Complex analysis of reflection coefficient for the characterization of layered materials using a line-focus-beam acoustic microscope

Line-focus-beam ultrasonic material property characterization system enables accurate measurements on the phase velocities of leaky surface acoustic waves(LSAW) or rayleigh-type leaky surface acoustic waves.But in view of the lens of the conventional acoustic microscopy is expensive and difficult to produce,new method of design and fabrication of two line-focus lens-less polyvinylidene fluoride(PVDF) transducers is presented for acoustic microscopy system utilizing a 40 ?m thick piezoelectric PVDF film to construct the active element.The transducers introduced here centre around 5 MHz and 11 MHz respectively.Different from high cost of equipments and measuring the oscillation period on an operation of tone-burst mode,an improved V(f,z) method based on tow-dimensional Fourier transform is developed for the broadband measurements.A defocusing system for measuring surface acoustic waves is established and a non-contact immersion experiment of a polished fused quartz is carried out.The LSAW velocity extracted by V(f,z) analytical technique is compared with the theoretical one and they both are in good agreement.

Surface acoustic wave (SAW) velocity and attenuation measurements have been performed on high-temperature superconducting YBa2Cu3O7−δ (YBCO) thin films (deposited on nonpiezoelectric substrates), and YBCO single crystals. A “bridge” technique is employed in which piezoelectric substrates, patterned with interdigital transducers, are bonded to the sample under investigation. The temperature dependence of 168 MHz SAW attenuation in a single crystal near its superconducting transition has been measured in applied magnetic fields up to 1.6 T. Features are seen above the superconducting transition temperature which may be indicative of a transition in the electric polarization of the material. A field-dependent attenuation peak, seen in the superconducting state, is discussed in terms of the theory of thermally activated vortex motion.

We discussed a method of evaluating surface compressive layer of tempered glasses by the ultrasonic microspectroscopy (UMS) technology. Chemically tempered glass specimens and the non-tempered specimen were prepared. Velocities of leaky surface acoustic wave (LSAW) and leaky surface skimming compressional wave (LSSCW) were measured by the line-focus-beam ultrasonic material characterization system. LSAW and LSSCW Velocities increased drastically caused by the surface compressive stress. Velocity differences caused by the difference of chemical strengthening process conditions were successfully detected. Resolution to depth of compressive stress layer by the LSAW velocity measurement was ±0.11 µm. Frequency characteristics of LSAW velocity have possibility of evaluating elastic properties distributions in the depth direction of tempered layer. Velocities changes were mainly caused by elastic constants changes by the bulk acoustic properties measurements.

A 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 thez‐stage, are investigated both theoretically and experimentally. The linear positioning and straightness errors of thez‐stage are estimated at 225 MHz and these effects on velocity measurements are successfully removed using the measured moving characteristics.

This paper reports the first determination of the phase velocity of surface acoustic waves (SAW) on the nanometer scale. With a scanning acoustic force microscope (SAFM) a maximum lateral resolution of 19.9 nm has been achieved. This is almost two orders of magnitude better than the resolution of standard quantitative acoustic microscopy. The key of measuring the phase of high frequency signals with a slowly responding SAFM cantilever is frequency mixing at its nonlinear force curve. For demonstrating its abilities SAW dispersion was studied on Au layers of different thicknesses by SAFM over a lateral distance down to 200 nm.

An acoustic microscope was used for determining the frequency dependence of surface acoustic wave (SAW) velocity on a specimen whose silicon single-crystal surface was machined under various conditions. Consequently, thickness of the damaged layers could be estimated from the curvature points of frequency dispersion curves of the SAW velocity. It was revealed that thicknesses of the damaged layers can be estimated through rough approximation by about one-half the wavelength determined by the frequency at curvature points. From specimens possessing two damaged layers, frequency dispersion curves with two curvature lines can be obtained. From the curvature point at high frequencies the thickness of the top damaged layer can be determined. On the other hand, from the curvature point at low frequencies, the thickness of the inner damaged layer can also be determined. By choosing an acoustic lens as the condition for exciting SAWs, images can be observed while varying the frequency. From observation results obtained with this method, the distribution in the depth direction can be clarified.

The velocities of longitudinal, transverse and leaky surface skimming compressional wave (LSSCW) of polyvinylchloride composite materials were measured as a function of the glass bead content. The velocities of longitudinal and transverse waves for composite materials increased with the concentration of glass beads, but the velocity of LSSCW for polyvinylchloride matrix did not change. The partial specific volume and specific compressibility of glass beads dispersed in polyvinylchloride were evaluated from sound velocity and density data. These results suggest that the boundary layer has a thickness of 1–5 micrometers, and is more rigid than the polyvinyl chloride (PVC) matrix.

