Noritaka Nakaso

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.

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.

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.

Angular spectral approach to reflection of focused beams with oblique incidence in spherical-planar-pair lenses

The characteristics of the output signal from spherical-planar-pair (SPP) lenses in the ultrasonic microspectrometer (UMSM) are described. The angular spectral approach is used to obtain the mathematical formula of the output signal. Although isotropic specimens are assumed, anisotropic materials can be treated as well with a small modification. A 2-D approximation is introduced to analyze the specular reflection, the Rayleigh critical angular phenomenon, and V(z) of the specular reflection. Numerical calculations are also performed for specimens with copper substrate, steel substrate, and layered structures, by numerically integrating the 2-D formulas of the output signal. It is found that a ray optical treatment is a good approximation of the wave propagation in the SPP lenses. The incident angular dependence of a reflection coefficient can be estimated by tilting the sensor unit.<<ETX>>

Surface Acoustic Waves on a Sphere —Analysis of Propagation Using Laser Ultrasonics—

Using the laser ultrasonic method, we investigated the distribution of surface acoustic waves (SAWs) on a sphere excited by a line source. Experimental results revealed that the SAWs were confined within a narrow path due to the balance of the diffraction and convergence effects. Moreover, the distribution had unique divergent, collimating and focussing forms, which had been predicted theoretically. These results were generally in good agreement with the results calculated using the approximate Greens function. It was shown that the distribution of SAWs is well defined by the half-aperture angle of the source and by the product of the wavenumber and the radius of the sphere.

Evaluation of Response Time in Ball Surface-Acoustic-Wave Hydrogen Sensor using Digital Quadrature Detector

Hydrogen leak detection sensors must have high sensitivity and a short response time of 1 s or less. A ball surface-acoustic-wave (SAW) hydrogen sensor has a high sensitivity and can detect hydrogen in a very wide concentration range of 10 ppm to 100%. Moreover, a fast response can be expected because of the very thin sensitive film used. In this study, we developed a digital quadrature detector (DQD) to measure responses of less than 1 s, and measure phases in 1 ms intervals with excellent sensitivity. We evaluated the response time of the ball SAW hydrogen sensor where the signal was averaged 256 times in 0.256 s using the DQD. As a result, the response time was found to be 1 s or less for 3.0 vol % hydrogen gas in nitrogen.

Temperature Compensation for Ball Surface Acoustic Wave Devices and Sensor Using Frequency Dispersion

We propose a new temperature compensation method for ball surface acoustic wave (SAW) devices using frequency dispersion. This method distinguishes the temperature effect independent of frequency and surface effects such as mass loading or elastic effects linearly dependent on frequency. After stating the principle of the method, we verify it by fabricating ball SAW devices with an interdigital transducer (IDT) that works at two frequencies, and apply them to the measurement of the coating of albumin on the surface of the device and on a hydrogen gas sensor with a surface-coated sensing film. Delay time measurements are carried out at two frequencies, and the difference in delay time is calculated to eliminate temperature-independent effects. Because the results clearly show surface effects without temperature disturbance, it is a difficult to realize sensors using ball SAW devices.

Analysis of spurious bulk waves in ball surface wave device

We analyzed the acoustic waves propagating in a sphere to establish a useful guideline for the design of NDE apparatus and ball surface acoustic wave (SAW) device exploiting the diffraction-free propagation of SAW on a sphere. First, we calculated the laser-generated acoustic displacements both under ablation condition and under thermoelastic condition and verified experimentally the validity of the calculation. Next, the acoustic waves excited by out-of-plane stress and those excited by in-plane stress were compared. The results showed that when the out-of-plane stress was applied, the relative amplitudes of the bulk waves to that of the SAW were larger and the number of bulk waves was larger than that when the in-plane stress was applied, while the SAW had similar waveforms in each case. The ratio of the relative amplitude of the bulk waves for the out-of-plane stress and the in-plane stress was 3.1:1 at phi(1)=90 degrees and 1.67:1 at phi(1)=0 degrees. The large amplitude for the out-of-plane stress can be explained by wide directivities of bulk waves. Consequently, we found that it is necessary for ball SAW device to select a piezoelectric material and form of interdigital transducer so that the in-plane stress becomes dominant.