Phase Velocity Of Acoustic Waves Research Articles

Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Characterization of Homogeneity of Langasite Wafers Using Bulk-Wave Measurement

A characterization technique for homogeneity of piezoelectric wafers utilizing bulk-wave measurement was established, and measurements were carried out on langasite (LGS: La3Ga5SiO14) wafers fabricated in our laboratory. The technique is capable of predicting the homogeneity of the phase velocity of a surface acoustic wave (SAW), which determines the deviation of the central frequencies of SAW devices, by measuring the distribution of the frequency constant of thickness-shear mode resonance. The frequency constant, measured by a combination of resonance frequency measurement and thickness measurement in a single wafer, yields a relative accuracy of about ±55 ppm. The distribution of the frequency constant is consistent with that of the phase velocity of SAW obtained by fabricating and measuring SAW devices. The method is both rapid and nondestructive, thus is suitable for routine characterization in the production of piezoelectric wafers. Characterization of langasite wafers using this technique correlated inhomogeneity to the growth conditions of the crystal.

Brillouin light scattering from surface phonons in hexagonal and cubic boron nitride films

Phase velocities of surface acoustic waves in several boron nitride films were investigated by Brillouin light scattering. In the case of films with a predominantly hexagonal crystal structure, grown under conditions close to the nucleation threshold of cubic BN, four independent elastic constants have been determined from the dispersion of the Rayleigh and the first Sezawa mode. The large elastic anisotropy of up to c11/c33=0.1 is attributed to a pronounced texture with the c-axes of the crystallites parallel to the film plane. In the case of cubic BN films, the dispersion of the Rayleigh wave provides evidence for the existence of a more compliant layer at the substrate-film interface. The observed broadening of the Rayleigh mode is identified to be caused by the film morphology.

Propagation Characteristics of Ion Acoustic Waves in an Ar/SF6Plasma

There has been a great interest in wave phenomena in a negative ion plasma. Usually sulfur hexafluoride (SF6) gas is used to produce a negative ion plasma, because SF6 gas has a large electron attachment cross-section for electron energies less than 1 eV. Furthermore, the negative ion concentration, r, can be controlled by partial pressure of the gas. However, the introduction of SF6 gas produces several ion species in discharge plasmas. In fact, there are many discussions on ion species in discharge plasmas. In the most of the experiment on the ion acoustic wave in a negative ion plasma, only F− or only SF6 was assumed as the negative ion species. 1, 2) On the other hand, in many experimental studies on the ion acoustic soliton in Ar/SF6 plasmas it was assumed that the light components, that is, F− contributes more effectively to the ion acoustic wave, 4) and the results of these experiments were consistent with this assumption. Cooney et al., however, obtained the soliton velocity that shifted gradually from the theoretical value for Ar and F− to that for Ar and SF6 as r increased. 5) Although they mentioned that this was perhaps due to an increase in the relative concentration of SF6 to F − as r increased, there was no evidence for the phenomenon. On the other hand, it was recently observed by the mass spectrometry that the relative concentration of SF6 to F− increases with increasing r in the double plasma (DP) device. Thus, in order to confirm whether the variations in the relative concentrations of ion species affect the propagation characteristics of the ion acoustic wave or not, we detected dominant ion species in an Ar/SF6 plasma with a quadrupole mass spectrometer (QMS) and measured the phase velocity of the ion acoustic wave for different values of r. While much work on the ion acoustic wave in two-positive ion plasmas have been done, 8) the study on the ion acoustic wave in two-negative ion plasmas hardly has been carried out. The purpose of this paper is to observe the dependence of the ion acoustic wave on ion species, especially negative ion species. The experiments were carried out using a multidipole DP device. The diameter and the length of the device

Precise velocity measurement of surface acoustic waves on a bearing ball

Using the photoacoustic effect of interference fringes scanned at the phase velocity of surface acoustic waves (SAW), we excited tone bursts of SAW with a center frequency of around 30 MHz on a 8 mm φ steel bearing ball. A surprisingly large number (around 20 turns) of round-trip propagations was observed. The time interval between the SAW at the first and the twelfth turn was as large as 93 μs, however it could be determined with a 2 ns resolution since an exact overlapping of the two wave forms was possible. Thus, we achieved a very high resolution of 0.002% in the velocity measurement, and a velocity change of 2 m/s due to the deposition of a 50-nm-thick Ag film was easily detected. Because of its noncontact nature, this method would be useful for nondestructive evaluation of bearing balls.