The acoustoelastic effect refers to the fact that elastic wave velocities vary with stress. Measurements of the change in stress induced velocity yield information which leads to the determination of stresses. Some work has been done to explore the possibility of using ultrasonic waves, including bulk longitudinal and shear waves as well as surface waves, for the nondestructive evaluation of stresses. This paper focuses on Rayleigh surface waves, which have the advantage of detecting both surface stresses and stress gradients. Piezoelectric or electromagnetic acoustic transducers have been used for the application of Rayleigh waves in the acoustoelastic measurement of stress. The size of these transducers and their fixtures limit the distance between transducers. Sharp edge wedges are usually bound to the pick-up piezoelectric transducers, known as the Rayleigh or surface wave device (SWD), to provide a better spatial resolution when the stress field is not uniform. The smallest distance reported is 11 mm. On the other hand, acoustic microscopes (AM) can measure localized stress for a very small area, from 30 {micro}m to 2 mm. Both SWD and AM method may lack the agility required for general applications. Laser ultrasonics (LU) is a method for optical generation and detection of ultrasound. The generation and detection areas can be focused to very small spot sizes, less than 1 mm, which allow velocity measurements to be made over a path length of a few millimeters. The LU technique is non-contact and can be applied remotely; it has many potential applications. The authors used the LU technique to measure the acoustoelastic behavior of an aluminum alloy, A16061-T6.

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

A method for precise measurement of the phase velocity of surface waves on highly dispersive substrates is reported. Lithium niobate covered with aluminum oxide, whose thickness-wavelength ratio was 0.026, was used for the substrate. An approximate value of the velocity was measured using tapped delay lines. The optimum trap numbers of the frequency responses were determined by substituting the approximate value into an equation. The optimum value of the velocity was calculated from the trap numbers and the measured trap frequencies of about 125 MHz. Experimental results for the velocity of about 4015 m/s showed that the optimum trap numbers were correct.

Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors

Optical coherence elastography (OCE) enables quantitative assessment of tissue biomechanical properties by analyzing surface acoustic wave (SAW) velocities. However, current estimation methods require significant expertise and lengthy image processing times, limiting the clinical translation of this promising technology. This study presents a SAW velocity prediction network (SP-Net), a novel end-to-end deep learning model designed for rapid and accurate prediction of SAW velocities from raw OCE phase data of human skin in vivo, addressing the need for rapid and high-accuracy analysis in clinical settings. By using a convolutional-transformer which combines the advantages of local feature extraction and global information processing, SP-Net can extract wave propagation features efficiently, achieving robust performance and high accuracy across diverse tissue types, including tissue-mimicking agar phantoms, healthy human skin and facial acne. The SP-Net has the closest matched predicted velocity compared to the time-of-flight method (10.18 ± 1.27 vs. 10.13 ± 1.53 m/s) in the facial acne case. The experiment results show that SP-Net has the best balance among accuracy, efficiency, and model complexity, significantly improving prediction accuracy by 17% over VP-Net. This method is beneficial for future skin research to allow the real-time tissue biomechanical properties visualization.

A theoretical investigation of the dispersion of leaky Rayleigh waves propagating along one-dimensional (1-D) rough fluid-solid interfaces was carried out by simulating the measurement process of a line-focus beam acoustic microscope. The interface profiles were described in terms of their rms, also known as the roughness of the profile, autocorrelation length, and autocorrelation function. The reflectivity of the interfaces was calculated by using a second-order perturbation approach in the profile roughness. Theoretical V(z) curves were generated and analyzed to yield values of the phase velocity of the Rayleigh waves. The dependence of the Rayleigh wave velocity on the profile and material parameters was examined. Significant variations of the phase velocity were found for values of the roughness which are small compared to the shortest of the wavelengths involved in the scattering. The dispersion relations also showed considerable sensitivity to changes in mechanical properties typical of materials of engineering interest. In the low-frequency range, simulations indicated the dispersion of Rayleigh waves to be rather insensitive to the spectral content of the profile.

The fracture toughness of structural materials are largely influenced by small flaws near the surface such as cracks, pores and inclusions. They had not been extensively investigated so far, because the observation technique has not been established. In this work, we describe two types of new applications of scanning acoustic microscope to the study of fracture and wear. First, detailed features of subsurface cracks in silicon nitride were observed and a new method to estimate the fracture toughness is proposed. It is also proposed that the measurement of attenuation of the leaky surface acoustic wave (leaky SAW) can be used to characterize the surface of steels with various heat treatment.

In recent years, nano-structured thin film systems are often applied in industries such as MEMS/NEMS device, optical coating, semiconductor or like this. Thin films are used for many and varied purpose to provide resistance to abrasion, erosion, corrosion, or high temperature oxidation and also to provide special magnetic or dielectric properties. Quite a number of articles to evaluate the characterization of thin film structure such as film density, film grain size, film elastic properties, and film/substrate interface condition were reported. Among them, the evaluation of film adhesive to substrate has been of great interest. In this study, we fabricated the polymeric thin film system with different adhesive conditions to evaluate the adhesive condition of the thin film. The nano-structured thin film system was fabricated by spin coating method. And then V(z) curve technique was applied to evaluate adhesive condition of the interface by measuring the surface acoustic wave(SAW) velocity by scanning acoustic microscope(SAM). Furthermore, a nano-scratch technique was applied to the systems to obtain correlations between the velocity of the SAW propagating within the system including the interface and the shear adhesive force. The results show a good correlation between the SAW velocities measured by acoustic spectroscope and the critical load measured by the nano-scratch test. Consequently, V(z) curve method showed potentials for characterizing the adhesive conditions at the interface by acoustic microscope.