface skimming bulk wave (SSBW), both as (nearly pure) S-H waves in these devices, seriously restricts the extent to which it is possible to maximize the time delay modulation sensitivity by reducing electrode gap spacing as done in similar SAW devices. The LSAW and surface-skimming body wave (SSBW) propagate at nearly the same velocity on a free surface, and perturbation of their velocity and relative attenuation rates by surface electrodes causes pronounced interference effects between the two modes for some device geometries.     

A simple closed form dispersion equation for shear types of surface waves propagating in periodic structures

A closed-form dispersion relation is found for shear surface acoustic waves (SAWs), namely, Bleustein-Gulyaev waves (BGWs), surface transverse waves (STWs), and leaky waves, propagating in periodic structures in the frequency range corresponding to the Bragg stopband. Changes in the spatial structure of the waves mutually reflecting on the grating as well as bulk wave scattering are considered. A comparison with numerically obtained dispersion curves for leaky waves on 36-LiTaO (3) shows good agreement.     

The power flow angle of acoustic waves in thin piezoelectric plates

The curves of slowness and power flow angle (PFA) of quasi-antisymmetric (A0) and quasi-symmetric (S0) Lamb waves as well as quasi-shear-horizontal (SH0) acoustic waves in thin plates of lithium niobate and potassium niobate of X-,Y-, and Z-cuts for various propagation directions and the influence of electrical shorting of one plate surface on these curves and PFA have been theoretically investigated. It has been found that the group velocity of such waves does not coincide with the phase velocity for the most directions of propagation. It has been also shown that S0 and SH0 wave are characterized by record high values of PFA and its change due to electrical shorting of the plate surface in comparison with surface and bulk acoustic waves in the same material. The most interesting results have been verified by experiment. As a whole, the results obtained may be useful for development of various devices for signal processing, for example, electrically controlled acoustic switchers.     

Longitudinal critical angle singularities and their effect on V(Z)of the line-focus-beam acoustic microscope

In order to investigate a mechanism which causes a velocity difference between the longitudinal wave and leaky surface skimming compressional wave (LSSCW) observed in a line-focus-beam acoustic microscope, the analytic property of an acoustic reflection coefficient and its effect on a V(z) analysis were studied. A pole hidden in the unphysical Riemann sheet close to the longitudinal branch point is found to be responsible for the abrupt phase change at the longitudinal critical angle. This, together with an effect of a dominant Rayleigh wave pole, affects the V(z) measurement of the LSSCW. A method to estimate the longitudinal and shear wave velocities is discussed     

The Thermal and Elastic Properties of Zinc Oxide-Based Ceramics at High Temperatures

The thermal conductivity, thermal expansion coefficient (TEC), and the propagation velocity of longitudinal and transverse ultrasonic waves in ZnO-based ceramics are investigated in the temperature range from 300 to 1200 K with a porosity from 1.5 to 21%. The Young, shear, and bulk moduli and the Poisson ratio are calculated from the data on the propagation velocities of ultrasonic waves. Formulas are suggested to calculate the investigated parameters as a function of temperature and porosity.