Propagation of acoustic waves in disordered flows composed of many vortices. II. Examples

The theory of acoustic wave propagation through systems of many vortices randomly distributed, developed in Part I, is applied to specific examples in two and three dimensions. Two classes of vortex blobs are considered; vortices with an axisymmetric distribution of vorticity, such as disks or tubes, and vortices with a nonvanishing dipolar moment such as dipoles or rings. The index of refraction and attenuation length are numerically computed as a function of wavelength for various values of vortex parameters. The asymptotic behavior of the dispersion relation for very short and very long wavelengths is also derived analytically. At short wavelengths λ the attenuation length scales as λ−2 in all examples studied. At long wavelengths the scaling depends on the lowest nonvanishing multipole moment of the vorticity distribution; say, for vortex rings, it is λ−4 as in Thomson scattering. For an ideal gas, the phase velocity of the coherent acoustic wave is greater than in the undisturbed flow for long wavelengths and smaller than in the undisturbed flow for short wavelengths. This appears to be a robust feature. When properly normalized, the attenuation length does not depend very strongly on the ratio l/ε, where l is a vortex length scale and ε the thickness of the vorticity bearing region, both in two and three dimensions. The effective index of refraction, however, does depend on this ratio. The conditions of applicability of the results, which rely on a Born approximation scheme, are also determined. The expressions obtained in this paper for the scattering cross sections are used to discuss the properties of sound localization in two dimensional disordered flows.

Reversal Phenomenon in Phase Velocity of Surface Acoustic Waves on Free and Metallized Surfaces by Proton-Exchange

Reversal Phenomenon in Phase Velocity of Surface Acoustic Waves on Free and Metallized Surfaces by Proton-Exchange

Group velocity of acoustic waves in strongly scattering media: Dependence on the volume fraction of scatterers

We study, both experimentally and theoretically, the ballistic propagation of ultrasonic wave pulses through a random strongly scattering medium as a function of the volume fraction of the scatterers. The scattering medium consists of a liquid suspension of monodisperse glass beads, whose concentration is varied by controlling the upward flow of the liquid in a fluidized bed. At intermediate frequencies, where the acoustic wavelength \ensuremath{\lambda} is comparable to the size of the glass bead scatterers, very strong scattering is observed, with the scattering mean free path reaching values as low as \ensuremath{\lambda}/2. At high volume fractions of scatterers, the scattering results in pronounced dispersion, as demonstrated experimentally by the strong frequency dependence found in both the phase and group velocities. However, as the volume fraction is lowered, the dispersion is substantially reduced, in marked contrast to recent predictions for electromagnetic waves. Our experimental results are explained quantitatively by a theoretical model, based on a spectral function approach, that accounts for the renormalization of the scattering within the medium, an effect that is greatest when the concentration of scatterers is largest. The mechanisms underlying the frequency dependence of the velocities and their dependence on volume fraction are further elucidated by examining the ultrasonic energy density, both inside the scatterers and in the surrounding fluid. This allows us to show that the velocities are substantially slowed down both by (i) resonant scattering from the glass spheres, where energy is trapped within the solid scatterers, and by (ii) tortuosity effects, where the wave energy is largely confined to the tortuous fluid pathways. These results demonstrate convincingly why the phase and group velocities of acoustic waves vary strongly with frequency at high volume fractions of scatterers, but only show weak dispersive effects at low volume fractions. Furthermore, our microscopic picture of the dispersion gives a simple physical explanation of why its volume fraction dependence is opposite to that expected for light and other electromagnetic waves, where the velocity inside the scatterers is normally less than in the surrounding medium.

Anomalies of attenuation and phase velocity of surface acoustic waves in thin films in the vicinity of

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa2Cu3O YBa2Cu3O7‒δ films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa2Cu3O7‒δ thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned.

