Yusuke Tsukahara

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.

Excitation of high frequency surface acoustic waves by phase velocity scanning of a laser interference fringe

We present a novel method for generating 100 MHz band surface acoustic wave (SAW) by using a scanning interference fringe at the phase velocity of the SAW. The scanning interference fringe is obtained by intersecting two laser beams with different frequencies, and used as a thermoelastic source. The principle of this method is described, and experimentally demonstrated in the 110 MHz Rayleigh waves on an aluminum specimen generated by a long‐pulse (140 ns) Q‐switched Nd:YAG laser.

Effects of temperature on the multiple cracking progress of sub-micron thick glass films deposited on a polymer substrate

Influence of temperature on the fracture of a SiOx film (PVD) on a PET substrate was investigated. Parallel multiple cracks were observed in situ during tensile tests at different temperatures ranging from 26 to 150°C. Fewer cracks were observed at elevated temperatures. On the other hand, the crack onset strain was not affected by the temperature. The residual strain in the SiOx film remained constant in this temperature range. These observations were successfully predicted by the shear lag model with an assumption of a unique stress criterion.

Ultramultiple roundtrips of surface acoustic wave on sphere realizing innovation of gas sensors

A thin beam of wave usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is naturally collimated, realizing ultramultiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high-performance ball SAW sensors. The advantage of ball SAW is most fully appreciated when applied to a very thin sensitive film for which the multiple-roundtrip enhances the sensitivity, but the attenuation loss is not very large. It is exemplified in a hydrogen gas sensor that realizes a wide sensing range of 10 ppm to 100% for the first time, and realizes relatively fast response time of 20 s without heating the sensitive film.

Statistical analysis of multiple cracking phenomenon of a SiOx thin film on a polymer substrate

The progress of multiple cracking in a silicon oxide (SiOx) film deposited onto a polyethylene terephthalate substrate was analyzed using Monte Carlo simulation. The finite-element analysis, assuming elastoplastic behavior of the polymer substrate, was conducted to calculate the stress distributions in film fragments and was used in the simulation. The Weibull parameters of the film were determined from the scatter of crack onset strain. The simulation predicted successfully the crack density and the distribution of fragment lengths during the progress of multiple cracking. The validity of the shear lag analysis based on the unique stress criterion in a previous study was also evaluated.

Observation of diffraction-free propagation of surface acoustic waves around a homogeneous isotropic solid sphere

This letter shows an unexpected phenomenon where a surface acoustic wave (Rayleigh wave) excited by a line source with a finite length on a solid sphere propagates along the great circle in a direction perpendicular to the line source without beam spreading due to diffraction. In experiments, a piezoelectric transducer, 1.5 mm in width and 20 mm in length, was glued on a surface of a glass ball, 80 mm in diameter, as a line source with a finite length. A beam of Rayleigh waves with frequencies centered at 1.1 MHz was excited in either direction perpendicular to the transducer length. A receiving transducer with a circular aperture, 2 mm in diameter, was used in direct contact with the surface to detect the distribution of vibration over the surface of the ball. It was observed that the excited Rayleigh wave propagated along a great circle of the ball for at least four roundtrips. The beam was confined within a narrow path around the ball, the width of which was no more than 20 mm.

Measurement of acoustic reflection coefficients by an ultrasonic microspectrometer

An ultrasonic microspectrometer (UMSM) was developed in order to evaluate the elastic properties of a solid specimen at a small spot on its surface. In this system, spherical-planar-pair (SPP) lenses were used, by which the acoustic reflection coefficient of a liquid/solid interface was measured as a function of the incident angle in the frequency range from 20 to 140 MHz. Using a specimen of fused quartz whose material constants were well known, the measurement accuracy was examined. The phase velocity of a leaky Rayleigh wave was obtained from the phase change of the reflection coefficient with 0.4% accuracy in this frequency range. For a specimen of steel with a large acoustic attenuation, bulk attenuation factors and their frequency dependence were successfully estimated by computer-fitting of the reflection coefficient. As an example of anisotropic materials, the reflection coefficient of X-cut quartz was also measured. Measured phase of the reflection coefficient was in good agreement with numerical calculation.<<ETX>>

Surface acoustic waves on a sphere with divergent, focusing, and collimating beam shapes excited by an interdigital transducer

The surface acoustic wave (SAW) device on a ball is expected to provide high-performance sensors. Since we must deal with three-dimensional wave fields for precise design of ball SAW devices, numerical methods, such as the finite element method, will require too many elements. Therefore, we applied an analytical solution to a practical calculation of the full-field acoustic waves. For an IDT aperture of 6° and 80° with a wave number parameter of 45, the SAW has a divergent beam and a focusing beam, respectively. For an intermediate aperture of 30°, the SAW forms a collimating beam. This calculation confirmed the possibility of an extremely sensitive sensor with diffraction-free SAWs without disturbance by spurious bulk waves.

Analysis of excitation and coherent amplitude enhancement of surface acoustic waves by the phase velocity scanning method

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocity V is equal to the phase velocity νR of the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocity V, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity νf of the fringes is determined by the intersection angle and the frequency difference ωa of the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency ω depends upon a characteristic time t*, defined as a propagation time of the SAW across the laser beam spot. The ...

Ball SAW device for hydrogen gas sensor

We propose a novel gas sensor using surface acoustic waves (SAW) on a sphere, with a typical velocity change of 10 ppm, and sensitivity of 10 ppb. In the first implementation for hydrogen sensor, a long propagation path of 1.3 m at 41th turns of 45 MHz SAW around a 10-mm-/spl phi/ quartz ball realized the propagation time as long as 403 /spl mu/s. Consequently, a sampling period of 25 ps realized relative temporal resolution of 0.06 ppm. By virtue of this high resolution, when sensing film was as thin as 20 nm, 7 ppm velocity change when exposed to 3% hydrogen in Ar gas was precisely measured. The response time was 60s, shortest among reported SAW hydrogen sensors, due to the small Pd film thickness. We also found a SAW attenuation increase which may be used to enhance the gas selectivity.