Surface acoustic wave technology for poisson ratio sensors

The phase velocity of surface acoustic waves (SAWs) propagating in structures consisting of an isotropic substrate and a thin isotropic layer is considered theoretically. The dependence of the change in the phase velocity on the Poisson ratio of a thin layer is presented. It is shown that the Poisson ratio of the layer may be determined from the results of the phase-velocity measurements. An appropriate analytic expression is given. Numerical simulations for LiNbO 3 and SiO 2 substrates with thin Al and W layers demonstrate a change of SAW velocity by (5–8)% under variation of the Poisson ratio of the layer from 0.1 to 0.4.

Nanoscale determination of phase velocity by scanning acoustic force microscopy

We measured the phase velocity of surface acoustic waves (SAW's) with a scanning acoustic force microscope (SAFM) and achieved a maximum lateral resolution of 19.9 nm. The phase measurement of high-frequency waves with a slowly responding SAFM cantilever was perfomed by frequency mixing at its nonlinear force curve. For Au layers of different thicknesses the SAW dispersion was studied on a lateral scale of 200 nm and compared to calculated data.

Line-focus acoustic microscopy measurements of acoustic properties of LiTaO3 crystal plates with an inversion layer

Inversion layers of 2 μm and 30 μm thickness were formed at the negative dipole-end (−c) face of a Z-cut LiTaO3 crystal plate by proton exchange followed by heat treatment. The phase velocities of leaky surface acoustic waves (LSAWs) on both the positive (+c) and negative (−c) faces of the plate were measured by line-focus acoustic microscopy versus the angle of propagation, at a frequency of 225 MHz, and the results were compared with theoretical predictions. It is shown that for an inversion layer of 30 μm thickness the phase velocities of LSAWs on the +c and −c faces are the same, and almost equal to the velocities of the virgin plate. This suggests that the material constants, which are decreased due to the proton exchange, are restored to the original values by the heat treatment.

Brillouin spectroscopy on doped SmS

SmS becomes intermediate valent at an applied pressure of about 6.5 kbar. On the other hand, SmS doped with La or Tm is already intermediate valent at normal pressure and room temperature. The doping atoms (depending on their concentration) create new occupied states in the SmS gap which lead to the typical hybridised 4f i /4f i−1 −5d 1-states. The La-cation is always trivalent in LaS, whereas the Tm-cation has a valence between 2 and 3. Therefore, we expect to see a difference in the intermediate valent behaviour. A strong evidence of intermediate valence is a negative C 12 and a negative Poisson's ratio. Using high-resolution Brillouin-spectroscopy we measured the phase velocity of the surface acoustic waves in the (100)-plane of Sm 1− x La x S and Sm 1− x Tm x S. Applying a standard fit-algorithm we calculated all three elastic constants ( C 11, C 12 and C 44) from the angular dispersion relation. To get more reliable results the compression-moduli also have been determined and linked to the elastic constants C 11 and C 12 in the fit model.

Elastic and magnetic properties of Co/Cu multilayers studied by Brillouin spectroscopy

The Brillouin light scattering technique has been used to investigate the elastic and magnetic properties of periodic Co/Cu multilayers deposited by rf sputtering on sapphire substrates. Using measurements of the phase velocity of the Rayleigh surface acoustic wave in p-p Brillouin spectra, we have evaluated the effective elastic constant c 44. The values obtained for multilayer periods larger than 2 nm are in good agreement with those calculated from the elastic constants of the bulk materials. For lower periods (0.6 and 1.2 nm) the measured elastic constants exhibit marked decreases caused by interface disorder. Concerning the magnetic properties, in the case of high-period specimens, the detection of both surface and bulk spin waves in the p-s spectra has allowed us to determine the magnetic parameters of the multilayers.

Theoretical and experimental investigation for LFB acoustic microscopy characterization of proton-exchanged LiTaO3 optical waveguides

Line-focus-beam (LFB) acoustic microscopy has been recognized as a new analytical technique of quantitative material characterization, by measuring the phase velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface. Recently, application of this ultrasonic technique has been demonstrated successfully for characterization of proton-exchanged layers for LiNbO3 and LiTaO3 optical waveguides and for evaluation of the device fabrication processes. It is necessary and important to calculate LSAW propagation characteristics, viz., velocity and attenuation, for interpretation of experimental results and for evaluation of physical properties, such as optical and elastic properties, in proton-exchanged layers. In this paper, LSAW propagation characteristics for the structural model of water/piezoelectric layer/piezoelectric substrate are analyzed. The propagation characteristics are interpreted with calculations of spatial distributions of mechanical displacements and electric potential. Specimens of Z-cut LiTaO3 substrates, with a layer thickness of 0.5 μm, were prepared. Experimental data of LSAW propagation characteristics, measured in the frequency range 100 to 300 MHz, are discussed in detail comparing with the calculated results.

Dependence on Waveguide Width of Elastic Convolver Efficiency

Theoretical and experimental results are reported for the optimum width of a (Δv/v) waveguide which has high convolver efficiency. Instead of controlling the waveguide width in the micron range which is equivalent to varying the wave guide effect of a (Δv/v) waveguide, we applied an Al2O3 thin film to an Al waveguide in order to vary its waveguide effect by increasing the phase velocity of surface acoustic waves. The optimum width of the waveguide for convolver efficiency is close to one input wavelength.

プラズマ中のイオン音波測定システムの開発

We developed the measuring system for the ion acoustic wave in the plasma. The phase velocity of the ion acoustic wave is obtained by phase delay between the excitation signals and the received signals in the glow discharge with parallel electrodes.

Scanning acoustic microscopy and its applications to material characterization

The scanning acoustic microscope is a powerful new tool for the study of the physical properties of materials and has been successfully used for imaging interior structures and for nondestructive evaluation in materials science and biology. Its principles of operation, resolution, penetration ability, and contrast mechanisms are simply described in this paper. Recent progress in the application of acoustic microscopy to material characterization in solid materials is summarized. The experimental elastic microanalysis of bulk materials is carried out by measuring $V(z)$, which includes examining the reflectance function of solid material, measuring the phase velocity and attenuation of leaky surface acoustic waves at the liquid-specimen boundary, and determining the elastic constants of the material. The layer thickness and mechanical properties of layered solids are studied by examining the dispersion properties of surface acoustic waves. A knowledge of the propagation properties of acoustic waves on the surface of materials is essential for understanding the contrast mechanisms and quantitative measurements in acoustic microscopy; these propagation properties are thus also briefly described in this paper. Finally, further developments of the scanning acoustic microscope aimed at improving its performance for quantitative evaluation are presented. These could expand the scope of the acoustic microscope as a diagnostic tool in many areas of science and technology.

A method of measuring moving characteristics of precision mechanical‐translation stages using ultrasonic plane waves and applying it to a line‐focus‐beam acoustic microscopy system

AbstractA method of measuring moving characteristics of precision mechanical‐translation stages using ultrasonic plane waves is presented. The moving characteristics are obtained by analyzing the phase variation of ultrasonic waves reflected from a reflector set on the translation stage. The resolution is 4 nm at an ultrasonic frequency of 225 MHz; higher resolution is expected at higher frequencies. The method is demonstrated for a line‐focus‐beam (LFB) acoustic microscopy system. Measurement errors of the phase velocity of leaky surface acoustic waves, caused by the positioning errors of the z‐stage, are investigated both theoretically and experimentally. The linear positioning and straightness errors of the z‐stage are estimated at 225 MHz and these effects on velocity measurements are successfully removed using the measured moving characteristics.

Propagation of surface acoustic waves across the randomly rough surface of an anisotropic elastic medium

The propagation of surface acoustic waves across the randomly rough surface of an anisotropic solid is studied. In the small roughness limit the dispersion relation for such waves is derived. The frequency dependences of the dispersion and attenuation of surface acoustic waves are obtained for the case when the wavelength λ is long compared to the transverse correlation length a of the roughness. In this limit the numerical curves for the relative change of the phase velocity of surface acoustic waves versus the angle of propagation are calculated for the various cuts of anisotropic media. Crystals of cubic, tetragonal, trigonal, hexagonal, and orthorhombic symmetry, adopted in applications, are considered. The effects of piezoelectricity are taken into account through the approach of ‘‘partially stiffened elastic constants.’’